9789241596350_eng_opt.pdf

Laboratory Equipmentfor


MAINTENANCE
Manual


2nd Edit ion




WHO Library Cataloguing-in-Publication Data


Maintenance manual for laboratory equipment, 2nd ed.
1.Laboratory equipment. 2.Maintenance. 3.Manuals. I.World Health Organization. II.Pan American Health Organization.
ISBN 978 92 4 159635 0 (NLM classifi cation: WX 147)


© World Health Organization 2008
All rights reserved. Publications of the World Health Organization can be obtained from WHO Press, World Health Organization, 20 Avenue Appia, 1211 Geneva 27, Switzerland
(tel.: +41 22 791 3264; fax: +41 22 791 4857; e-mail: bookorders@who.int). Requests for permission to reproduce or translate WHO publications – whether for sale or for
noncommercial distribution – should be addressed to WHO Press, at the above address (fax: +41 22 791 4806; e-mail: permissions@who.int).


The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of the World Health
Organization concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted lines on
maps represent approximate border lines for which there may not yet be full agreement.


The mention of specifi c companies or of certain manufacturers’ products does not imply that they are endorsed or recommended by the World Health Organization in
preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital
letters.
All reasonable precautions have been taken by the World Health Organization to verify the information contained in this publication. However, the published material is being
distributed without warranty of any kind, either expressed or implied. The responsibility for the interpretation and use of the material lies with the reader. In no event shall the
World Health Organization be liable for damages arising from its use.


Design and Layout: L’IV Com Sàrl, Morges Switzerland


Printed in Spain


Contact:
Dr G. Vercauteren, Coordinator, Diagnostics and Laboratory Technology, Department of Essential Health Technologies, World Health Organization, 20 Avenue Appia, 1211 Geneva
2, Switzerland


This document is available at www.who.int/diagnostics_laboratory




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


iii


TABLE OF FIGURES viii


ACKNOWLEDGEMENTS x


INTRODUCTION xi


CHAPTER 1 • MICROPLATE READER 1
Photograph of microplate reader 1
Purpose of the microplate reader 1
Operation principles 1
Installation requirements 3
Routine maintenance 3
Troubleshooting table 4
Basic defi nitions 5


CHAPTER 2 • MICROPLATE WASHER 7
Photograph of microplate washer 7
Purpose of the microplate washer 7
Operation principles 7
Installation requirements 9
Routine maintenance 9
Troubleshooting table 11
Basic defi nitions 12


CHAPTER 3 • pH METER 13
Purpose of the equipment 13
Photograph and components of the pH meter 13
Operation principles 13
pH meter components 14
Typical circuit 15
Installation requirements 16
General calibration procedure 16
General maintenance of the pH meter 17
Basic maintenance of the electrode 18
Troubleshooting table 18
Basic defi nitions 19
Annex: The pH theory 20


Table of Contents




TA B L E O F C O N T E N T S


iv


CHAPTER 4 • BALANCES 21
Photographs of balances 21
Purpose of the balance 22
Operation principles 22
Installation requirements 26
Routine maintenance 27
Troubleshooting table 28
Basic defi nitions 29


CHAPTER 5 • WATER BATH 31
Diagram of a water bath 31
Operation principles 31
Water bath controls 32
Water bath operation 32
Troubleshooting table 34
Basic defi nitions 34


CHAPTER 6 • BIOLOGICAL SAFETY CABINET 35
Illustration of a biological safety cabinet 35
Purposes of the equipment 35
Operation principles 35
Biological safety 39
Installation requirements 39
Using the safety cabinet 39
Routine maintenance 40
Functional evaluation (alternative) 41
Table of functional evaluation of biological safety cabinets 42
Troubleshooting table 43
Basic defi nitions 44


CHAPTER 7 • CENTRIFUGE 45
Photograph of centrifuge 45
Purpose of the centrifuge 45
Operation principles 45
Components of the centrifuge 46
Installation requirements 48
Routine maintenance 48
Appropriate management and storage recommendations 48
Troubleshooting table 50
Basic defi nitions 52


CHAPTER 8 • WATER DISTILLER 53
Diagram of a water distiller 53
Purpose of the water distiller 53
Operation principles 54
Installation requirements 54
Routine maintenance 55
Troubleshooting table 56
Basic defi nitions 57




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v


CHAPTER 9 • DILUTOR 59
Diagram of a dilutor 59
Purpose of the dilutor 59
Operation principles 60
Installation requirements 61
Routine maintenance 61
Troubleshooting table 63
Basic defi nitions 64


CHAPTER 10 • DISPENSER 65
Photograph and diagram of the dispenser 65
Purpose of the dispenser 65
Requirements for operation 67
Routine maintenance 67
Troubleshooting table 68
Basic defi nitions 68


CHAPTER 11 • SPECTROPHOTOMETER 69
Photograph of spectrophotometer 69
Purpose of the equipment 69
Operation principles 69
Spectrophotometer components 72
Installation requirements 73
Spectrophotometer maintenance 73
Good practices when using the spectrophotometer 75
Troubleshooting table 77
Basic defi nitions 79


CHAPTER 12 • AUTOCLAVE 81
Photograph of the autoclave 81
Purpose of the autoclave 81
Operation principles 82
Operation of the autoclave 84
Installation requirements 87
Routine maintenance 88
Maintenance of specialized components 90
Troubleshooting table 91
Basic defi nitions 92


CHAPTER 13 • DRYING OVEN 93
Photograph of drying oven 93
Purpose of the oven 93
Operating principles 93
Installation requirements 94
Oven operation 94
Oven controls 95
Quality control 96
Routine maintenance 96
Troubleshooting table 97
Basic defi nitions 98




TA B L E O F C O N T E N T S


vi


CHAPTER 14 • INCUBATOR 99
Photograph of incubator 99
Operating principles 99
Incubator controls 101
Installation requirements 101
Routine maintenance and use of the incubator 101
Troubleshooting table 103
Basic defi nitions 104


CHAPTER 15 • MICROSCOPE 105
Photographs of microscopes 105
Purpose of the equipment 106
Operation principles 106
Installation requirements 108
Description of potential problems with microscopes 109
General maintenance of the microscope 111
Troubleshooting table 115
Basic defi nitions 116


CHAPTER 16 • PIPETTES 119
Photographs of pipettes 119
Purpose of the pipettes 120
Operation principles of the pipette 120
Requirements for use 120
Using the pipette 121
Routine maintenance 122
Troubleshooting table 125
Basic defi nitions 126


CHAPTER 17 • STIRRING HEATING PLATE 127
Photograph of the stirring heating plate 127
Operation principles 127
Controls of the stirring heating plate 127
Installation requirements 128
Operation of the stirring heating plate 128
Routine maintenance 128
Troubleshooting table 129
Basic defi nitions 129


CHAPTER 18 • REFRIGERATORS AND FREEZERS 131
Photograph of a refrigerated storage unit 131
Purpose of refrigerated storage units 132
Operation principles 132
Installation requirements 133
Refrigerator control circuit 134
Refrigerator operation 134
Refrigerator routine maintenance 135
Troubleshooting table 137




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vii


Operation of ultralow freezers 138
Turning the unit on 138
Routine maintenance 139
Troubleshooting table 140
Basic defi nitions 141


CHAPTER 19 • CHEMISTRY ANALYSERS 143
Photographs of chemistry analysers 143
Purpose of chemistry analysers 144
Operation principle 144
Components 144
Installation requirements 145
Operation of the dry chemistry analyser 145
Operation of the wet chemistry analyser 146
Routine maintenance of chemistry analysers 146
Non-routine maintenance and troubleshooting 147
Troubleshooting table 148
Basic defi nitions 148


CHAPTER 20 • COLORIMETERS 149
Photograph of colorimeter 149
Purpose of the colorimeter 149
Operating principle 149
Components 150
Installation requirements 150
Operation of the colorimeter 150
Operation of the haemoglobinometer 151
Routine maintenance 151
Troubleshooting table 154
Basic defi nitions 155


BIBLIOGRAPHY 157




TA B L E O F F I G U R E S


viii


Table of Figures


Figure 1 Equipment used for ELISA tests 2
Figure 2 Microplate washer 8
Figure 3 Well profi les 8
Figure 4 Diagram of a pH meter 14
Figure 5 Types of electrodes 15
Figure 6 Example of a typical pH meter control circuit 15
Figure 7 Spring balance 22
Figure 8 Sliding weight scale 22
Figure 9 Analytical balance 22
Figure 10 Upper plate balance 23
Figure 11 Substitution balance 23
Figure 12 Components of the electronic balance 24
Figure 13 Compensation force principle 24
Figure 14 Classifi cation of balances by exactitude 25
Figure 15 Analytical balance control panel 26
Figure 16 Water bath 31
Figure 17 Immersion and external resistors 31
Figure 18 Water bath controls 32
Figure 19 Biological safety cabinet 35
Figure 20 Centrifugal force concept 46
Figure 21 Water distiller 53
Figure 22 Dilutor diagram 59
Figure 23 Dilutor controls 60
Figure 24 Syringe and dispenser 61
Figure 25 Dispenser 65
Figure 26 Dispenser and accessories 66
Figure 27 Interaction of light with matter 70
Figure 28 Absorbance phenomenon 71
Figure 29 Spectrophotometer components 72
Figure 30 Refraction of light 79
Figure 31 Diff raction grid 80
Figure 32 Vapour circuit of an autoclave 83
Figure 33 Space required for autoclave 87
Figure 34 Compressed air connection 87
Figure 35 Vapour connection 88
Figure 36 Vapour generator 89
Figure 37 Electronic control of the oven 95
Figure 38 Electrical circuit of the oven 95
Figure 39 Heat transfer systems used in incubators 100




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ix


Figure 40 Incubator controls 101
Figure 41 Positive (convergent) lens 106
Figure 42 Optics of the convergent lens 106
Figure 43 Diagram of a microscope 107
Figure 44 Cross-section of a microscope 108
Figure 45 Binocular head 109
Figure 46 Lighting system 109
Figure 47 Platform, plate or mechanical stage 110
Figure 48 Revolving, objective holder 110
Figure 49 Body of the microscope 111
Figure 50 Diagram of a pipette 119
Figure 51 Types of pipettes 120
Figure 52 Phases of pipette use 121
Figure 53 Disassembly of a pipette 123
Figure 54 Stirring heating plate controls 127
Figure 55 Induction motor 129
Figure 56 Refrigeration circuit 132
Figure 57 Control circuit of the refrigerator 134
Figure 58 Blood bank refrigerator controls 135
Figure 59 Ultralow temperature freezer control 138
Figure 60 Basic diagram of refl ectance photometry on a test strip 144
Figure 61 Ulbricht’s sphere 145
Figure 62 Basic components of a photometer 145
Figure 63 Controls of a portable colorimeter 150




P R E FA C E


x


Acknowledgements


This manual is a revised edition of “Manual de mantenimiento para equipo de laboratorio” (PAHO, 2005) translated from
Spanish into English. Revisions include additional chapters on laboratory equipment commonly used in some laboratories
and updates allowing global use of the manual.


The revised version has been prepared under the direction of Dr Gaby Vercauteren, World Health Organization, Geneva,
Switzerland and in coordination with Dr Jean-Marc Gabastou, Pan-American Health Organization/World Health Organization,
Washington, DC, USA; translated by Ms Christine Philips; reviewed by Ms Mercedes Pérez González and adapted, revised and
edited by Mrs Isabelle Prud’homme.


WHO kindly expresses thanks to those who have participated at all levels in the elaboration of this manual. WHO wishes to
acknowledge the original contribution of Dr Jorge Enrique Villamil who wrote the fi rst edition of this manual in 2005 (Manual
para mantenimiento de equipo de laboratorio, ISBN 92 75 32590 1) and Dr Jean-Marc Gabastou and Mr Antonio Hernández,
Reviewers at Essential Medicines Vaccines and Health Technologies at PAHO.


WHO also thanks manufacturers who have granted permission to use their images in this publication.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


xi


Introduction


This manual has been developed to support personnel employed in health laboratories. Its purpose is to give a better
understanding of the technical requirements regarding installation, use and maintenance of various types of equipment which
play an important role in performing diagnostic testing. The manual also aims to provide support to personnel responsible
for technical management, implementation of quality management and maintenance.


Due to the diversity of origins, brands and models, this manual off ers general recommendations. Equipment-specifi c details
are explained in depth in the maintenance and installation user manuals from manufacturers. These should be requested
and ordered through the procurement processes of the individual agencies and professionals responsible for the acquisition
of technology, or directly from the manufacturer.


This manual was originally developed by the Pan-American Health Organization (PAHO) to support improved quality
programmes which PAHO promotes in regional laboratories. The English version was produced by WHO to further expand
support for quality programmes in other regions. The revised edition now includes 20 equipment groups selected to cover
those most commonly used in low to medium technical complexity laboratories across the world. Given the diff erences in
technical complexity, brands and existing models, each chapter has been developed with basic equipment in mind, including
new technology where relevant. The following information is included in each chapter:
• Groups of equipment, organized by their generic names. Alternative names have also been included.
• Photographs or diagrams, or a combination of both to identify the type of equipment under consideration.
• A brief explanation on the main uses or applications of the equipment in the laboratory.
• A basic description of the principles by which the equipment operates with explanations of principles or physical and/or


chemical laws which the interested reader can – or should study in depth.
• Installation requirements with emphasis on the electrical aspects and the requirements for safe installation and operation,


including worldwide electrical standards.
• Basic routine maintenance, classifi ed according to the required frequency (daily, weekly, monthly, quarterly, annually


or sporadically). The procedures are numbered and presented in the actual sequence in which these should take place
(model-specifi c procedures can be found in the manuals published by the manufacturers).


• Troubleshooting tables with the most frequent problems aff ecting the equipment with possible causes and actions that
may resolve these problems.


• A list of basic defi nitions of some of the specialized terms used.
• For some equipment, additional themes related to calibration, quality control and design (with operational controls).


This information, along with good use and care, helps to maintain laboratory equipment in optimal condition.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


1


The microplate reader also known as “Photometric
micro-plate reader or ELISA reader” is a specialized
spectrophotometer designed to read results of the ELISA
test, a technique used to determine the presence of
antibodies or specifi c antigens in samples. The technique
is based on the detection of an antigen or antibodies
captured on a solid surface using direct or secondary,
labelled antibodies, producing a reaction whose product
can be read by the spectrophotometer. The word ELISA is
the acronym for “Enzyme-Linked Immunosorbent Assay”.
This chapter covers the use of microplate readers for ELISA
testing. For additional information on the instrument
principles of operation and maintenance, consult Chapter
11 discussing the spectrophotometer.


PHOTOGRAPH OF MICROPLATE READER


PURPOSE OF THE MICROPLATE READER
The microplate reader is used for reading the results of ELISA
tests. This technique has a direct application in immunology
and serology. Among other applications it confi rms the
presence of antibodies or antigens of an infectious agent in
an organism, antibodies from a vaccine or auto-antibodies,
for example in rheumatoid arthritis.


OPERATION PRINCIPLES
The microplate reader is a specialized spectrophotometer.
Unlike the conventional spectrophotometer which facilitates
readings on a wide range of wavelengths, the microplate
reader has filters or diffraction gratings that limit the
wavelength range to that used in ELISA, generally between
400 to 750 nm (nanometres). Some readers operate in the
ultraviolet range and carry out analyses between 340 to 700
nm. The optical system exploited by many manufacturers
uses optic fi bres to supply light to the microplate wells
containing the samples. The light beam, passing through
the sample has a diameter ranging between 1 to 3 mm.
A detection system detects the light coming from the
sample, amplifi es the signal and determines the sample’s
absorbance. A reading system converts it into data allowing
the test result interpretation. Some microplate readers use
double beam light systems.


Test samples are located in specially designed plates with
a specifi c number of wells where the procedure or test is
carried out. Plates of 8 columns by 12 rows with a total of
96 wells are common. There are also plates with a greater
number of wells. For specialized applications, the current
trend is to increase the number of wells (384-well plates)
to reduce the amount of reagents and samples used and a
greater throughput. The location of the optical sensors of the
microplate reader varies depending on the manufacturers:
these can be located above the sample plate, or directly
underneath the plate’s wells.


Nowadays microplate readers have controls regulated by
microprocessors; connection interfaces to information
systems; quality and process control programs, which by
means of a computer, allow complete test automation.


Chapter 1


Microplate Reader
GMDN Code 37036


ECRI Code 16-979


Denomination Photometric micro-plate reader


Ph
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C H A P T E R 1 M I C R O P L AT E R E A D E R


2


Equipment required for ELISA testing
In order to perform the ELISA technique, the following
equipment is required:
1. Microplate reader.
2. Microplate washer (Chapter 2).
3. Liquid dispensing system (multi-channel pipettes may


be used).
4. Incubator to incubate the plates.


Figure 1 illustrates how this equipment is interrelated.


Mechanical phases of the ELISA technique
Using the equipment
When an ELISA test is conducted, it typically follows these
steps:
1. A fi rst washing of the plate may be done using the


microplate washer.
2. Using a liquid dispenser or the multi-channel pipettes,


wells are fi lled with the solution prepared to be used in
the test.


3. The plate is placed in the incubator where at a controlled
temperature, a series of reactions take place.


Stages 1, 2 and 3 can be repeated several times depending
on the test, until the reagents added have completed their
reactions.


Finally, when all the incubation steps have been completed,
the plate is transferred to the microplate reader. The reading
of the plate is done and a diagnosis can be deduced.


Biochemical phases of the ELISA technique1


The ELISA technique from a biochemical point of view:
1. The plate wells are coated with antibodies or antigens.
2. Samples, controls and standards are added to the wells


and incubated at temperatures ranging between room
temperature and 37 °C for a determined period of
time, according to the test’s characteristics. During the
incubation, the sample’s antigen binds to the antibody
coated to the plate; or the antibody in the sample binds
to the antigen coated on the plate, according to their
presence and quantity in the sample analyzed.


3. After incubation, the unbound antigen or antibodies are
washed and removed from the plate by the microplate
washer using an appropriate washing buff er.


4. Next, a secondary antibody, called the conjugate, is
added. This harbours an enzyme which will react with
a substrate to produce a change of colour at a later
step.


5. Then begins a second period of incubation during
which this conjugate will bind to the antigen-antibody
complex in the wells.


6. After the incubation, a new washing cycle is done to
remove unbound conjugate from the wells.


7. A substrate is added. The enzyme reacts with the
substrate and causes the solution to change in colour.
This will indicate how much antigen-antibody complex
is present at the end of the test.


8. Once the incubation time is completed, a reagent
is added to stop the enzyme-substrate reaction and
to prevent further changes in colour. This reagent is
generally a diluted acid.


9. Finally, the plate in is read by the microplate. The
resulting values are used to determine the specific
amounts or the presence of antigens or antibodies in
the sample.


Note: Some of the wells are used for
standards and controls. Standards allow
the cut-off points to be defi ned. The
standards and controls are of known
quantities and are used for measuring
the success of the test, evaluating data
against known concentrations for each
control. The process described above
is common, although there are many
ELISA tests with test-specifi c variants.


1 More detailed explanations must be consulted in
specialized literature.








Dispensing
System


ELISA Plate
Washer


Incubator


ELISA
Reader


Computer


Figure 1. Equipment used in ELISA tests




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


3


INSTALLATION REQUIREMENTS
In order for the microplate reader to operate correctly, the
following points need to be respected:
1. A clean, dust free environment.
2. A stable work table away from equipment that vibrates


(centrifuges, agitators). It should be of a suitable size so
that there is working space at the side of the microplate
reader. The required complementary equipment for
conducting the technique described above is: washer,
incubator, dispenser and computer with its peripheral
attachments.


3. An electrical supply source, which complies with the
country’s norms and standards. In the countries of the
Americas for example, 110 V and 60 Hertz frequencies
are generally used, whereas other regions of the World
use 220-240V, 50/60HZ.


Calibration of the microplate reader
The calibration of a microplate reader is a specialized process
which must be executed by a technician or trained engineer
following the instructions provided by each manufacturer.
In order to do the calibration, it is necessary to have a set
of grey fi lters mounted on a plate of equal geometric size
to those used in the analyses. Manufacturers provide these
calibration plates for any wavelength the equipment uses.


Calibration plates are equipped with at least three pre-
established optic density values within the measurement
ranges; low, medium, and high value. In order to perform
the calibration, follow this process:
1. Place the calibration plate on the equipment.
2. Carry out a complete reading with the calibration plate.


Verify if there are diff erences in the readings obtained
from well to well. If this is the case, invert the plate (180°)
and repeat the reading to rule out that diff erences are
attributed to the plate itself. In general, it is accepted
that the instrument does not need further calibration if
the plate results are as expected at two wavelengths.


3. Verify if the reader requires calibration. If so, proceed
with the calibration following the routine outlined by
the manufacturer, verifying that the reading’s linearity
is maintained as rigorously as possible.


4. If the instrument does not have a calibration plate, verify
it by placing a coloured solution in the wells of a plate
and immediately carry out a complete reading. Then
invert the plate 180° and read the plate again. If both
readings display identical, average values in each row,
the reader is calibrated.


5. Verify that the reader is calibrated, column by column.
Place a clean, empty plate and carry out a reading. If
there is no diff erence between each of the average
reading of the fi rst to the last column, it can be assumed
that the reader is calibrated.


ROUTINE MAINTENANCE
Maintenance described next focuses exclusively on the
microplate reader. The maintenance of the microplate
washer is described in Chapter 2.


Basic maintenance
Frequency: Daily
1. Review that optical sensors of each channel are clean.


If dirt is detected, clean the surface of the windows of
the light emitters and the sensors with a small brush.


2. Confi rm that the lighting system is clean.
3. Verify that the reader’s calibration is adequate. When


the daily operations begin, let the reader warm up for
30 minutes. Next, do a blank reading and then read a
full plate of substrate. The readings must be identical.
If not, invert the plate and repeat the reading in order
to determine if the deviation originated in the plate or
the reader.


4. Examine the automatic drawer sliding system. It must
be smooth and constant.


Preventive maintenance
Frequency: Quarterly
1. Verify the stability of the lamp. Use the calibration plate,


conducting readings with intervals of 30 minutes with
the same plate. Compare readings. There must be no
diff erences.


2. Clean the detectors’ optical systems and the lighting
systems.


3. Clean the plate drawer.
4. Verify the alignment of each well with the light emission


and detection systems.




C H A P T E R 1 M I C R O P L AT E R E A D E R


4


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The reader gives a reading that does not make sense. The illumination lamp is out of service. Replace the lamp with one with the same
characteristics as the original.


The reader’s readings vary from row to row. Dirty optical sensors. Clean the sensors.


The illumination system’s lenses or parts are dirty. Clean the lighting system’s lenses.


Lack of calibration in one or more channels. Verify the calibration of each one of the channels.


The reader displays high absorbance values. Reagents expired and/or incorrectly prepared. Check to see if the TMB is colourless and the
preparation adequate.


Contamination with other samples. Repeat the test verifying the labelling, the washer
and how the pipette was used.


Incorrect wavelength fi lter. Verify the recommended wavelength for the test.
Adjust if it is incorrect.


Insuffi cient or ineffi cient washing. Verify the washing method used. Use an appropriate
quality control test.


Very long incubation time or very high temperature. Check incubation times and temperatures.


Incorrect sample dilution. Check process for sample dilution.


Some reagent was omitted. Verify that the test has been carried out according to
the established procedure.


The reader displays low absorbance values. Very short incubation time and very low
temperature.


Check temperatures and incubation times.


The reagents were not at room temperature. Check that the reagents are stable at room
temperature.


Excessive washing of the plate. Adjust the washing process to what the test
manufacturers indicate.


Incorrect wavelength fi lter. Verify the wavelength selected. Use wavelength
recommended for the test.


Expired or incorrectly prepared reagents. Check the used reagents. Test the dilutions.


A reagent was omitted. Verify that the test was done according to the
established procedure.


The plate displays scratches at the bottom of the
wells.


Prepare a new plate and repeat the test.


Incorrectly selected or dirty plate. Verify the type of plate used. Prepare a new plate
and repeat the test.


The plate wells have dried up. Change the manner in which the plate is washed.


The plate is incorrectly placed or is seated unevenly
in the reader.


Check the placement of the plate. Repeat the
reading.


Humidity or fi ngerprints on the outer part of the
bottom of the plate.


Verify that the plate under the bottom of the wells
is clean.


Residual quantities of washing buff er in the wells
before adding the substrate.


Confi rm that the washing buff er is completely
removed.


The substrate tablets do not dissolve completely. Verify that the tablets dissolve correctly.


The substrate tablet has been contaminated by
humidity or metal clips or is not complete.


Test the integrity and handling of substrate tablets.


The position of the blank well could have been
changed and an incorrect quantity has been
subtracted at each reading.


Verify that the plate set-up is correct.


The reader displays unexpected variation in the
optical density readings.


The reader’s lamp is unstable. Replace the lamp with one that has similar
characteristics as the original.


The reader displays a gradual increase or decrease
from column to column.


Inappropriate calibration of the plate’s advance
motor.


Calibrate the advance so that at each step the wells
remain exactly aligned with the lighting system.


The optical density readings are very low compared
to the operator’s optical evaluation criteria.


The reading is being carried out with a diff erent
wavelength than required for the test.


Verify the wavelength used when conducting
the reading. If this is the problem, adjust the
wavelength and repeat the reading. Verify that the
recommended wavelength fi lter has been selected.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


5


Low reproducibility. Sample homogeneity. Mix the reagents before use. Allow these to
equilibrate to room temperature.


Incorrect pipetting procedure. Ensure pipette’s tips are changed between samples
and that excessive liquid inside is removed.


Reader not calibrated. Check the calibration. Use an appropriate quality
control set.


instrument.
Wait until the reader has warmed up to its operating
temperature.


Expired reagents. Verify the expiry dates of the reagents.


when washed.


The data are not transferred from the reader to the
microprocessor.


Verify selected codes.


transmission plugs.
manufacturer.


Misaligned light beam. The reader was transferred or moved without using
the necessary precautions.


Call the specialized service technician.


The light source – lamp – has been changed and
the replacement has not been installed or aligned
correctly.


Verify its assembly and alignment.


The plate was incorrectly loaded.
reading carrying out the adjustments.


the reader. reading carrying out the adjustments.


Computer fails to indicate the error codes. The programme which controls the activation of


not validated by the manufacturer.


Call the specialized service technician.


The blank sample shows high absorbance. Contaminated substrate. Check that TMB is colourless and its preparation.


The reader demonstrates failure in detecting errors. Various components of the system display failure,
such as the liquid level detection system.


Call the specialized service technician.


BASIC DEFINITIONS


Chemiluminescence. Emission of light or luminescence resulting directly from a chemical reaction at environmental temperatures.


ELISA (Enzyme-Linked Immunosorbent Assay). Biochemical technique used mainly in Immunology to detect the presence of an antibody or an antigen in a
sample.


ELISA plate. Consumable standardized to carry out the ELISA technique. Generally, plates have 96 wells in a typical confi guration of 8 rows by 12 columns. There
are also ELISA plates with 384 wells or up to 1536 wells for specialized high throughput testing in centres with high demand.


Microplate washer. Equipment used for washing plates during specifi c stages of an ELISA test with the aim of removing unbound components during reactions.
Microplate washers use special buff ers in the washing process.


Enzyme. Protein that accelerates (catalyses) chemical reactions.


Fluorophore. Molecules absorbing light at a determined wavelength and emitting it at a higher wavelength.


Microplate reader. The name given to spectrophotometers with the capacity to read microplates.


TMB. Tetramethylbenzidine, a substrate for the horseradish peroxidase (HRP) enzyme.


alarms and warning messages is defective or is




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


7


Chapter 2


Microplate Washer


The microplate washer or “plate or ELISA washer” is designed to
perform washing operations required in the ELISA technique.
The microplate washer performs the washing of the ELISA
plate’s wells during the diff erent stages of the technique.


PHOTOGRAPH OF MICROPLATE WASHER


PURPOSE OF THE MICROPLATE WASHER
The microplate washer has been designed to supply cleaning
buffers required for the ELISA technique in a controlled
manner. In the same fashion, the equipment removes from
each well, substances in excess from the reaction. Depending
on the test performed, the washer can intervene from one
to four times, supplying the washing buff er, agitating and
removing the unbound reagents1 until the programmed
times and cycles are completed. The washer has of two
reservoirs; one for the washing buff er, the other for the waste
generated during the washing process.


OPERATION PRINCIPLES
The microplate washer has been designed to perform
washing operations in the ELISA technique. The equipment
possesses at least, the following subsystems which vary
depending on the manufacturer’s design.
• Control subsystem. Generally, the washer is controlled


by microprocessors allowing programming and
controlling steps to be performed by the washer such
as: number of washing cycles2 (1–5); expected times;
supplying and extracting pressures; plate format
(96–384 wells); suction function adjustment according
to the type of well3 (fl at bottom, V bottom or rounded
bottom or strips used); volumes distributed or aspirated;
the soaking and agitation cycles, etc.


• Supply subsystem. In general, this comprises a reservoir
for the washing solution; one or several pumps; usually
a positive displacement type syringe and a dispenser
head that supplies the washing solution to the diff erent
wells by means of needles. The head usually comes
with eight pairs of needles for washing and aspirating
simultaneously the wells of the same row (the supply
and extraction sub-systems converge on the head).
There are models with twelve pairs of needles and others
that conduct the washing process simultaneously in all
the wells. Some washers off er the possibility of working
with diff erent types of washing solutions, performing
the solution changes according to the program entered
by the operator.


1 See a brief explication of the ELISA technique in Chapter 1, Microplate
Reader.


2 The exact number of washing operations required depends on the assay
used. This is explained in each manufacturer’s test instruction manual.


3 If the bottom is fl at, the suction needle is located very close to one of well’s
faces; if it is rounded or V-shaped, the suction needle is centered.


GMDN Code 17489


ECRI Code 17-489


Denomination Micro-plate washer


Ph
ot


o
co


ur
te


sy
o


f B
io


Ra
d


La
b


or
at


or
ie


s




C H A P T E R 2 M I C R O P L AT E WA S H E R


8


• Extraction or suction system. This requires a vacuum
mechanism and a storage system for gathering the fl uids
and waste removed from the wells. The vacuum may be
supplied by external and internal pumps. Extraction is
done by a set of needles mounted on the washer/dryer’s
head. The number of needles varies from one to three,
according to the washer model used.


If it uses only one needle, the washing and
extraction operation is done with this single needle.
If it uses two needles, one is used for supplying the
washing solution and the other for extraction. If it uses
three needles, the fi rst is used for supplying the washing
solution, the second for extraction and the third for
controlling (extracting) any excess volume in the well.
Generally, the extraction needle is longer than the
supply needle, which enables it to advance (vertically)
up to a height ranging between 0.3 and 0.5 mm from
the bottom of the well.


• Advance sub-system. This is composed of a
mechanism which moves the supply and extraction
head horizontally to reach each well in the ELISA plate.
When the horizontal movement to the following row
occurs, there is a vertical movement towards the well
to dispense or extract the washing solution. There
are washers which carry out these operations in a
simultaneous manner.


The sub-systems previously described are shown in Figure
2. Figure 3 shows the diff erent types of wells most commonly
found in microplates. Each kind of well is suitable for a
particular type of test.


Washing process
The washing of the microplate is one of the stages of the
ELISA technique. Special solutions are used in the washing
steps. Among those most commonly used is phosphate
buff er solution or PBS. The phosphate buff er solution has a
stability of 2 months if kept at 4 °C. It is estimated that 1 to
3 litres of solution is required for washing one microplate
and that 300 µl is used in each well per cycle. Washing can
be done manually, but it is preferable to use an automated
microplate washer for a better throughput and to minimize
handling of potentially contaminated substances.


Among the washing processes used by microplate washers
are featured:
• Aspiration from top to bottom. When the aspiration


phase is initiated, the needles move vertically and the
aspiration is initiated immediately as these enter into
the liquid. The process continues until the needles
reach their lowest position very close to the bottom
of the wells. At this point they are stopped in order to
avoid suctioning the air that should fl ow against the
interior lateral walls of the wells. This type of aspiration
prevents air currents from drying the bound protein on
the surface of the wells.


Waste Receptacle


Extraction Pump


Supply and
Extraction Head


Horizontal and
Vertical
Displacement


Wells
ELISA Plate


Washing
Solution


Supply Pumps


Positive Displacement Pumps


Figure 2. Microplate washer


Flat
Bottom


Round
Bottom


V-shaped
Bottom


Easy
Wash


Figure 3. Well profi les




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


9


• Simultaneous distribution and aspiration. In certain
types of washer, the washing and aspiration systems
operate simultaneously, generating a controlled
turbulence inside the well which removes the unbound
substances during the incubations.


• Aspiration from the base of the wells. In this system,
the aspiration of the fluid contained in the wells is
performed initially with the aspiration needles in a
position very close to the bottom, immediately
beginning a suctioning cycle, usually time-controlled.
This system may aspirate air if there are diff erences in
the levels of the tanks.


Washer calibration
The microplate washer is critical for guaranteeing that the
ELISA technique performs as expected. The alignment to
be taken into account for the eff ective functioning of the
equipment is presented next:
• Position of the needles (supply and aspiration head).


The horizontal and vertical position adjustment with
respect to the wells must be verifi ed carefully. If the
plate has fl at bottom wells, the supply needle must be
checked to see that it is situated very close to the well’s
wall. If the bottom is round or V-shaped, the suction
needle should be located in the centre of the well:
upon the vertical movement, a needle-base distance
is maintained in the well, usually between 0.3 to 0.5
mm. The needles must never be allowed to touch the
bottom of the wells to avoid mechanical interferences
between the needle point and the well’s base during
the aspiration function.


• Aspiration time. Appropriately adjust the aspiration
time so that a solution fi lm adhered to the well’s wall
can fl ow towards the bottom. Avoid very long time
lapses to prevent the coating on the wells from drying
up. Check that the suction system’s needles are clean
(free of obstructions).


• Distributed Volume. Check that the volume distributed
is as close as possible to the maximum capacity of the
well; confi rm that all the wells are fi lled uniformly (at
the same level). Verify that the distributing needles are
clean (free of obstructions).


• Vacuum. The suctioning system must be calibrated
effi ciently. If the vacuum is too strong, the test can
be altered. In fact, it could dry out the wells and
considerably weaken the enzyme activity in the wells
and completely alter the test result. The majority of
washers function with a vacuum ranging between 60
and 70% of atmospheric pressure. In some washers, the
vacuum is made in an external pump which operates as
an accessory of the washer. Its operation is controlled
by the washer, which means that the vacuum pump
operates only when required.


Washing process verifi cation
To verify that the washing process is done according to
the specifications of ELISA techniques, manufacturers
of ELISA tests have developed procedures to be carried
out regularly. One of the controls1 is based on using the
peroxidase reagent, which is dispensed using a pipette in
the plate wells to be read at 405, 450 and 492 nm. At once
the wells are washed and a colourless substrate is added
(TMB/H2O2–Tetramethylbenzidine/Hydrogen Peroxide).
Whatever conjugate remains will hydrolyze the enzyme
and the chromogen will change to blue. After stopping
the reaction with acid, the TMB will turn yellow again. The
resulting colour intensity is directly related to the washing
process effi ciency.


INSTALLATION REQUIREMENTS
For the microplate washer to operate correctly, the following
is necessary:
1. A clean, dust-free environment.
2. A stable work table located away from equipment


that generates vibrations, (centrifuges, and agitators).
It must be of a suitable size to locate the necessary
complementary equipment: reader, incubator,
distributor and computer with its peripheral attachments
at the side of the microplate washer.


3. An electric outlet in good condition with a ground pole
and, an electrical connection which complies with the
country’s or the laboratory’s norms and standards. In the
countries of the Americas, the 110 V and 60 Hz frequency
is generally used. In other parts of the World, the 220-240
V and 50/60 Hz frequency is generally used.


ROUTINE MAINTENANCE
The routine maintenance described next focuses exclusively
on the microplate washer. Maintenance of the microplate
reader is dealt with in the Chapter 1.


Basic maintenance
Frequency: Daily
1. Verify the volume distributed.
2. Test the fi lling uniformity.
3. Verify the aspiration sub-system’s effi ciency.
4. Confirm the cleaning of the supply and extraction


needles.
5. Clean the washer with distilled water after use, to remove


every vestige of salt in the supply and extraction sub-
systems’ channels. The needles may be kept submerged
in distilled water.


6. Verify that the body of the washer has been cleaned.
If necessary, clean the exterior surfaces with a piece of
cloth, moistened with a mild detergent.


1 Procedure developed by PANBIO, ELISA Check Plus, Cat. Nº E-ECP01T.




C H A P T E R 2 M I C R O P L AT E WA S H E R


10


Preventive maintenance
Frequency: Quarterly
1. Disassemble and clean the channels and connectors.


Verify their integrity. If leaks or any vestiges of corrosion
are detected, adjust and/or replace.


2. Verify the integrity of the mechanical components.
Lubricate according to the manufacturer ’s
instructions.


3. Test the adjustment of each one of the sub-
systems. Calibrate according to the manufacturer’s
recommendations.


4. Confi rm the integrity of the electrical connector and the
inter-connection cable.


5. Clean the washer with distilled water after using it in
order to remove every vestige of salt in the supply and
extraction subsystems’ channels.


6. Verify the integrity of the fuse, and that its contact
points are clean.


Note: Trained technical personnel must carry out
maintenance of the control system. If necessary, call the
manufacturer or representative.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


11


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


Upon completion of washing, residual solution
remains in the wells.


The washer extraction system demonstrates failure. Verify if the vacuum system is functioning at the
appropriate pressure.


The conducts/pipes of the vacuum system are of a
diff erent diameter than that recommended.


Check that the diameter of the channels corresponds
to the recommendation by the manufacturer.


The suction line shows obstructions. Verify that the vacuum lines are clean.


The container for storing the waste is full. Confi rm the waste recipient’s level.


The line fi lter is damp or blocked. Verify the state and integrity of the suctioning
system’s fi lter.


The needles’ points are not placed correctly and do
not reach the bottom of the wells.


Examine the placement of the needles’ points.


A diff erent microplate is used in the test. Verify the type of plate required for the test.


The washer has not been purged suffi ciently. Check the purging process.


The operator has not followed the manufacturer’s
instructions correctly.


Examine the process recommended by the
manufacturer. Carry out the required adjustments.


The plate placed in the washer is incorrectly aligned. Check the placement of the plate in the washer.


The washing cycle is performing inadequately. The washing solution reserve is exhausted. Examine the cleaning solution storage receptacle.
Replace the volume missing.


The washer was not purged suffi ciently at the
beginning of the work cycle.


Clean adequately in order to homogenize the
humidity in each one of its components and to
eliminate air bubbles.


The volume of washing solution distributed has been
programmed erroneously.


Verify the required volume for each type of test and
for each plate.


The plate was placed incorrectly in the washer. Check the correct installation of the plate in the
washer.


The cycle setting was incorrectly selected. Review the cycle setting recommended for each type
of plate.


The plates used are diff erent from those
recommended by the manufacturer.


Verify that the plates used are completely
compatible with the washer.


The fl uid level in the wells is inadequate.


The washing solution supply tube is not of the
diameter or thickness specifi ed by the manufacturer.


Check the manufacturer’s specifi cations. If necessary,
correct.


The pressure is insuffi cient for delivering the
adequate amount of washing solution.


Check the supply system and supply channels, there
might be an obstruction in the fi lling line.


The washing container shows fungal and bacterial
growths.


The system is not used frequently. Check the procedures used for preventing fungal and
bacterial growth.


An adequate control procedure (disinfection) is not
used.


Check the procedures used for preventing fungal and
bacterial growth.


The tubes and connectors are not changed with the
required frequency.


Verify the change frequency suggested by the
manufacturer and or the technical department.


The washing solution has been contaminated. Confi rm the procedures used in the preparation
and management of the washing solution with the
aim of determining the cause of contamination and
eliminate it.


Maintenance has not been carried out according to
its schedule.


Check the dates planned for carrying out
maintenance. Inform those responsible.




C H A P T E R 2 M I C R O P L AT E WA S H E R


12


BASIC DEFINITIONS


Buff er. A solution containing either a weak acid and its salt or, a weak base and its salt, which makes it resistant to changes in pH at a given temperature.


PBS. One of the solutions used to perform washing operations in ELISA tests. PBS is the acronym for Phosphate Buff er Solution. This is made of the following
substances: NaCl, KCl, NaHPO42H2O and KH2SO4. The manufacturers supply technical bulletins which indicate the proportions and instructions for preparing PBS. In
general, one part of concentrated PBS is mixed with 19 parts of deionised water.


Plate (ELISA). Consumable with standard dimensions, designed to hold samples and reactions for the ELISA technique. In general, these have 96, 384 or 1536 wells
and are made of plastics such as polystyrene and polypropylene. There are plates specially treated to facilitate the performance of the tests.


Positive displacement pump. A pump adjusted by a plunger moving along a cylinder. The mechanism is similar to that of a syringe. It is equipped with a set of
valves for controlling the fl ow to and from the pump.


TMB/H2O2. (Tetramethylbenzidine/hydrogen peroxide). A set of reagents used for verifying the quality of washing done on the wells used in the ELISA technique.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


13


Chapter 3


pH Meter


The pH meter is used for determining the concentration of
hydrogen ions [H+] in a solution. This equipment, provided
it is carefully used and calibrated, measures the acidity of
an aqueous solution. pH meters are sometimes called pH
analysers, pH monitors or potentiometers.


PURPOSE OF THE EQUIPMENT
The pH meter is commonly used in any fi eld of science
related to aqueous solutions. It is used in areas such as
agriculture, water treatment and purifi cation, in industrial
processes such as petrochemicals, paper manufacture,
foods, pharmaceuticals, research and development, metal
mechanics, etc. In the health laboratory, its applications
are related to the control of culture mediums and to the
measurement of the alkalinity or acidity of broths and
buff ers. In specialized laboratories, diagnostic equipment
microelectrodes are used to measure the pH of liquid
blood components. The plasma pH allows the patient’s
health to be evaluated. It normally measures between 7.35
and 7.45. This value relates to the patient’s metabolism
in which a multitude of reactions occurs where acids and
bases are normally kept in balance. Acids constantly liberate
hydrogen ions [H+] and the organism neutralizes or balances
acidity by liberating bicarbonate ions [HCO


3
–]. The acid-base


ratio in the organism is maintained by the kidneys, (organs
in which any excesses present are eliminated). The plasma
pH is one of the characteristics that vary with factors such
as age or state of health of the patient. Table 1 shows typical
pH values of some bodily fl uids.


PHOTOGRAPH AND COMPONENTS OF THE
pH METER


OPERATION PRINCIPLES
The pH meter measures the concentration of hydrogen ions
[H+] using an ion-sensitive electrode. Under ideal conditions,
this electrode should respond in the presence of only
one type of ion. In reality, there are always interactions or
interferences with other types of ions present in the solution.
A pH electrode is generally a combined electrode, in which
a reference electrode and an internal glass electrode are
integrated into a combined probe. The lower part of the
probe ends in a round bulb of thin glass where the tip
of the internal electrode is found. The body of the probe


Fluid pH Value


Bile 7.8 – 8.6


Saliva 6.4 – 6.8


Urine 5.5 – 7.0


Gastric Juice 1.5 – 1.8


Blood 7.35 – 7.45


pH values of some bodily fl uids


GMDN Code 15164


ECRI Code 15-164


Denomination pH Meter


Ph
ot


o
co


ur
te


sy
o


f C
on


so
rt


1 Electrode carrying arm and electrode
2 Digital display
3 Control panel with temperature adjustment control, mode


selection (Standby/mV/pH) and calibration controls


1


3


2




C H A P T E R 3 p H M E T E R


14


contains saturated potassium chloride (KCl) and a solution
0.1 M of hydrogen chloride (HCl). The tip of the reference
electrode’s cathode is inside the body of the probe. On the
outside and end of the inner tube is the anodized end. The
reference electrode is usually made of the same type of
material as the internal electrode. Both tubes, interior and
exterior, contain a reference solution. Only the outer tube
has contact with the measured solution through a porous
cap which acts as a saline bridge.


This device acts like a galvanized cell. The reference electrode
is the internal tube of the pH meter probe, which cannot lose
ions through interactions with the surrounding environment.
Therefore as a reference, it remains static (unchangeable)
during the measuring process. The external tube of the
probe contains the medium which is allowed to mix with
the external environment. As a result, this tube must be
fi lled periodically with a potassium chloride solution (KCI)
for restoring the capacity of the electrode which would
otherwise be inhibited by a loss of ions and evaporation.


The glass bulb on the lower part of the pH electrode acts
as a measuring element and is covered with a layer of
hydrated gel on its exterior and interior. Metallic sodium
cations [Na+] are diff used in the hydrated gel outside of
the glass and in the solution, while the hydrogen ions [H+]
are diff used in the gel. This gel makes the pH electrode
ion-selective: Hydrogen ions [H+] cannot pass through the
glass membrane of the pH electrode. Sodium ions [Na+] pass
through and cause a change in free energy, which the pH
meter measures. A brief explanation of the theory on how
electrodes function is included in the appendix at the end
of the chapter.


pH METER COMPONENTS
A pH meter generally has the following components:
1. The body of the instrument containing the circuits,


controls, connectors, display screens and measuring
scales. The following are among some of its most
important components:
a) An ON and OFF switch. Not all pH meters have an


on and off switch. Some simply have a cord with a
plug which allows it to be connected to a suitable
electrical outlet.


b) Temperature control. This control allows
adjustments according to the temperature of the
solution measured.


c) Calibration controls. Depending on the design,
pH meters possess one or two calibration buttons
or dials. Normally these are identifi ed by Cal 1 and
Cal 2. If the pH meter is calibrated using only one
solution, the Cal 1 button is used; making sure
that Cal 2 is set at a 100%. If the pH meter allows
two point calibrations, two known pH solutions
covering the range of pH to be measured are used.
In this case, the two controls are used (Cal 1 and Cal
2). In special cases, a three-point calibration must
be done (using three known pH solutions).


d) Mode selector. The functions generally included
in this control are:
I. Standby mode (0). In this position the electrodes


are protected from electrical currents. It is the
position used for maintaining the equipment
while stored.


II. pH mode. In this position the equipment can
take pH measurements after performing the
required calibration procedures.


KCI KCI


High Impedance
Voltmeter


Temperature
Regulator


Reference
Terminal


Saline Mesh Bridge Solution Under Analysis


Special Glass Permeable to Ions


Active Termimal


Ag/AgCI Electrode


Figure 4. Diagram of a pH meter




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


15


III. Millivolt mode (mV). In this position the
equipment is capable of performing millivoltage
readings.


IV. ATC mode. The automatic temperature control
mode is used when the pH is measured in
solutions for which the temperature varies. This
function requires the use of a special probe. Not
all pH meters have this control.


2. A combined electrode or probe. This device must
be stored in distilled water and stay connected to the
measuring instrument. A combination electrode has a


reference electrode (also known as Calomel electrode)
and an internal electrode, integrated into the same body.
Its design varies depending on the manufacturer.


TYPICAL CIRCUIT
Figure 6 features a typical circuit adapted to the control
system of the pH meter. Each manufacturer has its own
designs and variations.


110 VAC


1N 4002
7812


3,300
mfd


0.1
mfd


3,300
mfd


0.1
mfd


7912


10K
Variable
resistor


12V
Lamp


Entrance
Reference


10K


30K


pH


560K


mV


9,09 K


1,00 K


TL081


110 V AC/ 12 V DC
Transformer


10K
Zero


3


2 7


6


Exit


pH


mV


4
5


1


Figure 6. Example of a typical pH meter control circuit


Combined Electrode


Silver Wire (Ag)


Reference Electrode


Semi-Permeable Mesh


Buffer Solution


Figure 5. Types of electrodes


Platinum Wire (Pi)


Reference Electrode (Calomel)


h


Mercury [Hg]


Mercury Chloride [Hg CI]


Potassium Chloride


Porous Stopper




C H A P T E R 3 p H M E T E R


16


System Element Description


Electric feeding and correction.


110 V/12 V AC transformer.* A device converting the voltage of the 110 V to 12 V
AC network.


1N4002 rectifi er diodes. Diode controlling the type of wave and guaranteeing
that is positive.


Electrolyte condensers 3300 microfarads (µfd) (2). Condensers absorbing the DC voltage to the diodes.


Tri terminal regulators (7812, 7912). A device regulating the voltage resulting from the
interaction between diodes and condensers.


0.1 microfarad (µfd) (2) electrolyte condensers. Devices used to achieve stability at high frequency.


12 V D C signal light. Light indicating if the equipment is ON.


Measurement of pH and millivolts. TL081 non-inverted type dual amplifi er. Millivolts circuits.


(R1) 9.09 K Ω (ohm) resistors.


(R2) 1 K Ω (ohm) resistors.


(R3) 560 K Ω (ohm) resistors. pH circuits.


(R4) 10 K Ω(ohm) variable resistors.


(R5) 30 K Ω (ohm) resistors. Ground resistance.


The circuit gain is governed by means of the
following equation:
Gain = 1+ (R3+PxR4)/R5+ (1–P) xR4.


Outlet section. Low cost DC voltmeter. Permits readings in millivolts. The voltage read is 10
times that of the cell, allowing a resolution of 0.1
millivolts.


The reading is done by using carbon/quinhydrone
electrodes.


Description of typical control circuit elements


* Diff erent voltage specifi cations are applicable in certain regions of the World.


INSTALLATION REQUIREMENTS
The pH meter works using electric current with the following
characteristics.


Power: Single phase Voltage: 110 V or 220-230 V Frequencies;
50-60Hz depending on the World region.


There is also portable pH meters powered with batteries.


GENERAL CALIBRATION PROCEDURE
pH analyzers must be calibrated before use to guarantee
the quality and accuracy of the readings following these
procedures:
1. One point calibration. This is carried out for normal


working conditions and for normal use. It uses one
known pH reference solution.


2. Two point calibration. This is done prior to performing
very precise measurements. It uses two known pH
reference solutions. It is also done if the instrument is
used sporadically and its maintenance is not carried out
frequently.


Description of the process
Frequency: Daily
1. Calibrate the pH meter using one known pH solution


(one point calibration).
1.1 Connect the equipment to an electrical outlet with


suitable voltage.
1.2 Adjust the temperature selector to the


environmental temperature.
1.3 Adjust the meter.
1.4 Remove the electrodes from the storage container.


The electrodes must always be stored in a suitable
solution. Some can be maintained in distilled
water, others must be kept in a diff erent solution
as their manufacturers recommend1. If for some
reason, the electrode becomes dry, it is necessary
to soak it for at least 24 hours before use.


1.5 Rinse the electrode with distilled water in an empty
beaker.


1.6 Dry the electrode with material able to absorb
residual liquid on its surface, without impregnating
the electrode. To avoid possible contamination,
the electrodes must be rinsed between diff erent
solutions.


1 Verify the type of buff er solution recommended by the electrode
manufacturer.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


17


2. Place electrodes in the calibration solution.
2.1 Submerge the electrode in the standardization


solution in such a manner that its lower extremity
does not touch the bottom of the beaker. This
decreases the risk of breaking the electrode. If the
test requires that the solution be kept in motion
using the magnetic agitator, special care must be
taken so that the agitation rod does not hit the
electrode as this could break it. Buff er solution
is used as a calibration solution, because its pH
is known and therefore will still be maintained
even if a little contamination occurs. In general, a
solution of pH = 7 is used for this purpose1.


3. Turn the functions selector from Standby position to
pH position.
3.1 This action connects the electrode to the pH


measuring scale in the pH meter.
3.2 Adjust the meter to read the pH of the calibration


solution using the button marked Cal 1. This
enables the meter to read the pH of the calibration
solution.
For example: For a solution at pH = 7, the needle
can oscillate slightly in units of 0.1 pH; on average,
the reading should be 7. The reading of the meter
(reading scale) should be done perpendicularly,
to avoid or eliminate parallel-type errors (reading
errors produced by the shadow of the meter’s
needle, visible on the mirror of the reading scale).
The pH meter is then ready (calibrated), to carry
out the correct pH readings.


3.3. Put the functions selector in the Standby
position.


4. Measuring the pH of a solution.
4.1 Remove the electrode from the calibration


solution.
4.2 Rinse the electrode with distilled water and dry


it.
4.3 Place the electrode in the solution of unknown


pH.
4.4 Turn the functions selector from the Standby


position to the pH position.
4.5 Read the pH of the solution on the meter’s scale or


the screen. Register the reading obtained on the
control sheet.


4.6 Turn the functions selector again to the Standby
position.
If it is necessary to measure the pH of more
than one solution, repeat the previously described
procedures, rinsing the probe with distilled water
and drying with clean, lint-free paper between
readings. When the pH has to be measured


in numerous solutions, the pH meter must be
calibrated frequently, following the steps previously
described.


5. Turn off the pH meter.
5.1 Remove the electrode from the last solution


analyzed.
5.2 Rinse the electrode in distilled water and dry it with


a drying material that will not penetrate it.
5.3 Place the electrode in its storage container.
5.4 Verify that the functions selector is in the Standby


position.
5.5 Activate the off switch or disconnect the feed


cable, if it lacks this control.
5.6 Clean the work area.


GENERAL MAINTENANCE OF THE pH METER
pH meters have two general maintenance procedures:
one concerning the analyzer’s body, the other for the pH
detection probe (electrodes).


General maintenance procedures for the pH meter’s
body
Frequency: Every six months
1. Examine the exterior of the equipment and evaluate its


general physical condition. Verify the cleanliness of the
covers and their adjustments.


2. Test the connection cable and its system of connections.
Check that they are in good condition and clean.


3. Examine the equipment controls. Verify that these are
in good condition and activated without diffi culty.


4. Verify that the meter is in good condition. To do this, the
instrument must be disconnected from the electric feed
line. Adjust the indicator needle to zero (0) using the
adjustment screw generally found below the pivot of
the indicator needle. If the equipment has an indicator
screen, check that it is functioning normally.


5. Confi rm that the on indicator (bulb or diode) operates
normally.


6. Verify the state of the electrode carrying arm. Examine
the electrode attachment and assembly mechanism to
prevent the electrode from becoming loose. Check that
the height adjustment operates correctly.


7. Check the batteries (if applicable); change them if
necessary.


8. Test its function by measuring the pH of a known
solution.


9. Inspect the ground connection and check for escaping
current.


1 Verify the type of calibration solution recommended by the electrode
manufacturer.




C H A P T E R 3 p H M E T E R


18


BASIC MAINTENANCE OF THE ELECTRODE
Frequency: Every four months
The measuring or detector electrode requires periodic
maintenance of the conducting solution to obtain precise
readings.
The recommended steps for replacing the electrolyte
solution are the following:
1. Remove the detector electrode from the storage buff er


solution.
2. Rinse the detector electrode abundantly with distilled


water.
3. Remove the upper cover of the detector electrode.
4. Fill the conduit surrounding the internal electrode with


a saturated potassium chloride (KCI) solution. Use the
syringe or applicator supplied with the KCI solution.
Verify that the tip of the syringe does not touch the
inside of the electrode.


5. Close the electrode with its cover. Rinse the electrode
in distilled water.


6. Keep the electrode in storage buff er solution while not
in use.


Cleaning of the electrode
The type of cleaning required for electrodes depends of
the type of contaminant aff ecting it. The most common
procedures are summarized next:
1. General cleaning. Soak the pH electrode in a 0.1 M HCl


solution or 0.1 M HNO3, for 20 minutes. Rinse with
water.


2. Removal of deposits and bacteria. Soak the pH
electrode in a diluted domestic bleach solution (e.g.
1%), for 10 minutes. Rinse abundantly with water.


3. Cleaning oil and grease. Rinse the pH electrode with
a mild detergent or with methyl alcohol. Rinse with
water.


4. Cleaning of protein deposits. Soak the pH electrode
in 1% pepsin and 0.1 M HCl for 5 minutes. Rinse with
water.


After carrying out each cleaning operation, rinse with
deionised water and refi ll the reference electrode before
use.


Other precautionary measures
1. Do not strike the electrode. Given that the structure is


generally made of glass and very fragile, it is necessary
to manipulate it very carefully, preventing it from being
knocked off .


2. Remember that the electrode has a limited lifespan.
3. While not in use, keep the electrode inside the storage


buff er solution.


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The pH meter shows unstable readings. There are air bubbles in the electrode. Soak the electrode to eliminate the bubbles.


The electrode is dirty. Clean the electrode and recalibrate.


The electrode is not immersed. Verify that the sample covers the tip of the electrode
perfectly.


The electrode is broken. Replace the electrode.


The electrode’s response is slow. The electrode is dirty or greasy. Clean the electrode and recalibrate.


The screen shows an error message. Incorrect operating mode selected. Verify the operation mode selected. Select a valid
operation.


The screen shows a calibration or error message. There is a calibration error. Recalibrate the pH meter.


The calibration of the buff er value is erroneous. Verify the buff er values used.


The electrode is dirty. Clean and calibrate the electrode.


The pH meter is on, but there is no signal on the
screen.*


The batteries are badly installed. Verify the polarity of the batteries.


The batteries are worn out. Replace the batteries.


The battery indicator is fl ashing.* The batteries are worn out. Replace the batteries.


* Applicable to equipment equipped with batteries only.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


19


BASIC DEFINITIONS


Buff er. A solution containing either a weak acid and its salt or, a weak base and its salt, which makes it resistant to changes in pH at a given temperature.


Calomel electrode. A reference electrode used with the active electrode for determining the pH of a solution. This electrode is constructed with a mercury base
(Hg), a covering of dimercuric chloride (Hg2Cl2) and a potassium chloride solution of 0.1 M. It is represented as Cl2[Hg2Cl2, KCl]Hg.


Dissociation. A phenomenon through which a break in the molecules occurs. As a result it produces electrically charged particles (ions).


Electrolyte. A solute which produces a conducting solution, e.g. NaCl (sodium chloride) and NH4OH.


Gel. A semisolid substance (e.g. jelly) composed of a colloid (solid) dispersed in a liquid medium.


Ion. Neutral atom which gains or loses an electron. When the atom loses an electron, it becomes a positively charged ion, called a cation. If the atom gains or captures
an electron, it becomes a negatively charged ion, called an anion.


Ion-sensitive electrode. A device which produces a diff erence in potential proportional to the concentration of an analyte.


Molarity. Number of Moles (M) in a substance in a litre of solution. (Number of moles of solute in a litre (L) of solution). The brackets around the ionic symbol
indicate that it is treated as a molar concentration.


Mol. (abbreviation for molecule). A quantity of any substance whose mass expressed in grams is numerically equal to its atomic mass.


Mole (unit). The amount of a substance that contains as many atoms, molecules, ions, or other elementary units as the number of atoms in 0.012 kilogram of
carbon 12. It corresponds to the number 6.0225 × 1023, or Avogadro’s number, also called gram molecule.
The mass in grams of this amount of a substance, numerically equal to the molecular weight of the substance, also called gram-molecular weight.


pH. Measurement of the concentration of the hydrogen ion (H+) given in moles per litre (M) in a solution. The pH concept was proposed by Sørensen and Lindstrøm-
Lang in 1909 to facilitate expressing very low ion concentrations. It is defi ned by the following equation:
pH = –log [H+] or [H+] = 10-pH


It measures the acidity of a solution. Example, in water the concentration of [H+] is 1.0 x 10-7 M resulting in pH = 7. This allows the range of concentrations from
1 to 10-14 M, to be expressed from zero (0) to 14. There are diverse systems for measuring the acidity of a solution. An acidic substance dissolved in water is capable
of producing H+ ions. A basic substance dissolved in water is capable of producing [OH–] (hydroxides) ions.


An acid substance has a greater quantity of ions [H+] than pure water; a basic substance shows greater quantities of ions [OH–] than pure water. The concentrations
of substances are expressed in moles per litre.


In pure water, the ion concentration [H+] and [OH–] is 1.0 x 10–7 M, it is thus considered a neutral substance. In reality, it is a weak electrolyte that is dissociated
following the following equation:
H2O ' [H+][OH–]


In all aqueous solutions there is a balance expressed as:
[H+][OH–]


= K


H2O


If the solution is diluted, the concentration of the non-dissociated water can be considered constant:
[H+][OH–] = [H2O]K = Ka


The new constant Ka is called a constant of dissociation or ionic product of water and its value is 1.0x10–14 at 25 °C.
[H+][OH–] = 1.0 x 10-14


X x X = 1.0 x 10-14


X2 = 1.0 x 10-14


X = 1.0 x 10-7


In pure water the concentrations of H+ and OH– are 1.0 x 10–7 M, a very low concentration, given that the molar concentration of water is 55.4 mol/litre.


Solution. Homogenous liquid mixture (with uniform properties) of two or more substances. It is characterized by the absence of chemical reactions among the
components in the mixture. The component in greater proportion and generally in a liquid state is called solvent and that or those in a lesser quantity, the solutes.




C H A P T E R 3 p H M E T E R


20


Annex


The pH theory
pH electrodes ideally behave as an electrochemical cell and react to the concentration of ions [H+]. This generates an
electromotive force (EMF) which, according to the Nernst law is calculated using the following equation:



Given that:


pH = ln a
H L


where a is the eff ective concentration of ions (Activity)


If n = 1, the equation is then rewritten as:



E = E o −


R 'T
F


pH


E° is a constant dependant on the temperature. If E° is substituted by E’T, the calibration will be more sensitive. Real electrodes
do not always perform according to the Nernst equation. If the concept of sensibility (s) is introduced, the equation can be
rewritten as:


E = E 'T − s
R 'T
F


pH


The values of E’ and s are found when measuring the EMF in two solutions with known pH. S is the slope of E versus pH,
while E’ is found at the intersection with the axis y. When E’ and s are known, the equation can be rewritten and the pH can
be calculated as:


pH =
E 'T − E


s
R 'T
T


E = E o +
RT
nF


ln a
H


ln a
H




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


21


Chapter 4


Balances


The balance is an instrument which measures the mass of
a body or substance using the gravity force which acts on
that body. The word comes from the Latin terms bis which
means two and lanx, plate. The balance has other names
such as scale and weight. It must be taken into account that
the weight is the force which the gravitational fi eld exercises


on a body’s mass, this force being the product of the mass
by the local acceleration of gravity [F = m x g]. The term
local is used to emphasize that this acceleration depends
on factors such as the geographical latitude, altitude and
the Earth’s density where the measurement is taken. This
force is measured in Newtons.


GMDN Code 10261 10263 45513 46548


ECRI Code 10-261 10-263 18-449 18-451


Denomination Balances Electronic balances Analytical electronic
balances


Micro analytical,
microelectronic
balances


Ph
ot


o
co


ur
te


sy
o


f A
cc


ul
ab


C
or


p
or


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io


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Ph
ot


o
co


ur
te


sy
o


f O
ha


us
C


or
p


or
at


io
n


Ph
ot


o
co


ur
te


sy
o


f O
ha


us
C


or
p


or
at


io
n


PHOTOGRAPHS OF BALANCES


Mechanical balance Electronic balance




C H A P T E R 4 B A L A N C E S


22


PURPOSE OF THE BALANCE
The balance is used for measuring the mass of a body or
substance or its weight. In the laboratory, the balance is used
for weighing as part of quality control activities (on devices
like pipettes), in the preparation of mixtures of components
in predefined proportions and in the determination of
specifi c densities or weights.


OPERATION PRINCIPLES
There are diff erences in design, principles and criteria of
metrology amongst balances. At present, there are two large
groups of balances: mechanical and electronic balances.


Mechanical balances
The following are some of the more common ones:
1. Spring balance. Its function is based on a mechanical


property of springs as the force exercised on a spring
is proportional to the spring’s elasticity constant [k],
multiplied by its elongation [x] [F = -kx]. The greater
the mass [m] placed on the balance’s plate, the greater
the elongation will be, given that the elongation is
proportional to the mass and the spring’s constant. The
calibration of a spring balance depends on the force
of gravity acting on the object weighed. This type of
balance is used when great precision is not necessary.


2. Sliding weight balance. This type of balance is
equipped with two known weights which can be moved
on setting scales (one macro, the other micro). Upon
placing a substance of unknown mass on the tray, its
weight is determined by moving the weight on both
setting scales until the equilibrium position is reached.
At this point, the weight is obtained by adding both
quantities indicated by the sliding masses’ position on
the scale.


3. Analytical balance. This balance functions by comparing
known weight masses with that of a substance of
unknown weight. It is composed of a base on a bar or
symmetrical lever, maintained by a blade-like support
on a central point called a fulcrum. At its ends, there
are stirrups, also supported with blades which allow
these to oscillate smoothly. From there, two plates
are suspended. Certifi ed weights are placed on one
of the plates and unknown weights on the other. The
balance has a securing system or lock, which allows
the main lever to remain stable when not in use or
when it is necessary to modify the counter-weights. The
balance is inside an external box which protects it from
interferences, such as air currents. Analytical balances
can weigh ten thousandths of a gram (0.0001 g) or 100
thousandths of a gram (0.00001 g). This type of balance
generally has a capacity of up to 200 grams.


X


Spring Without Load


Displacement


Measuring Scale


Mass


F=mg


F=-kx


Spring With Load


m
F = F1
-kx = mg


Figure 7. Spring balance


Tray
Macro Scale
Micro Sliding Weight


Macro Sliding Weight


Micro Scale


Figure 8. Sliding weight scale


Figure 9. Analytical balance




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


23


It is necessary to have a set of certifi ed masses. The set is
generally composed of the following pieces:


4. Upper plate balance (Top loading or parallel guidance
balance). This type of balance has a loading plate located
on its upper part, supported by a column maintained in
a vertical position by two pairs of guides with fl exible
connections. The eff ect of the force produced by the
mass is transmitted from a point on the vertical column
directly or by some mechanical means to the loading
cell. The requirement with this type of mechanism is that
parallel guides must be maintained with exactitude of up
to ± 1 µm. Deviations in parallelism cause an error known
as lateral load (when the mass being weighed shows
diff erences if the reading is taken at the centre of the
plate or on one of its sides). The diagram shown below
explains the operation principle some manufacturers
have introduced in electronic balances.


5. Substitution Balance (Unequal-lever arm or two-
knife balance). This is a balance with a single plate.
An unknown mass is placed on the weighing plate.
It is weighed by removing known masses from the
counterweight side until it reaches a balanced position,
using a mechanical system of cams. The fulcrum is
generally off -centre in relation to the length of the load
beam and located near the front of the balance. When
a mass is placed on the weight plate and the balance’s
locking mechanism is released, the movement of the
load beam is projected through an optical system to a
screen located on the front part of the instrument.


Operation verifi cation
The procedure used for verifying the functioning of a typical
mechanical balance is described below. The described
process is based on the substitution balance.
1. Verify that the balance is levelled. The levelling is


achieved using a ring-shaped adjustment mechanism
located on the base of the balance or by adjusting a
bubble or knob on a scale located on the front of the
balance’s base.


2. Test the zero mechanism. Place the controls on zero
and free the balance. If the reading does not stay at zero,
adjust the zero mechanism (a grooved screw located in
a horizontal position near the fulcrum). To do this, it is
necessary to block the balance and slightly adjust the
mechanism. The process is to be continued until the
zero adjusts correctly on the reading scale.


3. Verify and adjust the sensitivity. This is always readjusted
whenever some internal adjustment is done. It is
performed with a known standard according to the
following steps:
a) Lock the balance.
b) Place a standard weight (equivalent to the optical


scale range) on the plate.
c) Position the micro setting to one (1).
d) Release the balance.
e) Adjust to the zero position.
f ) Position the micro setting to zero (0). The balance


should indicate 100. If the scale displays less
or more than 100, the sensitivity control must
be adjusted. This requires locking the balance,
opening the upper cover and turning the sensitivity
screw: If the scale registers more than 100; turn
the screw in a clockwise position. If the scale
registers less than 100, it is necessary to unwind
the screw anticlockwise. Repeat the process until
the balance is adjusted (adjusting the zero and the
sensitivity).


Type of mass Capacity


Simple pieces


1, 2, 5, 10, 20, and 50 g


100, 200 and 500 g


Fractional pieces


2, 5, 10, 20 and 50 mg


100, 200 and 500 mg


Mass


Plate


Flexible
Connections


Support Column
F


G


Figure 10. Upper plate balance


Figure 11. Substitution balance




C H A P T E R 4 B A L A N C E S


24


4. Verify the plate’s brake. It is mounted on a threaded
axis which touches the plate in order to prevent it from
oscillating when the balance is locked. In case of an
imbalance, the axis must be rotated slightly until the
distance between the break and the plate is zero when
the balance is locked.


Maintenance of the mechanical balance
The maintenance of mechanical balances is limited to the
following routines:
Frequency: Daily
1. Verify the level.
2. Verify the zero setting.
3. Verify the sensitivity adjustment.
4. Clean the weighing plate.


Frequency: Annually
1. Calibrate the balance and document the process.
2. Disassemble and clean the internal components. This


must be done according to the process outlined by the
manufacturer or a specialized fi rm must be contracted
to do so.


Electronic balances
The electronic balances have three basic components:
1. A weighing plate. The object to be weighed placed


on the weighing plate exercises a pressure distributed
randomly over the surface of the plate. By means of
a transfer mechanism (levers, supports, guides), the
weight’s load is concentrated on a simple force [F] which
can be measured. [F = ∫P∂a]. The pressure’s integral part
on the area allows the force to be calculated.


2. A measuring device known as “load cell” produces an
exit signal corresponding to the load’s force in the form
of changes in the voltage or frequency.


3. A digital analogous electronic circuit shows the fi nal
result of the weight digitally.


Laboratory balances operate according to the principle
of compensation of the electromagnetic force applicable
to displacements or torques. The combination of their
mechanical components and automatic reading systems
provides weight measurements at defi ned levels of accuracy
depending on the model.


Principle. The mobile parts (weighing plate, support
column [a], bobbin, position and load indicator [G] -the
object in the process of being weighed-) are maintained
in equilibrium by a compensation force [F] equal to the
weight. The compensation force is generated by an electrical
current through a bobbin in the air gap of a cylindrical
electromagnet. The force F is calculated with the equation
[F = I x L x B] where: I = electrical intensity, L = total length
of the wire of the coil and B = magnetic fl ow intensity in the
electromagnet’s air gap.


With any change in the load (weight/mass), the mobile
mechanical system responds by moving vertically a fraction
of distance. Detected by a photosensor [e], an electrical
signal is sent to the servo-amplifi er [f ]. This changes the
fl ow of electrical current passing through the bobbin of the
magnet [c] in such a manner that the mobile system returns
to the balanced position upon adjusting of the magnetic
fl ow in the electromagnet. Consequently, the weight of
the mass [G] can be measured indirectly at the start of the
electrical current fl ow, which passes through the circuit
measuring the voltage [V] by means of a precision resistor
[R], [V = I x R]. To date, many systems developed use the
electronic system for carrying out very exact measurements
of mass and weight. The following diagram explains how
electronic balances function.


Transfer
Mechanism


Load Cell


Screen and
Signal Processor


P


Figure 12. Components of electronic balances


G


b
a


e


f


c d


R V=I*R


I


Figure 13. Compensation force principle




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


25


The signal processing system
The signal processing system is composed of the circuit which
transforms the electrical signal emitted by the transducer
into numerical data which can be read on a screen. The
signal process comprises the following functions:
1. Tare setting. This setting is used to adjust the reading


value at zero with any load within the balance’s capacity
range. It is controlled by a button generally located on
the front part of the balance. It is commonly used for
taring the weighing container.


2. Repeatability setting control. During a reading, weighed
values are averaged within a predefi ned period of time.
This function is very useful when weighing operations
need to be carried out in unstable conditions, e.g. in
the presence of air currents or vibrations. This control
defi nes the time period allowed for a result to lie within
preset limits for it to be considered stable. In addition,
it can be adjusted to suit a particular application.


3. Rounding off. In general, electronic balances process
data internally at a greater resolution than shown on the
screen. The internal net value rounded off is displayed
on the screen.


4. Stability detector. This light indicator fades when the
weighing result becomes stable and is ready to be
read. Alternatively in other balance models, this feature
allows the display of the result on the screen when the
measure of the weight becomes stable.


5. Electronic signalling process. It allows the processing
and display of the weighing operation results. It may also
allow other special functions such as piece counting,
percentage weighing, dynamic weighing of unstable
weight (e.g. animals), and formula weighing, among
others. The calculations are done by the microprocessor
following the instructions entered by the operator on
the balance’s keyboard.


Classification of balances
The International Organization of Legal Metrology (OIML)
has classifi ed the balances into four groups:
• Group I: special exactitude
• Group II: high exactitude
• Group III: medium exactitude
• Group IV: ordinary exactitude


The graph in Figure 14 shows the above-mentioned
classifi cation.


In the metrological classifi cation of electronic balances, only
two parameters are of importance:
1. The maximum load [Max.]
2. The value of the digital division [d]1


The number of the scale’s divisions is calculated by means
of the following formula.


n =
Max
dd


The OIML accepts the following convention for laboratory
balances.
1. Ultramicroanalytics dd = 0.1 µg
2. Microanalytics dd = 1 µg
3. Semi-microanalytics dd = 0.01 mg
4. Macroanalytics dd = 0.1 mg
5. Precision dd ≥ 1 mg


1 Kupper, W., Balances and Weighing, Mettler Instrument Corp., Princeton-
Hightstown, NJ.


Figure 14. Classifi cation of balances by exactitude




C H A P T E R 4 B A L A N C E S


26


Electronic balance controls
A diagram of the typical controls on a modern electronic
balance is shown in Figure 15. From this diagram it is
necessary to point out the following:
1. Numerous functions are incorporated.
2. Various measuring units can be selected.
3. It is possible to know the day and hour when the


measurements were taken.
4. The processes done can be documented and printed.
5. It is possible to select the language.


INSTALLATION REQUIREMENTS
For the satisfactory installation and use of a balance, the
following is required:
1. An environment with no air currents or sudden changes


in temperature and free from dust.
2. A perfectly levelled table/counter. A platform of high


inertia, isolated from the structures located in its vicinity
is ideal to reduce the eff ect of vibrations from certain
equipment such as centrifuges and refrigerators.
There must be a large enough area for installing the
balance and any auxiliary equipment needed during
the weighing processes. Likewise, the space required
for cables such as the interconnection, electrical current
cables and the information system connection to the
printer must be anticipated.


3. Avoid installing equipment which produces elevated
magnetic fi elds or vibrations like centrifuges, electrical
motors, compressors and generators in its vicinity.


4. Avoid locating it directly under the air-conditioning
system (air currents) and sunlight.


5. An electrical outlet which complies with the current
electrical standards in the country or the laboratory. It
must be in good condition and equipped with a ground
pole and switches.


Electronic balance operation
The operation of a modern electronic balance is clearly
detailed in its operator’s manual from the manufacturer. In
general, it must conform to the following procedure:
1. Allow the balance to equilibrate with the environment


where it is installed.
2. Allow the balance to warm-up before initiating activities.


Normally it is suffi cient to have it connected to the
electrical feed system. Some manufacturers suggest at
least 20 minutes from the moment it is energized until
use. Analytical balances Class 1 require at least 2 hours
for warming before initiating use.


Verify that the balance is calibrated. Electronic
balances generally have a factory-made calibration
stored in memory which can be used if it does not
have calibration masses. If calibration is required, use
calibrated masses as indicated by the manufacturer. The
calibrated masses must conform or exceed the ASTM
tolerances. For general information, the following table
shows the accepted tolerance for the ASTM Class 11
masses.


3. Follow the instructions indicated in the manufacturer’s
operations manual.


Calibration of balances
The calibration of balances must
be done by personnel specially
trained for this activity. It should be
highlighted that it must be done
based on the alignments of the OIML
or an equivalent body such as the
American Society for Testing and
Materials (ASTM), institutions which
have developed methodologies for
classifying standard weights. The
reference weights classifi cation used
by the OIML is covered in the table
opposite.


Weight (grams) Higher limit (g) Lower limit (g)


100 100.0003 99.9998


200 200.0005 199.9995


300 300.0008 299.9993


500 500.0013 499.9988


1 000 1000.0025 999.9975


2 000 2000.0050 1999.9950


3 000 3000.0075 2999.9925


5 000 5000.0125 4999.9875


1 Field Services Handbook for High Precision
Scales, IES Corporation, Portland, Oregon, 2004.


Selector
Buttons


Menu


Tare Button


Screen


Level


Selection/
Mode Button


Printing Button


Menu Button


On/Off


Unit
Date Hour


Calibration


Figure 15. Analytical balance control panel




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


27


Class Description Tolerance Uncertaintyallowed
Frequency of
recalibration


E1


Stainless steel weights without marks or adjusting
cavity.


± 0.5 ppm per kg ± 1/3 of the tolerance 2 years


E2 Stainless steel weights without marks or adjusting
cavity.


± 1.5 ppm per kg ± 1/3 of the tolerance 2 years


F1 Stainless steel weights with screw button for protecting
the adjusting cavity.


±5 ppm per kg ± 1/5 of the tolerance 1 year


F2 Bronze plated weights. ± 15 ppm per kg ± 1/5 of the tolerance 1 year


M1 Bronze weights (that do not corrode or become stained)
or of cast iron weights with a high quality paint fi nish.


± 50 ppm per kg ± 1/5 of the tolerance 1 year


M2 Bronze or cast iron weights (commercial weights). ±200 ppm per 1 kg ± 1/5 of the tolerance 1 year


Table of OIML reference weights classifi cation1


Any calibration process must be done using standard
weights. The results obtained must be analyzed to determine
if these are within the acceptable tolerances. The standard
weights must be selected based on the balance’s capacity.
The above table complements the previous. It provides
guidance in determining the standard weights to use in the
calibration of a balance according to its capacity.


ROUTINE MAINTENANCE
The balance is characterized as an instrument of high
precision. For this reason, the operator is only responsible
for minimal maintenance limited to the following:


Daily Activities
1. Clean the weighing plate so that it is kept free of dust.


Cleaning is done by using a piece of clean cloth which
may be dampened with distilled water. If there is a stain,
a mild detergent can be applied. Also a paintbrush with
soft bristles can be used to remove particles or dust
deposited on the weight plate.


2. Clean the weighing chamber, externally and internally.
Verify that the glass is free from dust.


3. Verify that the adjustment mechanisms on the front
door of the weighing chamber works adequately.


4. Always use a clean, pre-weighed container for weighing
(glass container or weighing paper if possible). Note
that plastic can become electromagnetically charged
and is not recommended for weighing powdered or
granulated chemicals.


5. Any spill must be cleaned immediately to avoid corrosion
or contamination. Use 70% ethanol to disinfect the pan
of the balance.


Very important: Never lubricate a balance unless the
manufacturer has expressly indicated it. Any substance
interfering with the mechanism of the balance retards its
response or defi nitely alters the measurement process.


Note: In general, the manufacturer or the specialized
installation representative carries out the maintenance
of the balances, according to procedures which vary
depending on the type and model.


1 Guidelines for calibration in laboratories, Drinking Water Inspectorate by
LGC (Teddington) Ltd., December 2000.


Capacity
Resolution


100 g 10 g 1 g 100 mg 10 mg 1 mg 0.1 mg 0.01 mg


Up to 200 g – – – M1 M1 F2 F1 F2


200 g to 1 kg – – M1 M1 F2 F1/E2 E2 E2


1 to 30 kg M2 M2 M1 F2 E2 E2 E2 –


30 to 100 kg M2 M1 F2 F1 E2 – – –


More than
100 kg


M2 M1/F2 F1 E2 – – – –


Table of standard weights’ use according to the balance’s capacity




C H A P T E R 4 B A L A N C E S


28


FUNCTIONAL ERROR PROBABLE CAUSE


Readings not reproducible (hysteresis). The measurement cell is dirty.


The measurement cell is badly assembled.


Non-linear readings. Defective electronic system.


Mechanical system is in bad condition.


Digital reading continually goes up or down. Defective electronic system.


Change in room temperature.


The digital reading goes up and down continually. Dirty measuring cell.


Defective electronic system.


Environmental problems like air currents, static
electricity or vibrations.


The digital screen is blank or shows marks that make
no sense.


Defective electronic system.


The screen indicates an overload or negative
condition without a load being applied.


Measuring cell damaged by overload.


Measuring cell is inadequately assembled.


The balance cannot be calibrated. Defective calibration battery.


Electronic system is defective.


Measurement cell is inadequately assembled.


TROUBLESHOOTING TABLE


Electronic balance


PROBLEM PROBABLE CAUSE SOLUTION


The balance does not turn on. T he interconnection cable is disconnected or
maladjusted on the balance.


Check the connection. Adjust the cable connector if
this is the case.


Electrical outlet has no power. Check electrical feed.


The weight reading is incorrect. The balance was not adjusted to zero before the
reading.


Place the balance on zero; repeat the measurement.


The balance is incorrectly calibrated. C alibrate according to the procedure recommended
by the manufacturer.


The balance is not levelled. Level the balance.


The balance does not show the desired units of
measurement on the screen.


The units are incorrectly selected.
select the required measurement unit.


The unit required not available or not activated. Activate the measurement unit according to the


The menu may be locked. Check to see if the locking switch is activated. If this
is the case, deactivate it.


The balance is incapable of keeping the selections
or changes. process.


Verify that the changes and selections are done
according to the manufacturer’s instructions. Repeat
the selection or change.


The balance’s reader is unstable. There is vibration on the surface of the table/counter.


again.


The front door of the balance is open.


Place the balance on a stable surface.


Close the front door to measure.


The RS232 interface does not function. The interconnection cable is maladjusted. Check the connection of the interconnection cable.


The screen shows incomplete readings or is locked. The microprocessor is locked.
the situation persists, seek technical assistance from
the service representative.


The screen displays an error code. Various. Verify the error codes in the balance’s manual.




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29


BASIC DEFINITIONS


ASTM. American Society of Testing and Materials.


Calibration. Determination of the correct value of an instrument’s reading by measurement or comparison against a standard or norm. A balance is calibrated by
using standard weights.


Certifi ed masses. Masses conforming to the tolerance defi ned by the certifi cation bodies. The ASTM classes 1 to 4 standards are those most widely used and must
be used (a compulsory reference) for performing the calibration routines.


Exactitude. The sum of all the balance’s errors. This is called total error band.


Hysteresis. The diff erence in the results when the load in the balance is increased or decreased.


Lateral load. A balance’s ability to consistently read the value of masses, no matter where they are placed on the weighing scale. This is also called corner load.


Lateral load error. A deviation in the results when an object is weighed placing it in diff erent parts of the weighing plate, i.e. in the centre of the plate and on
one of its sides.


Linear error. A diff erence showed when the balance is loaded in a successive manner, increasing the quantity of weight in equal magnitude until it reaches its
maximum capacity and unloaded in an analogous process. The diff erences shown between the readings obtained and the arithmetic values corresponding to the
weights used are interpreted as non-linearity.


Linearity. Refers to the ability of a balance to perform accurate readings of weights throughout its weighing capacity . A graph showing weight compared to the
weight indication on a perfectly linear balance should generate a straight line. In order to determine the linear error of a balance, certifi ed masses must be used.
The procedure allows the linear diff erences to be calculated by reading certifi ed masses with and without preloading. The diff erence between the readings allows
the linear error to be calculated.


Mass. A physical property of the bodies related to the quantity of matter, expressed in kilograms (kg), these contain. In physics, there are two quantities to which
the name mass is given: gravitational mass which is a measure of the way a body interacts with the gravitational fi eld (if the body’s mass is small, the body
experiences a weaker force than if its mass were greater) and the inertial mass, which is a quantitative or numerical measure of a body’s inertia, that is, of its
resistance to acceleration. The unit for expressing mass is the kilogram [kg].


OIML. International Offi ce of Legal Metrology.


Sensitivity. The smallest mass detected by the balance or the smallest mass that the balance can measure correctly.


Sensitivity error. Constant deviation throughout the weighing range or capacity of a balance.


Traceability. The ability to relate the measurements of an instrument to a defi ned standard.




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31


Chapter 5


Water Bath


The water bath is an instrument used in the laboratory for
carrying out serological, agglutination, inactivation, bio-
medical, and pharmaceutical tests and even for industrial
incubation procedures. In general they use water, but some
baths use oil. The temperature range at which water baths
are normally used range between room temperature and
60 °C. Temperatures of 100 °C can be selected, using a cover
with special characteristics. Water baths are manufactured
with chambers of a capacity ranging from 2 to 30 litres.


DIAGRAM OF A WATER BATH
Below is a basic diagram of a water bath. In the
diagram, it is possible to observe the electronic
control, the screen, the cover (an optional
accessory) and the tank. Other components
can be installed, e.g. a thermometer and
an agitation unit to keep the temperature
constant (not shown).


OPERATION PRINCIPLES
Water baths are made of steel and are
generally covered with electrostatic paint
with high adherence and resistance to
environmental laboratory conditions. Water
baths have an external panel on which the
controls can be found. They also have a tank
made of rustproof material with a collection
of electrical resistors mounted on their lower
part. By means of these, heat is transferred to
the medium (water or oil) until reaching the
temperature selected with a control device
(thermostat or similar). The resistors may be
of the following types:


• Immersion type. These resistors are installed inside
a sealed tube and located on the lower part of the
container in direct contact with heating medium.


• External. These resistors are located on the lower part
but on the outside of the tank. These are protected by
an isolating material which prevents heat loss. This type
of resistor transfers the heat to the bottom of the tank
through thermal conduction.


GMDN Code 36754 16772


ECRI Code 15-108 16-772


Denomination Water bath Water bath, shaker


Figure 16. Water bath


External
Resistors


Immersion
Resistors


Figure 17. Immersion and external resistors




C H A P T E R 5 WAT E R B AT H S


32


Certain types of water bath have a series of accessories such
as agitation systems or circulators, generating carefully
controlled movement of the heating medium to keep the
temperature uniform. A table which describes the main
types of water baths is shown below.


WATER BATH CONTROLS
Water baths generally have very simple controls.
Some manufacturers have incorporated controls with
microprocessors. They vary depending on the type of
bath. The diagram of a basic water bath’s control panel is
shown next.


The control panel has these elements:
1. The on and off control switch
2. A Menu button for selecting the operation’s parameters:


operation temperature, alarm temperature, temperature
scale (°C, °F)


3. Two buttons for parameter adjustment
4. A screen
5. A pilot light
6. Pilots (2) for identifying the temperature scale (°C, °F).


WATER BATH OPERATION


Installation
1. Install the water bath close to an electrical outlet. The


outlet must have its respective ground pole in order
to guarantee the protection and safety of the operator
and the equipment. Water baths generally operate at
120 V/60 Hz or 230 V/60Hz. Its installation and use is
facilitated by a sink close by for supplying and draining
of water.


2. Verify that the location selected is levelled and has the
necessary resistance to safely support the weight of the
water bath when it is full of liquid.


3. Ensure that the location has a suitable amount of space
for putting the samples and the accessories required for
the normal operation of the water bath.


4. Avoid placing the water bath where there are strong air
currents which can interfere with its normal operation.
For example: in front of an air-conditioning unit or
window.


Safety
1. Avoid the use of the water bath in environments where


there are fl ammable and combustible materials. The
equipment has components (resistors generating very
high temperatures) which could start an accidental fi re
or explosion.


2. Always connect the equipment to an electrical outlet
with a ground pole to protect the user and the
equipment from electrical discharges. The electrical
connection must comply with the required norms of
the country and the laboratory.


3. Use the water bath exclusively with non-corrosive or
non-fl ammable liquids.


4. Use personal protective elements when working with
the water bath. The bath has resistors which can cause
burns if inadvertently touched, even a considerable
time after turning off the equipment.


5. When working with substances that generate vapours,
place the water bath under a chemical hood or in a well
ventilated area.


6. Remember that liquids incubated in the water bath tank
can produce burns if hands are inadvertently placed
inside it.


7. Take into account that the water bath is designed for
use with a liquid inside the tank. If the inside is dry, the
temperature of the tank can become very high. Use
the diff using tray for placing the container inside of the
fi lled tank of the water bath. This has been designed for
distributing the temperature in a uniform way.


8. Avoid using the water bath if any of its controls is not
working, e.g. the temperature or limit controls.


Class Temperature range


Low temperature Room temperature up to 60 °C


Room temperature up to 100 °C


High temperature Room temperature up to 275 °C. When it needs to
reach temperatures above 100 °C, it is necessary to use
fl uids other than water as the boiling point of water is
100 °C under normal conditions


This type of bath generally uses oils which have much
higher boiling points.


Insulated Room temperature up to 100 °C with accessories and/
or agitation systems (with water).


4. Screen 1. On and Off Switch 5. On Pilot


2. Menu Button
6. Temperature
Scale Pilots (oC/oF)


3. Parameter
Adjustment Buttons


Figure 18. Water bath controls




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


33


Using the water bath
Before using the water bath, verify that it is clean and
that accessories needed are installed. The steps normally
followed are:
1. Fill the water bath with fl uid to keep the temperature


constant (water or oil). Verify that once the containers
to be heated are placed, the fl uid level is between 4 and
5 cm from the top of the tank.


2. Install the control instruments needed, such as
thermometers and circulators. Use additional mounts
provided for this purpose. Verify the position of the
thermometer’s bulb or thermal probe to ensure that
the readings are correct.


3. If water is used as the warming fl uid, verify that it is clean.
Some manufacturers recommend adding products
which prevent the formation of fungus or algae.


4. Put the main switch Nº 1 in the ON position (the
numbers identifying the controls herein correspond
to those shown in the diagram). Some manufacturers
have incorporated controls with microprocessors which
initiate auto-verifi cation routines once the ON switch is
activated.


5. Select the operation temperature using the Menu Nº 2
button and the buttons for adjusting the parameters.


6. Select the cut-off temperature (in water baths with this
control). This is a safety control which cuts off the supply
of electricity if it exceeds the selected temperature.
This is selected also by using the menu button and is
controlled by the parameter adjustment buttons.


7. Avoid using the water bath with the substances
indicated below:
a) Bleach.
b) Liquids with high chlorine content.
c) Weak saline solutions such as sodium chloride,


calcium chloride or chromium compounds.
d) Strong concentrations of any acid.
e) Strong concentrations of any salt.
f ) Weak concentrations of hydrochloric, hydrobromic,


hydroiodic, sulphuric or chromic acids.
g) Deionised water, as it causes corrosion and


perforation in the stainless steel.


Maintenance


Warning: Before carrying out any maintenance activity,
disconnect the equipment from the electrical feed outlet.


Water baths are equipment whose maintenance is simple.
The recommended routines mainly focus on the cleaning
of external components. The most common routines are
featured next.


Cleaning
Frequency: Monthly
1. Turn off and disconnect the equipment. Wait until it


cools to avoid the risk of burns and accidents.
2. Remove the fl uid used for heating. If it is water, it can


be poured through a siphon. If it is oil; collect into a
container with an adequate capacity.


3. Remove the thermal diff usion grid located at the bottom
of the tank.


4. Disassemble the circulator and clean to remove scale
and potential algae present.


5. Clean the interior of the tank with a mild detergent.
If there is any indication of corrosion, use substances
for cleaning stainless steel. Rub lightly with synthetic
sponges or equivalent. Avoid using steel wool to remove
rust stains as these leave particles of steel which could
accelerate corrosion.


6. Avoid bending or striking the temperature control
capillary tube generally located at the bottom of the
tank.


7. Clean the exterior and interior of the water bath with
clean water.


Lubrication
Frequency: Daily
For water baths with an agitation unit or circulator
system:
Lubricate the axis of the circulator’s electric motor. Put a
drop of mineral oil on the axis so that a good lubricating
condition is maintained between the motor’s bearings
and its axis.


Periodic inspection
Frequency: Quarterly
Check the thermometer or temperature controls every three
months using known standards. If no reference standard is
available, use an ice/water mixture and/or boiling water.
Note that the thermometer or the water bath temperature
controls should also be checked when the equipment is fi rst
installed after purchase.




C H A P T E R 5 WAT E R B AT H S


34


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


There is no power to the instrument. The water bath is disconnected. Connect the water bath.


The switch is defective. Change the switch.


The fuse is defective. Substitute the fuse.


The water bath is not getting hot. The temperature control not set. Set the temperature control.


The resistor(s) is/are defective. Change resistor(s).


The limit control is not set Set the limit control.


The temperature is higher than that selected. The temperature control is defective. Change the temperature control if required.


Verify the selection of the parameters.


The samples are warmed slowly. The tank is empty or contains very little fl uid. Fill the tank up to the recommended level.


The temperature is increasing very slowly. The resistor(s) is/are defective. Change the resistor(s).


The temperature control is defective. Substitute temperature control.


BASIC DEFINITIONS


Circulator. An apparatus that shakes or stirs fl uids to keep their properties (temperature, color, density) homogenous. These are also called agitators.


Diff using tray. Device located at the bottom of the water bath to support the containers located inside the tank. It also allows thermal convection currents generated
in the fl uid contained in the tank to circulate from top to bottom and back to the top, maintaining the temperature homogeneous at the level selected by the operator.
In general the diff using tray is made of stainless steel.


Electrostatic painting. A painting process that uses the particle-attracting property of electrostatic charges. A potential diff erence of 80-150kV is applied to a
grid of wires through which the paint is sprayed to charge each particle. The metal objects to be sprayed are connected to the opposite terminal of the high-voltage
circuit, so that they attract the particles of paint. The piece covered with paint particles is then placed in an electrical oven to melt the particles, making them adhere
strongly to the piece.


Fuse. A safety device which protects the electrical circuits from excessive current. Fuses are made of materials whose dimensions and properties equip them to
work well within some predefi ned conditions. If for some reason the design parameters are exceeded, the material burns out and interrupts the passage of the
electrical current.


Immersion resistor. An electrical resistor (see defi nition below) inside of a sealed tube. These are generally used for heating fl uids as water or oil.


Resistance. Opposition that a material or electrical circuit imposes to the fl ow of electric current. It is the property of a circuit that transforms electrical energy
into heat as it opposes the fl ow of current. The resistance [R], of a body of uniform section such as a wire, is directly proportional to the length [l] and inversely
proportional to the sectional area [a]. The resistance is calculated by the following equation:


R = k ×
l
a


Where:


k = constant that depends on the units employed
l = Length of the conductor
a = sectional area of the conductor


The ohm (Ω) is the common unit of electrical resistance; one ohm is equal to one volt per ampere.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


35


Chapter 6


Biological Safety Cabinet


This equipment is designed for controlling aerosols and
microparticles associated with managing potentially
toxic or infectious biological material in laboratories in
activities such as agitation, centrifugation, pipetting, and
opening of pressurized containers. Safety cabinets have
been designed to protect the user, the environment and
the sample manipulated using appropriate ventilation
conditions. They are also known as laminar fl ow cabinets
and/or biosafety cabinets.


ILLUSTRATION OF A BIOLOGICAL SAFETY CABINET


PURPOSES OF THE EQUIPMENT
The biological safety cabinet is used for the following:
1. To protect the worker from risks associated with the


management of potentially infectious biological
material.


2. To protect the sample being analyzed from becoming
contaminated.


3. To protect the environment.


The cabinets are used for routine work related to pathogens
(parasites, bacteria, virus, fungus), cell culture and under
very precise conditions, the management of toxic agents.


OPERATION PRINCIPLES
The biological safety cabinet is a chamber generally
constructed of steel. It has a front glass window of adjustable
height, a ventilation system with an electrical motor, a
ventilator and a set of ducts which while functioning,
generate a negative pressure condition inside the cabinet.
This forces the air to fl ow from inside the cabinet through
the front opening to generate a curtain of air protecting
the operator. Internally, the air is conducted through a
series of grids and ducts to be finally treated in HEPA1
fi lters. Depending on the design of the cabinet, the air is
recycled inside the laboratory or extracted and renewed in
diverse proportions. The air fl ow, which in Class II cabinets
moves from the fi lter towards the work surface, is laminar. A
summary of the existing type of cabinets and their principal
characteristics is presented next.


1 HEPA: High Effi ciency Particulate Air.


GMDN Code 15698 20652 20653 20654


ECRI Code 15-698 20-652 20-653 20-654


Denomination Cabinets, biological
safety


Cabinets, biological
safety, class I


Cabinets, biological
safety, class II


Cabinets, biological
safety, class III


Figure 19. Biological safety cabinet




C H A P T E R 6 B I O LO G I C A L S A F E T Y C A B I N E T


36


Type of cabinet, with illustration Characteristics


CLASS I — TYPE A


1. Protection provided: to the operator and the
environment.


2. Air velocity on entering the cabinet: 38 cm/s.


3. Suitable for working with bio-safety level1 1, 2 or 3
agents.


4. Filtration HEPA, located in extraction system which
may or may not be connected to the exterior.


5. Disadvantage: Does not protect the sample
manipulated in the cabinet.


CLASS II — TYPE A


1. Protection off ered: To the operator, the product and
environment.


2. Air velocity on entering the cabinet: 38 cm/s.


3. Suitable for working with agents with biosafety level
1, 2 or 3.


4. Filtration system: two HEPA fi lters, one located on the
work surface; the second on the extraction system
which may or may not be connected to the exterior.
If they are connected to the exterior, it utilizes a bell
type connection.


5. They recycle approximately 70 % of the air volume and
renew 30 % of it.


Summary of biological safety cabinet types


1 See biosafety classifi cations levels of agents in the following section “Biological safety”.


HEPA Extraction Filter


HEPA Extraction Filter


Vertical Laminar Flow


Rear Plenum


Work Area


Rear Grid


Ventilator Motors


Ventilator Suction Mouth


HEPA Filtered Air


Front Window


Front Aperature


Air Entry


Front Grid


Potentially Contained Air




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


37


Type of cabinet, with illustration Characteristics


CLASS II — TYPE B1


1. Protection provided: to the operator, the product and
the environment.


2. Air velocity entering the cabinet: 50.8 cm/s.


3. Suitable for working with agents with biosafety level
1, 2 or 3.


4. Filtration system: Two HEPA fi lters. It extracts
potentially contaminated air (70 %) through a duct
and recycles inside of the cabinet, after fi ltering, air
taken from the exterior, through the front grid (30 %).


5. All biologically contaminated ducts have a negative
pressure.



6. Allows work with small quantities of toxic and


radioactive chemicals.


CLASS II — TYPE B2


1. Protection provided: to the operator, the product and
the environment.


2. Air velocity on entering the cabinet 50.8 cm/s.


3. Suitable for working with agents of biosafety level 1, 2
or 3.



4. Filtration system: Two HEPA fi lters. It is known as the


total extraction cabinet. It does not have any type of
recirculation.



5. All biologically contaminated ducts have a negative


pressure.


6. It has an extraction duct which allows work with toxic
and radioactive chemicals.


Plenum System


Exyraction Duct


HEPA Filters


Laminar Flow


Work Surface


V=100 PLm
[50.8cm/s]


Prefilter


Extraction Duct


HEPA
Extraction Filter


Posterior Duct with
Negative Pressure


Back Grid


Front Grid
Lateral View


HEPA
Supply Filter


V vert = 55 PLm - (28cm/s)


V = 100 PLm - (50.8cm/s)




C H A P T E R 6 B I O LO G I C A L S A F E T Y C A B I N E T


38


Type of cabinet, with illustration Characteristics


CLASS II — TYPE B3 OR A/B3


1. Protection provided: to the operator, the product and
the environment.



2. Air velocity on entering the cabinet: 50.8 cm/s.

3. Suitable for working with agents of biosafety level 1, 2


or 3.


4. Filtration system: Two HEPA fi lters.


5. All biologically contaminated ducts have a negative
pressure.


6. It is known as a combined cabin. It can be connected
by means of a duct. It is denominated as Type B3. If the
duct is missing, it is a Type A. It recycles 70 % of the air
volume inside the cabinet.


CLASS III


1. Protection provided: to the operator, the product and
the environment.


2. Filtration system: two HEPA fi lters in series in the
extraction; a HEPA fi lter in the admission.


3. Suitable for working with agents classifi ed biosafety
level 4.


4. Totally sealed cabinet. The intake and extraction
elements are conducted through a double -door pass-
through box. The manipulation of materials is done by
using sealed gloves at the front of the cabinet.


HEPA Extraction Filter


HEPA Supply Filter


V vert = 55 PLm (28cm/s)


V = 100 PLm - (50.8cm/s)


Front Grid


Rear Duct with Pressure [-]


Rear Grid


LATERAL VIEW




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39


BIOLOGICAL SAFETY1


Microorganisms have been classifi ed into four categories
based on factors such as pathogenicity, infectious doses,
transmission modes, and host range, availability of
preventive measures and eff ectiveness of treatment for
the disease caused.
1. Risk level 1 group is composed of biological agents


very unlikely to cause sickness in healthy humans or
animals. (No individual and community risk).


2. Risk level 2 group is composed of pathogens which
cause sickness in humans or animals but unlikely to
be dangerous to laboratory workers, the community,
domestic animals or the environment under normal
circumstances. Those exposed in the laboratory rarely
become seriously ill. There are preventive measures
and effective treatment available and the risk of
dissemination is limited. (Moderate individual risk,
limited community risk).


3. Risk level 3 group is composed of pathogens which
usually cause serious sicknesses to human beings and
animals and produce a serious economic impact.


However, infection by casual contact by one
individual to another is not common. The sicknesses
these produce are treatable by antimicrobial or anti-
parasitic agents. (High individual risk, low community
risk).


4. Risk level 4 group is composed of pathogens which
usually produce very serious sicknesses in human beings
or animals, frequently without treatments available.
These agents are easily spread from one individual
to another or from animal to human being or vice
versa, directly or indirectly or by casual contact. (High
individual risk, high community risk).


INSTALLATION REQUIREMENTS
The following are requirements for a cabinet to function
adequately:
1. A laboratory area protected from air currents from


windows or air-conditioning systems. The cabinet must
also be located far from the laboratory circulation zones
in order to avoid air currents that could affect the
curtain of air inside the cabinet. It must also be verifi ed
that the cabinet is not installed alongside other types
of cabinets such as chemical hoods.


2. An electrical connection equipped with the respective
control and safety elements; the electrical outlet with
a ground pole.


3. A levelled and fi rm table designed for supporting the
weight of the cabinet and allowing the operator to work
comfortably. There must be free space for placing the
feet and its height must be adequate.


4. The floor on which it is located must be flat and
levelled.


5. The free space around the cabinet recommended by the
manufacturer must be respected. Likewise, the height
of the room must be verifi ed (the ceiling must be of
recommended height so that it can function without
hindrance).


6. Type B cabinets must have an extraction duct equipped
with the following required control devices: regulating
valves that allow the flow of air to be isolated and
regulated.


7. Gas connections must be in the immediate vicinity of
the cabinet in order to facilitate the connection to these
service valves.


8. The cabinet must be certifi ed annually to verify that it
complies with the established requirements in the NSF
49 Regulation.


USE OF THE SAFETY CABINET
Correct utilization of the biological safety cabinet is achieved
by complying with the following instructions:
1. Plan the work to be done in the biological safety cabinet


in advance. Determine what procedure and equipment
will be used. Coordinate the time of the cabinet’s use
with the other laboratory professionals in order to avoid
interruption or undesired traffi c while it is in use.


2. Turn on the cabinet. Turn off the UV lamp if lit. Turn on
the fl uorescent light lamp and the cabinet’s ventilator.
Verify that the grids in front and behind are free of
obstructions. Prepare the work area. Allow the cabinet
to function for at least 15 minutes.


3. Wash hands and forearms with germicidal soap. Put on
the personal protective apparel: coat/overall with long
sleeves and adjustable cuff s, protective eyeglasses and
mask if the work requires it. Prepare the interior surfaces
of the cabinet applying 70% ethanol or a suitable
disinfectant. After this, let the air fl ow through.


4. Only load and install the materials and equipment
required for the test or manipulation. Distinguish
between the clean areas and dirty areas. Place the
material in such a way that the clean materials do not
mix or cross used or dirty materials or impede the
circulation of the internal air through the front and
back grids. Place a biosafety bag for disposing waste
materials, a container with disinfectant for the pipettes
and a container for storing sharps. Avoid locating very
large objects near one another. Upon fi nalizing the
placing of the materials, the fl ow of air must be allowed
to sweep through the cabinet for approximately 3 to 5
minutes in order to eliminate any particle produced or
freed during the loading of materials and equipment.


5. Initiate activities. Slowly introduce hands into the work
area. Carry on the processes and tasks in a methodical
and careful manner (from the clean areas to the 1 The Laboratory Biosafety Guidelines, 3rd. Edition-Draft, Health Canada, 2001.




C H A P T E R 6 B I O LO G I C A L S A F E T Y C A B I N E T


40


potentially contaminated areas). Keep the materials
at least 10 cm behind the front grid. Try to perform
the most risky and contaminating activities towards
the back of the cabinet’s work area. Avoid the use of
open fl ames of lighters since this breaks the laminar
fl ow pattern and may burn the fi lter. Avoid removing
hands from the work area until all procedures are
accomplished and the potentially dangerous materials
are disposed of in the biosafety bag or in the pipette
and sharp containers.


6. Clean the cabinet, allowing the air to fl ow freely for 3 to
5 minutes upon ending all the procedures.


7. Decontaminate the surfaces of all the materials
and equipment in contact with the biologically
contaminated material. Apply 70% ethanol or a suitable
disinfectant and allow drying. Lift the equipment and
materials and disinfect the area underneath. Cover the
open containers before removal from the work area.
Transfer materials to their appropriate place (incubator,
autoclave, etc.).


8. Discard the gloves and remove personal protective
elements. Dispose of these following the laboratory’s
established procedure. Wash hands with a lot of water
and soap.


9. Turn off the ventilator, the fl uorescent lamp, close the
front opening and turn on the ultraviolet light.


Note: In case of a leak or spill inside the cabinet while in
use, it must be kept in operation and all the objects or
equipment involved must undergo a process of surface
decontamination. This will prevent the cabinet from
releasing contaminants.


Decontamination of the cabinet
The decontamination of the biological safety cabinet is an
activity which must be done before any maintenance work
involving opening its surfaces or internal components.
Whenever any of the processes indicated next are needed,
decontamination of the cabinet must be done previously.
1. Changing of fi lters.
2. Conducting tests requiring access to the interior surfaces


or exposure of the cabinet.
3. Before conducting certifi cation tests when the cabinet


has been used with classifi ed agents such as level 2 or
3 biological risk agents.


4. Before moving the cabinet to a diff erent location.
5. After a spill of a material containing high risk agents.


The most suitable decontamination procedure must be
defined by the professional responsible for industrial
safety and professional risks. In annex G of the NSF 49
Standard, the procedure for decontaminating the cabinet
using depolymerised paraformaldehyde is described. Only
professionals who have received the relevant training must
conduct such procedures.


ROUTINE MAINTENANCE


Warning: The maintenance of internal components must
only be done by trained and qualifi ed personnel. In order
to carry out maintenance on the internal components,
decontamination must be done previously. Personal
protection must be worn to perform the routines.


General maintenance required for the biological safety
cabinet is for the most part simple to perform. The routines
and frequencies are shown below:
Frequency: Weekly
1. Decontaminate the work surface and the interior


surfaces of the cabinet with 70% ethanol.
2. Clean the front glass door and the surface of the


ultraviolet lamp, using a domestic cleaning solution.
3. Verify the precision of the manometer’s reading,


indicating any fall in pressure fl owing through the HEPA
fi lter. Register the date and the reading in the cabinet’s
log book.


Frequency: Monthly
1. Clean the exterior surfaces, especially the front and


the upper part using a piece of damp cloth in order to
remove the dust.


2. Disinfect the surface of the lower compartment with
70% Ethanol or a suitable disinfecting solution.


3. Verify the state of the service valves.
4. Do the tasks due on a weekly basis.


Frequency: Annually
1. Carry out the certification process according to


established outlines in the NSF 49 regulation.
2. Check the intensity of the UV lamp1 with a radiometer.


Substitute it if necessary.
3. Test the state of the fl uorescent lamp. Substitute it if


necessary.
4. Perform the tasks due on a monthly basis.


Removal of the work surface
For the removal of the work surface the following procedure
is required:
1. Decontaminate the surface before removing it.
2. Loosen and remove the attachment screws located on


the front part of the work surface.
3. Loosen, but do not remove the attachment screws


located on the back part.
4. Raise the front end and remove it, pulling it towards the


front part of the cabinet.
5. Decontaminate the interior part of the work surface.
6. To assemble it, perform the activities described in steps


2, 3 and 4 in reverse order.


1 UV lamps have irradiation capacity lasting approximately 7,500 hours. Some
manufacturers suggest annual substitution.




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41


Changing of the ultraviolet lamp
In order to change the ultraviolet lamp, the manufacturers’
instructions must be followed. In general, the following
procedures are done:
1. Turn on the cabinet and leave it working for 5


minutes.
2. Raise the front window to its maximum position.
3. Decontaminate the interior surfaces and the UV lamp.
4. Disconnect the electrical feed to the cabinet.
5. Disconnect the UV tube from its connectors turning


it 90 degrees. Next, install a spare part with the same
characteristics as the original. Some manufacturers have
installed the lamps on a plate located in the front of the
cabinet, which is necessary to unscrew and lift so that
the assembly of the lamp is kept visible. Once this is
done, the lamp can be substituted as indicated above.


Specialized maintenance
Eventually, the cabinet will require specialized maintenance.
The following are some procedures to be done according
to the manufacturer’s technical service manuals by a
specialized contractor.
1. Annual certifi cation in accordance with Regulation NSF


49 outlines.
2. Motor change. Generally, it uses maintenance-free sealed


rollers and function by induction through frequency
control. This motor does not have brushes. (*)1.


3. Replacing ventilators. (*)
4. Replacing the HEPA fi lter (*). The replacement frequency


depends on the use of the cabinet and the system of
environmental control installed in the laboratory. If
there is a good control of dust, the fi lter could last many
years.


5. Repair of the electronic control system: fl ow control
alarms, position of the window, velocity controls.


6. Repair/cleaning of the fl ow regulator valves, bell type
adjustment fi ttings.


Cabinet certifi cation
The certifi cation process of the biological safety cabinets
is regulated by Standard NSF 49, which applies to all Class
II cabinets. This defines materials, design criteria and
construction, operation parameters and tests which allow
the cabinet to be guaranteed as safe and suitable for the
work performed. The following is a list of tests, in which
standards mentioned are included. The standards must be
consulted for details. The certifi cation process comprises
the following tests:
1. Air tightness test. This is done on the exterior surfaces.


Determine if joints, seals, penetration and solderings are
free from leaks.


2. HEPA fi lter leak tests. Determines the integrity of the
supply and extraction of HEPA fi lters, their location and
mounted frames.


3. Temperature increase test. Determines the maximum
temperature increase in the cabinet when the ventilator
and lights are operating.


4. Noise test. Determines the level of noise produced by
the cabinet.


5. Luminous intensity test. Determines the luminous
intensity on the cabinet’s work surface.


6. Vibrations test. Determine how much vibration there
is in the cabinet when it is functioning.


7. Protection test to personnel, to the product and cross
contamination biological tests. The test determines
if aerosols are contained in the cabinet, if external
contaminants reach the work table area and if aerosols
are reduced by the cabinet.


8. Stability test. Determines if the cabinet has structural
stability. Analyzes the resistance to shaking, to distortion
by means of applied force, to defl ection of the work
surface subjected to load and resistance to the tilting
of the work surface due to heavy loading conditions.


9. Vertical fl ow velocity test. Determines the velocity of
the air moved vertically towards the work surface.


10. Entry fl ow velocity test. Determines the velocity at
which the fl ow enters the cabinet through the front
opening and the cabinet’s extraction volume.


11. Smoke test. Determines if the fl ow of air along the
entire perimeter of the front opening advances towards
the cabinet, and if the vertical fl ow moving towards the
bottom does not show dead points or fl ow backs on the
work surface.


12. Drainage escape test. Defi nes the contention capacity
for spills below the work surface.


13. Motor/ventilator system functioning test. Determines
if the system provides the necessary static pressure.


14. Electric system test. Determines if there are potential
risks of electrical discharges. Measures the escaping
currents, the polarity, the functioning of the ground
defect protection system and the ground circuit
resistance.


FUNCTIONAL EVALUATION (ALTERNATIVE)
In case there are biological safety cabinets in the laboratory,
but no authorized certification services available, the
personnel responsible for maintenance has the option of
conducting annual revision procedures based on Standard
NSF 49. Duly documented, it should identify with low levels
of uncertainty if the cabinet is in good condition and its
operation normal2. The following are outlines of how these
activities must be done.
1. Installation evaluation. Verify that the cabinet


installation conditions are in accordance with the
recommendations from the manufacturer.


1 (*) These require specialized decontamination beforehand.
2 The functional evaluation is essentially based on the availability


(institutional or zonal) of properly trained and experienced technicians and
engineers.




C H A P T E R 6 B I O LO G I C A L S A F E T Y C A B I N E T


42


2. Operational evaluation. Test to see if the cabinet is
working in accordance with its manufacturing and
design characteristics.


3. Performance evaluation. Verify the cabinet’s capacity to
provide an adequate work space in normal and critical
working conditions.


In the following table are featured the parameters to be
taken into account in the functional evaluation. These are
generally included in inspection forms1 designed for this
purpose.


Parameters Observation


Institutional identifi cation of cabinets Brand, model, type, series, location, inventory code, date.


ELECTRICAL


Voltage Voltage measurement. Requires a voltmeter.


Amperage Amperage measurement. Requires a voltmeter or amperemeter clip.


Motor/ventilator Verifi cation of operation temperature. Verify noise level and vibration.


Illumination – Fluorescent Confi rmation that the lamp is functional.


Illumination – Ultraviolet Confi rmation of the operational hours of the lamps and their light intensity. Requires a radiometer.


Electrical outlet Integrity revision, quality of the contact and available voltages.


Switches Control of state and integrity.


Integrity cables and connectors Visual verifi cation.


Alarms Testing of state and calibration.


PHYSICAL


Internal/external fi nishes Visual verifi cation.


State of fi lters and pre-fi lters Visual verifi cation. There must be no leaks, neither in the fi ltering material nor in the seals.


Seals/gaskets Visual verifi cation. There must be no leaks.


Sliding window Visual verifi cation. Must be able to be moved smoothly and maintain the selected positions.


OPERATIONAL


Flow velocity Control of velocity according to the class and type of cabinet. Requires an anemometer (wind gauge).


Noise level Requires audiometer.


Pressure diff erential in the HEPA fi lter. Take a manometer reading of the cabinet.


PERFORMANCE


Counting of particles Method defi ned in the Federal Standard 209D, E. Requires DOP generator, photometer and particle counter.


CONDITIONS OF THE INSTALLATION AREA


Temperature Requires thermometer: approximately 20–22 °C.


Humidity Requires hygrometer: approximately 45–55 %.


Cleanliness Must be adequate.


Air currents There must be no air currents to aff ect the working of the cabinet.


Table of functional evaluation of biological safety cabinets


1 Each institution designs its own formats for record keeping of technical
maintenance.




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43


TROUBLESHOOTING TABLE1


PROBLEM PROBABLE CAUSE SOLUTION


Neither the light nor the ventilation system in the
cabinet works.


The cabinet is disconnected from the electrical
outlet.


Verify that the cabinet is connected to an electrical
outlet and that the cable is well connected to the
cabinet’s electrical box.


There is no electrical feed in the connection. Confi rm that the electrical outlet is energized and
that the circuit breaker is not deactivated (thermo
magnetic protection). Restart switches.


The cabinet’s ventilator is functioning but the light
does not.


The lamp is defective. Replace the lamp. Use one with the same
characteristics of the original


The lamp is badly connected. Check the lamps connection. Adjust to the correct
position.


The thermo magnetic protection of the service
breaker is activated.


Reconnect the circuit breaker.


The lamp’s wire is disconnected. Check the lamp’s wire.


The lamp’s ballast is defective. Replace the ballast.


The ventilator is not blowing but the light is coming
on.


The front window is closed. Open the window until it reaches the work position.


The ventilator’s motor is defective. Replace the motor ventilator set.


The ventilator’s motor is disconnected. Check the motor’s connections.


The manometer indicates an increase in the fall of
pressure through the fi lter.


Retention of particles in the HEPA fi lter has
increased.


Normal process during the useful life of the fi lter.


There is blockage in the grids or return slots. Verify that the grids are not obstructed by
equipment or material.


The extraction pipe is obstructed. Test that there are no existing blockages or
restrictions in the extraction pipe.


There is a blockage or restriction under the work
surface.


Verify that the pipe below the work surface is free of
obstructions.


There is contamination in the samples manipulated
in the cabinet.


Work procedures are inadequate. Check that the cabinet is being used according to
procedures and good practices.


Restrictions in the return slots or blockage of the
extraction duct.


Test the return and extraction system to see if they
are free from obstructions.


The cabinet’s external factors aff ect its fl ow patterns
on the inside and cause contamination.


Verify the installation of the cabinet and the
procedures that are being carried out.


The HEPA fi lter is defective. Replace the HEPA fi lter and certify the cabinet.


1 Purifi er® Delta® Series, Biological Safety Cabinets, User’s Manual, Kansas City, Labconco Corporation, Part Nº 36960-20, Rev. A ECO B296.




C H A P T E R 6 B I O LO G I C A L S A F E T Y C A B I N E T


44


BASIC DEFINITIONS


Aerosol. A suspension of fi ne solid or liquid particles in the air. Their average diameter ranges between 10-4 and 10-7 cm.


Air supply. Air which enters the cabinet through the front or work opening and replaces the air extracted from the cabinet.


Biological Safety cabinet. Equipment with appropriate ventilation conditions protecting the user, the environment and the sample from aerosols and microparticles,
associated with the management of potentially infectious biological material in laboratories as a result of activities such as agitation, centrifugation, use of pipettes
and opening of pressurized containers.


Certifi cation. Procedure establishing that the biological safety cabinet’s functioning complies with criteria and minimum requirements to operate safely. Standard
NSF 49 applies to the Class II cabins, Type A, B1, B2 and B3.


Decontamination. Removal or destruction of infectious agents; removal or neutralization of toxic agents.


HEPA fi lter. A fi lter with the ability to remove particles with average diameters of 0.3 µm with 99.97 % effi ciency. These fi lters are constructed of Boron silicate
micro fi bres bonded together with a water resistant adhesive. The fi ltering material is folded inside of a frame with the aim of increasing the fi ltration area.


Laminar fl ow. Non-turbulent fl ow of a viscous fl uid (e.g. air) in layers near a boundary. It occurs when Reynolds number [Re] is less than 3000.


NSF. An acronym of the National Sanitation Foundation, a non-profi t organization dedicated to research, education and service, which seeks to resolve problems
related to human beings, promote health and enrichment of the quality of life through conservation and improvement of the environment. NSF standards supply
the basic criteria for promoting salubrious conditions and public health protection.


Toxic. A substance with a physiologically adverse eff ect on the biological systems.


Ultraviolet light (UV). This is electromagnetic radiation, the wavelength of which is between 200 and 390 nm. It is used in biological safety cabinets for its
germicidal properties.


Work surface. A surface used when performing work, operation or activity inside the biological safety cabinet in this case.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


45


Chapter 7


Centrifuge
GMDN Code 15115 10778 10778


ECRI Code 15-115 15-117 15-116


Denomination Centrifuges, standing, low velocity,
non-refrigerated, for blood bank


Centrifuge, standing,
refrigerated


Standing centrifuge


The word centrifuge comes from the Latin word centrum
which means centre and fugere which means to escape. The
centrifuge is designed to use the centrifugal force generated
in rotational movements to separate the constitutive
elements of a mixture. There is a wide range of centrifuges
capable of serving specifi c industry and research needs. This
chapter focuses on standing centrifuges normally used in
public health and clinical laboratories.


PHOTOGRAPH OF CENTRIFUGE


PURPOSE OF THE CENTRIFUGE
The centrifuge uses centrifugal force (the force generated
when an object rotates around a single point), for separating
solids suspended in a liquid by sedimentation, or liquids of
diverse density. The rotational movements allow forces much
greater than gravity to be generated in controlled periods
of time. In the laboratory, centrifuges are generally used in
processes such as the separation of solid components from
biological liquids through sedimentation and in particular
of blood components: red cells, white cells, platelets among
others and for conducting multiple tests and treatments.
There are several kinds of centrifuges. The most widely used
in public health, surveillance and clinical laboratories are the
table-top centrifuge, the ultracentrifuge, the haeamatocrit
centrifuge and the standing centrifuge.


OPERATION PRINCIPLES
Centrifuges represent a practical application of Newton’s
law of motion. When a body of mass [m] turns around
a central point [O], it is subjected to a centripetal force
[N] directed towards the rotation axis with a magnitude
N = mω2R, where [m] is the mass of the body, [R] is the radius
and ω is the angular speed. Centrifuges possess a rotating
axis on which is mounted a rotor with sample receiving
compartments. Tangential speed is defi ned by the following
equation: VT=ωR.


Ph
ot


o
co


ur
te


sy
o


f B
ec


km
an


C
ou


lte
r




C H A P T E R 7 C E N T R I F U G E


46


When the system spins at a speed of ω radians per second,
the samples are subjected to the centrifugal force Fp of the
same magnitude as N, but in an opposite direction. The
fi gure shown below1 features a diagram of the concept,
of its actual application and of the obtained result. This
Fp force acts on particles in the substance centrifuged,
causing them to separate as a result of diff erences in density.
Denser particles will settle at the bottom of the tube in
shorter periods of time, while lighter ones require longer
periods of time, settling onto those of greater density. The
relationship between the centrifugal acceleration [ω2r ] to a
given radius [r] and the force of gravity [g] is known as the
relative centrifugal fi eld or [RCF]2.


RCF =
rω 2


g


The RCF is the tool which allows rotors of different
specifi cations to be compared when equivalent centrifugal
eff ects are required.


COMPONENTS OF THE CENTRIFUGE
The most important components of a centrifuge are the
following3:
The electric/electronic control which generally has the
following elements:
1. On and off control, operation time control (timer),


rotation speed control (in some centrifuges), temperature
control (in refrigerated centrifuges), vibration control
(safety mechanism) and brake system.


2. Refrigeration system (in refrigerated centrifuges).
3. Vacuum system (in ultracentrifuges, not shown in the


fi gure).
4. Base
5. Lid/cover
6. Casing
7. Electric motor
8. Rotor. There are different types of rotors. The most


common are the fi xed angle, the swinging buckets, the
vertical tube and the almost vertical tube types, which
are explained next.


1 Newton’s law of movement, together with the explanation of the inertia
marks of reference can be consulted in books on physics, chapters on
uniform circular movement.


2 RCF. Relative Centrifugal Field.
3 The numbers identifying each component correspond to those in the


sectional diagram of the centrifuge.


Sectional diagram of a centrifuge (numbers correspond to
descriptions in the text above)


Figure 20. Centrifugal force concept




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47


Types of rotors
Centrifuges use many diff erent types of rotors. Among the
most commonly used are the following:


Type of rotor Characteristics Transversal cross-section


Fixed angle rotors. These are general purpose rotors. They keep tubes at a fi xed
angle [α] which by design, is specifi ed between 20 and 45
degrees. They are used for sediment sub-cellular particles.
The angle shortens the trajectory of the particles and the
centrifugation time compared to the swinging buckets
rotors.


Swinging buckets rotors. These are used for carrying out isopycnic studies (separation
by density) and rate-zonal studies (separation by
sedimentation coeffi cient), where maximum resolution of
the zones is required for the sample.


Vertical tube rotors. This type of rotor keeps tubes parallel to the rotational axis.
Thus, separate bands are formed across the tube’s diameter,
not its length. These rotors are used for carrying out
isopycnic studies and in some cases, zonal limit separations
where a short centrifugation time is important. These rotors
use specially designed tubes.


Almost vertical tube rotors. This type of rotor is designed for gradient centrifugation
when some sample components do not participate in
the gradient. The small angle of these rotors reduces the
centrifugation time in comparison to fi xed angle rotors.


Position in
Rotation


Position
at Rest


r




C H A P T E R 7 C E N T R I F U G E


48


Normally, manufacturers specify rotors to be used in
centrifuges by providing specialized publications of tables
with the following information:
1. Type of rotor. Specifi es the type of rotor for which the


technical information is being provided.
2. Nominal capacity of the rotor. Defi nes the capacity in


litres or litre submultiples. For example: 6 litres; 250 ml,
etc.


3. Maximum speed. This indicates the maximum speed
at which this particular rotor should be operated in
revolutions per minutes (RPM).


4. Maximum Relative Centrifugal fi eld (RCF) obtained by
that type of rotor.


5. k Factor, the sedimentation coeffi cient, defi ned by the
following equation:


Where:
ω= angular speed in radians per second
r


max
= maximum radius in mm, measured in the


centrifugation tube
r


min
= minimum radius in mm, measured in the


centrifugation tube
The time required for sedimentation can be calculated


in hours using this factor.
6. Information on the compatibility of the rotor with other


models of centrifuges from the same manufacturer.


Recently manufactured centrifuges have incorporated
numerous improvements into their design to provide
greater safety and longer operational life. Among advances
mentioned are controls based on microprocessors. By means
of software controlled by a keyboard, these have several
diff erent operational programs in memory. According to the
type of rotor being used and procedure conducted, these
programs control the centrifugation time, the required
temperature, the rotor’s revolutions, the acceleration and
deceleration, alarms warning the operator about any
anomaly during operation.


Manufacturers have also incorporated induction motors
(without brushes) in centrifuges. These have the advantage
of electronically controlling currents and magnetic
fields regulating the rotor’s speed which reduces the
frequency of maintenance. Operation and maintenance
of such equipment must be carried out according to the
manufacturer’s recommendations.


INSTALLATION REQUIREMENTS
Centrifuges require the following for normal operation:
1. An electrical connection with a capacity suitable for the


equipment providing stable single phase or triphase


type voltage (depending on the model and specifi cation
given by the manufacturer). In general, centrifuges use
110V or 220 V/60 Hz.


2. A clean, dust free environment with a fi rm levelled
fl oor.


3. If the centrifuge is refrigerated, it needs a free space on
the side of the condenser for adequate heat transfer.


4. A cabinet in which the centrifuge accessories such as
the alternate rotors can be kept.


ROUTINE MAINTENANCE
The routine maintenance required by a centrifuge depends
on multiple factors such as the incorporated technology,
usage intensity, training of users, quality of the electrical
feed and environmental conditions. The following are
general recommendations regarding adequate use and
most common maintenance for guaranteeing correct
operation. The routines or specialized repairs will depend on
manufacturers’ recommendations for each brand and model.
Always disinfect the rotor bowl, centrifuge head, buckets
and trunnion rings as applicable before any servicing of
centrifuges used to prepare clinical or infectious samples.


Priority recommendation. Verify that only qualifi ed personnel
trained and familiar with the use, care, risks and handling
of the centrifuge operates it. It is the laboratory directors’
responsibility to supervise and take necessary precautions
so that personnel operating centrifuges understand the
implications of working with such equipment.


APPROPRIATE MANAGEMENT AND STORAGE
RECOMMENDATIONS1


Rotors
1. Register the date of purchase of each one of the rotors,


including information related to the serial and model
number.


2. Read and understand the rotor manuals, equipment
and tubes before use. Comply with indications for use
and care specifi ed by the manufacturer.


3. Use rotors only in centrifuges for which these have
been manufactured. Do not interchange rotors without
verifying the compatibility with the centrifuge.


4. Register operation parameters for each rotor in a log
book in order to determine its remaining operational
life and to acquire its replacements when needed.


5. Use the recommendations regarding maximum speed
and sample density from the manufacturer. Each rotor
is designed for supporting a maximum level of eff ort;
these specifi cations must be followed rigorously.


1 http://www.sunysb.edu/facilities/ehs/lab/cs.shtml


k =
ln rmax rmin( )


ω 2
×


1013


3600




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


49


6. Obey the recommendation related to reducing the
operation speed when working with high density
solutions in stainless steel tubes or plastic adaptors.
Manufacturers provide the related information.


7. Use titanium rotors if working with saline solutions
frequently.


8. Protect the rotors’ coating in order to avoid the metal
base from deteriorating. Do not use alkaline detergents
or cleaning solutions which can remove the protective
fi lm. The rotors generally made of aluminium [Al] are
covered by a fi lm of anodized aluminium which protects
their metal structure.


9. Use plastic brushes when cleaning the rotor. Metal
brushes scratch the protective coating and generate
sources for future corrosion. Corrosion is accelerated
in operation conditions and shortens the rotor’s
operational life.


10. If there are spills of corrosive substances, wash the rotor
immediately.


11. Air dry the rotor once cleaned and washed with water.
12. Store vertical tube rotors and almost vertical tube rotors


with the larger side facing downwards and without their
covers.


13. Store rotors in a dry area. Avoid leaving them in the
centrifuge.


14. Store swinging buckets rotors without the compartments’
covers.


15. Lubricate spiral and O-rings, according to the
manufacturer’s recommendation.


16. Observe recommendations related to guaranteed times
and operational life of each type of rotor.


17. Avoid using rotors whose operational lives have
ended.


18. Use a shield if working with radioactive material.
19. Load or unload rotors inside a biological safety cabinet


if working with materials classifi ed as Biosafety level II
or higher.


20. Never try to open the cover of a centrifuge while it is
functioning and never try to stop the rotor by hand.


Tubes
Tube care includes aspects such as fi lling of the tubes,
adequate temperature selection, centrifugation speed
limitations, washing and sterilization. The principle
recommendations are the following:
1. Wash tubes, adaptors and other accessories by hand


using a 1:10 mild detergent solution in water and a soft
textured brush (not metallic). Avoid using automatic
dishwashers.


2. Avoid using alcohol and acetone since such liquids aff ect
the structure of the tubes. Manufacturers recommend
the solvent to be used with each type of centrifugation
tube material.


3. Avoid drying tubes in a drying oven. Dry always with a
stream of hot air.


4. Verify if the tubes are reusable or not. If they are
disposable, use them only once.


5. For sterilizing, it is necessary to verify the material from
which the tube is made, as not all can stand sterilization
by heat. Glass tubes are normally sterilized with vapour
at 121 °C for 30 minutes.


6. Store tubes and bottles in a dark, fresh, dry place,
far from chemical vapours or ultraviolet radiation
sources.


7. Verify maximum fi lling levels and the sealing of thin
wall tubes in order to avoid collapse inside the rotor
by the action of the centrifugal force. Comply with
manufacturers recommendations.


Preventive maintenance


Warning: Never carry out a technical intervention in a
centrifuge if it has not been previously decontaminated.


The most important maintenance routines performed on a
centrifuge are the following:
Frequency: Monthly
1. Verify that the centrifuge external components are free


of dust and stains. Avoid aff ecting the rotor with spills.
Clean the rotor compartment using a mild detergent.


2. Test that the rotors’ connecting and adjustment
mechanisms are in good condition. Keep the points
lubricated as the manufacturer recommends.


3. Verify the locking /safety mechanism of the centrifuge’s
cover. This is fundamental in guaranteeing operators’
safety as this mechanism keeps the cover of the
centrifuge closed while the rotor is turning.


4. Check the lubrication state of elements such as for
O-rings as the manufacturer recommends. Always use
lubricants according to the manufacturer’s instructions
(frequency and type of lubricants). In recently
manufactured centrifuges, there are sealed ball bearings
which do not require lubrication.


5. Verify the state of gaskets and watertight joints.


Frequency: Annually
1. Verify that electronic cards are clean and well connected.
2. Test operation controls needed for selection of the


diff erent parameters of the centrifuge: speed, time,
temperature, alarms selectors and analogous or digital
instruments.


3. Verify compliance with electrical standards. Use an
electric safety analyzer: earth resistance test, escaping
current test.


4. If the centrifuge is refrigerated, test the temperature by
using an electronic thermometer. The temperature must
not vary by more than ± 3 °C.


5. Examine the exactitude of the time controls. Use a timer.
The time measured must not vary by more than ± 10 %
of the programmed time.




C H A P T E R 7 C E N T R I F U G E


50


6. Verify the actual rotation speed against the selected
one using a normal load. The testing is done with
a tachometer or a photo tachometer. If the hatch
is not transparent, the procedure indicated by the
manufacturer must be followed.


7. Confi rm the functioning of the brake system.
8. Verify the functioning of the refrigeration system in


refrigerated centrifuges. The following are the most
important activities:
a) Check the selected temperatures. These should


not vary by more than 3 °C from the temperatures
measured on the digital thermometer.


b) Verify the state of the air intake filter. If the
fi lter is obstructed, clean or substitute with an
equivalent.


c) Conduct a detailed cleaning of the diff using wing of
the condenser to eliminate the fi lth deposited. This
maintains the heat transference rate according to
the design specifi cations. If abnormal functioning
is detected, seek assistance from a specialized
service technician.


Note: Avoid spilling liquids on control keys. The keys must
be operated with the fi ngertips: The operator should avoid
using fi ngernails, as this can result in the perforation of their
protective membrane.


Every six months:
Verify the state of the motor’s brushes, if the centrifuge has
a motor with brushes. Substitute with new ones (with the
same specifi cations as the original) if necessary. Perform this
routine every six months.


Tools and required instrumentation
In order to carry out the maintenance inspections normally
required for a centrifuge, the following tools or instruments
are necessary:
1. A key for tightening and slackening the rotor’s nuts.
2. An electrical safety analyzer or an instrument for


measuring escaping current.
3. A timer.
4. An electronic thermometer with exactitude of 0.5°C for


refrigerated centrifuges.
5. A tachometer or photo tachometer.


TROUBLESHOOTING TABLE


Rotors1


PROBLEM PROBABLE CAUSE SOLUTION


Severe vibration. The rotor is unbalanced. Balance the rotor’s load. Fill all the opposite tubes
with the same level of liquid of same density.


Distribute the weight of the opposite tubes
symmetrically.


Load fi xed angle or vertical tube rotors
symmetrically.


The speed selected is near the rotor’s critical speed
range.


Select a rotation outside of the critical speed range.


The rotor is incorrectly mounted. Verify the rotor’s assembly. Test that it is well
adjusted.


There is a lack of lubrication in the rotor’s supports. Lubricate the pivoting axis according to the
manufacturer’s recommendation. For e.g. each 250
centrifugation procedures.


Rotor covers, canister or cubes diffi cult to loosen
after centrifugation.


A vacuum is being produced during centrifugation. Open the ventilation line in the upper part of the
rotor or bucket to eliminate the vacuum.


The rings are contaminated with fi lth, dried
lubricants or metallic particles.


Perform routine cleaning of the rings and lubricate.
Use recommended products recommended by the
manufacturers.


1 Rotors and Tubes for Beckman Coulter J2, J6 and Avanti® J series centrifuges, User’s Manual, Palo Alto, California, The Spinco Business Center of Beckman Coulter, 2001.




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51


Tubes


PROBLEM PROBABLE CAUSE SOLUTION


The tubes leak. The covers are badly secured. Adjust the covers.


The tubes are too full. The meniscus must be lower in order to prevent
leaks.


The maximum recommended level has been
exceeded in the open tubes.


Verify the volume and speed recommendations for
the centrifugation.


A defi cient seal is presumed in the rapid seal tubes. Press lightly, after heat sealing (only if the contents
are not aff ected). If leaks are visible, seal again.


The tubes are cracked or broken. The tubes can be broken or become fragile if they are
used below the recommended temperature.


If the sample is frozen, warm to 2 °C before
centrifuging. Evaluate how the tubes behave at low
temperatures before centrifuging.


The tubes become fragile with age and use. Discard expired tubes, use new ones.


Various systems


PROBLEM PROBABLE CAUSE SOLUTION


The main switch is in the on position but the
centrifuge is not functioning.


There is no power to the instrument. Verify the power supply.


The centrifugue cover cannot be opened. The centrifuge is off . Turn the centrifuge ON. Press the handle and open
the cover.


The balance indicator is activated. The load to be centrifuged is unbalanced. Balance the load to centrifuge.


The centrifuge is not levelled. Level the centrifuge.


There is a vibration at low speed. The rotor adjustment mechanism is slack. Correctly adjust the fastening system.


The load is unbalanced. Verify the balance of the load to be centrifuged.


The selected speed is close to the rotor’s resonance
point.


Select a more elevated rotation speed or use a
diff erent type of rotor.


There are fl uctuations in the rotation speed. The transmission belts are in a bad condition (*). Turn off the centrifuge. Verify the condition and state
of the belts. The belts must be tempered.


The rotation speed does not reach the selected
speed.


The brushes are defective. Turn off the centrifuge. Verify the condition of the
brushes. If this is the problem, put new brushes with
the same specifi cations as the originals.


The speed control calibration is maladjusted. Adjust the speed control calibration.


The chamber is cold but the rotor is warm. The temperature is incorrectly selected. Verify the temperature selection.


The display which signals the state of the brushes
is on.


The brushes are in a bad condition. Turn off the centrifuge. Verify the condition of the
brushes. Substitute the brushes by others with the
same specifi cation.


(*) Valid procedure in centrifuges with potential belt transmission system.




C H A P T E R 7 C E N T R I F U G E


52


BASIC DEFINITIONS


Anodized coating. A hard, thin layer of aluminium oxide, which is deposited on the surface of a rotor by means of electrochemical processes with the aim of
preventing corrosion. The coating is often fi nished in various colours.


Angular speed. The turning rate of a body measured in radians per second. It is calculated using the following formula:


Where:
rpm = revolutions per minute
π = constant with a value of 3.1416


Brush. A device that transmits electrical energy between the external electrical connection (cables in a static state) and the internal components (in rotation) of a
motor. In general, brushes are manufactured in very soft textured graphite and, in motors, must be changed regularly (every six months).


Centrifugal force. Apparent force equal and opposite to the centripetal force, driving a rotating body away from the centre of rotation and caused by the inertia of
the body. It is one of the components of the inertia vector, which equals the set of forces acting on a body. Its magnitude is always [m x a


n
] and its direction radial,


moving away from the centre.


Density. A body’s mass by volume unit, generally expressed in gram per cm3.


Isopycnic separation. A method for separating particles based on the density of the particle’s fl otation. It is known as sedimentation in balance. The speed of a
particle due to diff erences in density is given in the formula:


Where:
v = speed of sedimentation


d = diameter of the particle
ρ


p
= density of the particle


ρ
c
= density of the solution


µ = viscosity of the liquid medium
g = gravitational force


Radian. A unit of angular measure equal to the angle subtended at the centre of a circle by an arc equal in length to the radius of the circle. It is expressed as the
ratio between the arc formed by the angle with its vertex in the centre of the circle, and the radius of that circle.


RCF (Relative centrifugal fi eld or force). A relationship between the centrifugal acceleration and a specifi c speed and radius, [rω2] given with the normal gravity
acceleration. It is calculated by means of the following equation:


Where:
R = radius in mm
ω= angular speed in radians per second


g = Standard gravity acceleration = 9 807 mm/s2


Resonance. A situation in which a mechanical system vibrates as a response to a force applied at the system’s natural frequency.


Sedimentation. Particles from a suspension settling at the bottom of the liquid as a result of the action of the gravitational force. During centrifugation, this process
is accelerated and particles move away from the rotational axis.


ω =
2π × rpm


60


D =
m
V


v =
d2 ρp − ρc( )


18µ

⎝ ⎜ ⎜



⎠ ⎟ ⎟ × g


dr
dt



⎝ ⎜



⎠ ⎟


ω =
2π × rpm


60


RCF =
rω 2


g




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53


Chapter 8


Water Distiller


The word distiller comes from the Latin word distillare
which means to vaporize liquids through heat. The water
distiller, also called distillation unit or water still, used in the
laboratory, purifi es running water by means of controlled
vaporization and cooling processes. Upon applying thermal
energy to water in a liquid phase by a warming process, it
is changed into vapour. This allows the water molecules to
separate from the molecules of other substances mixed
or diluted. The water vapour is collected and passed
through a condenser, where it is cooled and returned to
the liquid phase. Then, the condensed water is collected
into a diff erent storage tank. Distilled water shows pure
characteristics compared to running water; it is practically
free of contaminating substances.


PURPOSE OF THE WATER DISTILLER
The water distiller facilitates obtaining very pure water from
potable water normally provided by the aqueduct services
in urban centres. Distilled water is characterized by a lack
of solids in suspension. It is used in multiple applications
in centres which provide health services, especially in
laboratory units, in washing, sterilization and dietetics. The
more specialized the procedures are in the laboratory, the
greater will be the level of purity required. For example: the
preparation of reagents or biological material requires water
of the highest quality. Distillation is one of the fundamental
processes to achieve this (although it may not be the only
one required). Water used in laboratories must be free of
pyrogens, with a concentration of total solids no greater
than 1 ppm, a pH value between 5.4 and 7.2 and an electrical
resistance of at least 3 x 105 ohm/cm at 25 °C1.


1 Warming cabinets, sterilizers, and associated equipment, Division 11–
Equipment, USACE/NAVFAC/ AFCESA, UFGS-11710, July 2003.


DIAGRAM OF A WATER DISTILLER


GMDN Code 40478


ECRI Code 15-136


Denomination Distillation units


1. Vapour Generator


2. Water Level Gauge


3. Control Valve


4. Hydraulic Connection


5. Water Liquid Phase


6. Immersion Resistance


7. Cooling Water Exit


8. Condenser/Distiller


9. Activated Carbon Filter


10. Distilled Water Deposit


11. Cold Water Entry


Figure 21. Water distiller




C H A P T E R 8 WAT E R D I S T I L L E R


54


OPERATION PRINCIPLES
The function of a distiller is based on a phenomenon
demonstrated in nature known as the water cycle. The
energy coming from the sun heats the water from the seas
and transforms part of it into water vapour. This vapour is
concentrated in clouds. When atmospheric conditions are
suitable, these cool and condense the water which returns
to the surface of the Earth in the form of rain.


Functioning of the water distiller
The water distiller reproduces the natural phenomenon
described above. The configuration and design vary
depending on the volume of water required. The following
is a general explanation of the components of a distiller and
a description of how these function.
1. Vapour generator. Also known as the boiling tank,


this component is the container where the water to
be distilled is stored. In general, it has a hydraulic
connection which allows the water evaporated and
distilled to be replenished. It is generally made of glass
in small distillers or of stainless steel with copper, tin or
titanium coverings in large capacity machines. It can
have level, fl ow and water quality feed controls, which
protect the distiller in case some irregularity in the
water supply occurs. As a source of energy, it uses the
water vapour coming from a boiler or vapour generator,
or the thermal energy from electrical immersion
resistors through direct conduction. These cause the
water temperature to rise until, in normal conditions
(atmospheric pressure equal to an atmosphere and
gravity acceleration equal to 9.80665 m/s2) water in the
liquid phase is transformed into vapour at 100 °C.


2. Water level. Device which allows the quantity of water
to be regulated inside the vapour generator. It is joined
directly to the connection which supplies the water
used by the distiller. When the quantity of water in
liquid phase contained in the boiling tank decreases,
the device allows the quantity of liquid evaporated to
be recovered.


3. Control valve. Mechanical or electromechanical device
which allows the fl ow of water towards the vapour
generator tank to be regulated.


4. Hydraulic connection. Network which supplies water
in liquid phase to the vapour generator tank.


5. Water in liquid phase. Water inside the vapour generator
tank. It receives thermal energy from the immersion
resistors and it is converted to vapour when the required
temperature and pressure conditions are met.


6. Immersion resistors. Devices generating heat when
an electrical current circulates through them. These
are isolated by a ceramic cap and protected from the
external environment by a metal shield.


7. Refrigeration water outlet. Line carrying the water
used for condensing the water vapour thus removing
the thermal energy from it (cooling).


8. Condenser. Device in which the vapour loses thermal
energy, cools and returns to its liquid phase. In order
to accelerate the process, forced convection by low
temperature fl uid circulation (air or water) around the
line through which the vapour fl ows is used.


9. Filter. Distillers have activated carbon fi lters located at
the exit of the condenser or collector. These eliminate
fl avours or particles which may be present in the vapour
being condensed.


10. Distilled water container. Device in which the fl uid
completing the distillation process is collected. Distilled
water must be stored in special plastic containers to avoid
ionic contamination. Polyethylene, polypropylene or
polytetrafl uoroethylene containers are generally used.


INSTALLATION REQUIREMENTS
Depending on the design, capacity and type of distiller,
the required installation may vary. The most common
requirements are the following:
1. A well ventilated environment in which the equipment


can be installed. This is necessary because the distiller
transfers heat to a fl uid and increases the temperature
of the area where it is installed. It is necessary to leave
free space around the distiller so that the fl ow of air is
facilitated. Some distillers are assembled inside a metal
box and need to be installed on a support to facilitate
the circulation of air under them.


2. A potable water connection. Typically the required
hydraulic connection has a diameter of 1/2”. To ensure a
smooth operation, the quality of the water feeding the
distiller must be evaluated to determine if it is necessary
to install a treatment system1 to prevent the presence
of incrustations or sediments in the vapour generating
tank and on immersion resistors. Potable water is used
for feeding the vapour generator and for refrigerating
the condenser2.


3. A distilled water connection. The distilled water produced
is initially collected into a storage tank. In large capacity
equipment, it is distributed to consumption points from
the tank by means of a network. In small or medium
equipment, it is transferred to containers from which it
is used at the feed points.


4. Cleaning connection. This is used to drain impurities
which may accumulate in the vapour generator tank
using a siphon located near the distiller.


1 Water treatment has been designed for removing substances normally
present in water due to the great solvent capacity of water. The substances
in general are inorganic ions (anions and cations) such as bicarbonate,
sulphite, chloride, calcium, magnesium, sodium, potassium, magnesium,
iron, nitrates and traces of many others.


2 Some manufacturers cool the condenser through the use of ventilators
which make air circulate on the condenser’s fi ns, generating heat
transference processes by forced convection from the diff usion surface to
the environment.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


55


5. An electrical connection equipped with control
and safety devices complying with the national and
international electrical standards used in the laboratory,
adapted to the capacity of the resistive elements of the
distiller. In general, the voltage is 220-240 V, 50/60 Hz.


Note: Always verify manufacturer’s recommendations on
installation to ensure the distiller is operating according to
the specifi cations.


ROUTINE MAINTENANCE
The maintenance depends on the design and capacity of
the distiller. The maintenance described in this manual
focuses on a distiller equipped with a stainless steel vapour
generator tank with immersion resistors and a condenser
refrigerated through a ventilator impelling air (on or through
the condenser’s diff using fi ns).


Warning: Before carrying out an inspection or routine
maintenance, verify that the distiller is turned off and
disconnected from the electrical source.


Inspection and cleaning of the vapour generator tank
Frequency: Monthly
1. Remove the protective panel or open the door allowing


access to the boiling tank or vapour generator.
2. Remove the cover of the boiling tank.
3. Visually verify if the interior walls or the immersion


resistors show solid deposits or sediments. The quantity
of deposits present depends on the quality of water fed
to the distiller. If there is an accumulation of sediments,
it must be cleaned to avoid damaging the resistors1.


4. Clean accumulated deposits. In general, the cleaning
process requires a chemical product especially designed
for removing them. The product must be selected
according to the characteristics of the water used. This
is determined by a chemical analysis.


5. Drain water from the generator tank until its level is
approximately 10 cm above the location of the water
level probe or the immersion resistance (verify that the
water level is higher than the base of the tank to ensure
that all of the elements stay submerged in water).


6. Add the chemical product recommended for the type
of water used.


7. Mix well.
8. Allow the chemical to act overnight or as recommended


by the manufacturer.
9. Drain the contents of the tank on the following


morning.
10. Add clean water, wash and drain until the chemical


has been completely removed along with the mineral
residues from the aff ected surfaces.


11. Reinstall the cover.
12. Place the front panels or adjust the door.
13. Operate the equipment normally.


Warning: Under no circumstances, should the solution used
for removing sediments be distilled.


Change of the activated carbon fi lter
Frequency: Every three months
Normally, the activated carbon fi lter is submerged in water
below the dispenser system which comes from the distilled
water storage tank. It is assembled on a casing installed on
the distilled water distribution line. In general, it is a device
which can be easily substituted. The following process is
generally done:
1. Unscrew the top of the fi lter.
2. Remove the used fi ltering element.
3. Install a new element with the same characteristics as


the original.
4. Reinstall the top of the fi lter.


Warning: The fi lter is adjusted inside its casing by means
of O-rings or gaskets that must be installed carefully within
their grooves in order to avoid leaks of distilled water.


Cleaning of the condenser
Frequency: Annually
1. In order to clean the condenser, it is necessary to remove


the protective panels or open the door, giving access to
the condenser.


2. Verify that the distiller is disconnected from the electrical
outlet.


3. Remove the condenser. Disconnect the linkage system
for the entry of vapour and the connection which links
the condenser to the distilled product storage tank.


4. Remove screws joining the ventilator with the condenser.
Disconnect the ventilator terminals from its connection
points.


5. Remove the ventilator and clean the dirt accumulated
on the blades. Lubricate the rotation axis with mineral
oil (two drops).


6. Remove the condenser. Aspirate dirt, dust and fl uff
accumulated on the surface of the diffusing fins.
Compressed air or a brush dampened with soap and
water can also be used.


7. Rinse the parts.
8. Dry.
9. Assemble again in the reverse order to that described.
Sterilization of the distilled water storage tank


1 The minerals deposited on the cover of the immersion resistors are
particularly poor heat conductors in that they impede an effi cient transfer
of heat between the immersion resistance and the water in the distillation
process. This makes the temperature of the resistance rise above that it
would reach in normal operating conditions, deteriorating its condition and
integrity..




C H A P T E R 8 WAT E R D I S T I L L E R


56


Frequency: Occasionally
Before operating a new water distiller, it is recommended
to insure that the distilled water storage tank is sterile and
clean. To carry out the sterilization, use a chemical process
with domestic bleach (chlorine based), for example. The
procedure is as follows:
1. Verify that the main switch is off .
2. Open the front panel in order to access the storage tank


for the distilled product.
3. Remove the activated carbon fi lter from its housing.
4. Prepare a chlorine bleach solution with a concentration


of 200 ppm and add it to the storage tank.
5. Allow the solution to interact with the tank for at least


three hours.
6. Empty the storage tank using the drainage line.
7. Turn on the distiller and allow the storage tank to be


fi lled with distilled water.
8. Drain the storage tank again.
9. Install the activated carbon fi lter in its place.
10. Allow the distiller to fi ll the storage tank with distilled


water. The activated carbon filter will remove any
remnant of chlorine bleach used.


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The distiller does not produce distilled water. There is no energy supply. Verify that the electric connector is well adjusted in
the electrical outlet.


Confi rm that there is power in the circuit feeding the
distiller.


Verify that the main switch is in the on position.


Test to ensure that there is water in the vapour
generator or boiling chamber.


The immersion resistance is burnt out. Verify the integrity of the immersion resistance.
Measure electrical continuity or resistance in
ohms. Substitute with another that has the same
characteristics as the original.


There is water around the distiller. The distiller or some of its components are
incorrectly adjusted.


Test the fi lter to ensure that the activated carbon is
well installed and that water fl ows through it.


Verify that the collector tank of condensed liquid is
properly placed.


Confi rm that the drainage installation does not have
leaks.


There is vapour around the distiller. The distiller’s ventilation is inadequate. Verify that the distiller has free space around it and
at the back.


Test that there are no objects interfering with the
fl ow of air towards the distiller.


Remove any object aff ecting the fl ow of air


The refrigeration ventilation does not function. Verify the condition of the ventilator. If it is turned
ON and not functioning, substitute the ventilator
with another with the same characteristics as the
original.


The distilled water has a fl avour. The carbon fi lter is worn out. Replace the activated carbon fi lter.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


57


BASIC DEFINITIONS


Distillation. A process through which a fl uid in liquid phase is heated until converted into vapour and then cooled and condensed back into liquid phase. The
distillation process is used for separating mixed substances, taking advantage of their diff erence in volatility. To obtain very pure substances, consecutive distillation
cycles are performed with the aim of progressively eliminating other substances present in the mix.


Hardness (of water). A chemical characteristic of water determined by the carbonate, bicarbonate, chlorine, sulphate and occasionally calcium nitrate and
magnesium content. The resulting resistance is undesirable in some processes. There are two types of resistors in water.


• Temporary hardness. This is determined by the magnesium and calcium carbonate and bicarbonate content. It may be eliminated by boiling the water and
subsequently fi ltering out the precipitate. It is also known as carbonate resistance.


• Permanent hardness. This is determined by all the calcium and magnesium salts, except the carbonates and bicarbonates. It cannot be eliminated by the
boiling of water and it is also known as non-bicarbonate resistance.
Interpretation of resistance:
Resistance as CaCO3 interpretation
0–75 soft water
75–150 water with little resistance
150–300 resistant water
> 300 water with great resistance
In potable water, the maximum limit allowed is 300 mg /l.
In water for heaters, the limit is 0 mg / l.


• Calcium resistance or hardness (RCa++). Quantity of calcium present in water.
• Magnesium resistance or hardness (RMg++). Quantity of magnesium present in water.
• Total resistance or general hardness [TH]. Quantity in calcium [Ca] solution and magnesium [Mg] as cations, without taking into account the nature of the


anions present in the water. It is expressed as ppm (parts per million) of calcium carbonate (CaCo3).


Incrustation (scale). A name given to solids in suspension deposited in layers on the surface of water storage containers.


Solution. A homogenous mix of two or more substances characterized by the absence of chemical reactions between the components of the liquid mixture. The
liquid component which generally appears in greater proportion is called the solvent and that found in a lesser quantity in solution, the solute.




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59


Chapter 9


Dilutor


The dilutor is used for diluting substances. Dilute comes
from the Latin word diluere and means to add liquid to a
solution. Solutions are defi ned as homogeneous mixtures
of two or more components which may be gaseous,
liquid or solid. To dilute is to reduce the strength of a
fl uid in a solvent, generally water. The dilutor facilitates
the preparation of liquid mixtures, until these achieve a
proportion (concentration) suitable for use in different
diagnostic processes. The identification of this type of
equipment is generalized using the word dilutor.


DIAGRAM OF A DILUTOR


PURPOSE OF THE DILUTOR
The purpose of the dilutor is to prepare mixtures of
substances to achieve determined concentrations and
volumes as done with a pipette, but with the advantage of
an automated or programmed process. Dilutors vary in size
and complexity. Their capacity depends on the models and
manufacturers. They can control known volumes between
25 µl (microlitres) and 25 ml (millilitres).


GMDN Code 15133


ECRI Code 15-133


Denomination Dilutors


Propeller


Control Panel


Dispenser


Figure 22. Dilutor diagram




C H A P T E R 9 D I LU TO R


60


OPERATION PRINCIPLES
The dilutor has various components which interact in a
coordinated manner to handle liquids and mix volumes
with great precision, which allows known solutions of
between 1 µl and 25 ml to be prepared. The dilutor has in
general, the following components:
1. A propulsion system
2. A control system
3. A dispensing system


Propulsion system
This is generally constituted of positive displacement
systems as found in syringes. One or more selectable
syringes (with a varying capacity) is/are used in the dilutor
to control the volume to be mixed or diluted. The syringes’
pistons are moved by a mechanism which controls their
position. Aspirated volumes or deliveries are calculated by
means of the following equation:


Where:
∂ V = fraction of the volume delivered by the syringe
when the piston has a displacement ∂l.
A = piston area.


The total volume aspirated or delivered is the corresponding
integral:


where lo and l1 correspond to the positions that defi ne the
piston’s displacement.


Controlling how the pistons move facilitates good control
over the volumes handled. The displacement system is
activated by an electric motor which moves a very precise
nuts and screws system and changes the position of the
piston. A set of valves controlling the aspiration and supply
processes complements the syringes and their displacement
systems. The confi guration of the dilutor depends on the
model and manufacturers.


Control system
Modern dilutors have a control system which is automatic or
controlled by microprocessors. The latter allow the following
to be selected and controlled:
1. Mixing processes and/or dissolution of substances


(programmable)
2. Predefi ned volume supply
3. Supply or suction velocities
4. Number of required cycles
5. Size or volume of selected syringes
6. Time
7. Priming and cleaning cycles
8. Quality control procedures


In order to give a clearer idea of the technical complexity
achieved, a diagram of the control system based on a
microprocessor displaying some of the dilutor functions is
shown next. The controls for this type of device are generally
symmetrical if they control two injectors.


∂V = A∂l


=


∂l



V = A ∂l


lo


l1∫


Left Injector Screen


Increase, Decrease
Parameter Controls


Operation Mode Controls


Right Injector Velocity Control


Right Injector Screen


Right Syringe Size


Volume Control


Right Syringe Selector Size


Main Switch


Figure 23. Dilutor controls




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61


Dispenser system
The dispenser system is composed of a set of high precision
syringes and devices called dispensers, through which
fl uids are supplied according to their volumes and selected
velocities. These syringes are selected and installed in the
dilutor depending on the densities, viscosities, and volumes
of fluids to be manipulated. The fluids are transported
through flexible tubes, whose diameters, lengths and
chemical compatibility are taken into account in the design
and manufacturing process for suitability with the selected
activity. These tubes are linked using connections manually
adjustable. Normally, the syringes are classifi ed according to
their use (e.g. syringes for reagents, diluents, samples), and
the volume these manipulate. The following table shows an
example of how they are classifi ed according to their size
and managed volumes.


Opposite, the components of the dispensing system (syringe
and dispenser) are shown.


INSTALLATION REQUIREMENTS
The dilutor must be installed on a clean, dry and extremely
levelled counter or work surface, far from areas where there
may be vapours which can aff ect its functioning.


There must be free space around the equipment for
facilitating ventilation and the passage of cables and
interconnection lines and cables with the solvent containers,
computers or supply systems. The space around the dilutor
should be approximately 10 cm.


There must be a 115 V, 60 Hz electrical outlet in good
condition with a ground pole or alternatively one of 220–240
V, 50/60 Hz, depending on the manufacturer’s specifi cations
and/or the electrical norms in the country of use.


ROUTINE MAINTENANCE
The routine maintenance focuses mainly on eliminating
contaminants which may accumulate inside the fluid
mechanisms and/or lines. The most common routines are
the following:


Cleaning of exterior surfaces
Frequency: Daily


Warning: Disconnect the dilutor from the electrical feed
outlet before beginning the external cleaning process.


1. Clean the exterior surfaces using a clean piece of cloth
dampened with a mild detergent mixed with water.


2. Lightly rub the surfaces of the dilutor and the
accessories.


3. Dry the treated surfaces.


Warning: Avoid humidity from entering the compartment
of the electrical and electronic components.


Part No.
(Depending


on the
manufacturer)


Model
(Depending


on the
manufacturer)


Syringe size
Range


(Processed
volume)


Duct size1


Aqueous
solution Viscous liquids


DM DM 25 µl 2.5–25 µl 18 18


DM DM 50 µl 5–50 µl 18 18


DM DM 100 µl 10–100 µl 18 18


DM DM 250 µl 25–250 µl 18 18


DM DM 500 µl 50–500 µl 18 18


DM DM 1 ml 100–1 000 µl 18 18


DM DM 2.5 ml 250–2 500 µl 18 12


DM DM 5 ml 500–5 000 µl 12 12


DM DM 10 ml 1 000–10 000 µl 12 12


DM DM 25 ml 2 500–25 000 µl 12 12


Table of syringe size/volumes managed


1 Table 2.4, Microlab 501A, 503A, 504A, User’s Manual, Hamilton Company.


Syringe


Dispenser


Figure 24. Syringe and dispenser




C H A P T E R 9 D I LU TO R


62


Cleaning of syringes, hoses or lines

Warning: If the dilutor has been in contact with dangerous
substances, the safety and prevention procedures
implemented in the laboratory must be respected.


Frequency: Daily
1. Feed the system with a cleaning solution. Consult the


manufacturer to enquire about the solution to use. Verify
that each system’s elements come into contact with the
solution and that air bubbles have been eliminated. This
process is known as priming. In order to feed the system,
the dilutor is connected to a container in which the used
solution is present. Once the priming is complete; the
waste solution goes into another container for fi nal
disposal.


2. Clean the system. In order to carry out cleaning, a fl uid
which complements the cleaning solution is circulated
(consult the manufacturer’s recommendations). It is
common to use deionised water as a cleaning fl uid.
Depending on the substances processed in the dilutor,
other cleaning agents can be used such as ethanol, urea,
or a 10% bleach solution in deionised water.


Cleaning of the fluid conduction system
Frequency: Before putting into service for the fi rst time
1. Prepare a container with cleaning solution and place


the fi lling tube inside (manufacturers recommend using
cleaning agents compatible with the dilutor).


2. Place the waste line inside the waste container.
3. Run a feed or priming cycle until the fl uid’s lines becomes


clean.


4. Remove the fi lling tube from the cleaning solution and
place it inside a container with deionised water. Start a
feed or priming cycle again until the fl uid trajectory is
free of cleaning solution. Discard the fl uid and rinse the
waste container.


5. Suspend the feed cycle.
6. Place the fl uid propulsion system in the rest position.
7. Use the system as it is clean and ready.


Procedure for storing the dilutor
Frequency: Whenever stored for a prolonged period of
time
1. Purge and prime the system using methanol (facilitates


drying).
2. Remove the tubes and syringes.
3. Store the syringes in their original protective covers.
4. Cover the body of the dilutor in order to protect it from


dust.
5. Store.


Quality control
The quality control of dilutors is similar to that of pipettes.
In order to resolve uncertainties, please see the explanation
regarding how calibration is conducted in Chapter 16 on
pipettes.




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63


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The dilutor does not turn on. There is a fault in the electrical feed. Check the electrical connection.


The electrical feed is disconnected. Connect electrical feed cable.


The protection fuse is open. Check the protection fuse. Substitute with an
equivalent one if it is burnt.


The dilutor operates well, but there are no messages
or indications on the screen.


There is possible damage to the LCD screen or in the
emission diodes of the LED light.


Verify that the control is well connected to the
propulsion system.


Call the manufacturer’s service technician.


The control keys do not function. The dilutor is on the Pause mode. Press the start/end button to complete the path of
the piston.


The dilute is obstructed. There is an internal error. Press the start/end button to complete the path of
the piston and to restart the cycle.


Call the manufacturer’s service technician, if the fault
persists.


The dilutor does not aspirate nor dispense. The hydraulic systems’ tubes are defective or
blocked.


Verify that tubes, syringes and connectors are free
from blockages. Clean or substitute.


Incorrect connection of tubes and syringes Test that the tubes, joints, connections and syringes
used are well adjusted.


The propulsion system is defective. Call the manufacturer’s service technician.


The valves are defective. Remove the valves. Verify that their seals are clean
and reinstall. Substitute for an equivalent valve if
necessary.


The dilutor does not produce precise results. There is air in the fl uid circuit. Verify that the feeding tubes are completely
submerged inside the containers which contain the
reagents.


Confi rm that the diff erent connectors are adjusted.


Verify that the syringes are correctly installed and
there are no leaks.


Test to ensure that the tubes or valves have no leaks.


Reduce the operational speed of the syringe to
eliminate cavitation problems.


The delivery tube is incorrectly selected for the
syringe’s capacity.


Verify the recommended size of the tube used and its
connections. For small volumes, use the dimensions
recommended by the manufacturer.


A small air gap appears on the tip of the probe after
the fi nal aspiration.


The aspiration tube is dirty. Change or clean the aspiration tube.


The aspiration mode is incorrect. Reduce the aspiration speed.


Air is persistently present or there are constant leaks
in the fl uid trajectory.


Cavitations are present in the system. The aspiration
speed is very high.


Reduce the propulsion system’s speed. Remember
that the more viscous the fl uids, the lower the speed
must be used to manipulate them.


The connections are loose, worn out or defective. Adjust the connections by hand. Substitute to tubes
with dimensions corresponding with the fl uids
processed.


The piston is defective or the syringe is damaged. Replace the piston or the syringe.


There is a defective valve. Replace the valve.


The dilutor is heating. There is inadequate ventilation. Check the ventilation.


The room temperature is too high. Check the air conditioning system in the area.


The work cycle is very intense. Use the dilutor with less intensity.




C H A P T E R 9 D I LU TO R


64


BASIC DEFINITIONS


Cavitations. A phenomenon in fl uids when a vacuum is created upon emptying a vessel. The pressure decreases until it reaches the vapour pressure of the fl uid.
This produces diverse phenomena such as vaporization of gases dissolved in the liquid or, in the case of water, the formation of vapour bubbles collapsing after an
infi nitesimal time lapse, perforating the surfaces of conducts in the immediate vicinity. This occurs in dilutors when using large capacity syringes with elevated
propulsion speed.


Concentration. A quantity measurement of a chemical substance present in a solution. The concept is expressed as the quantity of a substance dissolved into a
solvent. Concentration is expressed in diverse forms; the most common are: molarity [M], molality [m], normality [N], percentage rate of solute.


Dilution. To reduce the concentration of a solution by adding other fl uids. The fl uid added is known as the diluent. Adding the molecules of a liquid substance with
the molecules of another liquid substance. In order to determine the volume V1 of liquid needed to obtain V2 volume at a concentration C2 from a stock solution of
concentration C1, the following equation is used:


Dispenser. A device used for distributing liquids.


Dispensing. Distributing a fl uid at a constant volume or in a progressive form.


Dissolution. Process by which a chemical in solid form is dissolved in a solvent (e.g. water or other liquid). The chemical now in solution is called the solute.


Equivalent – gram [Eq]. Mass in grams of solute divided by its equivalent weight [EW]:


Equivalent weight [EW] (of one substance). Results from dividing the molecular weight [MW] by its valency.


Molality [m]. Number of moles of a given substance, for every 1000 g of solvent. Thus an m molal solution is obtained by adding m moles of the substance to
1000 g of water.


Molarity [M] (of a solution component). Number of moles of solute for each litre of fi nal solution. A solution n Molar of a salt is obtained by adding n moles from
that salt to water until obtaining one (1) litre of solution. Normally, the formula employed is the following:


Mole. Molecular weight (MW) of the solute expressed in grams:


Normality [N] (of a solute). Number of moles of solute per litre of fi nal solution.


Solution. A homogeneous liquid mixture of two or more substances. The dissolved chemical(s) called the solute(s) usually name the solution. The substance in
which the solute(s) are now dissolved is called the solvent. There is a usually greater quantity of solvent than solute(s) in a solution.


Weight/Volume. Relationship in clinical biochemistry expressing the mass of the solution in grams or its submultiples per volume unit in litres or submultiples
of a litre. For example: g/l, mg/ml.


Note: Another type of notation known as “part per unit” is used for measuring extremely low concentrations. For example: parts per million (ppm) means that there
is a particle of a given substance for each 999 999 particles of other substances.


V1 =
V2C 2


C1


Eq =
mass(g)
EW (g)


EW =
MW (g)
valency


M =
moles


Vol(L )


moles=
mass(g)


EW


N =
Eq


Vol(L )




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


65


The dispenser is a piece of equipment in the pipette and
dilutor family. The word dispenser comes from the prefi x
dis which implies privation, and from the Latin word pensum
which means task. There are diff erent types of dispensers
such as, models meeting chemical work requirements and
others used in microbiology, bacteriology, immunology
and pharmacology. There are automated dispensing units
controlled by computer programs, which are used in
institutions where there is a high testing demand and thus
a need for automated procedures. This chapter features
manual dispensers, also called repeater pipettes, as these
are the most commonly used.


PHOTOGRAPH AND DIAGRAM OF THE DISPENSER


PURPOSE OF THE DISPENSER
The dispenser is a multi-purpose piece of equipment which
can be used in the laboratory for carrying out the following
activities:
1. To aspirate and dispense volumes of liquid or solutions


when it does not require great exactitude.
2. To distribute a volume of liquid or solution stored in


a recipient container in predefined partial volumes
(repetitive dispensing with a constant fi nal volume).


3. To mix a solution by successive aspiration and delivery,
using an aspiration and supply device.


4. To titrate a solution or a virus stock by dispensing the
material to be titrated by serial dilution into a diluent
until reaching the end point.


Chapter 10


Dispenser
GMDN Code 41663, 35734


ECRI Code 16-274


Denomination Dispenser, liquid, laboratory


1. Volume Selector


2. Digital Screen


3. Dosage Lever


4. Filling Lever


5. Expelling Lever


6. Dispensing Joint


7. Dosage Scale


8. Reservoir


9. Dispensing tip Adapter with
Built-in Plunger


Dispensing
Head


Figure 25. Dispenser


Ph
ot


o
co


ur
te


sy
o


f G
ils


on
S


.A
.S


.
Ph


ot
o


co
ur


te
sy


o
f G


ils
on


S
.A


.S
.


Dispenser




C H A P T E R 1 0 D I S P E N S E R


66


5. To dilute the concentration of a solution by mixing
defi ned volumes of this solution with a diluent.


6. To use similarly to a pipette (by aspirating a volume and
then dispensing it).


7. To distribute the culture mediums in Petri dishes.
Automated dispensers equipped with accessories for
moving the Petri dishes and storing them once the
culture medium is dispensed are often used. Precise
application (small scale) of culture medium is done
using disposable plastic syringes with Nº 161 needles.


The dispenser can normally be programmed for such
activities according to the manufacturer’s instructions
provided.


Operation principles
In general, modern dispensers are controlled by
microprocessors and have the following components (Note
that the numbering below corresponds to that in Figure 25).
1. Volume selector. This thumbwheel is used to regulate


the volume to be dispensed. The selection made is
shown on the dispenser’s screen.


2. Digital screen. This shows the data related to the
selected function, such as selected volume, type of tip
present on the dispensing head and information related
to alarm and error messages that may be generated
during operation e.g.: low battery or incorrectly selected
tip for the volume selected.


3. Dosage lever. This lever activates the plunger attached
to a syringe-like positive displacement adaptor, in which
a piston is activated along a cylinder to dispense the
selected volume of liquid.


4. Filling lever. A mechanical lever manually activated to
aspirate the liquid into the adaptor’s reservoir.


5. Eject button. A mechanism that releases the dispensing
element (adaptor) from the dosing device head.


6. Dispenser connector. This is the off shoot connecting
the setting element to the dispenser head. It contains a
system of gaskets and guides for ensuring its adequate
adjustment.


7. Dosage scale. This shows the maximum volume that can
be dispensed with the selected adaptor. In some cases,
it also indicates the remaining volume.


8. Dispensing adaptor. A container which holds the
solution aspirated or supplied in dispensation cycles.
There is a great variety, depending on the model of
dispenser. There are simple or combined ones with
adapted tips.


9. Dispensing tip. This facilitates supplying or drawing
solutions. The tip is located at the end of the dispenser’s
adaptor. Without it, it is impossible to use the dispenser.


10. An on and off switch. (Not shown in the fi gure).
11. A battery compartment. (Not shown in the fi gure).


Dispenser’s accessories
For the dispenser to perform specifi c tasks, the appropriate
accessories are needed. Examples of adaptors are shown in
the fi gure below.


1 Product Information Sheet. 3cc Syringes. For dispensing and plating
Methocult®. http://www.stemcell.com/technical/28230_28240-PIS.pdf


Head Adaptors and Tips


Multichannel Adaptor
with Dispensing Tips


Repeator Tips with
Built-in Plungers


Figure 26. Dispenser and accessories




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


67


Dispensed volume
Dispensers have been developed for working with
predefi ned volume ranges. Before use, the type of solution
to be used and volumes to be dispensed will have to be
considered. Manufacturers off er diverse models of adaptors.
A table with typical work ranges is shown next.


REQUIREMENTS FOR OPERATION
Depending on the type of dispenser, minimum conditions
are required for operation, some of which are as follows:
1. Verify that the dispenser has been designed for the


solutions to be used. Verify the compatibility of materials
in the user manual provided by the manufacturer.


2. A clean environment, equipped with suitably sized work
stations, well ventilated and lit.


3. Verify that the room temperature is stable, with a
variation range of ± 0.5 °C, between 4 and 40 °C and an
optimum temperature of 20 °C.


4. Use the appropriate personal safety protection if
working with toxic materials or materials posing a
biological risk.


5. Use tips specifi cally designed by the manufacturer for
each particular application.


ROUTINE MAINTENANCE
The maintenance of the dispenser is simple. The routines
detailed below feature the most important activities:
Frequency: Daily
1. Clean the dispenser with a damp cloth and mild


detergent.
2. Disinfect the dispenser using 60% isopropanol.
3. Prevent humidity from entering the interior of the


electronic control and/or the mechanisms.


Battery change (as needed)
1. Open the battery compartment. This is generally done


by simply sliding the lid from the “closed” position to
the “open” position.


2. Remove the worn out battery. Dispose of it according
to recommendations.


3. Install a battery with the same characteristics as the
original. Verify the electrical polarity so that it is properly
installed. Before inserting it, clean the contact surface
with a piece of clean cloth.


4. Close and adjust the lid.


Adaptor capacity Volume ranges dispensed


0.1 ml 1–20 µl


0.2 ml 2–40 µl


1 ml 10–100 µl


5 ml 50–500 µl


10 ml 100 µl to 2 ml


25 ml 250 µl to 5 ml




C H A P T E R 1 0 D I S P E N S E R


68


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


It is not possible to install the adaptor in the
dispenser’s head.


The component is defective. Seek assistance from a specialized service technician.


The dispensing component is contaminated. Observe if there is some type of obstruction. Clean
if necessary. Seek help from the specialized service
technician.


The adaptor cannot be removed from the dispenser’s
head.


There is a failure in the electronic system. Reinitiate the equipment. (Switch off and on). Select
manual extraction option.


There is a failure in the adjustment mechanism. Verify if the piston moves forward and backwards.
Remove the cylinder over a waste container.


The tip of the dispensing device (adaptor) drips. The tip is defective. Substitute the dispensing device.


The pipette type dispensing device drips. The dispensing tip is not well adjusted. Free the dispensing tip from the adjustment cone.
Adjust fi rmly.


The dispensing tip was incorrectly selected. Verify the type of tip recommended by the
manufacturer.


The piston or piston seal is damaged. Replace the piston and seals. Use replacement parts
supplied by the manufacturer.


The screen shows the low battery signal. The battery is worn out. Replace the battery.


The screen does not show any signals. The battery is worn out. Replace the battery.


The electronic system is defective. Seek the assistance of a specialized service
technician.


The screen shows error signals. Various Seek the assistance of a specialized service
technician.


The screen shows a fi lling error. Insuffi cient liquid for the dispenser. Verify that the volume available for dispensing is
adequate. If not, load or aspirate a volume adequate
for the quantity to dispense.


The screen shows complete volume error. More liquid was aspirated than the adaptor or tip is
able to receive.


Eject all liquid. Check operation attempted again.


The screen shows tip selection error. The tip installed is not designed for carrying out the
operation attempted.


Verify what type of tip is designed for performing
the operation. Substitute the tip.


The tip is defective. Place a new tip with the same specifi cations as the
original.


BASIC DEFINITIONS


Culture medium. Liquid or solid material developed for medical purposes for cultivating and identifying microorganisms capable of producing diseases (pathogens)
and for various other purposes.


Dispensing element (adaptor). Devices also called Combitips, attached to the dispensing head to dispense a solution. Diff erent sizes and shapes are available
according to the volumes to be dispensed and the characteristics of the solution used.


Petri dish. A shallow plate made out of glass or plastic used for microorganism cultures in the laboratory.


Mix. Addition of substances which does not produce a chemical reaction. In a homogenous mixture, the composition and appearance must be uniform.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


69


Chapter 11


Spectrophotometer


The word spectrophotometer is derived from the Latin word
spectrum, which means image, and the Greek word phos
or photos, which means light. The spectrophotometer
is one of the main diagnostic and research instruments
developed. It uses the properties of light and its interaction
with other substances. Generally, light from a lamp with
special characteristics is guided through a device, which
selects and separates a determined wave length and makes
it pass through a sample. The light intensity leaving the
sample is captured and compared with that which passed


through the sample. Transmittance, which depends on
factors such as the substance concentration is calculated
from this intensity ratio.


PURPOSE OF THE EQUIPMENT
The spectrophotometer is used in the laboratory for
determining the presence or concentration of a substance
in a solution, thus allowing a qualitative or quantitative
analysis of the sample.


OPERATION PRINCIPLES
As a basic principle, light is considered to be a form of
electromagnetic energy. In space, it has a constant and
universal velocity [C] of approximately 3 x 108 m/s. In any
other medium (transparent) through which light passes, its
velocity will be slightly lower and can be calculated by the
following equation:


Where:
v0= Velocity at which light passes through the medium
n = Medium refraction index: whose value oscillates, in
general, between 1.0 and 2.5.


v0 =
C
n


GMDN Code 36411 36411 36411


ECRI Code 15-082 15-083 15-084


Denomination Spectrophotometer,
ultraviolet


Spectrophotometer,
ultraviolet, visible


Spectrophotometer,
visible


Ph
ot


o
co


ur
te


sy
o


f B
ec


km
an


C
ou


lte
r


Ph
ot


o
co


ur
te


sy
o


f B
ec


km
an


C
ou


lte
r


PHOTOGRAPH OF SPECTROPHOTOMETER


Conventional spectrophotometer




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


70


The electromagnetic energy has a very wide range of
wavelengths. Some examples are shown in the following
table:


Upon passing or interacting with diverse mediums,
light undergoes a series of phenomena. Among these
are featured refl ection, refraction, diff raction, absorption,
diff usion, polarization and other phenomena measured by
various instruments and devices. The table below shows the
wavelength ranges used for carrying out spectrophotometry
tests.


With regard to the interaction of light with matter, Figure
27 assists in clarifying the complexity of phenomena that
occur.


The diagram in Figure 27 shows that the incidental radiation
[Io] can undergo a series of transformations. It can be
refl ected [Ir], transmitted [It], diff used [Id], absorbed and
directly emitted as fl uorescence [If ]. The phenomena on
which spectrophotometry is based are mainly absorption
and transmission. In order to understand how, it is necessary
to take Beer Lambert’s law into account.


Beer Lambert’s Law. Also known as Beer’s law or Beer
Lambert Bouguer’s law, it identifi es the relationship between
the concentration of the sample and the intensity of light
transmitted through it. With regard to the law mentioned,
there are two implicit concepts: transmittance [T] and
absorbance [A].


The transmittance [T] is the fraction of the incidental light of
determined wavelength passing through the sample.


Where:
It = intensity of the transmitted radiation
Io = intensity of the incidental radiation


Type of electromagnetic
energy Range of wavelength


Radio waves From a few meters to a few kilometres


Radar waves From 1 to 10 cm


Infrared waves From 1 to 10 microns (10-6 m)


Visible light From 300 to 700 nm (nanometres)


X rays From 0.1 to 0.5 Å (Angstrom)


Gamma rays Approximately 0.0012 Å (Angstrom)


Section of the lighting
spectrum Range of wavelength


Ultraviolet 10–200 nm (nanometres)


Near ultraviolet 200–280 nm


Visible light 380–780 nm


Near infrared 780–3 000 nm


Mid infrared 3 000–20 000 nm


Far infrared 30 000–300 000 nm


T =
I t
I o


Absorbed Radiation


Incidental
Radiation (Io)


Reflected
Radiation (Ir)


Transmitted
Radiation (It)


Diffused
Radiation (Id)


Fluorescence(If)


Figure 27. Interaction of light with matter




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


71


The percentage of transmittance [%T] can be expressed by
the following equation:


The concentration of light absorbing molecules in a sample
is proportional to the absorbance [A] of that sample. It is
expressed mathematically as:


Where:
A = Absorbance measured
ε = Molecule absorbance coeffi cient
[litres/moles/cm]
l = Distance of the trajectory traversed (path length)
by the light in the sample
c = Sample concentration [moles/litres]


Absorbance [A] is related to transmittance [T] through the
following equation:


The following diagram explains the phenomenon of
absorbance:


The graphs presented next demonstrate how absorbance [A]
and transmittance [T] vary as a function of the concentration
[C] according to Beer Lambert’s law.


In conclusion it can be inferred that by increasing the
concentration of a substance, the transmittance is decreased
and, upon increasing the concentration of the substance,
absorbance is increased.


The linearity of Beer Lambert’s law is aff ected if the following
conditions occur.
1. Displacement of the sample’s chemical balance as a


function of the concentration.
2. Deviations in the absorbance coefficients, greater


concentrations than 0.01 M due to electrostatic
interaction between nearby molecules.


3. Changes in the refraction index at high concentrations
of the analyte.


4. Diff usion of light due to particles in the sample.
5. Fluorescence or phosphorescence of the sample.
6. Non-monochromatic radiation.


%T = I t
I o


×100


A = ε × l × c


A = log10
1
T


= log10
I o
I t


= log1010
ε ×c× l


= ε × c × l


Incidental Light
lo


Transmitted Light
lt = lo x 10 -a( )*c*l


Pathlength


Absorbing Solution of
Concentration [C]


Moles/Litre


Figure 28. Absorbance phenomenon


Concentration


Tr
an


sm
itt


an
ce


A = Log 1
T


Transmittance graph


A = x l x c


Concentration


Ab
so


rb
an


ce
Absorbance graph




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


72


SPECTROPHOTOMETER COMPONENTS
The diagram shown in Figure 29 describes the relationship
between the diff erent components of a spectrophotometer.
The most important are the following.
1. The light source
2. The monochromator
3. The sample carrier
4. The detector system
5. The reading system


These are the basic spectrophotometer components, not
covering novel technology incorporated by manufacturers
in advanced models. A brief explanation of these basic parts
is shown in Figure 29.


Light source
Depending on the type of spectrophotometer, the light
source can be a tungsten lamp for visible light or a deuterium
arc lamp for ultraviolet light. Some manufacturers have
designed spectrophotometers with long lasting xenon
intermittent lamps emitting light in the visible and ultraviolet
ranges. The lamp(s) come factory-assembled on a base that
ensures a fi xed position, to maintain optical adjustment
and focus when operating or when replacing the bulb. The
typical radiating energy emitted from a tungsten lamp is
between 2600 and 3000°K (Kelvin degrees).


Monochromator
The monochomator is a set of elements used to disperse
white light into waves of diff erent wavelengths, one of which
is used in the sample reading. In general, it has an entry
crevice or groove which limits the light radiation produced
by the source and confi nes it to a determined area; a set
of mirrors for transmitting light through the optic system;
an element for separating the light radiation wavelengths
(which may be a prism or a diff raction (or transmission)
grating); and an exit opening for selecting the wavelength


required to illuminate the sample. Diff raction gratings have
the advantage of eliminating the non-linear dispersion and
being insensitive to changes in temperature.


Sample holder
This device holds the sample(s) to be analysed. There are
various sample holder types to accommodate diff erent
spectrophotometer models and sample volumes:
these come as cuvettes, microcells, microplates, test
tubes and continuous flow cells, etc. In conventional
spectrophotometers, the holder is a cell or cuvette of
rectangular shape. Cuvettes are made of glass to read in
the range of 340 to 1000 nm and others of silica to read in
the visible range of 220 to 340 nm. There are also cuvettes
and other sample holder types (e.g. microplates) in plastic
such as styrene or polystyrene which are disposable.

Detector system
The detection system can be designed with photocells,
phototubes, photodiodes or photomultipliers. This depends
on the ranges of wavelength, the sensitivity and the required
speed of response. The detection system receives light
from the sample and converts it into an electrical signal
proportional to the energy received. This electrical signal can
be processed and amplifi ed to be interpreted by the reading
system. A summary of advantages and disadvantages of
devices normally used in detection systems is included in
the following table (see opposite).


Reading system
The signal which leaves the detector goes through various
transformations. It is amplifi ed and transformed until its
intensity becomes a proportional transmittance/absorbance
percentage. There are analogous reading systems (displaying
results on a reading scale) or digital ones (showing results
on a screen).


Light Source


Entry Crevice


Monochromator


Prism


Exit Crevice


Mirror


Samples


Detector
System


Reading System


Figure 29. Spectrophotometer components




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73


Device Advantages Disadvantages


Photocells Economic. Limited wave lengths between 400 and 750 nm.


Small. Low sensitivity.


Robust. Respond slowly to change in light intensity.


Do not need energy sources nor signal amplifi ers. Wear out.


Signal is dependent on the temperature.


Phototubes Function between 190 and 650 nm. Also between
600 and 1000 nm.


Require calibrations depending on the temperature
of the environment where the equipment is
installed.


Wear out with high levels of illumination.


Photodiodes No movable mechanical parts.


Acquire spectral data simultaneously.


Wide dynamic range.


Excellent reproducibility of wavelengths.


Photomultipliers More sensitive than phototubes and photocells. Can burn if day light penetrates them while in
operation.


Work on wider ranges of wavelengths. Very expensive.


Rapid responses to changes in light intensity. Need a high voltage source.


Do not become worn out like photocells. Used only in specialized spectrophotometers.


Can be made with sensitivity in the whole range of
ultraviolet and visible light. (From 190 to 900 nm).


Advantages and disadvantages of common detection devices


Analogous indicators traditionally bear the name meters.
Their exactitude depends among other factors, on the
length and the number of divisions of the scale (the more
divisions, the more exact it is). Their main disadvantage
is that they can be incorrectly read, due to the operators’
fatigue or errors identifying scales when there are several.


Digital indicators usually show results on a screen as
illuminated alpha numerals. This makes reading errors less
likely.


INSTALLATION REQUIREMENTS
For the correct functioning of a spectrophotometer, the
following is required:
1. An electric supply source that complies with the


norms and standards used in the country. In American
countries, voltages of 110 V and frequencies of 60 Hz
are generally used. Other parts of the World require
220-230V/50-60 Hz.


2. A clean, dust free, environment.
3. A stable work table away from equipment that generate


vibrations (centrifuges, agitators).


SPECTROPHOTOMETER MAINTENANCE
Spectrophotometers are very specialized and costly
equipment. Their integrity depends to a great extent on the
way they are installed and used. Their direct environment
and the quality of the electricity services constitute factors of
prime importance for the equipment to function according
to specifi cations. Routine maintenance required vary in
complexity, ranging from careful cleaning of components to
specialized procedures carried out by a trained specialized
technician or engineer with the technical information for
diff erent manufacturers’ models and designs. Following
manufacturer’s instructions and careful use will guarantee a
prolonged operational life. In recent models, manufacturers
have incorporated automatic routines of calibration and
verifi cation.


In this document general maintenance recommendations
applicable to a wide range of spectrophotometers are
presented. It is emphasized that specialized routines can
only be performed according to the specifi c manufacturer’s
recommendations for each particular model. General routine
maintenance for a spectrophotometer in good condition
and the frequency of estimated checks are as follows:




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


74


Inspection of the instrument’s surroundings
Frequency: Annually
The area in which the spectrophotometer is installed must
be inspected visually and tested electrically in order to
guarantee the safety of the operator. The inspection covers
the electrical installation and the installation area (physical
infrastructure related to the spectrophotometer).


Electrical installation
It must be verifi ed and tested for ensuring the following:
1. There is an electrical outlet or receptacle with a ground


pole.
2. The receptacle is in good condition and is no further


than 1.5 m from the spectrophotometer.
3. The voltage is of an appropriate level and must not


vary by more than 5% of the voltage specifi ed on the
equipment’s plate.


4. The receptacle’s polarity is correct.


These tests must be done by an electrical technician or an
engineer and results must be recorded to allow follow-up
over time.


Installation area
1. Check that there is free space around the


spectrophotometer for two purposes. First, for the
connecting cables to pass without hindrances and
for other components or support equipment (e.g. the
voltage stabilizer). Second, to allow adequate ventilation
of the equipment when it is in operation.


2. Test the integrity of the counter, its state and
cleanliness.


3. Verify that there is no equipment installed that can
transmit vibrations in proximity. (E.g. centrifuges).


4. Verify that it is not affected by excessively humid
conditions, dust or high temperatures. The
appropriate room temperature for the operation of
the spectrophotometer generally ranges between 10
and 40 °C.


5. Avoid installing the equipment where it receives direct
solar radiation.


6. Do not install the equipment where there are magnetic
fi elds or intense electromagnetic radiation.


7. Ensure installation area is free from the infl uence of
gases and corrosive substances.


Visual inspection of the equipment
Frequency: Every six months
The spectrophotometer must be inspected visually to verify
that the state and integrity of its components are maintained
in accordance to the manufacturer’s specifi cations. The most
important aspects are cited next:
1. Check that the structure of the work table supporting


the spectrophotometer is in good condition.


2. Test the general structure of the spectrophotometer.
Verify that buttons or control switches and mechanical
closures are mounted fi rmly and that their identifi cation
labels are clear.


3. Ensure that accessories are clean, not showing cracks
and that their functional state is optimal.


4. Confi rm that mechanical adjustment parts (nuts, screws,
bolts, etc.) are adjusted and are in good condition.


5. Check that electrical connectors do not have cracks or
ruptures, that they are joined correctly to the line.


6. Verify that cables are not showing signs of splicing, that
they are not frayed and that they do not have worn-out
insulation.


7. Check that cables securing devices and terminals are
free of dust, fi lth or corrosion. These same cables must
not be worn out or show signs of deterioration.


8. Check that the grounding system (internal and external)
is standardized, of approved type, functional and
correctly installed.


9. Ensure that circuit switches or interrupters, the fuse box
and indicators are free from dust, fi lth and corrosion.


10. Check the external electrical components for signs of
overheating.


General maintenance
Cleaning of spills
In case of a leak in the sample holder or carrier, the spill must
be cleaned according to the following procedure:
1. Turn off the spectrophotometer and disconnect the


cable from the electrical feed.
2. Use a syringe for cleaning the sample holder. Absorb as


much liquid that can possibly be extracted.
3. Dry the sample holder with a medicinal cotton bud.
4. Use lens paper or a clean piece of soft textured cloth for


cleaning the window of the photocell.
5. Clean the exterior of the instrument with a piece of


cloth moistened with distilled water. Include the screen,
control and keyboard in the cleaning.


Cleaning of quartz cuvettes
It is recommended to carry out the following procedure to
maintain quartz cuvettes in good condition:
1. Wash the cuvettes using a diluted alkaline solution such


as NaOH 0.1 M and a diluted acid such as HCl, 0.1 M.
2. Rinse cuvettes several times with distilled water. Always


use clean cuvettes to take absorbance measurements.
3. Conduct rigorous and careful cleaning procedures


on cuvettes if samples used can deposit fi lms. Some
manufacturers recommend using special detergents
for cleaning cuvettes.


Battery changes
Various models of spectrophotometers use batteries to
memorize data associated with the analysis, such as date
and time. The procedure to change the battery is similar




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


75


in the various equipment. Following this procedure is
recommended:
1. Verify that the low battery indication appears on the


instrument’s screen.
2. Turn off the spectrophotometer.
3. Disconnect the electrical feed cable.
4. Open the battery compartment and remove the worn-


out batteries.
5. Clean the electrical contact points.
6. Install new batteries with the same specifi cations as the


originals.
7. Close the compartment.
8. Reconnect the equipment.
9. Adjust the date and time information.


Change of bulb/lamp
The bulb is a consumable with a limited operational life.
It must be foreseen that at some point in time, it will be
necessary to replace it. Most likely it will burn out, or suff er
from internal metallization and evaporation and the light
emitted will no longer meet the spectrophotometric
processes specifications. Lamp change steps differ for
each model and one must always follow the manufacturer’s
instructions. Common steps are as follows:
1. Verify that the bulb is not functioning or that there


is some indication of fl aw. In modern equipment, a
sign will appear on the screen or an error code. In old
equipment, the light will simply no longer work.


2. Turn off the spectrophotometer.
3. Disconnect the feed cable.
4. Undo the screws securing the top of the lamp’s


compartment.
5. Undo the screws keeping the lamp’s mechanism fi xed.
6. Undo the screws fastening the electrical connection


cable to the lamp (in some equipment, this might not
be necessary, as the assembly base has direct contact
mechanism to the lamp’s contact terminals).


7. Install a new lamp with the same characteristics as the
original. Use gloves to avoid getting fi ngerprints on the
surface of the lamp.


8. Reconnect the electrical feed cables to the lamp.
9. Reinstall the screws keeping the lamp in place.
10. Replace the screws securing the lamp’s compartment’s


cover.
11. Reconnect the spectrophotometer.
12. Turn the equipment ON and carry out the


equipment’s recalibration procedure stipulated by the
manufacturer.


Preventive Maintenance
Preventive maintenance of the spectrophotometer must
correspond with routines and frequencies recommended
by the manufacturer. A series of basic routines which can
be performed in the laboratory is presented next:


1. Clean the spectrophotometer externally, including the
controls, screens or measurement meters. This can be
done using a piece of fi ne cloth (similar to the texture
used in handkerchiefs) dampened with distilled water.


2. Inspect and clean the electrical feed cable.
3. Verify that the lamp is clean and in good state. If it is not


functioning, install a new one with the same specifi cations
as the original. In modern spectrophotometers, the
lamp’s state is detected automatically by software which
controls the state and functioning of the equipment
making it easy to determine when it is necessary to
change the lamp. Change the lamp and carry out the
subsequent adjustments following the manufacturer’s
recommendations.


4. Check the protection fuse. Before opening the
compartment where the fuse is housed, check that
the spectrophotometer is turned off and check
that its contacts are clean and in good condition. If
it is necessary, replace by a new one with the same
characteristics as recommended by the manufacturer.


5. Put the instrument in the operational confi guration.
6. Activate the “on” switch and allow it to warm up for fi ve


(5) minutes. Verify that:
a) The lights or pilot indicators work.
b) The reading indicators stay on zero (0).
c) The light source works.


7. Carry out an escaping current test in the “on” and “off ”
position.
a) Verify the ground pole and the correct polarity.
b) Verify the correct polarity without a ground pole.
c) Verify the inverse polarity without a ground pole.


8. Calibrate the front panel of the spectrophotometer
according to the manufacturer’s instructions.


9. Measure the equipment’s sensitivity.
10. Conduct a test according to Beer’s law.
11. Return the spectrophotometer to the initial confi guration


if the calibration has been successfully completed.


GOOD PRACTICES WHEN USING THE
SPECTROPHOTOMETER
1. Calibrate the spectrophotometer every time a set of


samples is to be analysed.
2. Keep the cover of the sample holder and compartment


closed during the measurement process to ensure
adequate reading.


3. Avoid reusing disposable cuvettes.
4. Only use quartz cuvettes for carrying out analysis under


310 nm.
5. Avoid the use of plastic cuvettes if using organic


solvents.
6. Use high quality boron silicate glassware for preparing


standards. Avoid the use of sodium glass (sodium oxide)
whenever possible, as prolonged contact with standards
can permeate it and produce erroneous results.




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


76


7. Carefully clean the glass cuvettes after use. Discard
those that show lines on the clear surface.


8. Use high quality reagents. Those of low quality can
cause contamination even in very low concentrations.
The diluents used (water or solvents) must be free of
impurities.


9. Verify that samples or standards did not degas inside
the cuvettes. This phenomenon produces bubbles on
the inner surface of the cuvettes and causes errors in
the readings.


10. Take into account that not all substances comply
with Beer’s law. Carry out linearity tests on the range
of concentrations to be used. It is recommended to


prepare a group of known high standard solutions and
verify the results. The phenomena that aff ects Beer’s law
are the following:
a) High concentration by molecular association of


ionic species.
b) Variation in hydration at low concentrations


changing the nature of complex ions.
c) Absorptions that do not comply with the Beer law


require graphing results of known standards. This
will indicate reading versus the concentration such
that the reading of the unknown concentrations
can be related to concentrations from the graph.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


77


TROUBLESHOOTING TABLE


Automated spectrophotometer1


PROBLEM PROBABLE CAUSE SOLUTION


The spectrophotometer is without power. The on and off switch is in the off position. Move the switch to the on position.


There is no electric energy in the feed outlet. Verify the general electric feed. Test that some safety
mechanism has not misfi red.


The electric feed cable is not connected well. Connect the feed cable fi rmly.


The keyboard’s buttons do not respond. The initialization of the equipment during start-up
is incomplete.


Turn off the equipment and switch on again.


An incorrect command was activated during start-up.


The serial port RS 232 does not respond. There was incomplete initialization of the equipment
during start-up.


Turn off the equipment and switch on again.


The interconnection cable is badly connected. Verify the connection.


The LCD screen is diffi cult to read. The contrast control is maladjusted. Adjust the contrast.


The base lighting system burnt out. Call the representative.


The printer is blocked. There is a paper jam in the printer. Remove the excess paper with fi nely pointed
tweezers.


Turn off the equipment, remove the paper and
reinstall again.


The printer’s paper does not auto feed or advance. The printer paper is installed erroneously. Turn off the equipment, reinsert the roll of paper.


The front edge of the paper is not aligned or folded. Turn off the equipment. Reinsert the roll of paper.
Cut the front edge and realign in the feed system.


The paper feed control does not respond. Call the representative.


The cuvette does not enter the sample holder
compartment.


The cuvette is of the wrong size. Use the size of cuvettes specifi ed by the
manufacturer.


The cuvette’s adjustment mechanism is incorrectly
placed.


Correct the position of the adjustment mechanism.


The reading shows fl uctuations. There are interferences in the light path. Verify that the cuvette is not scratched.


Verify that there are no particles fl oating in the
cuvette.


Rub the optic walls of the cuvette with a piece of
clean cloth.


Verify that the working range selected is appropriate
for the sample under analysis.


The reading shows negative values. There is no
absorbance reading.


There is no sample. Add a sample to the solution.


The cuvette is incorrectly positioned. Verify the orientation of the cuvette’s window.


The wavelength is erroneously selected. Adjust the wavelength to the range compatible with
the analysis.


The equipment was erroneously calibrated with a
sample instead of a blank solution.


Calibrate with a blank solution or with distilled
water.


1 Instruction Manual, Spectrophotometer, SmartSpecTM 3000, BIO-RAD Laboratories.




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


78


Non-automated spectrophotometer1


PROBLEM PROBABLE CAUSE SOLUTION


The source lamp does not light-up. The fi lament is broken. Replace the lamp.


The safety fuse is burnt out. Replace the lamp.


There is resistance in the lamp’s fi lament. Replace the lamp.


The voltage is erroneous. Review the voltage. Check the feed source.


Low readings in the meter or in the galvanometer. The source lamp is defective. Replace the lamp.


The photocell is dirty or defective. Clean or replace the photocell.


The amplifying circuit is defective. Change or repair the amplifying circuit.


The source lamp’s voltage is low. Adjust the voltage.


Unstable indication of the measurer. The Zener diode stabilizer is defective. Replace the Zener diode.


1 Operation seminar workshop and Maintenance of Spectrophotometers, Maintenance Subregional Project, RE-HS-02, OPS/OMS Agreement.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


79


BASIC DEFINITIONS


Absorption. A physical phenomenon occurring when atoms or molecules of a substance absorb light (photons). The energy of a photon is taken up by another entity,
e.g. by an atom whose valence electrons change between two electronic energy levels destroying the photon in the process. The energy absorbed is lost through
heat or radiation. Absorbance is a mathematical measure of absorption, expressed in optical density units (OD).


Angstrom. A unit of length equal to 10-10m. Its symbol is [Å]. It is used for carrying out measurements of X- or Gamma-rays.


Band width. A wavelength range that a monochromator can transmit.


Diff raction. Phenomenon caused by a change in the directions and intensities of a group of waves after reaching an obstacle, or through a narrow aperture whose
size is approximately the same as the wavelength of the waves.


Diff raction grating. A component of the monochromator, also called “transmission grating”. It diff racts light and is shaped as a series of parallel fi ssures carved
onto a refl ecting surface. It is made by tracer machines protected against vibrations and temperature variations. Gratings used in spectrophotometers are copies of
one master grating that usually has more than 1200 fi ssures per millimetre. Figure 31 demonstrates the phenomenon of diff raction.


If the refl ection angle [δ] is known as well as the width [d] of the fi ssures, the wavelength [λ] can be determined according to the following equation:


Intensity [IV]. The amount of light emitted by a source in a particular direction per unit of time. More generally, a measurement of the average energy fl ow per
unit of time. To get the intensity, the energy per unit of volume is multiplied by the speed at which the energy moves. The resulting vector is the energy by square
surface per unit of time.


Molar extinction or absorptivity coeffi cient [ε]. Measures how strongly a chemical species absorbs light at a determined wavelength. It is an intrinsic property
of the chemical species. When there is more than one absorbing species in a solution, the absorbance is the sum of the absorbance values for each individual species.
The absorbance at a given wavelength of a mixture of species X, Y ... is given by


Where A is the absorbance of the mixture.


Nanometre. A unit of length corresponding to 10-9 m (a thousand millionth of a metre). It is identifi ed by the symbol [nm]. It is used for measuring visible or
ultraviolet light wavelengths.


Path length. The distance covered by visible or ultraviolet light through a sample in an analytical cell (cuvette or well).


Refraction. A change of direction that occurs when a ray of light reaches the interface between two media.


The light cuts at an angle [a] and refracts at an angle [b] upon changing propagation medium.


Figure 30. Refraction of light


sinδ = nλ
d


A = C x ×εx + C y ×εy + ...[ ]∫




C H A P T E R 1 1 S P E C T R O P H OTO M E T E R


80


Spectrophotometry. A method of chemical analysis based on the absorption or attenuation of light of a specifi ed wavelength or frequency by matter. The light
interacts with specifi c features of the molecular species being analyzed: the light absorbed depends on the wavelength, the concentration of the species and the
trajectory. This allows determining properties such as the concentration of substances, which in the fi eld of basic health, serves to perform a multitude of analysis
for determining the health status of a patient.


Wavelength. The distance between crests of a wave. It determines the nature of the diff erent forms of radiant energy in the electromagnetic spectrum. For
electromagnetic waves, the wavelength in meters is calculated by the speed of light divided by frequency (number of peaks passing through a certain point in a
determined time).


∆ = diff erence in wavelength between two adjacent slots (fi ssures).


Sin = n
d


d


2


3


4


5


6


Figure 31. Diff raction grid




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


81


Chapter 12


Autoclave


The autoclave is a piece of equipment used for sterilizing.
The word sterilizing means the destruction or elimination
of all forms of life (microbial, including spores) present
in inanimate objects by means of physical, chemical or
gaseous procedures. The word sterilizer comes from the
Latin word sterilis which means not to bear fruit. This chapter
will focus exclusively on autoclaves as these are greatly
used in public health establishments, clinical and research
laboratories. This type of equipment is also known as a
sterilizer. Sterilization must be considered as a group of
very important interrelated processes for carrying out
health services, (sterilization of materials, culture medium,
instruments) within rigorous conditions of asepsis. The
processes associated in achieving sterile conditions of
inanimate objects are the following:


1. Cleaning
2. Decontamination
3. Inspection
4. Preparation and packing
5. Sterilization
6. Storage
7. Delivery of materials


PURPOSE OF THE AUTOCLAVE
The autoclave is equipment designed with the aim of reliably
eliminating1 microorganisms, which would otherwise
be present on objects used in diagnostic activities, in
treatment or surveillance in health institutions (hospitals,
laboratories). It is also widely used in the food processing
and pharmaceutical industries. In the laboratory, materials
and objects are sterilized for the following purposes:
1. To prepare materials for bacteriological cell cultures


(test tubes, pipettes, Petri dishes, etc.) in order to avoid
their contamination.


2. Prepare elements used for taking samples. (All must be
in sterile conditions: needles, tubes, containers).


3. Sterilize contaminated material.


Autoclaves are available in many sizes. The smallest are
the table-top type and the largest are complex equipment
that require a great amount of pre-installation for their
operation. The volume of the sterilization chamber is taken
as a reference and measured in cubic decimetres [dm3] or in
litres [l] in order to measure the autoclave’s size. Depending
on how their operation is controlled, it is possible to find
manual, semiautomatic or fully automatic models.


1 The Food and Drug Administration (FDA) classifies sterility of an article
based on statistical studies. An article is considered sterile if the probability
of encountering it not sterile in a set of articles submitted to the same
process of sterilization, is less than one in a million. This index is called
Sterility Assurance Level (SAL) and describes the theoretic potential of
microbial inactivation in a sterilization process.


GMDN Code 35366 35366 35366


ECRI Code 13-746 16-141 16-142


Denomination Sterilizing unit,
steam


Sterilizing unit, bulk Sterilizing unit,
tabletop


y
y


PHOTOGRAPH OF AUTOCLAVE




C H A P T E R 1 2 A U TO C L AV E S


82


OPERATION PRINCIPLES
Autoclaves work by taking advantage of the thermodynamic
properties of water which can be considered as a pure
substance. In normal conditions (at sea level and pressure
of 1 atmosphere) water (in liquid phase) boils and is
converted into vapour (gaseous phase) at a 100 °C. If the
pressure is reduced, it boils at a lower temperature. If the
pressure rises, it boils at a greater temperature. Through
the control of water vapour pressure, the autoclave can,
in its sealed chamber, reach temperatures higher than 100
°C; or inversely, by controlling the temperature, can achieve
pressures greater than atmospheric pressure. The following
graph demonstrates the behaviour of water depending on
conditions of pressure and temperature.


Autoclaves use pressurized saturated vapour (with a quality
greater than 98%) for transmitting thermal energy to
elements that require sterilization. In general, this method
is known by the terms steam or moist heat sterilization. This is
the sterilization method mostly used due to its effectiveness,
rapidity and low cost. However, not all materials can be
sterilized with moist heat; for those elements that are
affected by heat and humidity, alternative methods of
sterilization have been developed. In the laboratory, in order
to carry out sterilization processes, steam autoclaves as well
as drying ovens using dry heat (without the presence of
humidity) are used. See Chapter 13: Drying ovens.


Temperature / Volume Graphic Pressure / Temperature Graphic


1. This graph shows two defined lines: the saturated liquid (to the left) and the
saturated vapour (to the right) lines.


1. This graph shows the behaviour and relation between the solid, liquid and
gaseous phases of water depending on the pressure and temperature conditions.


2. As the pressure increases, so does the temperature. (See lines P1, P2, P3)
where:
P3 > P2 > P1.


2. The sublimation lines show that at determined conditions, if heat is
transferred to the solid phase, it can be converted directly into the vapour phase
(section E-E), without going through the liquid phase.


3. To the left of the saturated liquid line, the water is in a liquid state (plot A-B).
Upon heat transfer, the temperature of the liquid is raised from Temperature A
to B.


3. The fusion line shows that at determined conditions, upon transferring heat
to water, the solid phase is transformed into the liquid phase and, if more heat is
added, it is transformed to the vapour phase (section H-H’).


4. Between the line of saturated liquid and saturated vapour (section B-C) there
is a mixture of the vapour and liquid phases, and the temperature remains
constant. The closer it is to point C, the greater is the vapour’s quality1.


4. The vaporization line shows at which temperature conditions the water in
liquid phase is transformed into the vapour phase.


5. To the right of the saturated vapour line, all the water is in vapour phase
(section C-D).


5. The point at which the three lines are intercepted is called the Triple Point. In
such circumstances the three phases exist simultaneously in equilibrium.


1 Quality [X]. The relationship between total vapour mass and total mass (liquid mass plus vapour mass). Quality = 1: means that the vapour is saturated and that
any increase in temperature will overheat the vapour.


Te
m


pe
ra


tu
re


Saturated
Vapour Line


Volume


Saturated
Liquid Line


D


CB


A


P1


P2


P3


Critical
Point


Fusion Line
Liquid Phase


Solid
Phase


Critical Point


Vaporization Line


Vapour Phase


Sublimation line


E’E


G’G


H’H


Temperature




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


83


Cross-section diagram of the vapour autoclave
Figure 32 shows the main components of the vapour
system of an autoclave. For clarity, parts normally located
around the autoclave (their precise location depends on
the manufacturer), have been included on top and at the
bottom of the autoclave diagram.


Description of the components in the diagram
A brief description of the most common elements of the
vapour circuit of an autoclave is given next. The same
number identifying each component is used in Figure 32
and its description below. Note that the confi gurations vary
depending on each manufacturer’s design.
1. Safety valve. A device that impedes the vapour


pressure from rising above a determined value. The
manufacturers install these in the sterilization chamber
as well as in the jacket.


2. Chamber manometer. A mechanical device that
indicates the vapour pressure in the sterilization
chamber.


3. Jacket manometer (pressure gauge). A mechanical
device that indicates the vapour pressure inside the
autoclave’s jacket.


4. Autoclave door. A device which allows the sterilization
chamber to be isolated from the outside environment. It
normally has safety devices that prevent it from opening
when the chamber is pressurized. It also has seals for
preventing vapour from leaving the chamber when
the equipment is in operation. Autoclave doors can be
manually or electromechanically operated.


5. Door handle. A device which in some equipment,
allows the operator to open and close the door. The
larger capacity equipment in general has motorized
mechanisms for activating the door.


6. Sterilization chamber. The space where objects or
materials to be sterilized are placed. When the door is
closed, the chamber remains isolated from the exterior.
When the sterilization process is in progress, it is fi lled
and pressurized with vapour.


7. Chamber condensation evacuation line. A duct that
allows the collecting of condensation formed in the
sterilization chamber as a consequence of the heat
transference processes between the vapour and objects
being sterilized.


8. Thermometer. An instrument that indicates the
temperature at which the sterilization processes in the
autoclave chamber is done.


9. The jacket’s condensation evacuation line. A duct
that allows the extraction of condensation formed in
the casing as a result of heat transference between the
vapour and the jacket’s walls.


10. Vapour exit at the end of the cycle. When a sterilization
cycle is fi nished, vapour is extracted from the autoclave
by controlled procedures.


11. Vapour passage restriction for liquid sterilization
cycle. A mechanical device that restricts the passage
of vapour during a liquid sterilization cycle to allow the
temperature to decrease in a controlled manner and to
prevent sterilized liquids from boiling.


1. Safety Valve


2. Chamber’s Manometer


3. Jacket’s Manometer


4. Autoclave Door


5. Door Handle


6. Sterilization Chamber


7. Chamber’s Condensation
Evacuation line


8. Thermometer


Jacket’s Condensation Line


# Electrovalves


20. Drain


19. Vapour traps


18. Vapour feed line


17. Vapour Entry Regulation Valve


16. Jacket


15. Admission Valve with Filter


14. Chamber Vapour Feed Line


13. Rapid Sterilization Vapour
Evacuation Line


12. Liquid Sterilization Vapour
Evacuation Line


11. Liquid Sterilization Vapour
Evacuation Passage Restriction


10. End-of-Cycle Vapour Exhaust
1


4


2 3


65


Figure 32. Vapour circuit of an autoclave




C H A P T E R 1 2 A U TO C L AV E S


84


12. Vapour evacuation duct for sterilization of liquids.
A path followed by vapour when a liquid sterilization
process is being conducted and which passes through
the restriction described above.


13. Vapour evacuation line during the rapid sterilization
cycle. A path that follows vapour when a rapid
sterilization cycle is being carried out.


14. Vapour feed line. A conduct that feeds the autoclave
with vapour. This line has controls and accessories that
enable vapour to reach the autoclave at the conditions
stipulated for the sterilization cycle.


15. Air admission valve with fi lter. A device that allows the
entry of fi ltered air upon fi nishing the sterilization cycle.
The valve homogenizes the pressure of the sterilization
chamber to that of the atmosphere.


16. Jacket. A space located around the sterilization
chamber in which vapour circulates. Its purpose is to
transfer heat to the chamber and lessen the formation
of condensation. It is connected to the chamber and to
the drainage through lines controlled by electrovalves.
Not all autoclaves have jackets. Some manufacturers
substitute it by placing electrical resistors around the
sterilization chamber.


17. Vapour entry regulation valve. It is a mechanical
device which controls the pressure at which vapour
enters the autoclave. Depending on the cycle selected,
the pressure and the temperature will be diff erent. The
greater the pressure, the greater the temperature. The
lesser pressure, the lesser the temperature.


18. Vapour feed line. A duct that brings vapour from the
boiler or the vapour generator to the autoclave.


19. Vapour trap. A device designed to take maximum
advantage of vapour’s thermal energy. Its function is to
prevent vapour from leaving the system. The trap only
allows condensed liquid formed in the chamber, jacket
and autoclave conducts to leave.


20. Drain. A collection line for the condensed liquid
produced in the autoclave to exit.


Nowadays, autoclaves use microprocessor-controlled
systems and each one of their valves and accessories work
in accordance with pre-established programs stored in their
memory. Operations remain recorded in a registering system,
which allows the diff erent stages of the sterilization to be
checked. Each manufacturer has incorporated registering
systems which are indispensable for quality control.


Vapour production. The vapour autoclaves use is generated
in devices which transfer thermal energy to water using
electrical energy or fossil combustible. These are called
boilers or vapour generators and constitute a fundamental
component of the autoclave. Depending on their size
and the frequency of use, autoclaves have vapour feed
systems that originate from a central system of boilers or
from their own vapour generator. These generally function


with electrical resistors and come already incorporated
into the equipment or are supplied as an accessory by the
manufacturers.


OPERATION OF THE AUTOCLAVE
The general operation of an autoclave is described next.
Some procedures will vary according to the degree of
automation incorporated into the equipment:
1. Verify that the registering system has forms and/or


paper required for documenting the development of
the sterilization cycle. Supply any missing element (ink,
form, etc.).


2. Turn the autoclave ON.
3. Open the door of the autoclave. In large capacity


autoclaves, this process is done electromechanically. It is
often manual in medium and low capacity autoclaves.


4. Place the sterilization baskets or containers containing
the previously prepared material (cleaned, washed,
dried, classifi ed and packaged) into the sterilization
chamber, according to the manufacturers’ recommended
distribution instructions.


5. Close the door of the autoclave1.
6. Select the required sterilization cycle depending on the


type of objects or materials to be sterilized2. In general,
a labelled button corresponding to the cycle required
is pressed and automatically initiates the programmed
cycle. From this moment on, the process proceeds as
indicated next3:
a) The pre-treatment phase is initiated. In this


phase, short alternate cycles of emptying and
injecting of vapour into the sterilization chamber
are performed so that air is extracted from it and
packets protecting the material are sterilized.


b) When the air has been removed, filling and
pressurization of the sterilization chamber is
initiated. At this time, the vapour enters into
contact with objects to be sterilized and a process
of heat transference is initiated between high
temperature vapour and articles to be sterilized.
Upon transferring thermal energy, a portion of
vapour is converted into liquid water (condensed
liquid) in the exterior layers of the material used
for packing, simultaneously decreasing its volume
in a signifi cant way. More vapour can then enter
the sterilization chamber, which penetrates even
further inside the packages to be sterilized. Vapour
eventually completely surrounds these and the
pressure and temperature are established.


1 Before loading the autoclave, the jacket is pressurized so that the interior
of the chamber is hot to reduce the formation of condensed liquid at the
beginning of the sterilization cycle.


2 See the information on the sterilization cycles included further on.
3 A typical cycle of a sterilizing autoclave, equipped with an exhaust system


activated by an electro hydraulic pump is described.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


85


c) Once these conditions are attained, the countdown
for completing the sterilization (depending on the
type of objects or materials being processed) is
initiated. The higher the temperature and pressure,
the lesser the time required for sterilizing.


d) Once the programmed sterilization time has
ended, post treatment process is initiated. This
includes depressurization of the chamber normally
done with the help of the exhaust and drying
system using the supply of heat transferred from
the jacket to the sterilization chamber. Upon
decreasing the pressure, the required temperature
for evaporating any liquid residue that may have
formed on objects during depressurization is
attained. A vacuum of 10 % of the atmospheric
pressure is created and maintained steady for
a period of time. When liquids are sterilized, no
vacuum is created; rather, vapour extraction is
controlled through a restrictive mechanism to
prevent boiling inside the containers autoclaved.


e) Finally, controlled entry of air through valves with
high efficiency filters will be allowed until the
pressure in the sterilization chamber is equal to
the atmospheric pressure. The sterilization cycle
has ended.


7. Open the door of the autoclave.
8. Unload the sterilized material.
9. Close the door once the sterilized material is unloaded


to conserve the heat in the sterilization chamber and
facilitate the next sterilization cycle.


10. Store the sterilized material appropriately.


Note: The sterilization cycles must be supervised and
submitted to quality control procedures through the use
of physical, chemical and biological type indicators for
ensuring their eff ectiveness.


Warning: Not all objects can be sterilized with moist heat.
Some require sterilization procedures at low temperature.
Verify which procedure must be used according to the type
of material to be sterilized.


Sterilization cycles
The sterilization processes follow predefined cycles
according to the type of load to be sterilized. There are
diff erent sterilization cycles for porous materials, surgical
instruments, liquids or heat sensitive material. The main
ones known as clinical sterilization cycles are carried out
under the following conditions: 121 °C / 1.1 kg /cm2 or
134 °C / 2.2 kg /cm2. Their main characteristics are featured
in the table on the next page.


Note: The sterilization cycle times are adjusted to the
altitude where the autoclave is located. Manufacturers
supply compensation tables to be taken into account. In


general, the higher the altitude of the equipment’s location,
the longer the sterilization time will be.


Quality Control
In order for a product to be considered sterilized, it is
necessary to verify that all the stages of the sterilization
process have been carried out correctly. To verify that these
have been fulfi lled, a series of tests have been developed
to evaluate the characteristics of the process and its
infl uence on the activity of microorganisms. Evaluations
of the temperature, pressure, time, humidity and general
equipment behaviour are carried out to certify that it
complies with, and functions according to procedures that
demonstrated its validity and reliability. There are also tests
or indicators that allow the death of the microorganisms
to be certifi ed in order to guarantee the quality of the
sterilization processes. Diff erent categories of tests have
been developed. Some are featured next:
1. Sterilization process indicators. These are designed


for supervising the functioning of the autoclaves.
They include instruments that control parameters
like temperature, time and pressure (thermometers,
manometers and chronometers) and register the
development of the process. The registering systems
of modern autoclaves (microprocessor) register all
the parameters of the sterilization cycle and also halt
the cycle in case some anomaly occurs. There is also
the Bowie-Dick test in this category: it evaluates the
effi ciency of the exhaust pump using a test sheet which
changes in colour uniformly if the process has been
completed satisfactorily. If it is not the case, the colour
of the sheet is uneven.


2. Chemical indicators. These are typical chemical
tests changing colour or state when exposed to the
diff erent phases of the sterilization process. Chemical
indicators allow the diff erentiation of articles submitted
or exposed to a successful sterilization process from
those that have not. Among the best known are the
adhesive tapes or strips that go inside a component or
on packages. The ISO Nº 11140-1 standard describes
categories of chemical indicators. One has to keep in
mind that chemical indicators by themselves do not
guarantee that the sterilization process complied with
all the requirements: personnel who use these must
receive precise training to allow them to determine if
the result obtained is coherent with the evolution of the
whole sterilization process.


3. Biological indicators. These are considered the best
methods for controlling the quality of a sterilization
process. They are made of live microorganisms which
have a greater resistance to a determined sterilization
process, or of chemical reagents which react in the
presence of the specifi c proteins of this type of organism.
In order to control the sterilization process by saturated
vapour, (hydrogen peroxide) or formaldehyde, spores




C H A P T E R 1 2 A U TO C L AV E S


86


Cycle no. Materials Temp. ˚C Pressure kg/cm2 Typical graph
1


1 • Porous loads
• Textiles
• Wrapped


instruments
• Tubes


135 2.2


2 • Open
instruments


• Utensils
• Glassware
• Open containers


135 2.2


3 • Heat sensitive
materials


• Rubber
• Plastic


121 1.1


4 • Liquids in open
or semi-closed
containers.(*)


121 1.1


Convention A: Pre-treatment. Alternate cycles of injection / vacuum of vapour.
Pre-treatment. (Processes 1, 2, 3).


Process 4: Sterilization.


C: Post-treatment (Process 5: vacuum and drying).


D: Internal and external pressures completely mixed.


Note: The liquid process does not have vacuum after sterilization. The cooling is natural.


1 The graphs included correspond to an autoclave with an emptying pump, Getinge brand GE-660 autoclave.
(*) Times depend on the volume of the load. There is no vacuum during cooling.


(+)


Pr
es


su
re


/T
em


pe
ra


tu
re


(-)
A


1 2 3 5


C


135o C, 7 min


< 50 mb, 5 min


D


Time
Atmospheric
Pressure


Atmospheric
Pressure


Pr
es


su
re


/T
em


pe
ra


tu
re


(-)


A


1 2 3 5


C


135o C, 4 min


< 50 mb, 2 min


D


(+)


4


< 50 mb, 5 min


Atmospheric
Pressure


Pr
es


su
re


/T
em


pe
ra


tu
re


(+)


(-)


A


521 3


4


121o C, 20 min


D
Time


121o C, 20 minTime


Atmospheric
Pressure


(+)


(-)


Pr
es


su
re


/T
em


pe
ra


tu
re


Time




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


87


of Bacillus stearothermophilus are generally used. To
control sterilization by dry heat (a process that drying
ovens perform) and by ethylene oxide, spores of the
Niger variety of Bacillus subtilis are used. The spore
indicator is placed in the sterilizing load. After the
process, it is incubated, analyzed and it is determined
if the cycle meets with the sterilization requirements.
Generally a change of colour is observed. These tests
are standardized and manufacturers indicate how to
use them and interpret the results. Biological indicators
by themselves do not guarantee that the sterilization
cycle complies with all the requirements. The only way
to do this is by controlling all the sterilization cycle’s
parameters.


Frequency of the quality control processes
A table summarizing the suggested frequency with regard
to the use of quality control indicators in the sterilization
processes is shown next.


INSTALLATION REQUIREMENTS
To be able to function, autoclaves require the following
services:


A well ventilated area for removing heat and humidity
generated while in operation. It also requires free space
around the back and sides, to accommodate technical


servicing. This space should be at least 0.8 m. Depending
on the design of the autoclave, complementary
infrastructure must be anticipated so that it can operate
satisfactorily. The diagram in Figure 33 explains the
space required around the autoclave. The temperature
in the immediate vicinity of the equipment may increase
to more than 70 °C when it is in operation. The fl oor
should be well levelled and constructed with materials
resistant to humidity and heat.


2. An electrical outlet in proportion to the equipment’s
consumption. If the autoclave is autonomous, meaning
that it has its own vapour generator, the electrical
connection must be studied in detail as the required
power could be significantly higher. Typical power
demands are 21, 38, 48 kW and higher, for the vapour
generator to function. The connection must be equipped
with required safety and protection elements. The
typical voltages required for autoclaves are 220 V, 60
Hz, or 380 V, 60 Hz triphase.


3. Water connection proportional to the equipment’s
consumption in volume and pressure: the larger the
equipment, the greater the consumption. The water
which the autoclave consumes must have received
required treatments for eliminating solids in suspension
as these may negatively aff ect the functioning of the
electrovalves as well as that of the electro hydraulic
devices.


4. Some sterilizers require compressed air, as their controls
are managed by pneumatic pressure. In general, the
required pressure varies from 5x105 to 9.9x105 Pa. The
following diagram shows the minimum installation
requirements (cut-off valve, fi lter and manometer).


5. A drainage system designed for collecting hot water.
6. A vapour connection. If the autoclave does not have


its own vapour generator, it must be fed from the
institution’s vapour generating system (machine room,
boiler). The supply installation must meet the necessary


Type of indicator Frequency of use


Process In each sterilization cycle.


Chemical In each package.


Biological Weekly, in all the sterilization equipment; in the
packets that contain implants.


1.


Figure 33. Space required for autoclave


Figure 34. Compressed air connection




C H A P T E R 1 2 A U TO C L AV E S


88


requirements: a cut-off valve, fi lter, manometer as well as
an appropriate installation for collecting the condensed
liquid with a fi lter and vapour trap, as indicated in the


Figure 35.


6. The autoclave must be operated exclusively by
personnel specially trained and qualifi ed in these types
of processes.


ROUTINE MAINTENANCE
The autoclave is equipment which demands supervision
and continuous preventive maintenance due to its multiple
components and systems. Maintenance is focused on the basic
routines that can be performed by the operators. In order to
carry out detailed maintenance, the instructions described in
the manufacturer’s service manuals must be followed.


Daily verifi cations
Before initiating the sterilization processes, the following


verifi cations will have to be carried out:
1. Place a new form on the registration device in order to


document the development of the sterilization cycle.
2. Ensure that the cycle-recording pen or that the printing


module of the autoclave has ink and recording paper.
3. Ensure that the cold water, compressed air and vapour


supply valves are open.
4. Activate the switch that triggers the autoclave’s jacket


heating. Upon activating this control, vapour is allowed
to enter the sterilization chamber’s jacket. When vapour
enters the sterilization chamber, the heating process
begins. To avoid heat loss, keep the autoclave’s door
closed until it is time to add the load for sterilization.


5. Verify that the pressure from the vapour supply line is
at least 2.5 bar.


6. Test the condition of manometers and thermometers.
7. Ensure that there are no vapour leaks in any of the


systems functioning in the autoclave.
8. Clean the front of the autoclave, controls, indicators and


handles with a damp cloth.
Weekly maintenance
Responsible: The equipment operator
1. Clean the sterilization chamber drainage fi lter. Remove


any residue retained inside.
2. Clean the inside of the sterilization chamber using


cleaning products that do not contain chlorine. Clean
the guides used for placing the baskets as well.


3. Clean with an acetifi ed solution, if solutions with chlorine
are being sterilized. The chlorine causes corrosion even
on stainless steel implants. Next, wash with plenty of
water.


4. Clean the external rust-proof surfaces with a mild
detergent. A solvent like ethylene chloride can be used,
avoiding touching any surface with painted coverings,
markings or plastic coverings.


5. In autoclaves with manually activated doors, verify
that these mechanisms are well adjusted and that their
operation is smooth.


6. Drain the vapour generator (if the equipment has one).
To do this, open a valve located on the lower part of
the generator which allows its contents to be drained.
Generally this is done at the end of weekly activities.
Follow the manufacturer’s recommendations.


7. Never use steel wool for cleaning the inside of the
sterilization chamber.


8. Check adequate functioning using a biological or
chemical indicator. To check the temperature, use
chemical test strips checking time and temperature of
exposure sold for this purpose.


Quarterly maintenance
Responsible: The autoclave technician
1. Check that the manometers function as expected.
2. Activate the safety valves manually to verify that they


are operating well. Use a large screwdriver to move
the activation lever normally located in the upper part
of the valve. Make sure that the face and body of the
operator are not in the vapour’s path. Once the valve is
activated, ensure that there are no vapour leaks. If there
are any leaks, the valve must be activated again until it
is well sealed.


Warning: If vapour leaks are not eliminated, this will
deteriorate the seal rapidly and the whole safety valve
system will have to be replaced.


3. Lubricate the door’s gasket. Use the lubricant and
the procedure recommended by the equipment’s
manufacturer. Some manufacturers recommend the
following procedure:
a) Remove the gasket. To do this, it is necessary to


dismount from the groove, loosening the retention
mechanisms (screws and plates).


Cut-0ff Valve


Manometer


To the Autoclave


Vapour Connection


Filter


Condenser


Vapour Trap


Alternative Floor Level Vapour Connection


Figure 35. Vapour connection




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


89


b) Clean the gasket and the groove with alcohol so that
there is no foreign material to aff ect the seal. The
surface of the gasket must stay smooth and clean.


c) Apply the lubricant recommended by the
manufacturer to the body of the gasket until
it is completely protected. Many autoclave
manufacturers use graphite lubricant resistant to
high temperatures.


d) Reinstall the gasket. In rectangular chamber
autoclaves, this is normally installed placing
the gasket in the middle of one of the assembly
groove’s sides and adjusting the remaining portion
towards the sides, until it is well adjusted inside the
groove. The same procedure is repeated for each
remaining side. In round chamber autoclaves, the
gasket assembly begins on the upper part and is
adjusted progressively into the groove without
pulling it, until the whole gasket is installed. Next,
assembly elements are adjusted.


4. Verify that the seals of the safety valves are in good
condition.


5. Clean the points of the registration pen system with
water or alcohol and restore the ink levels. Generally, the
pressure is registered with red ink and the temperature
with green.


6. Clean the inside of the vapour generator (for equipment
with this accessory). For the vapour generator, the
cleaning procedure involves carrying out the following
activities:
a) Disconnect the electrical supply to the


equipment.
b) Discharge the vapour pressure and wait for the


equipment to reach room temperature.


c) Remove the front cover of the generator.
d) Disconnect the electrical terminals of the heating


resistors (immersion).
e) Remove the screws that secure the front plate


where the heating resistances are installed and
dismount the front plate.


f ) Check the gasket and substitute it if necessary.
g) Remove dirt accumulated on the surface of the


heating resistors. Use products recommended1.
h) Re-assemble in the reverse order.


Figure 36 shows the vapour generator and its components.


Annual maintenance
Responsible: The autoclave technician
1. Clean all the fi lters.
2. Test and adjust the water level of feed tank so that it is


within 20 mm of the maximum level.
3. Verify and adjust the tension of diaphragm valves’


springs.
4. Dismount, clean and adjust the safety valves.
5. Change the air fi lter.
6. Conduct a general sterilization process testing in


detail the pressure, temperature, required times for
completing each phase of the cycle, state of the process’
signal lamps and functioning of the registration system.
Verify that it is functioning within tolerances defi ned by
the manufacturer.


7. Perform the quarterly routines.


1 Incrustations are seen when the water used by the vapour generator has
not received adequate treatment.


Water Level Control


Resistor Terminals


Flange Mounted Resistors


Vapour Exit Line


Float


Immersion Resistances


Vapour Generator Cover


Water Feed Line


Vapour Generator Drainage Line


Figure 36. Vapour generator




C H A P T E R 1 2 A U TO C L AV E S


90


MAINTENANCE OF SPECIALIZED COMPONENTS
Included next are some specialized routines requiring
a service technician and applicable to equipment
components. Given that autoclaves have multiple designs,
routines stipulated here are only applicable to certain
equipment models.


Maintenance of solenoid valves
1. Verify the sound made by the bobbins or solenoids


(humming). Excessive noise is a warning of overheating
due to abnormally high electric currents through the
solenoid. Current alternates rise when the impedance [Z]
of the circuit decreases. This occurs when the solenoid
is not adequately surrounded by a closed iron cover.
An air gap in the magnetic circuit can be caused by dirt
which prevents the protective cover from reaching its
fi nal position when the solenoid is energized. Carefully
clean the housing of the bobbin and its nucleus so that
the piston’s movement is not impeded by fi lth.


2. Replace the O-rings between the solenoid and the body
of the valve once these have been disassembled.


3. Before any disassembly, verify how the solenoid valve
is installed. Some possess clear installation indications
but others lack such information.


4. When dismounting a servo-assisted solenoid valve,
control the position of the orifi ces that put it in contact
with the work environment, so as to be able to re-
assemble the valve.


Cleaning of the vapour fi lter


Warning: Before disassembling the vapour fi lter, dissipate
the vapour pressure in the system.


1. Lift the cover.
2. Remove the mesh.
3. Clean carefully.
4. Reinstall the mesh.
5. Replace the cover.


Here are some of the most common problems. Given the
diversity of brands, models and available technology, it
is advisable that users follow instructions from the user
manual for the autoclave used.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


91


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The sterilization indicator did not indicate the
successful end of the sterilization cycle.


The sterilization chamber is incorrectly loaded or
over-loaded.


Check the load distribution and the load
quantity. Adjust according to the manufacturer’s
recommendations.


The vapour trap is defective. Check the vapour trap. Repair or substitute it.


The sterilization time is insuffi cient. Check the sterilization time. Adjust to the cycle type.


The autoclave does not reach the temperature and
sterilization pressure selected.


Check the temperature selection. Check the vapour
pressure corresponding to the selected cycle.


Check for possible vapour leaks in the door (gasket)
or in the passage control devices.


There is insuffi cient vapour penetration. Reduce the quantity of packets to be sterilized; this
allows a better vapour fl ow.


The pre-treatment is inadequate. Too much air has
remained inside the chamber.


Seek the assistance of a specialized service
technician to check the exhaust system.


The biological indicator is inappropriate for the cycle
conducted.


Check the user specifi cations of the biological
indicator. Repeat the sterilization cycle.


The sterilization cycle is interrupted without any
apparent reason.


Inadequate vapour, water or air pressure. As a result,
the regulation and servo-assisted control devices are
not activated.


Check vapour, water and air feed pressures. Adjust
the regulation systems.


The sterilized material comes out damp. The vapour trap is defective. Check/clean the vapour trap. Substitute the trap.


The sterilization chamber drainage is blocked. Check the drainage system. Clean.


The autoclave is overloaded. Reduce the load quantity in the chamber. Repeat the
sterilization cycle.


The autoclave is not levelled. Level the autoclave.


The biological indicator is positive. The biological indicator was incorrectly selected. Use a biological indicator of another lot or
manufacturer. Carefully register the parameters.


Vapour pressure too low. The door’s gasket is defective. Check the gasket; replace it.


The internal vapour leaks into another autoclave
component.


Check the traps, electrovalves etc.


There is excessive vapour pressure. The autoclave is overloaded with textile material. Reduce the autoclave’s load.


Autoclave is not calibrated. Calibrate the autoclave.




C H A P T E R 1 2 A U TO C L AV E S


92


BASIC DEFINITIONS


Asepsis. A set of procedures necessary to eliminate microorganisms.


Atmosphere. An old unit of pressure equivalent to 101 325 Pa (Pascals) or to 14.69 pounds per square inch.


Bar. A unit of pressure equivalent to 105 Pa (Pascals).


Cleaning. Mechanical removal of all foreign material located on the surface of inanimate objects; in general, it implies the use of clean water combined with a
detergent. It is a basic procedure performed before submitting the objects to their respective sterilization processes. Cleaning can be done manually or by using
automatic methods. It must be understood that it is not a procedure destroying microorganisms, but only decreasing their quantity.


Decontamination. A procedure to decrease the quantity of microorganisms of an object or substance so that its use or/and manipulation is safe. For example,
objects used in patient care procedures in possible contact with fl uids, bodily substances or organic materials require decontamination or even sterilization (see
defi nition below).


Disinfection. A process that uses physical or chemical means to destroy any form of life in a vegetative state from inanimate objects (excluding spores).


Inspection. A visual evaluation of washed articles, with the purpose of fi nding defects or dirt that may interfere with the sterilization processes. It is a process of
great importance which may be done using a magnifying glass to discern minute details.


Jacket. Enclosed space around the sterilization chamber through which vapour circulates. Its function is to transfer heat to the sterilization chamber in the pre-
treatment stages (air removal) and post treatment (drying of the sterilized material).


Moist heat. A sterilization method that eliminates microorganisms by denaturation of the proteins which is accelerated by the presence of water vapour (steam).


Pascal (Pa). A unit of pressure from the International system, which corresponds to the force of a Newton (N) that acts on a (1) square meter:


Quality. Thermodynamic property identifi ed in general with the letter [X] and defi ned as the relationship existing between the vapour mass and the total mass of
the substance under saturated conditions.


Servo-assisted valves. Solenoid-type valves that depend on the surrounding pressure to close or open. In general, these have membranes with small openings
through which the working medium is supplied.


Solenoid valves. Electromagnetic control devices used in multiple applications also known as electrovalves. The position of a piston is controlled by a bobbin which
is energized or at rest. The piston permits or impedes the passage of a fl uid inside of a determined circuit. They are used in hydraulic, pneumatic, vapour and vacuum
systems. Manufacturers have developed a great number of designs for specialized applications.


Sterilization. A set of actions by means of which all forms of life are destroyed (including spores) on inanimate objects using physical, chemical and gaseous
procedures.


Sterilization chamber. The area where objects requiring sterilization are placed. When the sterilization process is being carried out, the chamber is fi lled with
pressurized vapour, reaching temperatures directly related to the selected pressures. During the sterilization cycle, it is sealed by a door by a safeguarding system
which can only be opened once the sterilization process has been completed and the internal pressure has reached that of the atmosphere.


Sterilization indicator. A chemical or biological indicator that allows checking if an object or material has been submitted to a sterilization process successfully.
The most commonly known are the thermosensitive tape (it changes colour when the determined temperature conditions are reached) and B. stearothermophilus
spores.


Vapour trap. A device designed to restrict the passage of vapour and allow the passage of condensed liquid.


Pa =
1N
m2




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93


Chapter 13


Drying Oven


The drying oven is used in the laboratory for drying and
sterilizing glass and metal containers. Manufacturers have
developed several types of drying oven for that purpose:
some operate by natural convection or by forced convection,
others by gravity convection. In general, the ovens operate
between room temperature and 350 °C. They are also
known as hot air oven, or poupinel or pupinel.


PURPOSE OF THE OVEN
The drying oven is used for sterilizing or drying glassware
and metal materials used for examinations or tests
performed in the laboratory. Dry heat sterilization of clean
material is conducted at 180 °C for two hours in the oven.
Upon being heated by high temperature dry air, humidity
is evaporated from glassware and thus the possibility of any
remaining biological activity is eliminated.


OPERATING PRINCIPLES
Generally, drying ovens have an internal and an external
chamber. The internal chamber is made of aluminium or
stainless steel material with very good heat transference
properties. It has a set of shelves made of stainless steel
grids so that air circulates freely around objects requiring
drying or dry heat sterilization. It is isolated from the
external chamber by insulating material which maintains
high temperature conditions internally and delays the
transference of heat to the exterior. The external chamber
is made of steel laminate, covered with a protective fi lm
of electrostatic paint. Heat is generated through sets of
electrical resistors transferring this thermal energy to the
chamber. These resistors are located in the lower part of the
oven and heat is transferred and distributed by natural or
forced convection (in oven with internal ventilators).


GMDN Code 21086 21087


ECRI Code 21-086 21-087


Denomination Oven, laboratory Oven, laboratory,
forced-air


PHOTOGRAPH OF DRYING OVEN


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C H A P T E R 1 3 D R Y I N G O V E N


94


The power (energy by a unit of time) dissipated by an
electrical resistor can be calculated by means of the
following equation:


Where:
I = Intensity of the electric current in amps [A]
R = electrical resistance in ohms [Ω]


Given that the energy is neither created nor destroyed but
transformed, it is possible to calculate the thermal energy
equivalent to the resistive elements. In the case of a resistive
wire, the quantity of heat [q] dissipated can be calculated
by the following equation1:


Where:
R = resistance of resistive wire
I = intensity of the electrical current
r0 = outer radius of the wire
L = length of the resistance wire
= is the heat generated per unit volume


Resistance [R] can be calculated by the following
equation:


Where:
ρ = resistivity of the resistor’s material
A = surface of the resistance wire


The oven has a metallic door with its own thermal insulation
equipped with a similarly insulated handle to prevent burns
on hands. The door is installed on the front part of the oven
by a set of hinges which allow it to open at a 180° angle.


The modern oven is controlled by a module with a microprocessor.
It allows selection of the equipment’s operation parameters and
its alarms; and the programming of cycles or thermal processes
through which are controlled, not only the temperatures but
also the way in which they need to vary in time through phases
of heating/cooling (natural) or through stable temperatures
maintained within certain time intervals. Ovens operate normally
from room temperature up to 350 °C. Some models have limited
ranges of operation. Older ovens simply have a set of resistors,
whose operation is controlled by a thermostat.


The following table features the temperature/time relationship
required for dry heat sterilization in drying ovens.


INSTALLATION REQUIREMENTS
In order to be used, the drying oven requires the
following:
1. A large, strong, levelled work table.
2. Free space of at least 5 cm around the oven and enough


space to place the material to be processed.
3. An electrical outlet with a ground pole of appropriate


size for supplying electrical power to the oven. It must
be in good condition and comply with the national
or international electrical standards used in the
laboratory and must not be more than 1 m away from
the equipment. The typical voltage used is 110 V or 220
V/60 Hz.


4. Verifying that the electrical circuit has the necessary
protection devices for guaranteeing an adequate
electrical feed.


OVEN OPERATION
A series of precautions must be taken into account for the
correct operation of the oven. Among the most important
are the following:
1. Do not use fl ammable or explosive materials in the


oven.
2. Avoid spills of acid solutions or corrosive vapours


inside the oven to prevent corrosion of the surfaces
and interior shelves.


3. Use personal protection elements (insulated gloves,
safety glasses and tongs for placing or removing
substances or materials inside the drying oven).


Operation routine
In general, the following procedure is performed:
1. Activate the main switch, pressing the button usually


identifi ed by the symbol [I].
2. Press the key identifi ed as Program.


1 This example of heat transference equation is for a wire-type resistor of
circular shape. For other shapes, diff erent equations must be used.


2 Time is counted from the moment that the corresponding temperature is
reached.


Table of temperature/sterilization time by dry heat


Temperature °C Time in minutes2


180 30


170 60


160 120


150 150


140 180


121 360


P = I 2R


I 2R = q̇ πr0
2L




R = ρ L
A




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95


3. Select the operational temperature by pressing the key
marked by the sign (+) until the selected temperature
appears on the screen. The oven will start the heating
process until reaching the selected temperature.


4. For programmable ovens, instructions must be followed
as defi ned by the manufacturer for setting additional
parameters such as time, types of warming and alarms.


OVEN CONTROLS
A diagram of controls regulating modern drying ovens is
shown in Figure 37. It is possible to identify the following
elements:
1. The main switch.
2. Screens for controlling the current and selected


temperatures.
3. The parameter selection button (menu).
4. The button for programming operation cycles.
5. Buttons for increasing and decreasing the


temperatures.


Each manufacturer supplies detailed instructions to operate
these controls. In general, they are located on the lower
part of the oven and are cooled by a ventilator which
circulates ambient air inside the assembly space where
other electronic components are installed.


Electric circuit
Figure 38 shows the basic electrical circuit of the drying
oven. The following elements are outlined:
1. Main switch. It energizes or turns off the oven.
2. Control. It controls the oven’s functions (temperature,


time, type of heating and cooling, selected operation
modes such as preheating, sterilization, dehydration,
preparation, drying and even baking).


3. Resistors. Heating elements transforming electrical
energy into thermal energy.


4. Indicator systems. Devices complementing the
general control. These indicate if the oven is ON and in
operation.


Selected Temperature


Menu Programme


On Position


Main Switch


Off Position


Figure 37. Electronic control of the oven


General
Oven Control


Energized Resistance
Indicator


Resistances


Main Switch


Energized Oven
Indicator


Connector


Ground Pole


Figure 38. Electric circuit of the oven




C H A P T E R 1 3 D R Y I N G O V E N


96


QUALITY CONTROL
Quality control of drying ovens is slightly demanding since
sterilization by dry heat has temperature and time as critical
parameters. Generally, spores of Bacillus subtilis (Niger
variety) are used as biological indicators. These must be
incubated for several hours after the sterilization process.
The initial spore load of the biological indicator ranges
between 5 x 105 and 1 x 106. The eff ectiveness of the cycle
depends on the diffusion of heat, its amount available
and the amount lost. Its microbicidal action is aff ected
by the presence of organic material or fi lth on the article.
Sterilization by dry heat must be limited to materials which
cannot be sterilized in autoclaves.


ROUTINE MAINTENANCE
The maintenance required by a drying oven is simple and
no complex routine maintenance is necessary. General
maintenance routines to carry as necessary are described
next. The procedures vary depending on the type of oven
and designs from diff erent manufacturers.


Warning: Before carrying out any maintenance routine
on the oven, verify that it is at room temperature and
disconnected from the electrical feed outlet.


Access to electronic components
Frequency: Whenever necessary
The oven’s electronic components are usually located in
its lower part. In order to be able to check them, proceed
as follows:
1. Disconnect the oven from the electrical feed outlet.
2. Move the oven forward until the front part of the base


is aligned with the edge of the working space.
3. Place two wedges of approximately 3 cm in thickness


below each front support. This will elevate the front part
of the oven and facilitate the inspection of electronic
elements once the lower cover is removed.


4. Remove the screws securing the lower cover and lift
it. Next, check the electronic control components. In
general, the following elements are located in this
compartment.
a) The programmable control panel
b) A safety release
c) The main switch and circuit breaker (combined)


5. Replace the cover once checking has been completed.


Changing of the heating resistors
Frequency: Whenever necessary
The procedure explained next must be performed by
personnel with a good knowledge of electricity.
1. Disconnect the oven from the electrical feed outlet.
2. Remove the thermometer from the upper part of the


chamber.
3. Open the door and remove the shelves.


4. Disconnect the thermometer’s probe.
5. Remove the screws that secure the lower panel.
6. Remove the lower panel.
7. Remove the screws that secure the resistor’s electrical


feed cables and disconnect the terminals fastening
these to the resistors.


8. Remove the screws that secure the resistors as well as
the external resistors.


9. Install new resistors with the same characteristics as the
originals.


10. Reinstall the parts and reconnect the electrical
components.


Changing the cooling ventilator
Frequency: Whenever necessary
To change the cooling ventilator (generally located in the
lower part), these procedures must be followed:
1. Proceed as explained for opening the electronic


compartment.
2. Disconnect the ventilator’s electrical feed terminals.
3. Undo the screws that secure the ventilator.
4. Install a ventilator with the same specifi cations as the


original; connect the wires feeding the ventilator to the
terminals.


5. Replace the protective cover.


Changing of the door gasket
Frequency: Whenever necessary
The door’s gasket is usually made of silicone.
1. Turn off the oven and open the door.
2. Loosen the safety devices that keep the gasket in


place.
3. Remove the gasket using a screwdriver for disengaging


it from the retention guide. Avoid using excessive force
which can distort the housing.


4. Install the replacement gasket starting from the upper
part. Next, move the rest of the gasket towards the sides,
securing it with the assembly elements which fasten it
to the door. Finish the procedures on the lower part of
the door in the same fashion.


Changing of the thermocouple
Frequency: Whenever necessary
1. Open the electronic control compartment.
2. Remove the thermocouple’s connecting cables from


their connection points on the control card.
3. Loosen the thermocouple assembly from the upper


part of the oven. Move it towards the front part until a
free length of at least 15 cm of connector cable is left
exposed.


4. Cut the cable from the thermocouple to remove its
wrapping.


5. Secure the cut ends of the defective thermocouple with
the cables from the replacement. Use tape to prevent
these from becoming loose.




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97


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


There is no power to the oven. The oven is not connected. Connect the oven to the electrical outlet.


The main switch is off . Activate the start switch.


The circuit breaker is defective. Change the circuit breaker.


The control card is defective. Substitute the control card.


The connector cable is defective. Check/repair connector cables.


Erratic elevated temperature. The thermocouple is defective. Substitute the thermocouple.


The control is defective. Substitute the control.


The oven shows heating errors. A temperature lower than that selected. Change the temperature selection. Wait until it
reaches the selected temperature.


The thermocouple is defective. Substitute the thermocouple.


The heating resistor is defective. Substitute the heating resistor.


The relay is defective. Substitute the relay.


The control is defective. Replace the control.


The screen displays the message “open”. The thermocouple circuit is open. Verify the thermocouple connection or substitute the
thermocouple.


6. Gently pull the defective thermocouple outside of
the electronic compartment while keeping the
electric wiring attached to use as a guide during its
replacement


7. Disconnect the wires of the old thermocouple and place
those of the new thermocouple into their respective
connection terminals. Check that the original polarity
is maintained.


8. Reassemble the protective cover.


Changing of the door hinges
Frequency: Whenever necessary
To change the door hinges, proceed as explained next:
1. Open the door and lift it from the hinges.
2. Remove the assembly screws of the defective hinges.
3. Remove the defective hinge(s).
4. Put the new hinge(s) in place and tighten with the


assembly screws.
5. Reinstall the door.




C H A P T E R 1 3 D R Y I N G O V E N


98


BASIC DEFINITIONS


Circuit breaker. An electrical control device which allows a piece of equipment or a device to be ON or OFF. It is also called a switch.

Electric Thermocouple. A device used for accurate measurement of temperature. It consists of wirings of two diff erent metals joined together at one end, producing
a small voltage proportional to the diff erence in temperature between the two ends. This phenomenon is known as the “Seebeck eff ect” in honour of its discoverer,
the German physician Thomas Seebeck.


Heat. A form of energy transferred from one system at a given temperature to another at a lower temperature by means of the diff erence in temperature between
the two. When a system of great mass [M] is put in contact with another of small mass [m’] at a diff erent temperature, the resulting temperature is close to the initial
one of the greater mass system. It is said, then, that a quantity of heat ∆Q has been transferred from the system of higher temperature to that of lower temperature.
The quantity of heat ∆Q is proportional to the change in temperature ∆T. The proportion constant [C] or heat capacity of the system, allows the following relationship
to be established: ∆Q = C∆T, which infers that one of the consequences of the change in temperature in a system is heat transference.


Resistance. Opposition that a material or electrical circuit imposes to the fl ow of electric current. It is the property of a circuit that transforms electrical energy into
heat as it opposes the fl ow of current. The resistance [R], of a body of uniform section such as a wire, is directly proportional to the length [l] and inversely proportional
to the sectional area [a]. The resistance is calculated by the following equation:


Where:
k = constant that depends on the units employed
l = Length of the conductor
a = sectional area of the conductor


The ohm (Ω) is the common unit of electrical resistance; one ohm is equal to one volt per ampere.


Thermostat. A device which regulates the temperature of a system. It usually operates by expansion of one of its components which mechanically activates another
element, for example a switch which controls a particular function.


R = k ×
l
a




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99


Chapter 14


Incubator


The word incubator comes from the Latin word incubare
which means to brood. The incubator is designed as a
chamber of controlled temperature, atmosphere and
humidity for the purpose of maintaining live organisms
in an environment suitable for their growth. Among its
most common uses are incubation of bacteriological,
viral, microbiological and cellular cultures; determination
of the biochemical demand for oxygen (BOD) and
biological storage. Incubators vary in complexity and
design. Some only control temperature while others
control the atmospheric composition as well. Some have
the capacity to achieve temperature conditions below
room temperature with refrigeration systems. Depending
on the design and specifications, incubators control


temperatures from -10 °C and go up to 75 °C or slightly
more. Some incubators have CO


2
injection for achieving


special atmospheric conditions at which the growth of
diverse species of organisms and cells is favoured.


OPERATING PRINCIPLES
The incubator uses diverse means of heat transference
and environmental control to achieve conditions for
specialized laboratory procedures. In general, these have
a system of electrical resistors controlled by thermostats
or microprocessors. As for the heat transference systems,
the incubators use conduction and natural or forced
convection.


Thermal conduction
In incubators functioning by thermal conduction, a set
of electrical resistors transfers heat directly to the wall of
the chamber where samples are incubated. The resistors
constitute a region of high temperature, while the chamber
is one of lower temperature. Transference of thermal energy
always occurs from the region of higher temperature
towards the region of lower temperature according to the
following basic equation by Fourier:


Where:
q = quantity of heat transferred by conduction
k = thermal conductivity of the material
a = area of heat transference
∂T= temperature gradient in the direction of the heat fl ow


The minus sign (–) is introduced into the equation to fulfi l
the second law of thermodynamics.


q = −kA
∂T
∂x


GMDN Code 35482 35483


ECRI Code 15-151 15-152


Denomination Aerobic incubator Anaerobic incubator


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PHOTOGRAPH OF INCUBATOR


Standard incubator




C H A P T E R 1 4 I N C U B ATO R


100


Thermal convection
In incubators with thermal convection, heat generated by
the system of resistors is transferred through air circulating
in the incubation chamber, transferring it to the samples.
The effi ciency of this process depends on air fl ow patterns.
In general, air enters from the bottom of the incubator
and is heated in a compartment from which it fl ows into
the incubation chamber according to uniform patterns. It
fi nally exits through a pipe located in the upper part of the
incubator.


The basic equation which explains convection is1:


Where:
q = Quantity of heat transferred by convection
h = Convection coeffi cient for heat transfer
A = Area by which heat is transferred
T


W
= Temperature on the surface of the resistor


T
θ
= Temperature of the fl uid (air)



Some incubators also have ventilators to circulate air by
forced convection. In the following diagram, three designs
used for incubators are shown in Figure 39: thermal
conduction, natural convection and forced convection.


When a temperature lower than room temperature [Ta] is
needed in the incubation chamber, it is necessary to have a
refrigeration system. This allows heat to be extracted to keep
the incubation chamber cooler. The refrigeration system is
operated by the incubator’s temperature control system.
Water in liquid state has a great capacity of absorption and


thermal retention. Some manufacturers have incorporated
water chambers surrounding the incubation chamber into
their designs. This is particularly useful for guaranteeing
very stable temperature conditions inside the incubation
chamber.


Incubators designed to inject and maintain concentrations
of gases such as carbon dioxide (CO2) in the incubation
chamber between 3 % and 5 % are available.


The incubator temperature control system is based on
the use of thermostats (bi-metallic or fluid expansion);
thermocouples, thermistors or diverse semi-conductor
elements. These use electronic circuits which control,
through microprocessors, the temperature as well as the
incubator’s functions. Each manufacturer has developed its
own design. Actual or programmed incubator temperature
and other information are shown on light emitting diodes
(LED) displays.


In order for an incubator’s temperature to be properly
regulated, there must be a diff erence of at least 5 °C between
the temperature of the chamber [Tc] and room temperature
[Ta]. If the chamber’s temperature [Tc] must be lower than
room temperature [Ta], a refrigeration system is required. In
consequence, the acquisition of incubators depends on the
type of procedures carried out in the laboratory. Technical
specifi cations must be studied and carefully defi ned in
order for the acquisition to meet the actual needs of each
laboratory.


1 Heat transference by convection equation, developed by Isaac Newton (law
of cooling).


q = hA Tw − Tθ( )


Conduction Natural Convection Forced Convection


Figure 39. Heat transfer systems used in incubators




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101


INCUBATOR CONTROLS
The diagram shown in Figure 40 illustrates the type of
controls normally found in recent models of incubators.
1. A main switch for turning the equipment on or off.


Some manufacturers include a protection breaker. The
switch has two positions: ON position [I], the incubator is
energized. In position [O], the incubator is turned OFF.


2. A screen displaying the selected parameters. For
example: selected temperature, alarm temperature
(maximum and minimum).


3. Two control buttons are normally identifi ed as Menu
and Selection or Set. These allow the incubator to be
programmed and to determine the alarm thresholds.


4. Two selection buttons for temperature adjustment. The
selection buttons are used in combination with control
buttons.


5. A set of LED displays signalling the operational state.
If the heating system is in operation, the LED reads as
“Heat”. If the incubator is being programmed, the LED
display reads as “Program”.


6. The selection and control buttons are located on the
control panel.


Each manufacturer uses controls suitable for the incubator’s
design: in general, incubators have the controls mentioned
above. Instructions are found in user manuals provided by
the manufacturers.


In general, the parameter desired is selected by using the
Menu button. Using the selection button(s), parameters are
adjusted until reaching the desired point. The selection is
then confi rmed by using the Selection or Set button.


INSTALLATION REQUIREMENTS
Incubators require the following conditions for their
functioning:
1. An electrical connection complying with the electrical


standards used in the country. The electrical outlet
feeding the incubator must not be more than 1.5 m
away from the incubator. The electrical connection must
supply a voltage of 120 V, 60 Hz or 220-240 V, 50/60 Hz
and have its own ground connection.


2. Free space on the sides and back of the equipment
to allow a passage for cables and ventilation required
for the incubator’s normal functioning. This space is
estimated between 5 and 10 cm.


3. An area in the laboratory where the temperature
variation is minimal.


4. A fi rm, levelled table or counter, capable of supporting
the incubator’s weight. The weight of an incubator with
three shelves is between 60 and 80 kg.


5. Pressure regulators, hoses and connections for
incubators using carbon dioxide (CO2), as well as anchors
permitting the high pressure CO2 tank to be secured.


ROUTINE MAINTENANCE AND USE OF THE
INCUBATOR
The general operation and routine maintenance for
incubators are featured next. The specifi c procedures must
be followed according to the recommendation of each
manufacturer.


Recommendations for operation
1. Do not use an incubator in the presence of fl ammable


or combustible materials as components inside of
this equipment could act as ignition sources during
operation.


LED Indicators (5)


Control Panel (6)


Control Buttons (3) Selection Buttons (4)
Main Switch (1)


On Position


Off Position


Display Screen (2)


Menu


Programme
Alarm


Selec


C


Heat


Figure 40. Incubator controls




C H A P T E R 1 4 I N C U B ATO R


102


2. Avoid spilling acid solutions inside the incubator. These
cause the incubation chamber material to deteriorate.
Whenever possible, try to use substances whose pH
is neutral. Avoid incubating substances generating
corrosive vapours.


3. Avoid placing receptacles on the lower cover which
protects the resistive heating elements.


4. Use personal protective elements when using the
incubator: safety eyeglasses, gloves, tongs for placing
and removing containers.


5. Avoid staying in front of an open incubator. Some
substances emit vapours that should not be inhaled.


6. Calibrate the incubator where it is installed to establish
its uniformity and stability.


7. Verify the operational temperature of the incubator in
the morning and evening hours, with certifi ed calibrated
instruments (thermometer, thermocouple, etc.).


8. Register in the appropriate document or form each
excursion detected in the incubator (i.e. temperature,
humidity or CO2 level) and any corrective action
necessary.


9. Daily: Verify that the temperature in the incubator does
not vary more than one degree centigrade (+/– 1 °C).
Record temperature.


10. Add a non-volatile microbial inhibiting agent if water is
needed inside the incubator to maintain a certain level
of humidity.


Cleaning recommendations
Clean cell culture or bacterial incubators regularly, at least
every 14 days and after any infectious material spill, using
appropriate disinfectants.
1. Disconnect the incubator before initiating the cleaning


processes.
2. Use non-abrasive cleaning agents: a piece of cloth


dampened with mild detergent for cleaning easily
reached interior and exterior surfaces.


3. Avoid contact between cleaning agents and electric
elements.


4. Wait until the incubator is dry (free of humidity) before
connecting it again.


Routine Maintenance
A well installed and operated incubator has few maintenance
demands and many years can elapse before it requires any
technical intervention. When any maintenance activity is
performed, it must be done according to the manufacturer’s
recommendations.


Warning: Before performing any repairs, verify that
the incubator has been decontaminated, is clean and
disconnected from the electrical feed line.


The routine maintenance presented next must be carried
out only by approved personnel with technical training on
the incubator that are aware of the risks run in this type of
activity. These routines focus on verifying the conditions
and correct functioning of the following components:
1. The door gasket. This is generally made of a silicone base


for which several years of use are guaranteed. In order
to substitute the gasket, it is necessary to dismount the
door and remove the mechanisms that fasten it to the
door. In general, the gasket is mounted in a groove. The
new gasket must have the same specifi cations as the
original. Its mounting is done using the gasket’s housing
on the door and the fastening mechanism which can be
as simple as a set of screws in some incubators.


2. Heating elements (system of resistors). The heating
elements are generally located in the lower part of the
incubator. In order to substitute them, it is necessary
to dismount the panels and the lower covers of the
incubator. In some incubators, the doors need to be
dismounted as well (the exterior, metal, the interior,
glass). Once the protective covers are removed, the
resistors and the temperature sensor systems are
disconnected and substituted by new ones with the
same specifications as the originals. All removed
elements are then reassembled, and a calibration is
performed.


3. Cooling ventilator. In case of damage, this component
must be substituted by a ventilator with the same
characteristics as the original. To install, the compartment
in which it is housed must be opened. In some
incubators, it is necessary to dismount the doors and
some protective panels. Once this is done, the damaged
ventilator is disconnected and replaced by the new one,
verifying that the air blows in the right direction. All
dismounted elements are then reassembled.


For replacing the components mentioned below, proceed
similarly as described for the previous components. It is
very important to use replacement parts with the same
specifi cations as the originals.
4. Internal circulation ventilator.
5. Electronic control.
6. Electronic components.
7. Thermocouples.
8. Glass door (internal).
9. Handle.
10. Body of the incubator (internal and external elements).




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103


The common situations presented in the following table must be resolved by approved personnel with specialized training in incubator
operation and maintenance. Special cases must be treated according to the manufacturers’ recommendations.


TROUBLESHOOTING TABLE


Standard incubator


PROBLEM PROBABLE CAUSE SOLUTION


The incubator does not function. There is no power in the electrical feed network. Check the condition of the electrical connection.


The on/off switch is in the off position. Place the switch to the ON position.


The electrical feed cable is defective. Check the cable or replace it.


The incubator displays heating errors. The temperature control is defective. Check and adjust or replace the temperature control.


The heating resistor is defective. Replace the resistor with a spare one with the same
characteristics as the original.


The heating resistor connection is defective. Clean connection points. Adjust the connection.


The electric thermocouple is defective. Replace the electric thermocouple.


The temperature selected is lower than room
temperature.


Check the incubator’s specifi cation. Only refrigerated
incubators can operate in these conditions. Normally
the ambient temperature is lower than that of the
incubator.


The relay is defective. Replace the relay.


The door gasket(s) is/are defective. Change the door gasket(s).


The alarm remains on and the temperature is higher
than that selected.


The temperature selected was changed to a lower
value than the maximum limit of the alarm.


Wait until the temperature of the incubator goes
down to the selected temperature.


The temperature control is defective. Replace the temperature control.


The relay is defective. Replace the relay.


The screen continually shows an error sign. Usually
the LED displays the letters EEE.


The alarm diode is fl ashing. Allow the incubator to cool until it stabilizes at the
selected operational temperature.


Low temperature incubator


PROBLEM PROBABLE CAUSE SOLUTION


The incubator control does not function. The switch is turned off . Turn on the main switch.


There is no electrical feed. Verify the electrical feed circuit.


The temperature readings are erratic. (It is higher or
lower than selected).


There is an accumulation of frost around the
evaporator.


Defrost according to the procedure defi ned by the
manufacturer.


Reduce the cooling temperature.


The temperature in the incubation chamber is
uniform, but higher than selected.


There is an accumulation of frost around the
evaporator.


Defrost according to the procedure defi ned by the
manufacturer.


The fl ow of air in the interior is blocked by samples. Reorganize the content of the incubator to allow the
air to fl ow.


The temperature is higher or lower than selected. The temperature control could require calibration. Calibrate according to the procedure defi ned by the
manufacturer.


The control is disconnected while in operation. The voltage line is inadequate. Verify the voltage line, this must not vary by more
than 5% of the specifi ed voltage indicated on the
plate.


The electrical connection is defective.


The compressor does not function although the
cooling LED is on.


The thermal protector of the compressor is open. Verify the voltage; it must not vary by more than 5%
of the voltage specifi ed on the plate.


Temperature readings are higher than those selected
and set off the alarm over 40 °C.


The cooling relay is defective. Replace the cooling relay.


The compressor is defective. Replace the compressor. Load the refrigerant and
calibrate (this is a specialized procedure which
requires special tools).




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104


BASIC DEFINITIONS


Biochemical Oxygen Demand (BOD). Amount of oxygen required by aerobic microorganisms to decompose the organic matter in a sample. It is used as an indicator
of the degree of pollution of water. The (BOD) is measured as the mass in milligrams of oxygen used per litre of a sample when it is incubated at 20 °C over 6 days.


LED (Light-emitting diode). It is an electronic device which is widely used for displaying data on screens.


Resistance. Opposition that a material or electrical circuit imposes to the fl ow of electric current. It is the property of a circuit that transforms electrical energy into
heat as it opposes the fl ow of current. The resistance [R], of a body of uniform section such as a wire, is directly proportional to the length [l] and inversely proportional
to the sectional area [a]. The resistance is calculated by the following equation:


Where:
k = constant that depends on the units employed
l = Length of the conductor
a = sectional area of the conductor


The ohm (Ω) is the common unit of electrical resistance; one ohm is equal to one volt per ampere.


Thermal conduction. This is a form of heat transference within a substance when heat fl ows from the point of higher temperature to that of lower temperature.


Thermal convection. This is a form of heat transference through the movement of fl uid or air.


Thermistor. This is an electronic component, the resistance of which varies with temperature. They are low cost devices used in diverse applications; the most
common one is temperature control.


Thermocouple. A device for accurate measurement of temperature consisting of two dissimilar metals joined together at one end, producing a small voltage which
is proportional to the diff erence in temperature between the two when one of the connections has a higher temperature than the other. This phenomenon is known
as the “Seebeck eff ect” in honour of its discoverer, the German physician Thomas Seebeck.


Thermostat. This is a device which regulates the temperature of a system. In general, it operates by expanding one of its components which mechanically activates
another, for example a switch that controls a particular function.


R = k ×
l
a




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105


Chapter 15


Microscope
GMDN Code 36351


ECRI Code 12-536


Denomination Microscopes


The word microscope comes from the fusion of the Greek
words micros which means small and skopien, to see or
examine. This chapter presents the care and routine
maintenance of microscopes used in clinical practice.

Depending on the contrast system, microscopes are
given diff erent names. Among the most common are the
following:
• Clear fi eld optical microscope
• Dark fi eld optical microscope
• Fluorescence optical microscope
• Phase contrast optical microscope
• Interference optical microscope
• Polarized light optical microscope
• Inverted optical microscope
• Stereoscopic microscope


This type of microscope allows
tridimensional images or volumes to
be appraised by superimposing two
single images, one for each eye, over
each other.


This type of microscope uses
various systems of lenses and
controlled illumination to achieve
magnifi cation of an object.


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PHOTOGRAPHS OF MICROSCOPES


Stereoscopic microscope


Binocular microscope


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C H A P T E R 1 5 M I C R O S C O P E


106


PURPOSE OF THE EQUIPMENT
The microscope is a precision instrument with optical
subsystems (lenses, fi lters, prisms, condensers); mechanical
subsystems controlling the position of the sample in tri-
dimensional space X, Y, Z; electrical (transformers and light
source) and electronic subsystems (cameras, video, etc.)
interacting to amplify and control the image formation
of objects which are not detectable to the human eye. To
observe samples, it is essential to prepare these according
to techniques which emphasize details to be observed.


The microscope constitutes a diagnostic aid of fi rst order in
healthcare, in specialties such as haematology, bacteriology,
parasitology and in the training of human resources (there
are microscopes with specialized additions for students to
carry out observations directed by a professor). The technical
developments applied to microscopes have allowed the
design of numerous specialized models by the industry
and academia. These play a key role in developing human
knowledge and understanding the workings of nature.


OPERATION PRINCIPLES
The microscope is constructed using the physical properties
of lenses interacting with light. A lens is an optical element
usually made of glass which can refract light. It is of
calculated dimensions and in general has parabolic or
spherical surfaces. If light rays reaching one surface of the
lens converge in a common point F when exiting it, such
lens is known as positive or convergent. If it disperses the
light rays crossing it, it is divergent or negative. Positive
lenses (convergent) shown in Figure 41 constitute the
building blocks of microscopes.


In Figure 41, it is possible to identify the focus [F], (the point
where the light rays are concentrated) and how light is
refracted across the lens. The distance between the lens and
the focus is known universally as the focal distance [D].


Figure 42 summarizes concepts related to the functioning
of lenses applied to the design of microscopes.


Figure 41. Positive (convergent) lens


Figure 42. Optics of the convergent lens


Object [h’] located at a distance [a] from the
lens produces image [h] at a distance [b]
from the lens, where [h] = [h’]. The focus [F],
where the light waves are concentrated is at
the focal distance [f ] from the lens. See text
for additinal details and related equations.




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107


When an illuminated object [h’] is placed at a distance [a]
in front of a convergent lens, light rays cross the lens and
are refracted. A ray crossing the upper part of the object
crosses the optical axis of the lens at the focal point [F’].
It is refracted by both surfaces of the lens and exits in one
direction, parallel to the optical axis. The ray crossing the
upper part of the object in parallel with the optical axis
passes through the lens and is refracted. It then travels
through the focal point [F] on the image’s side until it
crosses the first ray at a distance [b] from the lens where
the image is formed. In the case shown in Figure 42, the
distance [a] is greater than the focal distance [f’], where a
real image is formed inverted at a distance [b] behind the
lens. The focal distance [f ] is related to the distances [a] and
[b] in the equation:


The magnification [M] of a lens, defined by the relationship
between the size of the object and the size of the image
formed is represented by the equation:


Where:
[h] and [h’] correspond respectively to the dimensions
of the image and the object; [a] and [b] to the distances
between the lens and the point where the image is formed
and between the lens and the point where the object is
located.


1
f


1
a


1
b


a
b


h
h


M
'


/


Figure 43. Diagram of a microscope




C H A P T E R 1 5 M I C R O S C O P E


108


Components
The main components of the microscope subsystems are
shown in the table below.


INSTALLATION REQUIREMENTS
Normally, microscopes use 110 V/60 Hz or 220 V/60 Hz
power. Some have a regulated source which allows light


intensity adjustments. Other microscopes use a mirror
through which light is directed towards the slide located
on the platform rather than a lamp. Such microscopes are
mostly useful in regions far from urban centres, where there
are no electricity lines and are used by health brigades.
Certain types of microscopes require special installations;
a fl uorescence microscope needs a dark cabinet in order for
observations to be carried out.


No. System No. Components


1 Binocular head 1 Eyepiece


2 Binocular tube


3 Binocular head


2 Revolving
objective holders


4 Revolving objective holders


5 Objectives


3 Platform, plate or
mechanical stage
and condenser


6 Condenser


7 Aperture diaphragm


8 Filter holders


9 Wide range lens


21 Condenser control


23 Platform, plate or mechanical stage


4 Illuminator 10 Closing glass with fi lter holders


11 Settings lever of the diaphragm’s light fi eld


12 Concave mirror


13 Incandescent light


14 Light holder with adjustment ring


15 Collector lens


16 Mirror


5 Microscope’s
body


17 Internal transformer


18 Control rheostat


19 Feed cable


20 Macro/micro metric adjustment knob


22 Microscope’s arm


24 Base


1


23


4
5


6
7


8
9


10


11
12


13


14


1516171819


20
21
22


23


Figure 44. Cross-section of a microscope Legend




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109


DESCRIPTION OF POTENTIAL PROBLEMS WITH
MICROSCOPES


Eyepieces
The most frequent problem affecting eyepieces is the
presence of dust and grime, which may be on the external
or internal surfaces. Such dust or grime produce shadows
interfering with the sample under analysis, especially when
high powered lenses are used (40X–100X). If these are
external, cleaning the surfaces of the lenses solves the
problem. If internal, it is necessary to disassemble the
eyepiece, clean the internal surfaces, reassemble and verify
the fi nal state.


Scratches may be observed on the eyepieces’ lenses,
especially on those that have been in service for a long
time. These are produced by negligence during the
cleaning process due to the use of inadequate material for
cleaning. Scratches produce cobweb-like shadows in the
visual fi eld of the eyepieces. Unfortunately with this type
of damage, the eyepieces must be changed. Sometimes
the focus mechanisms of the eyepiece stick. To repair,
the eyepiece is disassembled; the appropriate solvent
is applied to its threading and the focus mechanism is
cleaned and reassembled. If the lenses of the eyepiece show
ruptures due to abnormal circumstances (marks due to falls,
unsuitable use), the eyepieces must be changed.


Binocular head
The state of the binocular head has a direct eff ect on the
quality of the microscope’s image. Its most important
components are the prisms and mirrors. Grime adhered
to the optical components of the head aff ects the quality
of the image. This component can even become dirty due
to normal work in the laboratory, such as changing the
eyepieces, installing accessories (e.g., cameras) or simply
by forgetting to place stoppers when the microscope is
not in use.


• Prisms. These have silver-plated reflective surfaces
which can become rusty over time and lose their
refl ecting capacity. Some prisms have only one coat
of refl ective paint on their surface through which light


enters and leaves. If the refl ective surface is damaged,
the prism can be removed, cleaned, polished or
repainted, installed and aligned in the binoculars head.
This kind of maintenance is highly complex and can
only be done by specialized laboratories or companies
off ering this maintenance service. The removal of prisms
without training and suitable tools can have a serious
impact on the quality of the image and even break the
component.


• Mirrors. These have refl ecting surfaces directly exposed
and are susceptible to rust. If repair is necessary, the
mirror is dismounted and removed from the binocular
head and substituted by a new one, cut, cemented and
aligned directly where it is being mounted.


This is a fundamental element of the microscope. If the
illumination system does not work well, the microscope is
out of order as light intensity and contrast are fundamental
to observe samples. Several factors may aff ect the lighting
system; the most common ones are grime and deterioration
of the mirrors and lenses, defects in the feed voltage, or
the use of bulbs other than those recommended by the
manufacturers. The anomalies mentioned produce small
shadows in the vision fi eld and insuffi cient light intensity,
or a lack of homogeneity in the lighting.


Internal dust and grime
This occurs when the lighting systems are not sealed to
prevent dust and particle infi ltration. Dust in the system
produces diff usion and a decrease in the quantity of light
projected onto the sample. Large particles produce shadows
rendering observations diffi cult. In order to correct the
problem, the illuminator is disassembled, its components
cleaned, reassembled and realigned.


Figure 46. Lighting system


Figure 45. Binocular head




C H A P T E R 1 5 M I C R O S C O P E


110


Mirrors
The mirrors have a refl ective coating directly on their surface.
In recently manufactured microscopes these generally have
a protective coat. In older equipment, the refl ective coat is
exposed to rust.


Incandescent bulb
The bulb is a consumable component with a determined
operational life. Its acquisition must be planned ahead to
ensure a replacement is always available in the laboratory
or in the institution where the equipment is installed. The
bulb installation is done according to the manufacturer’s
instructions. Some equipment, such as the fl uorescence
optical microscope, uses special bulbs (mercury or xenon
light) requiring mounting and calibrating procedures which,
although simple, must be carried out according to the
manufacturer’s recommendations. The voltage supplied
to the microscope must correspond to that specifi ed by
the manufacturer. Otherwise, unnecessary risks which
may aff ect the quality of lighting are taken. Note that some
microscopes use internal or external transformers and
voltage regulation systems.


Condenser
The condenser controls how the light is concentrated on,
or contrasted against the sample under observation. It is
composed of optical and mechanical elements. The optical
elements are the lenses and the mechanical ones those
which allow the control of the position of the lenses and the
quantity of light reaching the sample through a mechanical
diaphragm.


Normally, optical components are aff ected by the presence
of dust. These must be cleaned in a similar manner to lenses,
using a fi ne camel hair brush to remove dust deposited on
the surface. The mechanical components require adjustment
by tools with special characteristics and each manufacturer
has its own designs. The usual routines are focused on
cleaning, adjustment and lubrication procedures.


Plate or sample holders
The plate or sample holder comprises a series of components
interacting with each other. Their purpose is to control
the position of the sample under analysis. The plate has
movement capability in the direction X/Y, which the operator
controls with independent macro/micrometric buttons.
Beside it, the plate has tension devices to allow smooth
sliding using “milano tail” type guides, which are normally
lubricated. In its upper part, are installed plates or control
gripping devices for the specimen slides. Maintenance
seeks to keep these mechanisms clean, lubricated and well
adjusted.


The maintenance of the revolving objective holder is
simple. It has an internal catch mechanism which allows the
objective in use to be aligned with the optical microscope
equipment. It simply rotates smoothly until a trip mechanism
adjusts the correct position of the next objective. Each
manufacturer defi nes the number of objectives which can
be mounted on the revolver. The most common revolvers
can hold between three to fi ve objectives. Maintenance
seeks to keep the rotating mechanism clean, lubricated
and well adjusted.


The objectives should receive routine cleaning of their
external optical surfaces. Immersion type objectives require
that oil is cleaned off after each use to avoid the objective’s
internal optical structure from being contaminated with oil
through capillarity.


Figure 47. Platform, plate or mechanical stage


Figure 48. Revolving, objective holder




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111


The microscope’s body is designed to receive and support
the components already described (binocular head,
mechanical stage, condenser and revolving objective
holder, other components such as the transformer and
electrical/electronic elements of the microscope’s lighting
system).


Maintenance of the microscope’s body basically consists in
keeping its surface clean, removing grime, dust or elements
aff ecting its presentation and state. It is necessary to take
special care with chemical substances that may be corrosive,
including dyes used in the laboratories for staining slides.


GENERAL MAINTENANCE OF THE MICROSCOPE
Above all, it is necessary to emphasize that the microscope
is high precision equipment. The integrity of its optical
components, both mechanical and electrical, must be
preserved in order to preserve it in the best condition. Each
element of the microscope has been developed using the
most advanced manufacturing techniques. Its assembly
and adjustment are done in the factory using specialized
equipment. During this process the required tolerance of the
various components of the equipment is highly controlled
through advanced measuring techniques. The cleaning of
the microscope environment, its installation and careful
use are fundamental to achieve a long and operational life.
Humidity, dust and bad conditions of the electrical feed,
misuse or inadequate installation are counterproductive for its
conservation. Microscope maintenance involves a lot of care,
patience and dedication. It must only be carried out by trained
personnel using specialized tools. General recommendations
are presented next. These are required for installing and
maintaining a microscope in good working condition.


Installation and storage
1. Ensure that the area where the microscope is installed


is protected from dust and humidity. Ideally, there must
be an air-conditioning system which guarantees air free
from dust or particles, humidity control and permanent
temperature control.


2. Verify that the area is secure, having a door with a lock
to prevent unauthorized removal.


3. Confirm that the location of the microscope is far
from water supplies or where chemical substances are
handled in order to avoid spills or splashing. Also, areas
with direct sunlight must be avoided.


4. Verify that the area selected has an electrical outlet
compatible with the lighting system of the microscope.
It must be in good condition with voltage adjusted to
the magnitude and frequency of the electric codes and
standards. If the microscope uses a mirror, it must be
located near a window which allows good illumination,
but it should not be directly exposed to sunlight.


5. Install the microscope on a levelled surface of a rigid
structure, under which there is suffi cient room for the
user (the microscopist) to place his/her legs. His or
her body should be close to the microscope with the
head near the eyepieces without strain of the vertebral
column, neck and back.


6. To facilitate the microscopist work position, provide a
chair of adjustable height with good back support. If
there is no back support; provide support for the feet,
placing it at the front of the work space (not on the
chair). The purpose of this is for the vertebral column
to be as erect as possible and to reduce fl exing of the
shoulders and neck.


7. Avoid locating microscopes near equipment
which produce vibrations such as centrifuges or
refrigerators.


8. Try not to move the microscope from its installation
position, especially if it is used intensely each day.


9. Cover the microscope with a dust protector if not
used for long periods of time, taking precautions so
it is not aff ected by excessive humidity. The dryer the
environment, the lower the probability fungi will grow.
The protector can be of plastic or cloth of similar quality
to that of handkerchiefs which do not deposit lint.


10. In areas of high humidity, keep the microscope in a
box or cabinet lit with a bulb of no more than 40 W
during the night. This helps keeping the storage area
dry and reduces the probability of fungal growth. If this
alternative is used, verify that there are some openings
permitting ventilation inside.


Cleaning procedures
Cleaning of the microscope is one of the most important
routines and must be considered essential. The following
materials are required:


Figure 49. Body of the microscope




C H A P T E R 1 5 M I C R O S C O P E


112


1. A piece of clean cloth with a similar texture to that of a
handkerchief.


2. A bottle of lens cleaning solution which can be obtained
from opticians. Normally, it does not aff ect the lenses’
protective coating nor the adhesives or cements used
in their assembly. Among widely used cleaning liquids
are ethyl ether, xylene and white gasoline.


Warning: Some manufacturers do not recommend using
alcohol or acetone as these can aff ect (dissolve) the cements
and adhesives used for attaching lenses.


3. Lens paper. This can normally be obtained from
opticians. If it is not possible to obtain this material, it
can be substituted with soft absorbent paper or with
medicinal type cotton. Also a piece of soft silk can be
used.


4. A piece of very fi ne chamois. This can be obtained from
shoe shops.


5. A rubber (nasal) bulb for blowing air. A device can be
made in the laboratory by connecting a Pasteur pipette
to the rubber bulb.


6. A plastic cover to protect the microscope from its
external environment when not in use. A cloth bag with
a texture similar to handkerchief material can also be
used.


7. A soft camel hair brush or a fi ne paint brush. Importantly,
the brush’s hair should be natural, of uniform length
with a very soft texture, dry and free from grease. It
is possible to obtain this in photography stores. Also,
it is possible to fi nd an equivalent in shops supplying
cosmetics.


8. A 250 g packet of desiccant (silica gel). This is used to
control the humidity in the microscope’s storage box
if it is airtight. It changes colour when it is saturated
by humidity to detect when it needs to be substituted
or renewed. When it is in good condition, the colour is
generally blue; when it is saturated with humidity, it is
pink.


9. Bulbs and replacement fuses. These should be of the
same model as those installed by the manufacturer or
of equivalent characteristics.


Note: All required materials for cleaning must be kept
clean and stored in containers that protect them from their
external environment.


Cleaning of the optical elements
In a microscope, there are two types of optical elements:
those external in contact with their outside environment and
those internal, inside the body of the microscope and more
protected (objectives, eyepieces, mirrors, prisms, condenser,
illuminator, etc.). The cleaning procedures, although similar,
diff er with regard to the care and precautions.


1. The external optical elements of eyepieces, objectives,
condenser and illuminator are cleaned by gently
brushing their surfaces with the camel hair brush. This
removes dust particles. The rubber bulb is then used to
blow streams of air onto the lenses’ surface to ensure
that these are free from dust. If dust is found adhered to
the optical surface, a piece of very soft clean cloth is used
with small circular movements, without exercising too
much pressure on the lens. The nasal bulb is used again
to blow air on the lens to remove adhered particles. A
piece of fi ne chamois can also be used. If so, place the
chamois at the end of a small cylindrical object with a
slightly smaller diameter than that of the lens. Without
exercising much pressure, rotate gently on the lens
surface. Finally, air is blown onto the lens surface with
the nasal aspirator. This is suffi cient to clean the external
surfaces. The piece of chamois can be humidifi ed with
distilled water if necessary.


2. Under adequate conditions of installation, interior
surfaces of optical elements should not be aff ected
by dust or particles. If for some reason, particles are
detected, it is necessary to open them to carry out the
cleaning process. An eyepiece or objective must never
be opened if there is not a clean environment to carry
out the cleaning procedure. Clean with a camel hair
brush and with the nasal aspirator according to the
procedure explained previously. It is not recommended
to dismount the objectives for any reason as this could
alter the tolerances achieved by the manufacturer.
If dismounted, it would be necessary to realign the
elements and this is only feasible if the manufacturer’s
precise instructions are followed. Cleaning of the
objectives will be limited to keeping the front and back
lenses clean.


3. If immersion oil residues are detected on the lens
surface, remove using lens paper or medicinal type
cotton. The lens’ surface must be then cleaned with a
solution composed of 80 % ether petroleum and 20 %
2-Propanol.


Cleaning of the microscope’s body
1. The microscope’s body can be cleaned with a detergent


solution to remove external fi lth and cut the grease and
oil. This must be applied with a small brush. After the
grease and fi lth have been removed, the microscope’s
body must be cleaned with a 50/50 solution of distilled
water and 95% ethanol.


Note: This solution is not adequate for cleaning optical
surfaces.


2. The parts integrated in adjustment mechanisms for
the macro/micrometric (thick and fi ne) adjustment, the
condenser and the stage or platform must be lubricated
periodically with refi ned machine oil to facilitate smooth
movement.




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113


Microscope maintenance
Among the most important steps for maintaining
a microscope in suitable operation conditions are the
following:
1. Verify the adjustment of the mechanical stage. It must


move gently in all directions (X-Y) and must stay in the
position selected by the microscopist.


2. Test the focus adjustment mechanism. The focus
selected by the microscopist must remain stable. The
height must not change from that assigned by the
microscopist.


3. Verify the functioning of the diaphragm.
4. Clean all the mechanical components.
5. Lubricate the microscope according to the


manufacturer’s recommendations.
6. Confirm the adjustment of the specimen holder


(gripping device).
7. Verify the optical alignment.


Precautions
1. Avoid cleaning optical components with ethanol


because it aff ects the optical elements. Also, do not
clean the base of the platform with xylene or acetone.


2. Do not use ordinary paper to clean lenses as it could
scratch their surface.


3. To prevent leaving fi ngerprints, do not touch lenses with
bare fi ngers.


4. Do not clean the eyepieces’ lenses or objectives with
cloth or paper, because the coating covering the optical
elements could deteriorate. Clean these surfaces with
a camel hair brush or by blowing air with a nasal
aspirator.


5. Avoid leaving the microscope without the eyepieces.
Place stoppers on these to prevent dust and particles
from entering the binocular head.


6. Do not leave the microscope stored inside a box in
humid environments.


7. Avoid pressing the objective against slides as it could
damage the thin lamina or its front lens. Adjust the focus
slowly and carefully.


8. Keep the platform or mechanical stage clean.
9. Do not disassemble optical components since this


can produce misalignments. Optical surfaces must be
cleaned fi rst with a camel hair brush and then with a
chamois or lens paper.


10. Use both hands for lifting the microscope, one
hand supporting the microscope arm and the other
supporting its base.


11. Avoid touching the surface of the bulb with fi ngers
when changing it. Fingerprints decrease the light
intensity.


12. Verify that the feed voltage is correct in order to
prolong the life span of the bulb. Whenever possible,
use the lowest light intensity needed for carrying out
observations.


13. Connect the microscope to a voltage stabilizer if the
feed voltage is not stable.


Special care in warm climates
In warm climates as well as in dry ones, the main problem
affecting the microscope is dust since it affects the
mechanical and the optical systems. This problem can be
controlled by the following steps:
1. Always protect the microscope with a plastic cover


when not in use.
2. After use, clean the microscope by blowing air using a


nasal aspirator.
3. Clean the lenses with a camel hair brush or with an air


brush. If the dust stays adhered to optical surfaces, try
to remove it with lens paper. However, rub the surface
very gently to avoid scratches.


Special antifungal care in humid climates
In humid and generally warm climates, microscopes can be
aff ected by fungi growing mainly on the surface of lenses,
in the grooves of screws and under the protective paint. If
the equipment is not adequately protected, it could become
useless in a short period of time. The following care instructions
will assist in preventing the formation of fungus.
1. At night, store the microscope in a box equipped with


an electric light of no more than 40 W. The bulb must be
installed in the upper part of the box, near the binocular
head and must be kept on during the night. The box
must have some openings to allow the air to circulate.
The temperature inside the box must not exceed 50 °C
so that properties of the microscope’s lubricants are not
aff ected.


2. If it is not possible to use a box with electric light, as an
alternative, a drying agent such as silicone gel or rice
can be used. When a drying agent is used, verify that
the microscope is kept in a protected box or under a
cover made of fabric similar to that of a handkerchief.
Verify that the drying agent is in good condition. If this
is not the case, substitute it.


3. Clean the microscope periodically. Use latex gloves if
lenses must be touched. This will prevent leaving any
fi ngerprint and decrease the risks of fungal growth.


4. If none of the mentioned alternatives is feasible, put the
microscope in a place with good air circulation. When
the microscope is not in use, it may be located under
direct solar light, for short periods. This reduces the
humidity and the risk of fungi growing on the surfaces
of the equipment.


5. Air conditioning (temperature and humidity control)
signifi cantly prevents fungal growth on microscopes.
However, this is not an option for a great number
of laboratories. If the air conditioning service is not
continuous in the area where the microscope is installed,
precautions must be taken to control the humidity.




C H A P T E R 1 5 M I C R O S C O P E


114


Removal of fungal hair
1. Check and clean the microscope frequently using the


procedures mentioned in this chapter. Control the
humidity conditions where the microscope is stored.
If adequate ventilation is maintained, it decreases the
possibility of fungal growth on the microscope.


2. If fungal growth is detected, use a small piece of cotton
dampened in an antifungal solution, normally ether or
xylol (xylene). Rub gently making circular motions on
the entire surface of the lens. An oscillatory movement
can also be used, towards the front and back or left-
right-left, exercising a very moderate pressure on the
surface of the lens. If necessary, repeat the procedure
with a new piece of cotton.


3. When removal of the fungal hair is completed, clean
with a small piece of clean cotton.


Microscope care
Frequency: Daily (after use)
1. Clean the immersion oil off from the 100X objective.


Use lens paper or, if not available, use medicinal type
cotton.


2. Clean the sample holders.
3. Clean the condenser.
4. Place the light intensity control rheostat in the


lowest position and then turn off the lighting system
completely.


5. Cover the microscope with a protective cover (of plastic
or cloth). Ensure that it is kept in a well ventilated place
where the humidity and temperature are controlled. If it
has a ventilated storage box equipped with a light bulb
for humidity control, place the microscope inside, turn
on the light and close the box.


Frequency: Each month
1. Remove dust particles from the microscope’s body. Use


a piece of cloth dampened with distilled water.
2. Remove dust particles from the eyepieces, objectives


and condenser. Use a rubber bulb for blowing air. Next,
clean the lenses’ surface with lens cleaning solution. Do
not apply this solution to lenses directly, but on lens
paper and then rub their surfaces gently with the wet
paper.


3. Remove the slide holder mechanism, clean carefully and
reinstall.


Frequency: Every six months
As a complement to the monthly maintenance routines, the
following are recommended:
1. Carry out a general visual inspection of the microscope.


Verify that each component is in good condition, clean
and mechanically well adjusted.


2. Verify that good ventilation conditions, temperature
and humidity control are maintained in the place of
installation.


3. Test the quality of the electric system that feeds the
microscope. Verify the integrity of the connectors, fuses
and of the incandescent light.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


115


TROUBLESHOOTING TABLE


Lighting system


PROBLEM PROBABLE CAUSE SOLUTION


The lighting system does not come on. The electrical feed cable is disconnected. Connect the electrical feed system.


The protection fuse is burnt out. Replace the protection fuse.


The bulb is burnt out. Replace the light bulb. Ensure it is well aligned.


The lighting switch is defective. Replace the switch.


The lighting system is not producing uniform light. The electrical system shows voltage errors. Check and repair the electrical system. Connect the
microscope through a voltage stabilizer.


The microscope’s connector to the wall outlet is
slack.


Connect the plug to the outlet. If any of the elements
are defective, replace it.


The bulb is badly installed and is not making good
contact.


Reinstall the bulb.


There are metal or black specks on the bulb’s surface. Replace the light bulb.


The sample is not illuminated in a uniform manner. The light source is not centred. Rectify the alignment of the condenser.


The objective is not well centred. Slowly turn the revolving objective holder until the
adjustment catch sound.


The sample is poorly illuminated. The diaphragm’s iris is almost closed. Open the diaphragm’s iris until the lighting is
adequate.


The condenser is very far (very low). Bring the condenser closer.


The condenser’s lenses show dust or fungal growth. Clean the condenser. Remove the dust with a brush.
Remove the fungi with a lens cleaning solution.


There is excessive contrast in the image. The diaphragm’s iris of the condenser is almost
closed.


Open the iris of the diaphragm slightly.


The image is slightly too clear and shiny. The diaphragm’s iris of the condenser is very open. Close the diaphragm’s iris slightly.


Optical/mechanical system


PROBLEM PROBABLE CAUSE SOLUTION


The mechanical stage does not stay in position and
the image is continually going out of focus.


The adjustment tension of the mechanical stage is
slack.


Adjust the tension mechanism of the mechanical
stage.


The mechanical stage cannot be raised to its higher
limit.


The mechanical stage is locked very low. Loosen the locking mechanism of the mechanical
stage. Adjust to the desired height. Readjust the
locking mechanism.


There is poor quality of the image with objective
40X.


The lenses show fungi. Remove the fungi using a cleaning solution. Follow
the manufacturer’s instructions regarding the device.


The lenses are damaged. Check the objective. Verify if the lenses show
scratches, punctures or nicks. Replace the objective.


The lenses are accidentally smeared with immersion
oil.


Remove the oil carefully with lens paper.


The immersion objective does not give clear images. The objective is being used without immersion oil. Place immersion oil on the slide.


The immersion oil is of a low refraction index. Use good quality oil.


There is immersion oil in the interior of the objective. Clean the lenses with lens paper. If cleaning the
outside is not the solution, send the objective to
a specialized laboratory for repair. (Dismount the
lenses, clean, change the seals, cement, realign and
assemble).


Dust or visible dirt is in the fi eld of vision. Dust present on the collector lens of the light source. Remove particles of dust with a camel hair brush.


Dust present on the upper lens of the condenser. Remove the dust particles with a camel hair brush.


There is dust on the eyepiece. Remove the particles of dust with a camel hair
brush.




C H A P T E R 1 5 M I C R O S C O P E


116


NA = nsin φ( )


BASIC DEFINITIONS


Acetone. This is a colourless, fl ammable liquid with an excellent capacity to mix with water; a solvent used for a great number of organic substances. Boiling point:
56 °C. Chemical formula:
CH3 – CO – CH3


Diaphragm. This is a device which controls the fl ow of light through the microscope. There are two types of diaphragms: the aperture diaphragm which adjusts
the angle of the aperture in the microscope, and the fi eld diaphragm which regulates the size of the image. The purpose of the diaphragms in optical microscopes
is to prevent rays of light with severe aberrations from reaching the image formation levels and to ensure an adequate distribution of light in the sample as well as
in the image’s space.


Ethanol. This is a colourless liquid also known as ethylene alcohol. A widely used industrial solvent, for example in the pharmaceutical industry. Its density is 0.806
g/cm3, boiling point 78.3 °C and chemical formula:
CH3 – CH2OH


Ether. This is a liquid substance derived from alcohol by eliminating one molecule of water between two molecules of alcohol. It is an excellent solvent which is not
very soluble in water and is very volatile and fl ammable. Its boiling point is 35 °C and chemical formula:
CH3 – CH2 – O – CH2 – CH3


Eyepiece. Set of lenses through which the microscopist observes the image (real or virtual image depending on the relationship that exists with other sets of
microscope lenses).


Field depth. The specimen or sample’s compactness which is reasonably clear at a determined level of focus.


Field of vision. The surface area seen when looking through the microscope. The area decreases with increasing power of magnifi cation. The diameter of the fi eld of
vision is measured in millimetres (mm) on the intermediate plane of the image. The fi eld of vision in an optical microscope at a particular magnifi cation is expressed
as its diameter in mm or simply as a number.


Focus. The point where, as a result of the light’s refraction, the light rays passing through a lens are concentrated. If the light rays converge in one point, the lens is
positive and the focus is real; if the light rays diverge, the lens is negative and the focus virtual.


Focus depth. A range at which the image plane can be moved maintaining clarity.


Numerical aperture. This is a measurement of the capacity of an objective to concentrate light and distinguish minute details of an object. Normally, the value of
the numerical aperture is recorded on the side of the objective’s body. Greater values of numerical aperture allow a greater number of oblique rays of light to pass
through the objective’s front lenses, producing a higher resolution of the image. It is expressed mathematically as:


Where:
NA = numerical aperture
n = refraction index (n = 1 air; n = 1.52 immersion oil)
Φ = aperture angle. At a greater the angle, a greater
thenumerical aperture, a greater resolution


Numerical aperture Mathematical expression


NA = n x Sin Φ
0.27 = 1 x
Sin (16°)
Magnifi cation
approx. 10X


NA = n x Sin Φ
0.42 = 1 x
Sin (25°)
Magnifi cation
approx. 20X


NA = n x Sin Φ
0.68 = 1 x
Sin (43°)
Magnifi cation
approx. 40X




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


117


Propanol. Also known as isopropyl alcohol and prepared by the hydration of propylene. It is used as a solvent as well as in the preparation of acetone. Its boiling
point is 83 °C and chemical formula:
CH3 – CHOH – CH3


Range of useful magnifi cation. [RUM] of an objective/eyepiece combination is defi ned by the numerical aperture of the system. For perceiving the details of
an image, a minimum magnifi cation traditionally between 500 and 1000 times the numerical aperture [NA] of the objective is required. {Acceptable from RUM =
(500) x [NA] to (1 000) x [NA]}.


Refraction index. Value calculated by comparing the speed of light in space and in a second medium of greater density. It is normally represented by the letter [n]
or [n´] in technical literature or in mathematical equations.


Resolution. The ability to distinguish the fi nest details from a slide or particular sample. Among factors which most infl uence achieving a good resolution are the
numerical aperture, the type of sample, the lighting, the aberration correction and the type of contrast used. It is one of the most important characteristics of the
microscope.


Revolving objective holder. Mechanical device designed for mounting the objectives and allowing rapid interchange by means of a rotational movement. Its
capacity depends on the type of microscope. In general, it varies between three and fi ve objectives.


Xylene. Ethyl benzene isomer obtained from coal. It is used as a solvent and also in the preparation of dyes and lacquers. Its boiling point is 138 °C / 144 °C and
chemical formula:
CH3
C6H4
CH3




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


119


Chapter 16


Pipettes
GMDN Code 15166


ECRI Code 15-166


Denomination Pipettes


Pipettes are devices used for measuring or transferring small
volumes of liquid from one container to another with great
precision. There are many pipette models. Initially, they were
made of glass; at present, there is a wide range of options.
Fixed volume and variable volume pipettes with mechanical
controls are highlighted herein. Recently, pipettes with
electronic controls have been introduced into the market.
This chapter deals with aspects referring to the maintenance
and calibration1 of mechanical pipettes.


1 Calibration must be done exclusively by trained personnel according to
current international standards as BS ES ISO 8655-6:2002 or updated ones.
Reference work instruments must be suitably calibrated by national or
international institutions, responsible for verifying the compliance with
international measurement standards.


PHOTOGRAPHS OF PIPETTES


Single channel pipette Multichannel pipette


Figure 50. Diagram of a pipette


Ph
ot


o
co


ur
te


sy
o


f G
ils


on
S


.A
.S


.


Ph
ot


o
co


ur
te


sy
o


f E
p


p
en


do
rf


A
G




C H A P T E R 1 6 P I P E T T E S


120


PURPOSE OF THE PIPETTES
Pipettes are devices widely used in clinical and research
laboratories to supply very exact quantities of fl uids.


OPERATION PRINCIPLES OF THE PIPETTE
The mechanical or piston pipette generally functions by
manually transmitting force exercised on a plunger. The
plunger is an axis connected to a piston which moves along
a fi xed length cylinder, forcing a predetermined volume of
liquid outside or inside the pipette.


There are two types of piston pipettes: the fi xed volume
type with a predetermined liquid volume known as nominal
volume [Nv] and the variable volume type, which allows
adjusting of the volume dispensed within a determined
range depending on the pipette’s specifi cations. Volume
adjustment is achieved by modifying the range of the
piston’s movement inside the plunger. In variable volume
pipettes, the nominal volume is the maximum volume
the pipette can hold according to the manufacturer’s
specifi cations.


Fixed volume and variable volume pipettes can be
subdivided into two types: A and B. Pipettes of the type
A are named air displacement pipettes due to the fact that
there is a volume of air between the head of the piston and
the liquid in the cylinder (see pipette No. 1, Figure 51). Type
B pipettes are called positive displacement pipettes or direct
displacement pipettes as the piston is in direct contact with
the liquid (see pipette No. 2). Figure 44 shows diff erences
between these types of pipettes.


Air displacement pipettes have the advantage of presenting
less risks of contamination when heavily used. However,
they are not as precise as positive displacement pipettes
when working with very small volumes of liquid due to the


compressibility of air. All piston pipettes have disposable tips
for minimizing risks of contamination. It is recommended
to exclusively use tips provided by the manufacturer or
compatible with the specifi c pipette to guarantee their
correct adjustment to the pipette’s body as well as volumes
dispensed. In order to facilitate identifying these volumes,
some manufacturers have adopted a colour code which
simplifi es the identifi cation of the volumes to be dispensed.
The following table demonstrates this colour convention.


REQUIREMENTS FOR USE
To use a pipette, the laboratory must be suitably clean and
well lit. The general conditions are the following:
1. Verify that room temperature is stable with an optimum


temperature of 20 °C with a variation range of ± 5 °C
(between 15 °C and 30 °C).


2. Confi rm that the relative humidity is higher than 50 %.
The pipettes and samples or liquid materials must be
stabilized to the conditions of the laboratory. Typically
it is recommended to equilibrate these in the laboratory
two to three hours before the work is performed.


3. Avoid working with pipettes under direct sunlight.
4. Use the appropriate protective elements if working with


toxic materials or those carrying a biological risk.


Volume range in microlitres (µl) Colour


0.1–2.5 µl Black


0.5–10 µl Grey


2.0–20 µl Grey/Yellow


10–100 µl Yellow


50–200 µl Yellow


100–1000 µl Blue


500–2500 µl Red


Table of pipette colour coding


Figure 51. Types of pipettes




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


121


USING THE PIPETTE
In order to obtain precise, exact and reliable results, it is
necessary for pipette operators to know in detail correct
pipetting procedures. This is achieved by training and
detailed follow-up regarding the use of pipettes. The general
outlines for the appropriate use of pipettes are as follows:


Warning: Before using a pipette, verify that it is correctly
calibrated and suitable for the transfer of liquid volume to
be performed.


General recommendations
1. Verify that the pipette is in a vertical position to aspirate


a liquid. The vertical position guarantees that there is no
uncertainty due to minimal variation at the surface of
the liquid.


2. Use the recommendation outlined by the manufacturer
for the minimum immersion depth of the pipette’s tip
to aspirate liquids. The depths vary according to the
pipette type and capacity. A general guide is shown in
the following table1:


3. Humidify tips of air displacement pipettes for better
pipetting accuracy. To humidify the tip, draw working
solution several times, dispensing its contents into
the waste container. This reduces the possibility of air
bubbles being aspirated when dense or hydrophobic
liquids are aspirated. The process mentioned allows
humidity to be homogeneous in the pipette’s air
chamber (area between the piston’s head and the
liquid’s surface). Pre-humidifying is not necessary in
pipette dispensing volumes lower than or equal to
10 µl. Neither is humidifying necessary for positive
displacement pipettes.


4. After fi lling the pipette tip, remove any drop on the tip
by gently sliding the pipette tip against the wall of the
original tube. Absorbent material may be required to
avoid touching the pipette’s tip and taking necessary
precautions in case the material shows any sign of
contamination.


5. Dispense the liquid drawn by letting the tip touch the
wall of the receiving tube. The pipette’s tip must form
an angle ranging between 30 and 45° with the tube at
8 to 10 mm above the surface of liquid.


Correct pipetting technique
The following is a description of the general steps required
when using a mechanical air displacement pipette. The
operator must take into account specifi c recommendations
of the manufacturer. This observation must also be respected
when using electronically-controlled pipettes. The diagram
in Figure 52 shows the description of the process.


Volume range of the pipette
(µl)


Depth of the immersion
(mm)


1–100 2–3


100–1000 2–4


1 000–5000 2–5


Table of tip immersion depth according to the pipette
volume range


1 Blues, J., Bayliss, D., Buckley, M., The calibration and use of piston pipette, UK,
National Physical Laboratory, Teddington, Middlesex, 2004, page. 6.


(www.npl.co.uk)


1 2 3 4 65A


     


B
C


Position
F


Figure 52. Phases of pipette use




C H A P T E R 1 6 P I P E T T E S


122


1. Place a new tip according to the pipette specifi cations
on the pipette tip holder. Avoid contaminating the
tip with other substances. Verify that it remains well
adjusted.


2. Press the plunger gently until it reaches the fi rst limit.
Until this point, the tip of the pipette must not touch
the liquid.


3. Put the extremity of the tip in the liquid. Verify the
recommended depth included in table 2 or use the
recommendation provided by the manufacturer.
Confi rm that the pipette is in a vertical position. This
process corresponds to the position 1B (fi rst to the left)
in the fi gure.


4. Release the plunger gently for the pipette to aspirate the
liquid (position 2A). Verify that the plunger is completely
released. Wait at least two seconds before removing the
pipette’s tip from the liquid.


5. Place the pipette’s tip against the wall of the receiving
tube. Verify that the angle formed between the pipette’s
tip and its wall is between 30 and 45°. If the receiving
tube already contains liquid, avoid the pipette’s tip from
being submerged (position 3A).


6. Dispense the contents of the pipette by pressing the
plunger gently but fi rmly, until reaching the fi rst limit
(position 4B). At all times, maintain contact between
the pipette’s tip and the wall of the receiving container.
Gently slide the tip against the inside wall at 8 to 10 mm
from the tube edge to ensure that there are no drops of
liquid left on the pipette tip.


7. Press the plunger gently until it reaches the second
limit on the piston’s path (position 5C). This expels any
fraction of liquid still in the pipette’s tip, by forcing out
the air in the chamber through the opening of the tip.
Keep the plunger pressed at the second limit while the
pipette is removed from the receiving tube. Once the
pipette is removed, gently release the plunger to the
higher limit position.


8. Discard the pipette’s tip. To do this, press the expulsion
mechanism’s button (position 6).


Note: If a variable volume pipette is used, the volume to
be dispensed must fi rst be selected. To do this, instructions
indicated by the manufacturer must be followed. Normally
the volume controls are found in the upper part of the
pipette. It is necessary that the operator understands and
learns to diff erentiate the scales.


ROUTINE MAINTENANCE
General outlines of the required routine maintenance for
mechanical pipettes are featured next. Specifi c maintenance
must be carried out on the diff erent models according to
the instructions manuals provided by the manufacturers.


Inspection:
Frequency: Daily
Pipettes require frequent inspection in order to detect
abnormal wear and tear or damage and/or to verify that
they are in good working condition. Inspection must cover
the following aspects:
1. Verify the integrity and adjustment of the mechanisms.


These must move smoothly. The piston must move
smoothly.


2. Confi rm that the tip holder is not displaying distortions
or signs of being worn out, as it is essential for the
exactitude of measurements. Verify the adjustment of
the tips.


3. Put on a tip and fi ll it with distilled water. The pipette
must not show any leak.


Cleaning and decontamination
1. Every day, verify that the pipette is clean. If dirt is


detected, it must be cleaned using a suitable solvent
or a mild detergent solution. Check the manufacturer’s
recommendation regarding the compatibility of the
pipette with solvents to select the appropriate one.


2. Sterilize the pipette according to the manufacturer’s
indications. Some pipettes can be sterilized in an
autoclave using a cycle of 121 °C for approximately
20 minutes. Some will need to be disassembled for
the vapour to come into contact with their internal
components1. Disassembly consists of liberating and
unscrewing the central body of the pipette according
to the procedures indicated by the manufacturer. To
disassemble or assemble some pipettes, a set of tools
(keys) provided by the manufacturers with the pipette at
the time of sale must be used. After the sterilization cycle,
the pipette must only be reassembled once at room
temperature. Prior to assembly, it should be verifi ed
that the components are dry. Some manufacturers
recommend sterilizing the pipette using a 60 %
isopropanol solution and washing the components
with distilled water, drying and assembling.


3. If a pipette has been used with harmful substances,
it is the responsibility of the user to ensure that it
is completely decontaminated before it is used in
other procedures or removed from the laboratory. It is
advisable to expeditiously prepare a report indicating its
brand, model, serial number, contaminating substances
and substances or procedures with which it was treated
or cleaned.


1 Pipettes which can be sterilized with vapour have a mark with such
identifi cation; the manufacturer supplies the requirements for disassembly.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


123


Maintenance
Frequency: Bi-annually
A pipette used daily must be submitted to the following
procedures for guaranteeing its correct functioning:
1. Disassemble the pipette. Follow the procedure described


by the manufacturer in the user manual (the procedure
varies depending on the brand and model). Normally,
the main body of the pipette is disassembled from the
tip ejector system unscrewing the body of the pipette
from the cylinder.


2. Clean the O rings, the plunger and the inside of the
cylinder before lubricating. If the internal components
were contaminated accidentally, all the surfaces should
be cleaned with a mild detergent and then with distilled
water. If the O rings or gaskets need to be changed,
replacement parts with the same characteristics as the
original should be used. The type of ring or gasket varies
depending on the pipette brand, type and model.


3. Lubricate the plunger and piston with silicone grease1
specially developed for pipettes. Always use the
lubricant recommended by the manufacturer. Remove
any excessive lubricant with absorbent paper.


4. Assemble following the reverse process to that of
disassembly.


5. Calibrate the pipette before use.


Concepts of pipette calibration
Calibration of pipettes is done using standardized
procedures.


The calibration method depends mainly on the volume the
pipette handles. The smaller the volume range of the pipette,
the more demanding and costly the calibration process is.
A brief description of the gravimetric process used with
pipettes dispensing volumes between 20 µl (microlitres)
and 1 ml (millilitre) is explained in this chapter.


Required materials and equipments2


1. Analytical balance.
2. Electronic thermometer with a 0.1 °C or greater


resolution, of suitable temperature range with a
submersible probe


3. Hygrometer with a standard uncertainty of 10 % or
less.


4. Barometer with a standard uncertainty of 0.5 kPa or
less.


5. Timer.
6. Micropipettes of various volumes.
7. Disposable tips of various volumes.
8. Flat bottom vials.
9. Bi- or tri-distilled degassed water.
10. Trained operator.


Recommended Pipette Calibration Frequency
(Quarterly)


Principle
The procedure is based on measuring the volume of a water
sample from the mass of water dispensed by the pipette
and dividing that mass by the water density. In practice,
a group of measurements is done, to which corrections
are applied to compensate for any variation due to non
standard temperature and atmospheric pressure and to any
signifi cant evaporation during test.


1 There are diff erent specifi cations for silicone grease; therefore the grease
recommended by the pipettes manufacturer must be used.


2 The equipment used in pipette calibration must be certifi ed by an
accredited calibration laboratory.


Handle
Cover


Handle


Piston
Assembly


Piston


O Type Seal


Spring Support


Piston Spring
Filter


Conical Tip


Secondary Spring


Tips Ejector Collar


Volume Display Window


Tips Ejector


Tip Ejector Spring


Button


Figure 53. Disassembly of a pipette




C H A P T E R 1 6 P I P E T T E S


124


This type of test allows the following:
1. To compare diff erent types of pipettes to each other to


detect if there are diff erences among them.
2. To check the precision and exactitude of a pipette.
3. To check the exactitude and precision of a batch of


pipettes.
4. To check factors attributable to the use of one pipette


by several users.


Procedure1


The procedure explained next is valid for air displacement
pipettes. It includes the following steps:
1. Install a new tip on the pipette.
2. Pipet distilled water and empty into the waste container.


Repeat at least 5 times in order to stabilize the humidity
of the air inside the pipette.


3. Add water to the weighing receptacle until the level of
liquid reaches at least 3 mm.


4. Register the temperature of the water, environmental
pressure and relative humidity.


5. Cover the weighing receptacle, if this applies.
6. Register the weight shown on the balance or press tab


so that the reading is zero (0).
7. Fill the pipette with water from the storage container


and dispense it into the weighing receptacle expelling
all the water. This is done in the same way pipettes are
used on a daily basis (see step 7 of the Correct pipetting
technique).


8. Register the new weight detected by the balance.
9. Repeat steps 7 and 8 nine (9) additional times, recording


the weight registered by the balance at the end of each
cycle.


10. Register the temperature of the liquid inside the weighing
receptacle at the end of the tenth cycle and measure the
time elapsed since the measurements started.


11. Evaluate if evaporation has been signifi cant (this is critical
when working with pipettes of very small volumes). If
this is the case, an additional period of time [Ta] equal
to the time used during the ten measurements must be
allowed to elapse, and when completed, a new reading
has to be carried out.


12. The mass of water lost by evaporation in the additional
time [Ta] is divided by the total number of samples
analyzed (ten). This will give an indication of the average
mass of liquid lost due to evaporation per cycle. This
fi gure must be added to each of the mass readings.


Calculations
Proceed as follows:
1. Calculate the mass of water dispensed by the pipette


in each cycle. Subtract the reading registered at the
end of the previous cycle to the reading registered
in the current cycle. Repeat for all measurements. If
appropriate, add the average mass corresponding to
the calculated evaporation per cycle.


2. Convert each mass value to a volume at 20 °C, dividing
the mass by the density of water adjusted to the
mentioned temperature.


3. Calculate the average of the volumes calculated in
step 2. (The sum of volumes, divided by the number
of samples). Apply the adjustments per phenomenon
such as the air pressure onto the mass (fl otation). To
accomplish this, multiply each mass by a correction
factor [Z].


4. Calculate the standard deviation of the sample.


5. Calculate the coeffi cient of variation.


A table containing a summary of the mathematical formulae
mentioned is shown next.


1 The procedure presented is a general guide. For complete details, consult
the standards BS ES ISO 8655-6:2002 or current updates.


2 The values Z depend on the temperature and pressure of distilled
water. Refer to specialized publications such as the Standard BS EN ISO
8655-6:2002, Attachment A.


Vi =
M i
D


X = Vi n × Z∑


SD =
1


n −1
× X i − X AV( )2


i=1


n∑


Cv[ ]CV (%) = S
X AV


×100




Conventions: Conventions:


X =average volume
SD = standard deviation
Z = adjustment factor in
(µl / mg)2


CV(%) = variation coeffi cient
D(%) = error


Table of mathematical formulae


X = Vi n × Z∑


SD =
1


n −1
× X i − X AV( )2


i=1


n∑


E s = X − Vn


Cv[ ]CV (%) = S
X AV


×100


Vi =
M i
D


Nom


NomAV


X


XX
D




=%




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125


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The pipette displays leaks. The tip is placed incorrectly on the pipette. Install the tip according to the procedure indicated
by the manufacturer.


There are foreign bodies between the tip and the
adjustment cone.


Clean the joint. Remove the tip and clean the
adjustment cone. Install a new tip.


There are foreign bodies between the piston and the
O-ring in the cylinder.


Disassemble and clean the cylinder/piston set.
Lubricate and assemble.


There is insuffi cient lubricant in the piston and/or
the O-ring.


Disassemble and lubricate adequately.


The O-ring is twisted or damaged. Replace the O-ring. Disassemble, clean, replace
gasket, lubricate and assemble.


The piston is contaminated. Clean the piston and lightly lubricate.


The lower cone is slack. Adjust the lower cone.


There are visible drops inside the pipette’s tip. There is non-homogeneous humidifi cation of the
plastic wall.


Install a new tip on the pipette.


The pipette shows inaccuracies. Incorrect operation of the pipette. Check the pipetting technique and correct the
detected errors.


There are foreign bodies under the activation button. Clean the button’s assembly mount.


The pipette tip is incorrectly mounted. Check the fi t of the pipette’s tip. Install a diff erent tip
suitable for the pipette’s specifi cation.


There is interference in the calibration. Recalibrate according to standardized procedure.
Check use procedure.


The tip is contaminated. Use a new tip.


The tip shows inaccuracies with determined liquids. The calibration is inadequate. Recalibrate the pipette using standardized
procedure.


Adjust the calibration if liquids of high viscosity are
used.


The control button does not move smoothly or shows
high resistance to its activation.


The piston is contaminated. Clean and lightly lubricate.


The gasket is contaminated. Disassemble the pipette, clean all the gaskets, or
replace them if necessary. Lightly lubricate.


The piston is damaged. Replace the piston and the piston’s gaskets. Lightly
lubricate.


Solvent vapours have entered into the pipette. Unscrew the central joint of the pipette. Ventilate,
clean and lightly lubricate the piston.




C H A P T E R 1 6 P I P E T T E S


126


BASIC DEFINITIONS


Coeffi cient of variation [%CV]. A statistical parameter representing the ratio of the standard deviation of a distribution to its mean.


Density. Relationship between a body’s mass and the volume which it occupies. The average density of an object is equal to its total mass divided by its total volume.
It is identifi ed by the Greek letter Ro [ρ]. In the International System of Units, density is measured in kilograms by cubic metres [kg/m3].


Error (of a measurement). A diff erence shown between the value measured and the correct value.


Exactitude. A concept related to errors shown in measurements. It is said that an instrument is exact when the value of a group of measurements are suffi ciently
close to the real value.


Mass. A physical property of the bodies related to the quantity of matter these contain, expressed in kilograms (kg). In physics, there are two quantities to which the
name mass is given: gravitational mass which is a measure of the way a body interacts with the gravitational fi eld (if the body’s mass is small, the body experiences
a weaker force than if its mass were greater) and the inertial mass, which is quantitative or numerical measure of a body’s inertia, that is, of its resistance to being
accelerated.


Microgram [µg]. A unit of weight equivalent to 1 x 10-6 grams (g).


Microlitre [µl]. A unit of capacity equivalent to 1 x 10-6 litres (l). One (1) µl of water weighing exactly one (1) mg and has a volume of 1 mm3.


Milligram [mg]. A unit of weight equivalent to 1 x 10-3 grams (g).


Millilitre [ml]. A unit of capacity equivalent to 1 x 10-3 litres (l). One (1) ml of water weighing exactly (1) g and has a volume of 1 cm3.


Precision. A concept related to errors shown in measurements. An instrument or method is precise when upon repeating a measurement in independent tests,
the results obtained are similar.


Range. A diff erence between the maximum and minimum value which an instrument reads or measures.


Standard deviation [SD]. Measure of the dispersion of a set of data from its mean. The more spread apart the data is, the higher the deviation. It is used as a
statistical parameter for determining the global error of a sample set.


Volume. A quantity of physical space that a mass occupies. It is calculated by dividing the mass by its average density.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


127


Selected / Actual Temperature
Display Screen


Temperature Mode
On/Off Button


Temperature
Selection Button


HEAT AGIT


RESET


Temperature
Scale Button


Reset Button


Selected / Actual Speed
Display Screen


Agitation Mode On/Off Button


Agitation Speed Button


Figure 54. Stirring heating plate controls


Chapter 17


Stirring Heating Plate
GMDN Code 36815


ECRI Code 16-287


Denomination Heating Plates


The stirring heating plate or heated stirring heating plate
has been developed to heat and mix fluids contained
in laboratory receptacles such as fl asks, test tubes and
beakers.


PHOTOGRAPH OF THE STIRRING HEATING PLATE


OPERATION PRINCIPLES
Generally, the stirring heating plate has a fl at surface on
which are placed receptacles containing fl uids to be heated
and agitated. Its surface is made of good thermal conductors
such as aluminium [Al] or ceramic materials. Some heating
plates exclusively use radiation sources such as infrared
(infrared light) for heating. Stirring hot plates have a heating
element (an electrical resistor), a control system (on and off ,
temperature control, agitation control and its respective
motor). The motors used in these types of instruments are
generally of single phase induction named shaded pole1.
The speed range depends on the number of poles and the
frequency of the feed voltage.


Temperature:
Room Temperature up to approximately 500 °C.
Rotation speed:
From 60 RPM up to approximately 1200 RPM.


CONTROLS OF THE STIRRING HEATING PLATE
The diagram in Figure 54 includes a typical control found on
a stirring heating plate. The diagram shown corresponds to
a microprocessor-regulated heating plate which is found in
most modern equipment.


1 The power of these motors is
approximately 1/20 hp; these
are characterized by having a
low torque and being low in
price. They are called shaded pole
induction motors.


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C H A P T E R 1 7 S T I R R I N G H E AT I N G P L AT E


128


The control has buttons for selecting the temperature
and the stirring heating plate’s speed. These can be used
independently or in combination. To select the parameters,
only the corresponding control button needs to be activated
and the temperature and speed selected, whichever is
required.


INSTALLATION REQUIREMENTS
The stirring heating plate needs to be connected to an
electrical outlet in good condition with a ground pole.
The outlet must be compatible with the equipment and in
compliance with the national and international electrical
standards. In general, stirring heating plates operate with
voltages of 120 V/60 Hz, or 230 V/50-60 Hz.


For normal operation it is required to have an appropriately
levelled surface with suffi cient resistance to support the
weight of the stirring heating plate together with that of
the receptacles and liquids these may contain.


OPERATION OF THE STIRRING HEATING PLATE


Precautions
1. Always connect the stirring heating plate to an electrical


outlet in good condition which has a ground pole.
2. Disconnect the equipment before carrying out any


maintenance routine.
3. Avoid using the equipment in the presence of


combustible or flammable materials. Avoid using
equipment in environments with corrosive vapours.


4. Carefully check if substances have a low ignition point
(Flash point). It could start a fire or an explosion if
the vapour touches the surface of the heater at this
temperature.


5. If working with fl ammable liquids, use personal protective
elements: gloves and protective eyeglasses.


6. Take into account that the surface of the equipment
can stay hot for a long period after being turned off or
disconnected.


7. Avoid placing on the heating surface:
a) Metallic laminates
b) Materials with insulating properties
c) Low melting point glassware


8. Maintain a free space around the equipment to facilitate
its connection and placing materials or substances
needed with the equipment. Some manufacturers
recommend a free space of approximately 15 cm.


9. Avoid placing combustible materials near the
equipment.


10. Avoid using containers whose weight exceeds the
capacity indicated by the manufacturer.


ROUTINE MAINTENANCE
The stirring heating plate is designed to work under
normal conditions and requires minimal maintenance.
This equipment should work without problems for several
years if well installed and operated. This document
presents the general routine maintenance recommended
by manufacturers. Specialized procedures must be done
carefully following manufacturers’ recommendations.


Cleaning
Frequency: Monthly
1. Clean the equipment in a vertical position to avoid


cleaning agents from reaching internal components.
2. Use a mild detergent. Apply to the external surfaces


using a piece of cloth of similar texture to that of a
handkerchief.


3. Verify that the equipment is completely dry before
connecting it again.


Replacement of the ceramic surface
Frequency: Whenever necessary
General recommendations applicable to the substitution of
the ceramic surface are presented next.
1. Verify that the heating plate is disconnected and cold.


This prevents the risk of electric shock or burns.
2. Handle the equipment with extreme care since a broken


ceramic surface has dangerously sharp edges.
3. Place the unit with its heating surface facing


downwards.
4. Remove the screws which secure the lower cover and


remove it.
5. Locate and disconnect the cables which feed the


electrical resistors (in models with such elements).
6. Disconnect the cables connecting the equipment’s


control and the resistors.
7. Remove the screws which fasten the upper cover to the


base. Verify that they do not aff ect the connection to the
heating resistors.


8. Place the new ceramic surface in its appropriate
location.


9. Observe how the safety devices of the damaged ceramic
cover are positioned. Remove the safety devices and
place the heating and insulating elements inside the
new surface, maintaining the same alignment and
distribution of the original. Put the new safety devices
back.


10. Reconnect the components in the reverse order to that
described above.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


129


Main Bobbin


Secondary Bobbin


Stator


Rotor


Poles


Figure 55. Induction motor


Replacement of fuses
Frequency: Whenever necessary
If the stirring heating plate is connected and the main
switch is in the on position but it is not warming up, it is
possible that a fuse needs to be changed. The following is
the process for changing the fuse:


1. Place the main switch in the off position and disconnect
the electrical feed cable.


2. Remove the top of the fuse compartment with a fl at
screwdriver.


3. Replace the fuse by a new one with the same
specifi cations as the original.


4. Replace the fuse’s compartment cover.


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


There is no electrical power. There is a failure in the protection fuse. Substitute the protection fuse.


There is a failure in the electrical connection feeding
the equipment.


Check the state of the electrical connection.


The equipment is disconnected from the electrical
feed outlet.


Connect the equipment to the electrical outlet.


The electrical feed cable is defective. Substitute the electrical feed cable.


The plate shows no sign of warming up. The heating function has not been selected. Activate the heating function on the control panel.


The heating resistor is out of service. Substitute the heating resistor. Install replacement
parts with the same characteristics as the original.


There is no rotation. The rotation function has not been selected. Activate the rotation control on the control panel.


BASIC DEFINITIONS


Erlenmeyer. A glass container used in laboratories to put or measure substances.


Shaded pole motor. An induction motor used in small machines. It is characterized by having a bobbin (squirrel cage rotor) requiring a rotating magnetic fi eld for
starting. Each fi eld pole has a shading coil (copper ring) which induces currents causing the magnetic fl ow to become imbalanced in relation to the fl ow in the other
portion, producing a torque in the rotor. These motors are low cost and low effi ciency. Their speed can be calculated by means of the equation:




Where:
[n] = synchronous speed in revolutions per minute
[f] = frequency of voltage applied
[p] = number of poles in the stator


A diagram is included showing the inner part of the electrical circuits.


Ignition point. The temperature at which molecules of a substance react with oxygen in the air, initiating combustion. This temperature is called Flash Point.


n(rpm) = 120 f
p




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


131


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PHOTOGRAPH OF A REFRIGERATED STORAGE UNIT


Chapter 18


Refrigerators and Freezers
GMDN Code 13315 13315 17157 35486 40513 15145


ECRI Code 13-315 15-170 17-157 15-171 22065 15-145


Denomination Refrigerators Biological
refrigerators


Laboratory
refrigerators


Blood bank
refrigerators


Freezer,
laboratory,
ultralow


Freezer,
laboratory


REFRIGERATORS AND FREEZERS
Refrigerators and freezers are among the most important
pieces of equipment in laboratories. They maintain a
temperature controlled (refrigerated) environment for
various fl uids and substances. At lower temperatures, less
chemical and biological activity is present so that fl uids
and substances are better preserved. To achieve this, the
temperature of the refrigerated storage unit needs to be
lower than ambient temperature. In the laboratory, diff erent
kinds of refrigerators and freezers are used. They can be
grouped by temperature ranges:
• Conservation refrigerators in the range of
2 to 8 °C.
• Low temperature freezers in the range of
–15 to –35 °C.
• Ultralow temperature freezers in the range of
–60 to –86 °C.


A unit with appropriate functions must be selected
depending on the activities carried out in the laboratory. For
example: if it is necessary to conserve whole blood, it will be
appropriate to use a Blood bank refrigerator which provides
temperatures between 2 and 8 °C. On the other hand, if it is
required to conserve a particular viral or microbial stock, an
ultralow temperature freezer is required. Refrigerators and
freezers are essential for conserving biological substances
and reagents. This chapter deals with the operational and
maintenance aspects of the conservation refrigerators and
ultralow temperature freezers.


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C H A P T E R 1 8 R E F R I G E R AT O R S A N D F R E E Z E R S


132


High Pressure
Zone


Exterior Ta
Qh Qh Qh Qh Qh Qh Qh 2


Condenser3
Expansion
Valve


Filter


4


Low Pressure Zone
[4-1]


Evaporator
Refrigerator Environment Ta


QI QI QI QI QI QI QI


[2-3]


Refrigerant Fluid: Ideal Compression Cycle


2


2’3


44’


S


T


Thermal
Isolation


Compressor


1


Figure 56. Refrigeration circuit


PURPOSE OF REFRIGERATED STORAGE UNITS
Refrigerators and freezers are used for the conservation
of blood and its derivatives, biological liquids and tissues,
reagents, chemicals, and stocks. In general, the higher the
temperature the more chemical and biological activity
is present. By reducing temperature, one can control the
eff ects on the composition and structure of substances to be
preserved. In the laboratory, systems of refrigeration are used
for conserving substances such as reagents and biological
elements which would otherwise decompose or lose their
properties. Refrigeration, as a technique off ers conditions
which renders possible the conservation of elements such as
blood and its derivatives needed for diagnosis, surveillance
and for providing health services. It is possible to achieve
extremely low temperature ranges, such as those used for
master stocks conservation (–86 °C) or temperatures within
the range of 2 and 8 °C, which is suffi cient for conserving
reagents and diverse biological products.


OPERATION PRINCIPLES
Refrigerators and freezers function according to laws of
physics regulating the energy transfer where temperature
diff erences exist. From the second law of thermodynamics
it is known that, if thermal energy needs to be transferred
from a point with low temperature to another with high
temperature, a mechanical task needs to be carried out.
Modern refrigerators and freezers are thermal systems
which function mainly using a cycle called compression,
where refrigerant gas with special properties achieving heat
transference is used. This chapter focuses on explaining how
refrigerators and freezers using compression operate.


Refrigeration circuit
The basic circuit shown in Figure 56 demonstrates how
a refrigerator functions. On the left side it is possible
to distinguish the following components: evaporator,
condenser, compressor, expansion valve, filter and
interconnection tubing. Within each one of these
components, refrigerant gas circulates.


On the right side of the figure is shown a graph of
temperature [T] versus entropy [S], which demonstrates the
functioning of an ideal1 refrigeration cycle. The numbers on
the basic diagram on the left show points of the adiabatic
processes (compression [1-2] and choking [3-4]) and the
processes involved in heat transference (in the evaporator –
refrigerated environment [4-1], in the condenser [2-3] on the
exterior). The complete cycle is described as the sequence
of processes [1-2-3-4-1].


Evaporator. Contains a network of channels through which
the refrigerant gas circulates. In the evaporator, a process of
heat transference [Ql] occurs at a constant pressure. In order
for the refrigeration process to occur, the environment to
be refrigerated must be surrounded by a system of thermal
isolation. This is to prevent thermal energy from entering
the evaporator’s zone of infl uence at the same rate as the
refrigerant gas absorbs it. The refrigerant gas enters into
a liquid phase in the evaporator by point [4] (ideal) or [4’]
(real) and while it passes through the network of evaporator
channels, it absorbs heat [Ql] and progressively transforms
into vapour. When the refrigerant gas reaches point [1],
it is under the form of vapour. It is then suctioned by the
compressor through a tube or line.


1 The real cycle diff ers from the ideal cycle by some irreversible processes not
indicated in the graph for the sake of clarity and simplicity.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


133


Compressor. Usually propelled by an electric motor, the
compressor suctions the vaporized refrigerant from the
evaporator (saturated) at low pressure and by means of
a piston or set of pistons, exercises a process of reversible
adiabatic compression on it (without heat transfer) between
points [1-2]. Upon being discharged from the compressor,
the vapour is hot as a result of the compression process and
is delivered to the condenser in point [2].


Condenser. Similar device to the evaporator, which has
a network of channels through which the refrigerant gas
circulates. As the temperature of the refrigerant is higher
than ambient temperature [Ta], a heat transference process
[Qh] is produced from the refrigerant to the environment
at constant pressure. To facilitate heat transference, the
condenser tubes have thin fi ns which increase the transfer
surface. As heat continues to be lost [Qh] as a result of the
process of transference, the refrigerant returns to its liquid
phase until it reaches point [3] as saturated liquid where it
enters the expansion valve.


Expansion valve. Allowing the refrigerant to flow in a
controlled manner, the valve exercises a resistance on the
passage of the refrigerant to avoid any heat transference
by an adiabatic process. As a result, the pressure in the
valve is reduced in a drastic way in point [4]. A fi lter is
generally installed at the exit of the expansion valve. Some
manufacturers replace the expansion valve by a capillary
tube which has an equivalent restrictive effect on the
passage of the cooling fl uid.


Filter. Retains humidity and impurities which may be
present in the refrigerant. At the back of the fi lter, the system
is connected again to the evaporator at point [4] and the
cycle described is repeated.


Liquid collector. Sometimes placed by manufacturers
before the refrigerant enters the compressor. Its purpose
is to retain any portion of that fluid in liquid phase to
guarantee that only vaporized refrigerant gas enters the
compressor (not shown in the refrigeration diagram).


Thermal insulation. Set of materials with the property
of slowing heat transference. Its function consists of
preventing thermal energy from the environment to reach
the refrigeration area at the same rate as the system extracts
the internal thermal energy. All refrigeration equipment has
adequate thermal isolation for this purpose. Among the most
commonly used insulation materials are polyurethane foam
and glass wool. Similarly, it is customary to manufacture
interior surfaces in materials such as ABS plastic.


Service valves. Valves used for loading the refrigeration
circuit with refrigerant gas. By means of these valves, the
draining and filling systems are connected so that the


refrigerated storage unit operates according to specifi cations
established by the manufacturer. Only the manufacturer
and specialized technical personnel have access to these
valves (not indicated in the refrigeration diagram).


Thermal protector. This is a protective device which is
activated and disconnects the compressor in case overloads
aff ecting the bobbins in the compressor’s fi eld occur (It
pertains to the electrical system and is not indicated in the
refrigeration system’s diagram).


Note: The evaporator, as well as the condenser are made
of materials with good thermal conduction properties
such as aluminium [Al] and copper [Cu]. To improve heat
transference, ventilation systems which induce forced
convection processes have been incorporated. To attain the
diff erent temperatures (refrigeration) required in laboratories
or in the industry, manufacturers have developed diverse
designs and refrigerants for the targeted results.


INSTALLATION REQUIREMENTS
For their functioning refrigerators and freezers require the
following precautions:
1. An electrical connection with a ground pole appropriate


to the voltage and frequency of the equipment. In
general depending on their capacity, refrigerators and
freezers can be obtained in versions with 115 V, 60 Hz
and 220-240 V, 50 Hz. Electrical connections complying
with international and national electric standards used
in the laboratory must be anticipated.


2. If more than one unit installed depend on the same
electrical circuit, it must be verifi ed that the capacity
(electrical power) and safety devices are adequate
for supplying the amount of power required by these
units.


3. Directly connect the unit to the electrical outlet. Never
connect a unit to an overloaded electrical outlet or one
with voltage defi ciencies. Avoid the use of electrical
extensions. The electrical outlet must not be more than
2 m from the unit.


4. Install the unit on a levelled surface, leaving free space
around the equipment. Refrigerators and freezers have
a levelling system at their base which allows them
to adjust to small diff erences in level of the fl oor. It is
customary to leave a free space of 15 cm at the sides
and at the back of the unit to facilitate ventilation of
the condenser.


5. Avoid installing the unit under direct sunlight or near a
heat source such as radiators or heaters. Remember that
the greater the diff erence in temperature is between the
environment and the condenser, the more effi cient will
the heat transference be.




C H A P T E R 1 8 R E F R I G E R AT O R S A N D F R E E Z E R S


134


115 V/ 60 Hz


1. Key Switch


2. Door Switch


3. Compressor


4. Evaporator Ventilators


5” Defrosting Resistor


6. Condenser Resistor


6. Frontal Resistor


Condenser Ventilator


5. Defrosting Switch


7. Thermostat


5” Defrosting Temporizer


5” Defrosting Control Limiter


5’ Defrosting Switch


5”’ Defrosting Motor
TemporizerCompressor


Release


2


Refrigerator Light


Evaporator
Ventilator Switch 2


Figure 57. Control circuit of the refrigerator


REFRIGERATOR CONTROL CIRCUIT
The scheme in Figure 57 is a typical control circuit installed
in refrigerators and freezers. Its purpose is to give an idea of
how their diverse subsystems are interrelated. The control
circuit of each model varies according to the characteristics
incorporated by the manufacturer.


The following are featured as central components:
1. The main switch. It energizes the refrigerator.
2. The door switch. It turns on the light when the door is


opened.
3. The compressor.
4. The evaporator’s ventilators.
5. The defrosting subsystem. The switch, resistors,


temporizer (5, 5’, 5’’, 5’’’, 5’’’’).
6. The resistor subsystem for defrosting or maintaining the


equipment’s components free from ice.
7. The thermostat.


REFRIGERATOR OPERATION


Conservation refrigerators
The operation of conservation refrigerators is generally very
simple. Each manufacturer gives basic recommendations.
Some of these are highlighted below.
1. Connect the refrigerator’s electrical feed cable to an


electrical outlet equipped with a ground pole and the
capacity to supply voltage at the required power.


2. Activate the on switch. Some manufacturers place key
switches on refrigerators. Wait for the refrigerator to
reach the operating temperature before storing any
product. The manufacturers adjust the temperature of
refrigerators at approximately 4 °C.


3. Select the temperature at which the alarm must be
activated. Follow the instructions provided by the
manufacturer.


4. Load the refrigerator according to the capacity
established by the manufacturer.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


135


Technical Service Indicator


Thermometer


Key Switch


Door Open Indicator


Low Battery Indicator


Temperature Decrease Button


Audio


Selector Button for Alarm,
Temperature Parameters


Temperature Increase Button


Display Screen


Figure 58. Blood bank refrigerator controls


5. Distribute the load homogeneously inside the
refrigerator. The temperature uniformity depends on
the free circulation of air within the refrigerator.


6. Avoid opening the door for long periods of time in order
to prevent thermal energy and humidity (from the air)
from entering into the refrigerated environment. This
forms ice and increases the working temperature of the
refrigeration system. Open only for placing or removing
stored elements.


Conservation refrigerator controls
A diagram of a recently developed control for conservation
refrigerators (e.g. a Blood bank refrigerator) is shown in
Figure 58.


The following controls can be seen in the diagram:
1. A main switch, activated by a key
2. Open door, low battery and abnormal technical


condition indicators
3. Buttons for adjusting parameters
4. Display screen


REFRIGERATOR ROUTINE MAINTENANCE
Refrigerators are generally not very demanding from a
maintenance perspective, although demanding with
regards to the quality of the electrical feed systems. If
connected to good quality electrical circuits and good
ventilation fl ows around the unit, they can function for
years without specialized technical service. The refrigeration
circuit is sealed during manufacturing and does not have
components requiring routine maintenance. The most
common maintenance routines are described next. Consult


WHO’s Manual on management, maintenance and use of cold
chain equipment, 2005, for care and preventive maintenance
schedules specific to Blood bank refrigerators, plasma
freezers and walk-in refrigerators and freezers used in the
blood cold chain.


Cleaning the interior
Frequency: Every quarter
1. Verify that the refrigerator’s inner shelves are clean.


These are generally made of rust proof metallic mesh.
Before cleaning, any material which can interfere must
be removed from the refrigerator. Move the empty
shelves towards the front. Dampen a piece of cloth with
a mild detergent and apply by rubbing surfaces gently.
Dry and place in their original position.


2. If the refrigerator has drawers, cleaning is done the
same way. Empty the drawers and dismount from
the adjustment devices. Remove them from the
refrigerator.


3. Once the shelves and drawers are dismounted, clean the
interior walls of the refrigerator, using a mild detergent.
Dry before mounting the internal accessories.


4. Apply a mild detergent with a damp piece of cloth to the
drawers. Rub carefully. Dry the drawers and put them
back on their mounts in the refrigerator.


Warning: Avoid using steel wool or other abrasive materials
for cleaning the shelves and drawers. Avoid using gasoline,
naphtha or thinners, as these damage the plastic, the
packing or the paint on the surfaces.




C H A P T E R 1 8 R E F R I G E R AT O R S A N D F R E E Z E R S


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Cleaning of the condenser
Frequency: Every six months
1. Disconnect the electrical feed cable.
2. Verify the position of the condenser. Manufacturers


usually place it at the lower back of the equipment. In
some refrigerators, it is installed on the top part.


3. Remove the condenser’s protective grids and the
protective fi lter (not all manufacturers provide a fi lter).


4. Remove the dust and grime deposited on the surface of
the condenser. Use an aspirator equipped with a suction
brush. Run it over the entire surface of the condenser
to remove grime or accumulated dust. Verify that the
tubes’ surfaces as well as those of the heat conducting
wings are clean. Vacuum the fi lter as well (if present).


5. Replace the cover.
6. Connect the refrigerator to the electrical connection.


Warning: If the condenser is not clean, this will interfere
with the heat transference process and the refrigerator
could “heat” or function at temperatures diff erent than
selected.


The door gasket verifi cation
Frequency: Quarterly
The door gasket is a component which must stay in a
good condition for the unit to work correctly. To verify its
condition, one must proceed according to the following
steps:
1. Open the door.
2. Insert a strip of paper of about 5 cm in width between


the door gasket and the edge of the refrigerator’s body
where the gasket is housed.


3. Close the door.
4. Pull the paper gently from the exterior. The paper must


put up resistance when being moved outwards. If the
paper can be moved without resistance, the gasket
must be substituted. Perform this procedure on 10 cm
of gasket at a time around the entire gasket housing.


Warning: A door gasket in bad condition produces various
problems in the functioning of cooling units:
1. It allows humidity to enter which condenses and freezes


inside the evaporator.
2. It increases the time needed by the compressor for


maintaining the selected temperature.
3. It aff ects the storage temperature.
4. It increases the operational costs.


Defrosting
Frequency: Every six months
Many modern freezers have automatic cycles for defrosting
the evaporator in order to avoid frost accumulation.
Normally, these cycles are done with a set of electrical
resistors which rapidly eliminate the frost present. Some
models do not have defrosting cycles and the process is
done manually on a scheduled basis. The following are the
recommended procedures for defrosting.
1. Verify that the thickness of the frost is more than


8 mm.
2. Remove the contents of the compartments.
3. Disconnect the freezer.
4. Leave the door open.
5. Remove the water while it is accumulating in the


compartments. Use a sponge or a piece of absorbent
cloth.


6. Place a towel to avoid the melting ice from wetting the
front and interior part of the refrigerator.


Warning: Never use sharp elements to remove ice or frost
from the evaporator. Such an action can perforate the wall
of the evaporator and allow the refrigerant gas to escape
causing a serious defect which can only be repaired by a
specialist.




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137


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The unit is not functioning. Blown fuse. Check fuse.


The equipment is disconnected. Verify the unit’s connection.


There is no or low electricity in the feed circuit. Test the electrical connection.


Verify the main switch (breaker).


The freezer is functioning continuously but is not
cooling.


The thermostat is adjusted too high. Confi rm the adjustment of the thermostat.


Adjust the thermostat to a lower temperature.


The unit contains excessive frost. Defrost the unit.


The unit is showing fl uctuations in temperature. The temperature control is not calibrated. Calibrate the operational temperature according to
the procedure defi ned by the manufacturer.


The condenser is dirty. Clean the condenser according to the procedure cited
in the maintenance routines.


The unit shows a high temperature. The door is open. Verify that the door is well adjusted and closed.


Poor door seal. Level cabinet and adjust door seal or replace gasket.


There is a defect in the electrical feed. Confi rm that the electrical connection functions
correctly.


A warm load (liquids or solids) was placed inside
the unit.


Wait for the unit to cool the load.


The compressor is not functioning. Verify the functioning of the compressor.


Test to see if one of the alarms is on.


The compressor is functioning but there is no ice in
the evaporator.


Verify if the evaporator’s ventilators are functioning.


The compressor is functioning, but there is no ice in
the evaporator and the evaporator’s ventilators are
functioning well.


A complete verifi cation of the refrigeration system is
required. Call in the specialized service technician.


Low refrigerant gas level. Call in the specialized service technician.


Upon operating the unit, noises similar to clicking
sounds can be heard.


The compressor’s thermal protector has been
disconnected.


Verify that the feed voltage is correct.


Noisy operation. Floor not stable or cabinet not levelled. Move to an adequate fl oor area or adjust casters as
appropriate.


Drip tray vibrating. Adjust tray or cushion it.


The cooling fan hitting cover or compressor is loose. Call in the specialized service technician.


The compressor runs continuously. Not enough air circulation around the unit. Move the unit to provide with suffi cient clearance.
Relocate if necessary.


Faulty thermostat. Call in the specialized service technician.


Poor door seal. Check seals and adjust.


Room too warm. Ventilate the room appropriately.


The door is being opened too often or is not closed. Restrict door opening or close door.


The light switch is defective. Check if light goes out after the door is shut.




C H A P T E R 1 8 R E F R I G E R AT O R S A N D F R E E Z E R S


138


Figure 59. Ultralow freezer temperature control


OPERATION OF ULTRALOW FREEZERS


Ultralow temperature freezers
Operation of ultralow temperature freezers implies following
a procedure recommended by their manufacturers to
achieve the conditions stipulated for the equipment. The
recommendations common to any ultralow freezer are
highlighted next:
1. Connect the unit to an electrical outlet with a ground


pole exclusively dedicated to the unit. This outlet must
be in good working condition and appropriate for
the electrical power required for the unit. It must also
be in compliance with the national and international
electrical standards. The voltage must not vary by more
than +10 % or –5 % from the voltage specifi cation on
the equipment. There are units which require power
of approximately 12 kW. It is then essential to have an
electrical connection which is of a suitable size for such
loads.


2. Select a location which has a fi rm and levelled fl oor (in all
directions). It should be well ventilated and away from
direct sunlight or heat sources. Some manufacturers
stipulate that suitable ambient temperature is between
10 °C and 32 °C. The free space at the sides and back
must be at least 15 cm. The door must open freely at an
angle of 90°. Normally, manufacturers include additional
devices at its base on the support wheels for levelling
the unit.


TURNING THE UNIT ON
In order to understand the freezer’s operational procedures,
a diagram of a control panel similar to those used
in such units is presented. The diagram in Figure 59 is
generic: diff erences in the controls used by the various
manufacturers will certainly be encountered. Included next
are recommendations common to all refrigerators.


Procedures
1. Connect the electrical feed cable to the electrical supply


outlet.
2. Turn the key to the on position. The screen must be


illuminated indicating the temperature of the cabinet.
A light transmitting diode display will indicate that the
unit is energized. This action will start the compressor,
ventilators of the evaporator and the condenser.


3. Select the unit’s operational temperature. In general,
various buttons are activated simultaneously; the
button corresponding to the temperature control and
those to adjust the temperature. Once the desired
temperature is selected, the controls are released. The
screen will show the operational temperature selected.
Wait a suitable time for the unit to reach the selected
temperature.


4. Select the limit temperatures which will activate the
alarms. These temperatures do not generally diff er by
more than 10 % from the operational temperature. In
general, the alarms are adjusted when the unit has
reached a temperature near its operational point. The
procedure consists of activating the alarms’ control
and selecting higher and lower temperature limits
so that the alarm is activated if these are exceeded.
The manufacturer’s recommended procedure must be
followed. Usually, the control has a button which allows
the alarms to be deactivated and also the option to test
their functioning.


5. Ultralow temperature units have another series of alarms
which warn the operators regarding the occurrence of
events which can aff ect the adequate functioning of the
unit. Among these are the following:
• A fl aw in the electric feed.
• Low voltage.
• Excessive room temperature.
• The lower temperature limit is exceeded.




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ROUTINE MAINTENANCE
The maintenance routines of the ultralow temperature
freezers are focused on the following elements described
below. Consult WHO’s Manual on management, maintenance
and use of cold chain equipment, 2005, for care and preventive
maintenance schedules specifi c to plasma freezers and
walk-in freezers used in the blood cold chain.


Cleaning of the condenser
Frequency: Every six months
1. Remove the protective grid.
2. Remove and clean the fi lter. If too obstructed, substitute


by a new one with the same characteristics as the
original.


3. Verify the functioning of the ventilator.
4. Vacuum the condenser and its diff usive fi ns.
5. Reinstall the protective grid and the fi lter.


Warning: A dirty condenser prevents normal heat
transference causing the unit to warm up or exceed the
selected temperature limits.


Integrity of the door gasket
Frequency: Recommended quarterly
It is recommended that periodically, the integrity of the
door gasket be verifi ed. It must remain in good condition
and not display cracks, punctures or tears.


Defrosting
Frequency: Recommended every six months
Whenever it is necessary to defrost the unit, it must be
conducted in the following manner:
1. Transfer the products kept frozen to another unit with


the same operational characteristics.
2. Turn off the unit and allow its interior to reach room


temperature.
3. Remove the ice and water accumulated inside the


unit.
4. If strange odours emanate, wash the inside of the unit


with sodium bicarbonate and warm water.
5. Clean the exterior with a mild detergent, dry and then


apply a protective wax if appropriate.


Warning: Never use sharp elements for removing ice or
frost from the evaporator. Such an action can perforate
the wall of the evaporator allowing the refrigerant gas to
escape, dangerous for the operator and causing a serious
damage which can only be repaired by a specialized repair
shop.


Maintenance of the alarm system battery
Frequency: Approximately every two to three years
The alarm system battery must be changed once worn out.
To substitute it, proceed as described next:
1. Remove the front cover. In general, the battery (batteries)


is (are) located immediately behind the front cover.
2. Disconnect the connection terminals.
3. Remove the worn out battery.
4. Install a battery with the same characteristics as the


original.
5. Connect the terminals.
6. Replace the cover.




C H A P T E R 1 8 R E F R I G E R AT O R S A N D F R E E Z E R S


140


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The low voltage indicator is on. There is inadequate voltage in the electrical feed
outlet.


Verify the feed voltage. Test the connection and its
protective systems.


The dirty fi lter indicator is on. Verify the cleanliness of the fi lter. Clean the condenser’s protection fi lter. If it is
saturated with grime, substitute it for another with
the same characteristics as the original.


The low battery indicator is on. The battery is worn out. Substitute with a battery of same specifi cations as
the original.


The unit is not functioning. The equipment is disconnected. Connect the equipment to the electrical feed outlet.


The fuse is burnt out. Substitute with a fuse of same characteristics as the
original.


The unit functions in a continuous manner. The operating temperature selected is very low. Increase the temperature selected.


The unit functions in a continuous manner without
getting cold.


The condenser is dirty. Clean the condenser.


There is inadequate ventilation. Verify and correct the ventilation.


There is an ice build-up aff ecting the insulation. Defrost the unit. Call in the specialized service
technician if the problem is not resolved.


Rapid frost accumulation on the evaporator. Leaking door gasket. Adjust door hinges. Call in the specialized service
technician if the problem persists.


The door on the freezer compartment is shut frozen. Faulty door seal heater. Call in the specialized service technician.


Noisy operation. Floor not fi rm or cabinet not level. Move to sound fl oor area or adjust casters as
appropriate.


Drip tray vibrating. Adjust tray or cushion it.


The cooling fan hitting cover or compressor is loose. Call in the specialized service technician.


The compressor runs continuously. Not enough air circulation around the unit. Move the unit to provide with suffi cient clearance.
Relocate if necessary.


Faulty thermostat. Call in the specialized service technician.


Poor door seal. Check seals and adjust.


Room too warm. Ventilate the room appropriately.


The door is being opened too often or is not closed. Restrict door opening or close door.


The light switch is defective. Check if light goes out after the door is shut.


Other additional maintenance procedures require specialized tools and instrumentation.




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141


BASIC DEFINITIONS


Adiabatic process. A process in which there is no transference of heat. This implies ∆Q= 0.


BTU. This is a unit for determining the heat transference in the English System. BTU is the acronym for the British Thermal Unit. One BTU is the quantity of heat that
must be transferred for increasing the temperature of one pound of water from 63 °F to 64 °F.


Calorie. This is a quantity of heat which must be transferred to a gram of water to raise the temperature by 1 °C. This defi nition applies when under normal conditions
(atmospheric pressure equal to 760 mm Hg, gravity acceleration equal to 9.81 m/s2); the temperature of a gram of water is increased from 14.5 to 15.5 °C.


Entropy. Measure of a system’s energy that is unavailable for work, or of the degree of a system’s disorder. The reversible diff erential changes of entropy are expressed
by means of the following equation:


Where:
dQ: heat absorbed from a reserve at temperature T during an infi nitesimal reversible change of the state.
T: temperature of the reserve.
The following equation must be carried out for any reversible cycle change.


If the cycle is irreversible, it must be:


Heat. This is a form of transferred energy over the limit of a system at a given temperature, to another one at a lower temperature by virtue of the temperature
diff erence between the two systems. When a system of great mass [M] is placed in contact with another of small mass [m’] at a diff erent temperature, the resulting
fi nal temperature is close to the initial temperature of the greater mass system. It is therefore said that a quantity of heat ∆Q has been transferred from the system
of higher temperature to the system of lower temperature. The heat quantity ∆Q is proportional to the change in temperature ∆T. The proportion constant [C], called
the system’s caloric capacity, allows the following relationship ∆Q=C∆T to be established, from which it is inferred that one of the consequences of the change in
temperature in a system is the transference of heat.


Latent heat. The quantity of thermal energy required for a change in phase to occur in a substance, for example: from liquid phase to vapour phase.


Refrigerant gas. A substance (i.e. coolant) used as a medium in the processes of heat absorption.


Specifi c heat. The quantity of heat required to increase the unit of mass by one degree.


Sensitive heat. The quantity of energy required for increasing the temperature of the refrigerant gas upon absorbing heat. For example: the quantity of heat
required for raising the temperature from 15 to 20 °C or from 30 to 40 °C.


Thermal system. A device which operates in a thermodynamic cycle and carries out a certain positive quantity of work as a result of the transference of heat
between a body at high temperature to a body at low temperature.


dS =
dQ
T


∆S = dS = dQ
T


= 0∫


∆S = dQ
T


∫ < 0




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143


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Chemistry analysers measure the concentration of analytes
in blood or other bodily fl uids based on specifi c chemical
reactions by photometry. Applications vary from clinical
diagnostic, drug abuse monitoring to forensic testing, etc.
Chemistry analysers comprise among others, dry chemistry
analysers using sample-impregnated dipsticks onto which
chemical reactions are detected, and wet chemistry analysers
testing analytes in solution. Various models of chemistry
analysers are available, some designed to measure a single
analyte, e.g. glucometers, haemoglobinometers; others
to measure up to more than ten. Chemistry analysers are
available as bench top instruments with various degrees
of automation or in portable formats. Some are adapted to
tropical conditions with electronic components protected
from high humidity. Chemistry analysers group a large
family of instruments including various photometers and
colorimeters (see Chapter 20). Other common terms used to
defi ne these are: general chemistry analyser, clinical analyser
or cholesterol meter, glucometer, haemoglobinometer (see
Chapter 20) etc. for single-analyte instruments.


Chapter 19


Clinical Chemistry Analysers
GMDN Code 35513 — 34549*


ECRI Code 15-551 18-505 15-551


Denomination Clinical chemistry
analysers


Analysers, point-of-
care (portable)


Dry chemistry
analyser


* Subcategory under GMDN code 35513




C H A P T E R 1 9 C L I N I C A L C H E M I S T R Y A N A LY S E R S


144


Reaction Zone


Test Strip


DetectorLED


Figure 60. Basic diagram of refl ectance photometry on a test strip. Arrows illustrate the
light path. The dashes represent the change in intensity due to the eff ect of the colour on
the reaction zone of the test strip.


PURPOSE OF CHEMISTRY ANALYSERS
In the clinical laboratory, the chemistry analyser is used to
measure one analyte or various analytes such as glucose,
urea, creatine, haemoglobin, cholesterol, etc., in blood,
urine, serum or plasma. It is also used to perform liver
function tests.


OPERATION PRINCIPLE


Dry chemistry analyser
A dry chemistry analyser is a reflectance photometer.
Figure 27 of Chapter 11 shows the interaction of light
with matter and light refl ection also called refl ectance.
Reflectance photometry quantifies the intensity of a
chemical or biochemical reaction generating colour on a
surface (e.g., slide, test strip, dipstick or test patch). Light is
emitted at a specifi c wavelength onto the test strip by the
instrument’s light source (e.g. light emitting diodes or LEDs).
The coloured product absorbs that wavelength of light.
The more analyte in the sample, the more product (colour)
and the less the light is refl ected. The instrument’s detector
measures the refl ectance of this colorimetric enzymatic or
chemical reaction on the test dipstick or strip and converts
it into an electronic signal. This signal is translated into the
corresponding concentration of analyte in the bodily fl uid
tested and the concentration is then printed and/or shown
on a LED digital display.


Wet chemistry analyser
The wet chemistry analyser is a photometer. As opposed
to a spectrophotometer, it does not have a prism or
transmission grating. One of several or a single colour fi lter
is used to measure the absorption of light in liquid samples


according to the Beer- Lambert law (see Chapter 11). The
wet chemistry analyser generally uses a light source such
as a halogen lamp with fi lters. More recent models use a
single LED or several LEDs at specifi c wavelengths. Tests
performed on wet chemistry analysers are based on the
production of a coloured compound of the analyte with
specifi c reacting reagents. The colour is directly proportional
to the concentration of analyte(s) in solution. Typically,
measurements are performed between 304 and 670 nm or
with additional fi lters. Some instruments have the capacity
to perform kinetic measurement through time.


COMPONENTS


Dry chemistry analyser
There are various designs of dry chemistry analysers. One
feature of these instruments is the compartment or window
where the test strip is placed. Designs vary according to
manufacturers. The compartment is either closed with a fl ap
cover or the strip is inserted into the instrument manually or
through an advance mechanism. The light source is usually
one Light Emitting Diode (LED) or several, with specifi c
wavelength(s). The approach for refl ectance measurement
varies in diff erent designs of dry chemistry analysers. It can
be performed directly as shown in Figure 60, or in a chamber
of square or spherical shape. The following Figures show an
Ulbricht’s sphere (also called integrating sphere) and how
it measures refl ectance.


In Ulbricht’s spheres, one or more LEDs of key wavelength(s),
e.g. 567, 642 and/or 951 nm act(s) as the light source(s) to
accommodate various tests. The receptors are two symmetrical
photodiodes, the reference (DR) and a measuring one, (D).




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


145


Figure 62. Basic components of a photometer. (Note that in some instruments, the fi lter is placed between the cuvette and
the detector.)


The light emitted by the LED is uniformly refl ected from
the white inner wall of the sphere. Photodiode DR measures
the intensity of the diff used light (I0 ) and photodiode D
measures the light intensity diff usely refl ected from the test
portion of the strip (I). The I0 /I ratio is proportional to the
refl ectance value R. The refl ectance measured is converted
into a concentration or activity value based on test-specifi c
standard curves.


Wet chemistry analyser
Wet chemistry analysers also widely vary in design. The
common basic features are the photometric components
described in the Figure below. Additional accessories
vary widely depending on the degree of automation and
sophistication of the instrument. Wet chemistry analysers
are often equipped with peripheral or integrated computer
and printer. Advanced instruments provide concentration of
the targeted analytes in the relevant units of measure.


INSTALLATION REQUIREMENTS
1. Unpack the chemistry analyser carefully.
2. Ensure that the instrument is placed away from direct


sunlight, stray light or heat sources.
3. Place the instrument on a fi rm bench near a power


outlet (if not battery operated).
a. The outlet must have its respective ground pole in


order to guarantee the protection and safety of the
operator and the equipment. Chemistry analysers
generally operate at 110-120 V/60 Hz or 220-230
V/50Hz.


b. If not battery operated, protect the chemistry
analyser from power surges using a voltage
stabilizer.


4. Follow the manufacturer specifications for the
installation of specifi c models.


5. Keep specialized packaging for future use or return for
repair.


6. For added safety, some instrument models may be
locked in a cupboard when not in use.


OPERATION OF THE DRY CHEMISTRY ANALYSER
Only staff trained and authorized to use the dry chemistry
analyser are allowed to operate the instrument. The
procedure below is based on the use of a particular
instrument. Refer to the instruction manual for other dry
chemistry analyser models.
1. Connect the instrument to its power supply and switch


on.
2. Warm up time should be displayed in seconds. For other


instruments, wait 15 minutes before use, or as indicated
by the manufacturer.


Figure 61. Ulbricht’s sphere.


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C H A P T E R 1 9 C L I N I C A L C H E M I S T R Y A N A LY S E R S


146


3. When READ appears on the screen or the appropriate
time has elapsed, proceed with the testing intended.


4. Take a reagent strip out of the vial.
5. Using a pipette, draw the appropriate amount of sample


(e.g. 32 μl) avoiding air bubbles in the tip.
6. Remove the aluminium foil from the application zone


of the strip without bending it.
7. Apply the sample to the centre of the red application


zone avoiding touching the strip with the pipette tip.
8. Open the fl ap, place the strip on the guide and insert


horizontally into the instrument until a click is heard.
9. Close the fl ap. The display confi rms that the correct


test-specifi c magnetic code is read by the instrument,
e.g. GLU for glucose.


10. The time before the results are to appear, is displayed
in seconds.


11. The concentration of the analyte is usually displayed in
mg/dl.


12. After use, open the fl ap and remove the strip.
13. Turn off by switching off at the wall socket if applicable


and removing the plug or disconnecting the battery
terminals.


OPERATION OF THE WET CHEMISTRY ANALYSER
Only staff trained and authorized to use the wet chemistry
analyser are allowed to operate the instrument. The
procedure below is based on the use of a portable semi-
automated wet chemistry analyser with inbuilt fi lters and
digital display. Refer to the instruction manual from the
manufacturers when using other models.
1. Connect the instrument to the power supply and switch


on.
2. Warm up time should be displayed in seconds.
3. Prepare all the solutions in test tubes in a rack, i.e. blank,


standards, test solutions.
4. Once the instrument is ready, blank the instrument.
5. Read each one of the test tubes.
6. Record the results.
7. Turn off by switching off at the wall socket if applicable


and removing the plug or disconnecting the battery
terminals.


ROUTINE MAINTENANCE OF CHEMISTRY
ANALYSERS
Some chemistry analysers require minimal maintenance
and automatically perform self-calibration routines. The
guidelines below are general procedures applicable to most
instruments. Always carefully follow the manufacturer’s
instructions for calibration, regular servicing and
maintenance of your analyser.


Frequency: Daily
1. Any spill on, or around the instrument should be cleaned


immediately.
2. At the end of the day, disconnect the power source


by switching off at the wall socket if applicable and
removing the plug or disconnecting the battery
terminals.


3. For dry chemistry analysers: Do not leave test strips
in the instrument. Regularly clean the window or
compartment where test strips are inserted and keep
it closed. Use a soft, clean damp swab.


4. For wet chemistry analysers: Keep the sample chamber
empty and closed when not in use.


5. Cover the instrument after use.
6. Store appropriately away from dust.


Frequency: As needed
1. Replace blown fuses and bulbs according to the


manufacturer’s instructions.
2. If the equipment is faulty, consult a qualifi ed biomedical


engineer.


Frequency: Monthly
The window and/or front surface of the photodetector
should be inspected and cleaned with lens tissue.


Frequency: Every six months
1. Inspect the instrument visually to verify the integrity


of its components according to the manufacturer’s
specifi cations.


2. Verify that the buttons or control switches and
mechanical closures are mounted fi rmly and that their
labels are clear.


3. Ensure that all the accessories are clean and intact.
4. Check the adjustment and condition of nuts, bolts and


screws.
5. Make sure the electrical connections do not have cracks


or ruptures. Test that they are joined correctly.
6. If applicable:


a. Verify that cables securing devices and terminals
are free from dust, grime or corrosion.


b. Verify that cables are not showing signs of splicing
or of being worn out.


c. Examine that the grounding system (internal
and external) is meeting the electric code
requirements.


7. Make sure the circuit switches, fuse box and indicators
are free from dust, corrosion and grime.


8. Check lamp alignment if recommended by the
manufacturer.




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147


Frequency: Annually
These tests must be performed by an electrician (for
instruments using main power), engineer or other trained
personnel. Results must be recorded and retained for follow-
up through time.
1. Check the installation location for safety of the electrical


(for instruments using main power only) and the physical
infrastructures.


2. For instruments using main power:
a. Check that the voltage is appropriate and does


not vary more than 5% from the voltage in the
equipment specifi cations.


b. Check that the polarity of the outlet is correct.
3. Check that there is sufficient space around the


instrument for the connecting cables and for adequate
ventilation.


4. Test the integrity of the counter and its cleanliness.
5. Verify that the instrument is away from equipment


generating vibrations and direct solar radiation.
6. Check that there is no excessive humidity, high


temperature or dust.
7. Ensure that there is no source of smoke, gas or corrosive


emissions nearby.


NON-ROUTINE MAINTENANCE AND
TROUBLESHOOTING
These instructions are general guidelines for troubleshooting
chemistry analysers. Since there are numerous models
available, always refer to the instruction manual from the
manufacturer and follow the steps recommended.
1. If there is no light passing through the system, or if its


intensity is not constant, change the bulb.
2. If there is light in the system but no display response,


change the photocell.
3. Always replace blown fuses and bulbs according to the


manufacturer’s instructions.
4. If the equipment is faulty, consult a qualifi ed biomedical


engineer.
5. If the chemistry analyser fails to switch on, check the


electric socket outlet. Plug and check the fuse or the
battery terminals.


6. In case of a major breakdown, consult a qualified
biomedical engineer.




C H A P T E R 1 9 C L I N I C A L C H E M I S T R Y A N A LY S E R S


148


TROUBLESHOOTING TABLE


PROBLEM PROBABLE CAUSE SOLUTION


The analyser does not start. The on and off switch is in the off position. Move the switch to the on position.


There is no electric energy in the feed outlet. Verify the general electric feed. Test that some safety
mechanism has not misfi red.


The electric feed cable is not well connected. Connect the feed cable fi rmly.


The batteries are worn out or not well connected. Check the batteries connection and status. Replace
or recharge if necessary.


The command buttons do not respond. The initialization of the equipment during start-up
is incomplete.


Turn off the equipment and switch on again.


An incorrect command was activated, during start-
up.


The serial port does not respond. There was incomplete initialization of the equipment
during start-up.


Turn off the equipment and switch on again.


The interconnection cable is not properly connected. Verify the connection.


The LCD screen is diffi cult to read. The contrast control is maladjusted. Adjust the contrasts.


Base lighting system burnt out. Call the company representative.


The printer is blocked. Paper jam. Remove the excess paper with fi nely pointed
tweezers.


Remove the paper and reinstall again.


The printer’s paper does not auto feed or advance. The printer paper is installed erroneously. Reinsert the roll of paper correctly.


The front edge of the paper is not aligned or is
folded.


Reinsert the roll of paper. Cut the front edge and
realign in the feed system.


The paper feed control does not respond. Call the company representative.


The cuvette does not fi t in the sample holder
compartment of the wet chemistry analyser.


The cuvette is of wrong size. Use the size of cuvette specifi ed by the manufacturer.


The cuvette’s adjustment mechanism is incorrectly
placed.


Correct the position of the adjustment mechanism.


The test strip is not read by the dry chemistry
analyser.


The strip was not placed correctly in the analyser. Make sure the usual click is heard when the strip is
placed if applicable.


Check that the strip was placed in the analyser in
the correct orientation and with the black underside
facing down.


The dry chemistry analyser does not perform as
expected.


The incorrect test strip was used. Check that the strip corresponds to the test required.
Repeat assay with the correct strip if needed.


The instrument is defective. Perform the instrument checks as recommended
by the manufacturer. Some instruments provide on
screen user guidance to follow and quality control
strips to check the optical system.


BASIC DEFINITIONS


Analyte. Component of a bodily fl uid (e.g. blood, urine, etc.) which itself cannot be measured, but with certain properties which can be measured using a medical
device designed for that purpose. For example lactate cannot be measured but lactate concentration can. Common analytes evaluated in clinical chemistry include
cholesterol, urea, creatin, glucose, etc., which are measured to assess the health status of patients.


Refl ectance (R). Ratio between the intensity of light refl ected (I0) on a surface with that of the incident light (I), I0/I.


Test strip. Flat testing device containing test reagents and materials used for diagnostic purposes. Test strips of various degrees of complexity have been developed.
These can simply consist of fi lter paper with bound reactive or of an elaborated system of reagent paper, transport fi bres, reagent/indicator layers and magnetic
strips with data encoded. The test or reaction zone is the area where the reaction takes place and where it is read by a dry chemistry analyser or directly by an
operator.


Note: Other relevant defi nitions may be found in Chapter 11.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


149


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Chapter 20


Colorimeters
GMDN Code 36910 38837 15146


ECRI Code 18-257 18-258 15-146


Denomination Photometer, fi lter,
automated


Photometer, fi lter,
manual


Haemoglobin analysers
(Haemoglobinometer)


PURPOSE OF THE COLORIMETER
A colorimeter is an electrically powered instrument which
measures the concentration of analytes in coloured
solutions. It is a simple version of a photometer. The
diff erence in the quality of its fi lters makes it less sensitive.
The colorimeter is used for clinical chemistry, namely for
determining haemoglobin concentrations. Colorimeters
are made by several manufacturers and include types
with inbuilt individual removable fi lters or fi lter wheels
for up to ten wavelengths. Some models are adapted
for hot and humid climates with gelatine fi lters encased
in glass to prevent fungal growth and coated individual
components to prevent corrosion. Colorimeters may be


manual or semi-automated. Absorbance readings are done
with needle or digital readouts. The haemoglobinometer is
a portable colorimeter designed to provide direct, accurate
haemoglobin concentration readings in g/dl or g/l. It will
also be covered in this chapter.


OPERATING PRINCIPLE
A colorimeter uses filters to produce light of a single
wavelength selected according to the colour of the solution
being measured. The coloured light passes through the
sample and the amount of light emerging is measured on a
scale of absorbance. The absorbance is directly proportional
to the concentration of the coloured compound in the
solution according to Beer-Lambert law (see Chapter 11). It
can usually measure reliably between 0 and 0.7 absorbance
units. Calibration factors are higher for colorimeters than for
photometers as they are less sensitive. Calibration factors
for specifi c methods or reagents are usually provided by
manufacturers or in the literature.


Haemoglobinometers measure the concentration of
haemoglobin in blood. The majority of models is manually
operated and uses main or battery cell power. New models
have rechargeable batteries and/or use solar energy as
a source of power. Most require dilution of blood before
haemoglobin measurement. Some models use a device
for collecting blood without dilution; these devices are
single use and disposable, thus increasing the cost of
haemoglobin estimation.


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PHOTOGRAPH OF COLORIMETER


Portable haemoglobinometer




C H A P T E R 2 0 C O LO R I M E T E R S


150


Figure 63. Controls on a portable colorimeter


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COMPONENTS
The basic components of colorimeters are similar to those
of a photometer as shown in Figure 62, Chapter 19. As
mentioned earlier in this chapter, these instruments
are simpler and due to the quality of their filters, less
sensitive. The light source may be a diode lamp emitting
monochromatic light. Alternatively light produced by a
tungsten or halogen lamp may be fi ltered to achieve the
required wavelength. Depending on the model, the controls
of the instrument may feature the following:
1. Display window
2. ON/OFF button
3. Cuvette chamber
4. Test button
5. Reference button
6. Various modes selection button, e.g., Absorbance/


%Transmittance, Kinetics (not on all models)


INSTALLATION REQUIREMENTS
1. A clean, dust, fume and smoke free environment, away


from direct sunlight is required.
2. Unpack carefully and assemble following instructions


from the manufacturer if applicable.
3. Place the instrument on a fi rm bench and, if required,


near (no more than 1.5 m away) an electric power outlet
with a ground pole.
a. The outlet must have its respective ground pole


in order to guarantee the protection and safety


of the operator and the equipment. Colorimeters
generally operate at 110-120 V/60 Hz or 220-230
V/50Hz.


b. If not battery operated, protect the instrument
from power surges using a voltage stabilizer.


4. Follow the manufacturer specifications for the
installation of specifi c models.


5. For added safety, the instrument may be locked in a
cupboard when not in use. This may not be possible
for large models, although these could be locked in
another fashion if judged necessary.


OPERATION OF THE COLORIMETER
Only staff trained and authorized to use the colorimeter are
allowed to operate the instrument. This section is based on
the use of the portable colorimeter model, equipped with
inbuilt fi lters and a digital display. Other models may require
diff erent procedures and manufacturer’s instructions should
always be followed.
1. Connect the unit to the power supply and switch ON.
2. Allow 15 minutes for the instrument’s optical and


electronic systems to warm up.
3. Select the correct wavelength for the compound to be


tested e.g. 540 nm for haemoglobincyanide.
4. Select “absorbance” using the Mode button.
5. Arrange all the required solutions in a test rack: blank


(reagent containing no sample); standard of known
concentration and test solutions (samples).


Display window


Absorbance / %Transmittance
mode button


ON/OFF Button


Cuvette chamber with cuvette


Test Button


Reference Button


Kinetic mode button




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


151


6. Carefully clean the cuvette using lint-free soft tissue
or lens paper to avoid scratches. Always hold by the
opaque ground side.


7. Transfer the blank solution into the cuvette and place it
into the sample compartment with the clear sides facing
the light path.


8. Close the chamber and set the display to zero using the
SET BLANK control.


9. Remove the cuvette from the compartment and pour
the solution back into its original test tube.


10. Pour the standard solution into the cuvette and read
the absorbance.


11. Repeat step 9.
12. Read the test solutions in the same fashion.
13. Using a table of values obtained from a calibration curve


derived from the instrument, read the concentration of
the test samples against the absorbance.


14. After use, switch off the power supply and cover the
equipment to protect it from dust.


15. Rinse the cuvette with distilled water, drain dry and
wrap in soft material. Store carefully into a small box to
prevent scratches and dust.


OPERATION OF THE HAEMOGLOBINOMETER
Only staff trained and authorized to use the
haemoglobinometer are allowed to operate the
instrument. This section describes the operation of a portable
haemoglobinometer with LED light source and digital
display. Diff erent models require diff erent procedures and
manufacturer’s instructions should always be followed.
1. Connect the instrument to the power supply and switch


ON or use the internal power source.
2. Place the ON/OFF switch on the ON position.
3. Choose readout to be used routinely, e.g. g/Dl.
4. Warm-up time should be displayed in seconds if


applicable. For other models wait 15 minutes or the
time recommended by the manufacturer.


5. Prepare all the solutions in test tubes in a rack, i.e. blank,
standards, test solutions.


6. Leave at room temperature for 10 minutes to
equilibrate.


7. Meanwhile, carefully clean the cuvette using a soft
tissue to avoid scratching.


8. Avoid touching the sides of the cuvette facing the light
path; hold the cuvette by the opaque sides that will not
face the light path.


9. Transfer the blank solution into the cuvette and place it
in the sample compartment with the clear sides facing
the light path.


10. Blank the instrument: close the cover and wait
approximately 3 sec and adjust the display knob at
0:00.


11. Remove the blank from the compartment and pour it
back into the original test tube.


12. Pour the standard solution into the cuvette and place
it in the compartment.


13. Close the cover and wait 3 sec. Register the reading from
the digital display.


14. Remove the standard from the compartment and pour
it back into the original test tube.


15. Pour the diluted sample solution into the cuvette and
place it in the compartment.


16. Close the cover and wait 3 seconds and register the
reading from the digital display.


17. Remove the sample from the compartment and pour it
back into the original test tube.


18. Repeat steps 16-17 for each sample to be tested.
19. Rinse the cuvette with distilled water. Drain dry, wrap


in soft material and store in a small box to prevent
scratches.


20. Turn off by switching off or disconnecting at the
wall socket if applicable. If not, remove the plug or
disconnect the battery terminals.


21. Store in a locked drawer or in another suitable
location.


ROUTINE MAINTENANCE
Maintenance should be performed by qualifi ed personnel.
This section describes general routine maintenance for
colorimeters and haemoglobinometers. Some models
may require diff erent procedures. Always carefully follow
the manufacturer’s instructions for regular servicing and
maintenance of the colorimeter or haemoglobinometer.


Frequency: Daily
1. Any spill on, or around the instrument should be cleaned


immediately.
2. At the end of the day, turn off the instrument or


disconnect the power source or the battery terminals
as appropriate.


3. Keep the cuvette chamber empty and closed when not
in use.


4. Cover the instrument after use. Store appropriately,
protected from dust.


Frequency: As needed
1. Replace blown fuses and bulbs according to the


manufacturer’s instructions.
2. If the equipment is faulty, consult a qualifi ed biomedical


engineer.


Frequency: Monthly
The window and/or front surface of the photodetector
should be inspected and cleaned with lens tissue.




C H A P T E R 2 0 C O LO R I M E T E R S


152


Frequency: Every six months
1. Inspect the instrument visually to verify the integrity


of its components according to the manufacturer’s
specifi cations.


2. Verify that the buttons or control switches and
mechanical closures are mounted fi rmly and that their
labels are clear.


3. Ensure that all the accessories are clean and intact.
4. Check the adjustment and condition of nuts, bolts and


screws.
5. Make sure the electrical connections do not have cracks


or ruptures. Test that these are joined correctly.
6. If applicable:


a. Verify that cables securing devices and terminals
are free from dust, grime or corrosion.


b. Verify that cables are not showing signs of splicing
or of being worn out.


c. Examine that the grounding system (internal
and external) is meeting the electric code
requirements.


7. Make sure the circuit switches or interrupters, fuse box
and indicators are free from dust, corrosion and grime.


8. Check lamp alignment if recommended by the
manufacturer.


Frequency: Annually
These tests must be performed by an electrician or engineer
and results must be recorded and archived for follow-up
through time.
1. Check the installation location for safety of the electrical


and the physical infrastructures.
2. For instruments using main power:


a. Check that the voltage is appropriate and does
not vary more than 5% from the voltage in the
equipment specifi cations.


b. The polarity of the outlet is correct.
3. Check that there is sufficient space around the


instrument for the connecting cables and for adequate
ventilation.


4. Test the integrity of the counter and its cleanliness.
5. Verify that the instrument is away from equipment


generating vibrations and direct solar radiation.
6. Check that there is no excessive humidity, dust or high


temperature.
7. Ensure that there is no source of smoke, gas or corrosive


emissions nearby.


General maintenance
Refer to the general maintenance of spectrophotometer
in Chapter 11 for the cleaning of spills and replacement of
batteries.


Cuvette use and maintenance
Cuvettes must be rigorously clean for accurate
measurements. Clean these as described in Chapter 11.
Additional recommendations are as follow:
1. Always hold cuvettes by their opaque, non-optical


walls.
2. Unless specified by the operator’s manual, do not


perform any measurements without performing a blank
determination.


3. Use a single cuvette or a set of matched cuvettes
for proper performance of the instrument. Note:
Absorbance of cuvettes should not exceed 0.01 when
measuring distilled water. To avoid incorrect results, a
cuvette exceeding this limit should not be used as part
of a set unless it is matched with one with the same
absorbance reading when measuring distilled water.


4. Remove bubbles present in the solution by gently
tapping the cuvette with the fi nger.


5. Ensure that there is a high enough level of solution in
the cuvette (above the light beam) so that the refl ection
of light from the surface does not interfere with the
reading.


6. All solutions used and the specimen to be measured
should be clear. If the mixed reagent solution and
specimen is turbid, the measurement must be repeated
after checking and confi rming the cuvette’s transparency
and cleanliness.


7. If a kinetic measurement is performed over a long
period of time, seal the cuvette to avoid evaporation
causing erroneously high readings.


8. When performing readings on a series of specimens,
readjust the zero every 5 to 10 measurements by
reading the blank solution to avoid a drift of the zero.


9. Do not leave the cuvette in the instrument.
10. If using semi-micro or micro-cuvettes, ensure correct


positioning in the light path to avoid false readings due
to partially refl ected light.


11. Store in a dust-free box to prevent damage as
scratched or damaged cuvettes can lead to incorrect
measurements.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


153


Optical fi lters use and maintenance
1. Handle removable fi lters by the circumference to avoid


contamination.
2. Keep spare fi lters in a dust-free box to insure protection


from breakage or scratches.
3. Ensure that a fi lter is in its slot when the lamp is turned


ON to avoid damage to the photocell. Store fi lters in the
appropriate storage box when the instrument is not in
use.


4. When the instrument is cool and turned OFF, clean the
fi lters and optical window with lens tissue as instructed
by the manufacturer.


Light source use and maintenance
1. Turn OFF the lamp after each use to maximize its life


span. Some manufacturers recommend keeping a
record log of the instrument lamp use.


2. Check lamp periodically. Replace if it is the cause of
instability in the absorption signal.


Lamp alignment
The following are procedures to align new lamps. Refer
to the instructions from the manufacturer to insure the
procedure is performed according to specifi cations of the
instrument model in use.


Realign the new lamp as follows:
1. Place a clean cuvette fi lled with distilled water in position


in the instrument.
2. Set the meter to a mid-scale reading, e.g. at 50%


transmission.
3. Move each optical component slightly in turn and check


if the reading was aff ected.
4. If needed, adjust the lamp alignment for maximum


transmission.
5. Alternatively, place a white card in front of the photocell


(some instruments will allow this). Observe the image of
the lamp on the card. It should be vertical and in focus.
If not, adjust the lamp alignment until the best image
is obtained.




C H A P T E R 2 0 C O LO R I M E T E R S


154


Troubleshooting tables containing problems sometimes encountered with colorimeters are presented below. Since instrument models vary
widely the following guidelines take precedence:
1. Always refer to the instruction manual from the manufacturer.
2. If an instrument fails to switch on, if applicable, check the electric socket outlet. Plug and check the fuse or the battery terminals.
3. In case of a major breakdown, consult a qualifi ed biomedical engineer.


TROUBLESHOOTING TABLE


Automated Colorimeter


PROBLEM PROBABLE CAUSE SOLUTION


The colorimeter does not start. The on/off switch is in the off position. Move the switch to the on position.


There is no electric energy in the feed outlet. Verify the main electric feed. Verify that some
electrical safety mechanism has not been misfi red.


The electric feed cable is not well connected. Connect the feed cable fi rmly.


The keyboard or buttons do not respond. The initialization of the equipment during start-up
is incomplete.


Turn off the equipment and switch on again.


An incorrect command was activated, during start-
up.


The serial port does not respond. There was incomplete initialization of the equipment
during start-up.


Turn off the equipment and switch on again.


The interconnection cable is not connected well. Verify the connection.


The LCD screen is diffi cult to read. The contrast control is maladjusted. Adjust the contrasts.


Base lighting system burnt out. Call the representative.


The printer is blocked. Paper jam. Remove the excess paper with fi nely pointed
tweezers.


Remove the paper and reinstall again.


The printer’s paper does not auto feed or advance. The printer paper is installed incorrectly. Reinsert the roll of paper.


The front edge of the paper is not aligned or folded. Reinsert the roll of paper. Cut the front edge and
realign in the feed system.


The paper feed control does not respond. Call the representative.


The cuvette does not enter in the sample holder
compartment.


The cuvette is of wrong size. Use the size of cuvette specifi ed by the manufacturer.


The cuvette’s adjustment mechanism is incorrectly
placed.


Correct the position of the adjustment mechanism.


The reading shows fl uctuations. There are interferences in the light’s path. Verify that the cuvette is not scratched.


Verify that there are no particles fl oating in the
cuvette.


Rub the optic walls of the cuvette with a piece of
clean cloth.


Verify that the working range (wavelength and
dilution) selected is appropriate for the sample
analyzed.


The reading shows negative values. There is no
absorbance reading.


There is no sample. Add a sample to the solution.


The cuvette is incorrectly positioned. Verify the orientation of the cuvette. Clear sides
should face the light path.


The wavelength is erroneously selected. Adjust the wavelength to the range compatible with
the analysis.


The equipment was calibrated with a sample in
place of a standard solution.


Calibrate with a standard solution or with distilled
water.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


155


Non-automated Colorimeter


PROBLEM PROBABLE CAUSE SOLUTION


The source lamp does not light up. The fi lament is broken. Replace the lamp.


The safety fuse is burnt out. Replace the fuse.


There is resistance in the lamp’s fi lament. Replace the lamp.


The voltage is incorrect. Review the voltage. Check the feed source.


Low readings in the meter or in the galvanometer. The source lamp is defective. Replace the lamp.


The photocell is dirty or defective. Clean or replace the photocell.


The multiplier is defective. Change or repair the multiplier.


The source lamp’s voltage is low. Adjust the voltage.


BASIC DEFINITIONS


Since these instruments are based on the photometry principles, relevant defi nitions may be found in Chapter 11.




M A I N T E N A N C E M A N U A L F O R L A B O R AT O R Y E Q U I P M E N T


157


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Copyright 2016, Engineering World Health