Microsoft Word - PPM

Equipment*Packet:*Balances(and(Scales*UDMS*#:(10261(&(13455(Date*of*Creation:(October(29,(2015(Creator:(Complied(by(Cassandra(Stanco(for*Engineering(World(Health((EWH)***Equipment*Packet*Contents:*This(packet(contains(information(about(the(operation,(maintenance,(and(repair(of(laboratory(balances(and(scales.((Part*I:*External*From*the*Packet:** 1. An*Introduction*to*Balances*and*Scales:*(PowerPoint(*Part*II:*Included*in*this*Packet:** 1. Operation*and*Use:*a. Brief(Introduction(to(Scales((p.(3)(b. Operation(and(Use(of(Balances((p.(4O10)(c. Calibration(of(Scales((p.(11O14)(2. Diagrams*and*Schematics:*a. Figure(1:(Schematics(of(Electromagnetic(Balances((p.(16)*b. Figure(2:(Schematics(of(Mechanical(Balances(Part(1((p.(17)*c. Figure(3:(Schematics(of(Mechanical(Balances(Part(2((p.(18)*3. Preventative*Maintenance*and*Safety:*a. Analog(Scale(Preventative(Maintenance((p.(20)*b. Digital(Scale(Preventative(Maintenance((p.(21)*c. General(Scale(Preventative(Maintenance(Table((p.(22)(*4. Troubleshooting*and*Repair:***a. Analog(Scale(Troubleshooting(Flowchart((p.(24O27)*b. Digital(Scale(Troubleshooting(Table(((p.(28O31)*c. General(Scale(Troubleshooting(Table((p.(32)*d. Balance(Troubleshooting(Table((p.(33)*5. Resources*for*More*Information*a. Resources(for(More(Information((p.(35)*b. Bibliography((p.(36)** (**




*1.*Operation*and*Use*of*Balances*and*Scales****Featured*in*this*Section:***DHT(Laboratory.(“Calibration(of(Scales.”(From(the(Publication:(Biomedical(Technician(Assistant((BTA)(Skills.(Duke(University.((2011).( *Strengthening(Specialised(Clinical(Services(in(the(Pacific.(User(Care(of(Medical(Equipment:(A(first(line(maintenance(guide(for(end(users.((2015).( *WHO.(“Chapter(4:(Balances.”(From(the(publication:(Laboratory(Equipment(Maintenance(Manual,((WHO:(2008).( *Wikipedia.(“Body(Weight.”(Wikipedia.(Retrieved(from:(https://en.wikipedia.org/w/index.php?oldid=444807901((** *(* ** *




User Care of Medical Equipment – First line maintenance for end users


54


Chapter 4.16 Scales


Function
Measuring patient weight is an important part of monitoring health as well as calculating drug and


radiation doses. It is therefore vital that scales continue to operate accurately. They can be used for all ages of
patient and therefore vary in the range of weights that are measured. They can be arranged for patients to stand
on, or can be set up for weighing wheelchair bound patients. For infants, the patient can be suspended in a sling
below the scale or placed in a weighing cot on top of the scale.


How it works
Mechanical scales have a spring deflected by patient weight. The spring pushes a pointer along a


display or rotates a disc to indicate weight. Electronic scales have a sensor that bends under patient weight and
the circuitry converts this to displayed digits.








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


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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 1 0 0. If the scale displays less
or more than 10 0, 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
S i m p l e pieces



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


100, 200 and 500 g
Frac tional 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-ampli fi 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.


Classifi cation 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 1 4 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 d d = 0.1 µg
2. Microanalytics d d = 1 µg
3. Semi-microanalytics d d = 0.01 mg
4. Macroanalytics d d = 0.1 mg
5. Precision d d ≥ 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 1 1
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/O ff


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
cavi t y.


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


E2 Stainless steel weights without marks or adjusting
cavi t y.


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


F1 Stainless steel weights with screw button for protec ting
the adjust i n g cavit y.


±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 qualit y 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 cation 1


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




Knowledge domain: Mechanical
Unit: Calibration
Skill: Scale


Tools and Parts Required:
1) Scale
2) Object of known weight


OR
3) Water
4) Syringe or graduated cylinder
5) Container for water (cup or glass)






Introduction
A weighing scale is a device which determines the weight of an object. Medical scales
can be used to measure the body weight of human beings. Scales are either analog or
digital. Analog scales represent weight by a pointer’s position on a dial. Digital scales
electrically display the exact weight. Analytical balances are common digital scales
found in hospitals. An analytical balance is used to accurately and precisely measure
the mass of an object. Analytical balances are usually used in laboratories. A weighing
scale must be occasionally calibrated to ensure accurate readings of the weight.

Example
Below are pictures of different types of scales.





Analog scale







Identification and Diagnosis
Analog and digital scales should read zero when nothing is being weighed. If the scale
displays a weight when there is nothing on the scale, you must tare the scale. Tare the
scale by setting the empty scale to zero.

Calibration should be verified after taring. Some scales can be calibrated by the
biomedical technician. Some scales are impossible to calibrate. Discard uncalibrated
scales that cannot be repaired.

Anytime a scale is repaired it should be calibrated. Anytime a scale is dropped it should
be calibrated. In most cases, when a scale is moved it should be calibrated


Procedure
Analog Scale: Place the scale on a stable, level surface. Place the weight boat or baby
sling on the scale. Allow the reading to stabilize. Find the adjustment dial. It is usually
located on the back or side of the scale. It can be moved either from side to side or up
and down.




Analytical Balance


Look on the back and
side of the scale to find
the adjustment dial




Turn the dial to set the scale to zero.




Verify calibration: Locate an object whose precise weight is known and within the range
of the scale. If you do not have an object of known weight, you must create one. Use a
syringe or graduated cylinder to measure a known volume of water. Water weighs one
gram for each milliliter. So, if you need a weight of 20 grams, measure 20 milliliters of
water.



Remove the object. The dial should now return to zero. The scale should now be
properly calibrated.


If the scale does not read the correct weight, the scale is not correctly calibrated. Many
mechanical scales cannot be calibrated. If you attempt to open the scale for
calibrations, remember to tare and verify correct calibration before returning the scale to
service.









Analytical Balance: Remove everything from the balance. Turn the balance on. Press
the button that reads “ON / TARE.” Some balances may read “ZERO” instead.







The digital balance should now only display zeros. The analytical balance is tared and
ready for use. Some analytical balances have a second knob or button for calibration.
Read the owner’s manual for calibration instructions.

Other Digital Scales: Some digital devices have a “calibration mode.” Calibration mode
will automatically calibrate the device. Use the owner’s manual to automatically
calibrate a digital scale.


Exercise
Your instructor will give you either an analog or digital scale. This may be a piece of
medical equipment from your hospital. Tare the scale using the procedure above. Use
water or an object of known weight to verify calibration.

Your instructor must verify your work before you continue.


Preventative Maintenance and Calibration

Scales should be tared before every use. Verify the calibration of the scale every six
months.




2.*Diagrams*and*Schematics*of*Balances*and*Scales*****Featured*in*this*Section:*****WHO.(Maintenance(and(Repair(of(Laboratory,(Diagnostic(Imaging,(and(Hospital(Equipment((WHO:(1996).*** *










3.*Preventative*Maintenance*and*Safety*of*Balances*and*Scales****Featured*in*this*Section:****((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Analog)(Preventative(Maintenance.”(From(the(publication:((Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Digital)(Preventative(Maintenance.”(From(the(publication:((Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Strengthening(Specialised(Clinical(Services(in(the(Pacific.(User(Care(of(Medical(Equipment:(A(first(line(maintenance(guide(for(end(users.((2015).(***




Scales'(Analog)'Preventative'Maintenance'!Preventative)Maintenance)
• Clean!plate!of!debris!after!each!use.!Use!a!brush,!if!possible,!to!avoid!placing!plate!under!excess!pressure!• Check!the!space!between!the!plate!and!the!base!for!dirt!and!debris.!If!debris!is!preventing!the!plate!from!displacing,!use!a!small,!thin!object!such!as!an!unfolded!paperclip!or!a!pin!to!clear!it!• Check!scale!regularly!for!levelness!• Check!scale!calibration!regularly'!




Scales'(Digital)'Preventative'Maintenance'!Preventative)Maintenance)
• Regularly!check!and!replace!batteries!• Check!power!cord!for!defects!• Check!calibration!regularly!• Check!for!levelness!• Ensure!that!device!is!free!of!debris,!corrosion,!dirt,!etc.!




User Care of Medical Equipment – First line maintenance for end users


56


User Care Checklist – Scales



Daily

Cleaning



9 Wipe off dust and dirt from exterior

9 Clear away any dirt or hair on controls and feet




Visual checks



9 If bent, cracked or damaged, send for repair




Function
checks



9 Check zero at start of day and before each patient



9 Replace dust cover after checks







Weekly

Cleaning



9 Clean exterior with damp cloth and dry off

9 Send for repainting if any exposed or rusted metal




Visual checks



9 Check all screws and parts are fitted tightly




Function
checks



9 Check reading is accurate using a known weight

9 Send for repair if inaccurate or sticking

9 Replace battery or charge if display shows low battery






Every six months
Biomedical Technician check required




4.*Troubleshooting*and*Repair*of*Safety*Balances*and*Scales** **Featured*in*this*Section:******Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Analog)(Troubleshooting(Flowchart.”(From(the(publication:(Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Digital)(Troubleshooting(Flowchart.”(From(the(publication:(Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Strengthening(Specialised(Clinical(Services(in(the(Pacific.(User(Care(of(Medical(Equipment:(A(first(line(maintenance(guide(for(end(users.((2015).***WHO.(“Chapter(4:(Balances.”(From(the(publication:(Laboratory(Equipment(Maintenance(Manual,((WHO:(2008).(** ******




Scales'(Analog)'Repair'and'Troubleshooting'!


!




Description+#! Text'box! Explanation'or'comment!
1! Begin! Begin!diagnostic!process!for!a!work!order!on!scales!(analog).!Analog!scales!contain!no!electrical!components,!so!nothing!needs!to!be!turned!“on.”!Make!sure!that!scale!plate!is!clear!of!any!objects!or!debris!and!any!weighted!elements!are!set!to!zero.!
2! Is!the!scale!body!free!of!dents?! By!looking!and!feeling!the!exterior!of!the!scale,!assess!for!major!external!damage.!
3! Do!dents!affect!scale!function?! Depress!the!scale!platter!gently.!Does!the!external!damage!appear!to!hinder!the!movement!of!the!scale’s!needle?!4! Proceed!to!step!8! Proceed!to!step!8!
5! Can!dents!be!hammered!or!pushed!out?! By!looking!and!feeling!the!exterior!of!the!scale,!gently!test!to!see!if!the!dents!can!be!removed!easily.!
6! Scale!requires!spare!parts!or!must!be!retired! The!external!damage!is!beyond!simple!repair!and!impairs!scale!function.!7! Remove!dents! Gently!push!or!hammer!out!the!dents!in!the!scale!body.!
8! If!the!scale!has!beams,!are!they!unbent?! Visually!assess!for!external!damage!in!beams.!
9! Can!the!beams!be!bent!back!to!a!straight!position?! Gently!try!to!push!beams!back!into!a!straight!position.!10! Straighten!beam! Gently!try!to!push!beams!back!into!a!straight!position.!
11! Place!a!spherical!or!cylindrical!object!on!surface!scale!sits!on! Any!object!that!rolls!will!suffice.!Ideal!objects!would!be!a!marble,!small!ball,!pen,!pencil,!dowel,!etc.!
12! Is!the!scale!on!a!level!surface?! If!the!object!rolls!of!its!own!accord,!the!surface!the!scale!sits!on!is!not!level.!
13! Move!to!a!level!surface! Move!scale!to!a!surface!on!which!the!spherical!or!cylindrical!object!does!not!roll!of!its!own!accord.!
14! Place!a!spherical!or!cylindrical!object!on!scale!platter! Repeat!test!for!levelness!on!scale!platter.!
15! Is!the!scale!level?! If!the!object!rolls!of!its!own!accord,!the!scale!platter!is!not!level!and!adjustments!need!to!be!made.!16! Are!the!feet!uneven?! To!see!if!the!problem!lies!in!the!interface!between!the!scale!and!the!surface!it!sits!on,!press!on!each!of!the!corners!of!the!




scale!body!and!verify!visually!and!audibly!whether!or!not!the!scale!wobbles.!
17! Are!the!feet!adjustable?! On!some!models,!the!feet!can!be!adjusted!like!knobs!to!raise!or!lower!a!corner!of!the!scale.!
18! Place!paper!under!uneven!foot!or!edge!until!level! If!the!scale!feet!cannot!be!adjusted,!the!scale!can!be!leveled!by!placing!paper!or!cardboard!under!the!problem!corner!until!the!scale!plate!is!level.!Use!spherical!or!cylindrical!object!to!determine!levelness.!
19! Adjust!until!scale!is!level! Twist!knobs!until!scale!is!level.!Use!spherical!or!cylindrical!object!to!determine!levelness.!
20! Return!to!step!15! In!order!to!double!check!the!level\ness!of!the!scale,!return!to!step!15.!
21! Is!the!plate!unbalanced?! If!the!problem!is!not!in!the!feet,!the!lack!of!levelness!is!due!to!either!the!plate!itself!or!the!connection!between!the!plate!and!the!rest!of!the!scale!body.!
22! Is!the!plate!removable?! Inspect!to!see!if!plate!can!be!removed.!May!need!to!slide,!twist,!remove!screws!or!pins,!etc.!See!BTA!skills!on!Mechanical!Attachment.!
23! Clean!connection!of!plate!to!scale!body!of!dust,!rust!and!debris! By!gently!blowing,!wiping!and/or!scraping!all!pieces!that!interface!between!the!plate!and!the!internal!mechanism.!See!BTA!skills!on!Mechanical!Cleaning.!24! Remove!plate! Remove!by!the!mechanism!detected!in!step!21.!
25! Reset!and!adjust!internal!brackets!and!levers!as!needed!


Some!of!the!pieces!inside!the!scale!body!may!have!been!dislodged!or!moved,!for!instance,!if!the!scale!has!been!dropped.!There!are!likely!to!be!grooves!or!marks!where!they!should!align,!both!inside!the!scale!body!and!on!the!underside!of!the!scale!plate.!26! Replace!plate! Replace!plate.!
27! Clean!plate! Using!a!wet!cloth,!wipe!down!plate.!See!BTA!skills!on!Mechanical!Cleaning.!
28! Does!the!needle!for!measurement!read!zero?! Visually!determine!if!the!needle!for!measurement!is!aligned!with!the!zero!tic\mark.!
29! Does!the!scale!have!a!radial!measurement!display?! Most!analog!scales!display!measurement!in!one!of!two!ways!–!with!a!radial!dial!or!a!system!of!beams!with!a!sliding!weight.!
30! Adjust!tare!block!until!needle!reads!zero! Tare!block!looks!like!a!small,!metal!bracket!with!an!adjustable!knob!on!the!back!that!holds!the!bracket!in!place.!Slowly!slide!




the!bracket!along!the!beam!until!the!needle!reads!zero.!
31! Remove!face! Remove!face,!may!need!to!twist.!
32! Clean!dust,!rust!and!debris!from!pivot! By!gently!blowing,!wiping!and/or!scraping!in!pivot.!See!BTA!skills!on!Mechanical!Cleaning.!
33! Lubricate!pivot! Using!WD40,!Vaseline,!or!an!acceptable!substitute.!See!BTA!skills!on!Lubrication.!
34! Place!on!an!object!of!a!known!weight!on!plate!


Suggested!items!would!be!any!pre\packaged!object!with!a!pre\scribed!weight,!though!this!rests!on!the!assumption!that!the!written,!packaged!weight!is!accurate.!An!acceptable!alternative!would!be!to!use!a!known!volume!of!water,!remembering!that!the!density!of!water!is!1!g/cm3!or!8.34!lb/gal.!
35! Does!the!reading!match?! The!display!should!read!the!value!of!the!known!weight!of!the!object.!
36! Find!and!adjust!tare!knob! Most!models,!whether!radial!or!beam,!have!a!tare!knob!located!on!the!back!or!base!of!the!scale!body.!This!can!be!turned!to!manually!move!display!needle.!
37! Does!the!scale!have!beams?! Most!analog!scales!display!measurement!in!one!of!two!ways!–!with!a!radial!dial!or!a!system!of!beams!with!a!sliding!weight.!
38! Adjust!tare!block!until!reading!matches!


Tare!block!looks!like!a!small,!metal!bracket!with!an!adjustable!knob!on!the!back!that!holds!the!bracket!in!place.!Slowly!slide!the!bracket!along!the!beam!until!the!needle!reads!the!desired!value.!
39! Remove!weight! Remove!weight.!
40! Ready!for!use! Scale!is!ready!for!use.!


!




Scales'(Digital)'Repair'and'Troubleshooting'Flowchart* !




!Description*#! Text'box! Explanation'or'comment!
1! Begin:!Turn!on!scale.!Wait!several!minutes! Start!the!diagnostic!process!for!a!work!order!on!Scales!(digital).!It!is!important!to!allow!scale!to!warm!up!before!use.!
2! Does!LCD!illuminate?! The!first!part!of!the!diagnostic!process!will!investigate!the!electronic!components!of!the!scale,!beginning!with!the!LCD!display.!Look!for!numbers,!a!decimal!point,!zeros,!dashes!or!a!weight!unit!of!measurement!on!display.!
3! Press!LCD!display!gently!and!firmly!into!console! If!connection!between!LCD!display!and!internal!circuitry!is!loose,!a!simple!push!may!suffice!to!reconnect.!
4! Is!the!scale!battery!powered?! Look!for!battery!cavity!covering!and!check!batteries!for!rust,!leakage!and!expiration.!See!BTA!skills!for!Batteries.!
5! Replace!batteries! If!the!batteries!are!too!old,!power!cannot!be!supplied.!See!BTA!skills!on!Batteries.!
6! Check!cord!connections! Power!cord!may!disconnected!from!scale!console,!it!is!important!that!it!stays!in!contact!with!internal!circuitry!to!maintain!power!supply.!7! Now!does!LCD!illuminate?! See!#2!
8! Problem!most!likely!in!circuitry,!if!able,!open!scale!and!check!internal!LCD!connections! Troubleshoot!circuitry!for!loose,!rusted!or!otherwise!damaged!connections.!See!BTA!skills!for!Connections.!
9! Make!sure!weigh!platter!is!clear!and!clean.!Press!tare!button! Tare!button!is!on!keypad!and!is!labeled!as!“ZERO”,!“0/T”,!“TARE”,!etc.!Pressing!this!button!resets!calibration!to!zero.!
10! Are!standard!calibration!weights!available?! Every!scale!should!come!with!official!calibration!weights!with!explicit!instruction!for!care!and!storage,!if!available,!these!should!be!used.!
11! Check!scale!labels!for!scale!capacity!and!magnitude! Text!on!scale!exterior!should!give!an!indication!of!the!precision!and!degree!of!magnitude!for!which!the!model!is!designed.!
12! Is!a!functional!scale!available!for!reference?! It!is!best!to!verify!calibration!of!scale!with!an!object!with!a!known!and!verified!weight!
13! Measure!out!a!mass!of!approx.!1/5!of!scale!capacity!of!a!known!medium!


If!a!functional!scale!is!available,!a!makeshift!calibration!weight!can!easily!be!found.!Suggested!mediums!for!weight!include!any!item!with!a!labeled!package!weight!(i.e.!bags!of!sugar!or!flour!for!scales!with!higher!magnitude!and!lower!precision!or!pre_




packaged!medical!substances!or!powders!for!scales!of!lower!magnitude!and!higher!precision),!or!plastic!containers!filled!with!water!(size!the!container!appropriately!and!proportionately!to!the!scale!magnitude).!It!is!important!to!record!the!weight!of!this!mass!for!reference.!
14! Find!an!object!with!a!known!weight!of!the!same!order!of!magnitude!but!less!than!capacity!of!scale!


Suggested!items!would!be!any!pre_packaged!object!with!a!pre_scribed!weight,!though!this!rests!on!the!assumption!that!the!written,!packaged!weight!is!accurate.!
15! Place!weight!on!scale!platter! Place!weight!on!scale!platter.!
16! Does!the!display!match!the!known!weight?! If!the!scale!is!properly!calibrated,!the!display!should!show!the!known!weight!for!the!item.!
17! Is!the!scale!on!a!level,!non_vibrating!surface?! If!the!scale!is!on!an!uneven!or!vibrating!surface,!it!is!likely!to!display!an!incorrect!weight!reading.!
18! Move!scale!to!a!level,!non_vibrating!surface! Examples!of!level,!non_vibrating!surfaces!include!a!paved!or!finished!floor,!a!counter_top,!or!a!table.!
19! Is!the!scale!in!an!environment!of!typical!temperature?!


If!the!scale!is!in!an!environment!with!an!abnormal!temperature,!the!circuitry!may!not!function!properly.!A!typical!temperature!is!approximately!within!the!range!of!10_32.2!degrees!C.!
20! Move!scale!to!a!level,!non_vibrating!surface!in!an!environment!with!a!typical!temperature!


If!possible,!store!and!use!scale!in!a!temperature_controlled!room.!Otherwise,!store!and!use!scale!in!shaded!area!away!from!moisture.!
21! Is!the!scale!far!from!all!electrical!and!air!currents?! Electrical!currents!affect!the!internal!circuitry!and!air!currents!can!distort!the!effective!mass!on!top!of!the!scale.!
22! Move!scale!to!a!level,!non_vibrating!surface!in!an!environment!with!a!typical!temperature!and!no!currents!


Electrical!currents!can!come!from!high_power!machinery!or!appliances.!Air!currents!can!come!from!open!windows,!open!doors,!heating!or!cooling!vents.!Store!and!use!scale!as!far!from!all!of!these!as!possible.!
23! Find!leveling!bubble!on!top!of!scale! This!should!be!on!the!console!as!opposed!to!the!scale!platter,!probably!towards!the!rear!of!the!scale.!There!should!be!two!concentric!circles!showing!fluid!and!an!air!bubble!beneath.!
24! Is!bubble!centered!in!circle_shaped!guide_lines?! The!bubble!should!be!about!the!size!of!the!smaller!circle,!the!scale!is!level!when!the!two!line!up.!
25! Adjust!scale!feet!until!bubble!is!centered! All!four!scale!feet!should!be!adjustable!by!rotation!to!slowly!change!the!height!of!each!supporting!leg!of!the!scale.!There!may!also!be!a!fifth!“phantom”!leg!towards!the!front!of!the!scale!




that!can!be!lowered!to!add!balance!while!adjusting!the!feet.!
26! Clean!scale!platter,!remove!any!dust!or!rust,!especially!in!crevices!


Scale!platter!should!be!cleaned!with!clean,!warm!water,!a!cloth!and!gentle!scrubbing.!A!cloth!can!typically!remove!dust!or!rust!from!crevices,!as!can!blowing!a!jet!of!air!gently!through!any!cracks!or!crevices.!See!BTA!skills!for!Mechanical!Cleaning.!
27! Go!to!#9! Restart!calibration!process!to!ensure!that!scale!is!properly!calibrated.!28! Ready!for!use! Scale!is!ready!for!use.!!




User Care of Medical Equipment – First line maintenance for end users


55


Troubleshooting – Scales




Fault Possible Cause Solution


1.



Zero point cannot be set



Scales are not level


Zero control broken or internal
part jammed



Set scales on level ground and
retest

Send for repair



2.



Movement is stiff or jerky



Dirt lodged inside


Internal blockage



Remove any visible dirt or
foreign body and retest

Send for repair



3.



Reading is inaccurate



Zero not properly set

Calibration error



Reset zero and retest

Recalibrate or send for repair



4.



Electronic display is blank



Battery / power failed


Internal error



Replace battery (if accessible) or
power supply and retest

Send for repair






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 dir t y.


The measurement cell is badly assembled.
Non-linear readings. D efec tive elec tronic system.


Mechanical system is in bad condition.
D igital reading continually goes up or down. D efec tive elec tronic system.


Change in room temperature.
The digital reading goes up and down continually. D ir t y measuring cell.


D efec tive elec tronic system.
Environmental problems like air currents, static
elec tricit y or vibrations.


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


D efec tive elec tronic 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. D efec tive calibration batter y.
Elec tronic system is defec tive.
Measurement cell is inadequately assembled.


TROUBLESHOOTING TABLE
Electronic balance


PROBLEM PROBABLE CAUSE SOLUTION
The balance does not turn on. T he interconnec tion cable is disconnec ted or


maladjusted on the balance.
Check the connec tion. Adjust the cable connec tor if
this is the case.


Elec trical outlet has no power. Check elec trical feed.
The weight reading is incorrec t. The balance was not adjusted to zero before the


reading.
Place the balance on zero; repeat the measurement.


The balance is incorrec tly calibrated. C alibrate according to the procedure recommended
by the manufac turer.


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


The units are incorrec tly selec ted.
selec t the required measurement unit.


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


The menu may be locked. Check to see if the locking switch is ac tivated. If this
is the case, deac tivate it.


The balance is incapable of keeping the selec tions
or changes. process.


Verify that the changes and selec tions are done
according to the manufac turer ’s instruc tions. Repeat
the selec tion or change.


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


The front door of the balance is open.


Place the balance on a stable sur face.


Close the front door to measure.


The RS232 inter face does not func tion. The interconnec tion cable is maladjusted. Check the connec tion of the interconnec tion cabl e.
The screen shows incomplete readings or is locked. The microprocessor is locked.


the situation persists, seek technical assistance from
the ser vice representative.


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




5.*Resources*for*More*Information*about*Safety*Balances*and*Scales****Featured*in*this*Section:*****Malkin,(Robert.(“Balances(Use(and(Operation.”(Medical(Instrumentation(in(the(Developing(World.(Engineering(World(Health,(2006.((((WHO.(“Balances.”(From(the(publication:((Maintenance(and(Repair(of(Laboratory,(Diagnostic(Imaging,(and(Hospital(Equipment((WHO:(1996).*((Weithoner,(Frank.(“Laboratory(Balances.”(Frank’s(Hospital(Workshop.(Retrieved(from:(http://www.frankshospitalworkshop.com/equipment/weighing_scales_equipment.html((( (**((((* *(((((((




*Resources*for*More*Information:(*((Internal*Resources*at*library.ewh.org:*For*More*Information*about*balances*and*scales,*please*see*this*resource*in*the*BMET*Library!*( 1. Malkin,(Robert.(“Balances(Use(and(Operation.”(Medical(Instrumentation(in(the(Developing(World.(Engineering(World(Health,(2006.((( 2. WHO.(“Balances.”(From(the(publication:((Maintenance(and(Repair(of(Laboratory,(Diagnostic(Imaging,(and(Hospital(Equipment((WHO:(1996).*(External*Resources:*3. Laboratory*Balances*Training:(This(article(explains(the(basics(of(laboratory(balances(including,(pictures,(calibration,(installation,(and(safety.(Weithoner,*Frank.*“Laboratory*Balances.”*Retrieved*from:*http://www.frankshospitalworkshop.com/equipment/weighing_scales_equipment.html((((




Balances*and*Scales*Bibliography:(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Analog)(Preventative(Maintenance.”(From(the(publication:((Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Digital)(Preventative(Maintenance.”(From(the(publication:((Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Analog)(Troubleshooting(Flowchart.”(From(the(publication:(Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((Cooper,(Justin(and(Alex(Dahinten(for(EWH.(“Scales((Digital)(Troubleshooting(Flowchart.”(From(the(publication:(Medical(Equipment(Troubleshooting(Flowchart(Handbook.(Durham,(NC:(Engineering(World(Health,(2013.(((DHT(Laboratory.(“Calibration(of(Scales.”(From(the(Publication:(Biomedical(Technician(Assistant((BTA)(Skills.(Duke(University.((2011).(* Strengthening(Specialised(Clinical(Services(in(the(Pacific.(User(Care(of(Medical(Equipment:(A(first(line(maintenance(guide(for(end(users.((2015).(((Weithoner,(Frank.(“Laboratory(Balances.”(Frank’s(Hospital(Workshop.(Retrieved(from:(http://www.frankshospitalworkshop.com/equipment/weighing_scales_equipment.html(( (WHO.(Maintenance(and(Repair(of(Laboratory,(Diagnostic(Imaging,(and(Hospital(Equipment((WHO:(1996).(*(WHO.(“Chapter(4:(Balances.”(From(the(publication:(Laboratory(Equipment(Maintenance(Manual,((WHO:(2008).(((Wikipedia.(“Body(Weight.”(Wikipedia.(Retrieved(from:(https://en.wikipedia.org/w/index.php?oldid=444807901((




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