ASTM E319-85(2008)
(Practice)Standard Practice for the Evaluation of Single-Pan Mechanical Balances
Standard Practice for the Evaluation of Single-Pan Mechanical Balances
SIGNIFICANCE AND USE
Monitoring Weighing Performance—This practice provides results in the form of control charts which measure the weighing capability at the time of the test. A series of tests at appropriate intervals will monitor balance performance over a period of time. A marked change from expected performance may result from a variety of causes including: maladjustment, damage, dirt, foreign material, and thermal disturbances. If the test results are to indicate future performance, any disturbances that occur exterior to the balance must be brought under control (2).
Acceptance Tests—This practice may also be used as acceptance tests for new balances. For this purpose, the tests should be conducted under favorable, but not necessarily ideal, conditions. Since systematic error in the course of the zero and the course of the sensitivity may be caused by disturbances external to the balance, limits on these errors are not ordinarily prescribed in acceptance requirements.
SCOPE
1.1 This practice covers testing procedures for evaluating the performance of single-arm balances required by ASTM standards.
1.2 This practice is intended for but not limited to sensitivity ratios of 106 or better and on-scale ranges of 1000xd or more where d = reability either directly or by estimation.
1.3 This practice can also be applied to other single-pan balances with mechanical weight changing of different capacities or sensitivities with appropriate test loads and calibration weights.
Note 1—Mechanical balances of this type have largely been replaced by automatic electronic balances incorporating a variety of operational principles. Nevertheless, some single-pan mechanical balances are still manufactured and many older balances will remain in service for years to come. One type of automatic electronic balance, the so-called “hybrid,” bears considerable similarity to single-pan mechanical balances of the null type. (1)
1.4 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E319 − 85(Reapproved 2008)
Standard Practice for the
Evaluation of Single-Pan Mechanical Balances
This standard is issued under the fixed designation E319; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The balance performs two basic functions: (1) it compares an unknown load with one or more
weights, and (2) it indicates the difference between the two loads for differences smaller than the
smallestweightsnormallyusedonthebalance.Thetestproceduregivenhereinmeasurestheprecision
withwhichthebalancecancomparethetwoloads,andtheratesatwhichsystematicerrorsmayaffect
the observed difference.
1. Scope 2.1.1 accuracy—the degree of agreement of the measure-
ments with the true value of the magnitude of the quantity
1.1 This practice covers testing procedures for evaluating
measured (2).
the performance of single-arm balances required by ASTM
2.1.2 correction for a weight—thecorrectionfortheerrorin
standards.
adjustment is:
1.2 Thispracticeisintendedforbutnotlimitedtosensitivity
Cr.W 5 A 2 N (1)
ratios of 10 or better and on-scale ranges of 1000xd or more
where d = reability either directly or by estimation.
where:
1.3 This practice can also be applied to other single-pan Cr.W = correction for the error in adjustment to nominal
balances with mechanical weight changing of different capaci- value,
A = actual value of the weight, and
ties or sensitivities with appropriate test loads and calibration
N = nominal value.
weights.
NOTE 2—In practice it is not possible to adjust weights exactly to their
NOTE 1—Mechanical balances of this type have largely been replaced
nominal values.
by automatic electronic balances incorporating a variety of operational
2.1.3 correction for error in scale indication— the correc-
principles. Nevertheless, some single-pan mechanical balances are still
tion for the scale indication, I, is:
manufactured and many older balances will remain in service for years to
come. One type of automatic electronic balance, the so-called “hybrid,”
Cr.I 5 A 2 I (2)
bearsconsiderablesimilaritytosingle-panmechanicalbalancesofthenull
NOTE 3—The correction for the scale is taken with reference to the
type. (1)
measured value of a weight used as a test load during calibration of the
1.4 This standard does not purport to address all of the
on-scale range.
safety problems, if any, associated with its use. It is the
2.1.4 index of precision—the standard deviation, computed
responsibility of the user of this standard to establish appro-
in any acceptable manner, for a collection of measurements
priate safety and health practices and determine the applica-
involving a given pair of mass standards (3).
bility of regulatory limitations prior to use.
NOTE4—Thestandarddeviationiscomputedfromthedataprovidedby
the instrument precision test (see Section 7) index of precision.
2. Terminology
2.1.5 null-type balance—a balance which requires, as the
2.1 Definitions(1):
final step in its operation, that the observer restore the angle of
the balance beam to its original (or null) position. The least
significant figures of the balance indication are obtained from
This practice is under the jurisdiction ofASTM Committee E41 on Laboratory
this operation.
Apparatusand is the direct responsibility of Subcommittee E41.06 on Weighing
2.1.6 optical-type balance—in this type the least significant
Devices.
Current edition approved Nov. 1, 2008. Published January 2009. Originally
figures of the balance indication are related to the deflection
approved in 1968. Last previous edition approved in 2003 as E319–85(2003).
angle of the beam from its original (or null) position. A scale
DOI: 10.1520/E0319-85R08.
placed on the moving beam is optically projected onto the
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. (stationary) balance case to provide this indication.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E319 − 85 (2008)
2.1.7 precision—the repeatability of the balance indication testresultsaretoindicatefutureperformance,anydisturbances
with the same load under essentially the same conditions. thatoccurexteriortothebalancemustbebroughtundercontrol
(2).
NOTE 5—The more closely the measurements are grouped, the smaller
the index of precision will be. The precision must be measured under 4.2 Acceptance Tests—This practice may also be used as
environmental conditions that represent the conditions under which the
acceptance tests for new balances. For this purpose, the tests
balance is normally used.
shouldbeconductedunderfavorable,butnotnecessarilyideal,
2.1.8 readability—the value of the smallest decimal subdi- conditions.Sincesystematicerrorinthecourseofthezeroand
vision of a scale division in terms of mass units, that can be the course of the sensitivity may be caused by disturbances
read, when the balance is read in the intended manner. externaltothebalance,limitsontheseerrorsarenotordinarily
prescribed in acceptance requirements.
NOTE 6—The readability of a particular instrument is not a measure of
its performance as a weighing device. The relationship between the
5. Preparation of Apparatus
numericalvalueobtainedbyreadingdevicesandtheabilityoftheoperator
to estimate the location of the reference device or index is important. It is
5.1 Balance (In all cases, the balance should be used in
possible to introduce a large number of readable subdivisions of the main
accordance with the manufacturer’s instructions):
scale divisions that would increase the “readability” as defined but if the
5.1.1 The results obtained will depend on the environment.
readingdevicecannotberesettothesamenumericalvaluewhenthebeam
Select an area which is free of excessive vibration and air
is in an immovable condition, or when the load on the beam is a constant
currents, where rapid changes in temperature and relative
value, the readability becomes meaningless. Readability substantially less
than 1 standard deviation as determined by repeated measurement with a
humidity will not be encountered, and where the floor is rigid
given test weight is usually superfluous.
enough to be free of a tilting effect on the balance indication.
Place the balance on a sturdy bench. If the balance has been
2.1.9 scale division—the smallest graduated interval subdi-
vided either by estimation or with the aid of a vernier. moved to a new location, permit it to come to thermal
equilibrium for at least 1 h before performing the test,
Subdivisions which appear as divisions on the vernier are not
considered to be scale divisions, but rather parts of scale preferably several hours.
5.1.2 Inspect and test the balance to make sure that it is in
divisions.
proper mechanical order.Arrest and release the beam to make
2.1.10 sensitivity weight—a small weight used to measure
sure that readings are approximately repetitive. Observe the
the “on-scale” deflection of the balance indicator.
indication during arrest and release to ensure that there is no
NOTE 7—With single-pan balances the sensitivity weight should be “kick” that would indicate that arrestment points might be out
equal to the value of the smallest built-in weight represented by the first
of adjustment. If necessary, have the balance adjusted by a
step on the dial for the smallest weights.
competent balance technician.
2.1.11 test load—a load chosen to represent the sample load 5.1.3 Make a few trial measurements of the interval from
in the test procedure. zero to the full-scale indication.
5.2 Reading the Balance—The balance should be read in
2.1.12 value of the division—the change in load required to
change the balance indication by one scale division. The accordance with the instructions supplied by the manufacturer.
Optical types should include the reading of verniers or mi-
reciprocal of the sensitivity is its most useful function.
crometres. Null types should include the indication of the
device for restoring to null including verniers or micrometres.
3. Summary of Practice
3.1 The accuracy of the direct-reading scale, the smallest
6. Preliminary Testing of Single-Arm Balances
weight of the set of built-in weights, and uniformity of
6.1 Summary of Method—With single-pan balances the
sensitivity between the upper and lower halves of full-scale
smallest built-in weight, indicated by the first step on the dial,
deflections are verified by preliminary tests.
is compared with a calibrated weight. The direct-reading scale
3.2 Estimatesofrateofchangeofthezerowithtime,rateof istestedforagreementwiththesmallestbuilt-inweightandthe
change in the value of the scale division with time, and a sensitivity is adjusted, if necessary, so that the indications of
quantitative measure of the variability or random error are the scale are precise in terms of the calibrated weight. A
provided by short tests for precision and bias. “fifty-fifty” test verifies the accuracy of the midpoint at
half-full scale. This test should be performed before proceed-
3.3 An overall test of the direct-reading capability is pro-
ing to other tests. After the accuracy of adjustment of the
vided by tests of the built-in weights.
smallest built-in weight is verified, this weight is used to test
full-scale deflections.Tests are also made for the uniformity of
4. Significance and Use
deflection over the lower and upper halves of the full-scale
4.1 Monitoring Weighing Performance—This practice pro- deflection.Thepreliminarytestsshoweitherthatthebalanceis
vides results in the form of control charts which measure the operating properly, or that discrepancies indicate the presence
weighing capability at the time of the test. A series of tests at of sources of error. Uncertainties of perhaps one millionth of
appropriate intervals will monitor balance performance over a the balance capacity may be caused by dirt or foreign material
period of time. A marked change from expected performance in the bearings, or by unskilled handling, while larger discrep-
may result from a variety of causes including: maladjustment, anciesmaybecausedbywornordamagedknife-edgesorother
damage, dirt, foreign material, and thermal disturbances. If the sources such as electrostatic effects.Any necessary cleaning or
E319 − 85 (2008)
servicing should be done at this point. If discrepancies 6.4.2 Compute S1 in scale divisions to verify the full-scale
continue, other possible sources of uncertainty should be value on the direct-reading scale as follows:
studied. There is no point in proceeding with routine test
S1 5 c 2 b1d 2 e /2 (4)
~ !
procedures until acceptable results can be obtained with the
where c, b, d, and e are taken from Table 1. Adjust the
preliminary tests.
balance sensitivity if necessary so that the full-scale reading
NOTE8—Withnull-typebalances(includingthehybrid)itispossibleto
equals D1.
use the flexure of a segment of metal, quartz, etc. as the main pivots
6.4.3 Compute average scale difference, A, for lower 50%
instead of knife edges.Aflexure pivot is by its nature free of problems of
of direct-reading scale as follows:
dirt. Flexures are also generally more robust than knives. The chief
problem associated with flexures is that they act like springs and thus add
A 5 g 2 f1j 2 k /2 (5)
~ !
arestoringforcewhichmayvarywithtimeortemperature.Thisdrawback
can be minimized by careful design and all but eliminated by the use of
6.4.4 Compute average scale difference, B, for upper 50%
servo-control in electronic balances.
of direct-reading scale as follows:
6.2 Materials:
B 5 h 2 g1i 2 j /2 (6)
~ !
6.2.1 A watch or clock which indicates seconds,
6.2.2 Pencils for recording data, A and B should agree within 3 standard deviations (see
6.2.3 Columnar data sheets (If balance performance will be 7.5.3).Anydiscrepancysmallerthan3standarddeviationsmay
monitored periodically, it may be useful to enter data directly beascribedtouncertaintyinthepreliminarymeasurementsand
into a personal computer which has been programmed for this does not necessarily indicate a real change in the value of the
task.), scale divisions.
6.2.4 A calibrated weight designated S1 which has the 6.4.5 Inspect the no-load readings, a, f, and k for agreement
nominal value equal to the smallest interval on the dial- or zero drift.
operated weights, and 6.4.6 See Table 2 and Fig. 1 for examples of calculations
6.2.5 Two weights of half of the nominal value of S1 and observation form.
1 1
designated ( ⁄2)1 and ( ⁄2)2. (These weights need not be
calibrated but they should bear distinguishing marks, prefer-
7. Instrument Precision (4)
ably one, and two dots.)
7.1 Summary of Method:
6.3 Procedure—Adjust the “no-load” readings to a point
7.1.1 Aset of four readings is repeated four times, or more,
near the center of the vernier so that zero drift or other to obtain pairs of readings with identical loads:
deviation will not cause a negative scale reading. Perform the
7.1.1.1 A reading near zero,
preliminary tests, loading the pan and changing the dial
7.1.1.2 A reading near the upper end of the scale,
settings according to the schedule in Table 1. Before releasing
7.1.1.3 Areading near the upper end of the scale with a test
thebeam,recordtheloadonthepanandthedialsettingsothat
load plus a small weight, and
the observation will be confined to the scale reading. Release
7.1.1.4 A reading near zero with the test load but with the
the balance and observe the scale reading. Record the indica-
small weight removed.
tion and verify the stability of the scale reading, then arrest the
7.1.2 Readings are taken at a steady pace as rapidly as
balance promptly.
practicable, consistent with good practice, and the time is
observed at the start of each set of observations and at the end
6.4 Calculations for Preliminary Tests:
of the test.
6.4.1 Compute D1, the value of the smallest built-in weight
7.1.3 The balance indications are plotted on a graph to
as follows:
provide a visual presentation of errors. The zero readings are
D1 5 a 2 b1f 2 e /2 1S1 (3)
@~ ! #
connected to show the course of the zero with time. The
where: a, b, f, and e are taken from Table 1, and S1=cali- response of the balance to the small weight is plotted. The
brated value of test weight. course of the sensitivity with time is represented by a plot of
TABLE 1 Schedule for Preliminary Tests of Single-Arm Balances
Observation Time Pan Load Dial Setting Scale Reading
a Record the time zero 0 .
A
b S1 1 .
A
c S1 0 .
A
dWait30s S1 0 .
A
e S1 1 .
f Record the
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:E319–85 (Reapproved 2003) Designation:E319–85 (Reapproved 2008)
Standard Practice for the
Evaluation of Single-Pan Mechanical Balances
This standard is issued under the fixed designation E319; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The balance performs two basic functions: ( 1) it compares an unknown load with one or more
weights, and (2) it indicates the difference between the two loads for differences smaller than the
smallestweightsnormallyusedonthebalance.Thetestproceduregivenhereinmeasurestheprecision
withwhichthebalancecancomparethetwoloads,andtheratesatwhichsystematicerrorsmayaffect
the observed difference.
1. Scope
1.1 Thispracticecoverstestingproceduresforevaluatingtheperformanceofsingle-armbalancesrequiredbyASTMstandards.
1.2 Thispracticeisintendedforbutnotlimitedtosensitivityratiosof10 orbetterandon-scalerangesof1000xdormorewhere
d = reability either directly or by estimation.
1.3 This practice can also be applied to other single-pan balances with mechanical weight changing of different capacities or
sensitivities with appropriate test loads and calibration weights.
NOTE 1—Mechanical balances of this type have largely been replaced by automatic electronic balances incorporating a variety of operational
principles. Nevertheless, some single-pan mechanical balances are still manufactured and many older balances will remain in service for years to come.
One type of automatic electronic balance, the so-called “hybrid,” bears considerable similarity to single-pan mechanical balances of the null type. (1)
1.4 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Terminology
2.1 Definitions (1):
2.1.1 accuracy—the degree of agreement of the measurements with the true value of the magnitude of the quantity measured
(2).
2.1.2 correction for a weight—the correction for the error in adjustment is:
Cr.W 5 A 2 N (1)
where:
Cr.W = correction for the error in adjustment to nominal value,
A = actual value of the weight, and
N = nominal value.
NOTE 2—In practice it is not possible to adjust weights exactly to their nominal values.
2.1.3 correction for error in scale indication— the correction for the scale indication, I, is:
Cr.I 5 A 2 I (2)
NOTE 3—The correction for the scale is taken with reference to the measured value of a weight used as a test load during calibration of the on-scale
range.
2.1.4 index of precision—the standard deviation, computed in any acceptable manner, for a collection of measurements
This practice is under the jurisdiction ofASTM Committee E41 on LaboratoryApparatus and is the direct responsibility of Subcommittee E41.06 onWeighing Devices.
´1
Current edition approved Aug. 30, 1985. Published October 1985. Originally published as E319–68. Last previous edition E319–68(1976) .
Current edition approved Nov. 1, 2008. Published January 2009. Originally approved in 1968. Last previous edition approved in 2003 as E319–85(2003).
The boldface numbers in parentheses refer to the list of references at the end of this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E319–85 (2008)
involving a given pair of mass standards (3).
NOTE 4—The standard deviation is computed from the data provided by the instrument precision test (see Section 7) index of precision.
2.1.5 null-type balance—a balance which requires, as the final step in its operation, that the observer restore the angle of the
balance beam to its original (or null) position. The least significant figures of the balance indication are obtained from this
operation.
2.1.6 optical-type balance—in this type the least significant figures of the balance indication are related to the deflection angle
of the beam from its original (or null) position. A scale placed on the moving beam is optically projected onto the (stationary)
balance case to provide this indication.
2.1.7 precision—the repeatability of the balance indication with the same load under essentially the same conditions.
NOTE 5—The more closely the measurements are grouped, the smaller the index of precision will be. The precision must be measured under
environmental conditions that represent the conditions under which the balance is normally used.
2.1.8 readability—thevalueofthesmallestdecimalsubdivisionofascaledivisionintermsofmassunits,thatcanberead,when
the balance is read in the intended manner.
NOTE 6—The readability of a particular instrument is not a measure of its performance as a weighing device. The relationship between the numerical
value obtained by reading devices and the ability of the operator to estimate the location of the reference device or index is important. It is possible to
introduce a large number of readable subdivisions of the main scale divisions that would increase the “readability” as defined but if the reading device
cannotberesettothesamenumericalvaluewhenthebeamisinanimmovablecondition,orwhentheloadonthebeamisaconstantvalue,thereadability
becomesmeaningless.Readabilitysubstantiallylessthan1standarddeviationasdeterminedbyrepeatedmeasurementwithagiventestweightisusually
superfluous.
2.1.9 scale division—the smallest graduated interval subdivided either by estimation or with the aid of a vernier. Subdivisions
which appear as divisions on the vernier are not considered to be scale divisions, but rather parts of scale divisions.
2.1.10 sensitivity weight—a small weight used to measure the “on-scale” deflection of the balance indicator.
NOTE 7—With single-pan balances the sensitivity weight should be equal to the value of the smallest built-in weight represented by the first step on
the dial for the smallest weights.
2.1.11 test load—a load chosen to represent the sample load in the test procedure.
2.1.12 value of the division—the change in load required to change the balance indication by one scale division.The reciprocal
of the sensitivity is its most useful function.
3. Summary of Practice
3.1 The accuracy of the direct-reading scale, the smallest weight of the set of built-in weights, and uniformity of sensitivity
between the upper and lower halves of full-scale deflections are verified by preliminary tests.
3.2 Estimates of rate of change of the zero with time, rate of change in the value of the scale division with time, and a
quantitative measure of the variability or random error are provided by short tests for precision and bias.
3.3 An overall test of the direct-reading capability is provided by tests of the built-in weights.
4. Significance and Use
4.1 Monitoring Weighing Performance— This practice provides results in the form of control charts which measure the
weighingcapabilityatthetimeofthetest.Aseriesoftestsatappropriateintervalswillmonitorbalanceperformanceoveraperiod
of time.Amarked change from expected performance may result from a variety of causes including: maladjustment, damage, dirt,
foreignmaterial,andthermaldisturbances.Ifthetestresultsaretoindicatefutureperformance,anydisturbancesthatoccurexterior
to the balance must be brought under control (2).
4.2 Acceptance Tests—This practice may also be used as acceptance tests for new balances. For this purpose, the tests should
be conducted under favorable, but not necessarily ideal, conditions. Since systematic error in the course of the zero and the course
of the sensitivity may be caused by disturbances external to the balance, limits on these errors are not ordinarily prescribed in
acceptance requirements.
5. Preparation of Apparatus
5.1 Balance (In all cases, the balance should be used in accordance with the manufacturer’s instructions):
5.1.1 The results obtained will depend on the environment. Select an area which is free of excessive vibration and air currents,
where rapid changes in temperature and relative humidity will not be encountered, and where the floor is rigid enough to be free
of a tilting effect on the balance indication. Place the balance on a sturdy bench. If the balance has been moved to a new location,
permit it to come to thermal equilibrium for at least 1 h before performing the test, preferably several hours.
5.1.2 Inspect and test the balance to make sure that it is in proper mechanical order.Arrest and release the beam to make sure
that readings are approximately repetitive. Observe the indication during arrest and release to ensure that there is no “kick” that
would indicate that arrestment points might be out of adjustment. If necessary, have the balance adjusted by a competent balance
technician.
5.1.3 Make a few trial measurements of the interval from zero to the full-scale indication.
E319–85 (2008)
5.2 Reading the Balance—Thebalanceshouldbereadinaccordancewiththeinstructionssuppliedbythemanufacturer.Optical
types should include the reading of verniers or micrometres. Null types should include the indication of the device for restoring
to null including verniers or micrometres.
6. Preliminary Testing of Single-Arm Balances
6.1 Summary of Method—With single-pan balances the smallest built-in weight, indicated by the first step on the dial, is
compared with a calibrated weight. The direct-reading scale is tested for agreement with the smallest built-in weight and the
sensitivity is adjusted, if necessary, so that the indications of the scale are precise in terms of the calibrated weight.A“fifty-fifty”
test verifies the accuracy of the midpoint at half-full scale. This test should be performed before proceeding to other tests. After
theaccuracyofadjustmentofthesmallestbuilt-inweightisverified,thisweightisusedtotestfull-scaledeflections.Testsarealso
madefortheuniformityofdeflectionoverthelowerandupperhalvesofthefull-scaledeflection.Thepreliminarytestsshoweither
that the balance is operating properly, or that discrepancies indicate the presence of sources of error. Uncertainties of perhaps one
millionth of the balance capacity may be caused by dirt or foreign material in the bearings, or by unskilled handling, while larger
discrepanciesmaybecausedbywornordamagedknife-edgesorothersourcessuchaselectrostaticeffects.Anynecessarycleaning
or servicing should be done at this point. If discrepancies continue, other possible sources of uncertainty should be studied. There
is no point in proceeding with routine test procedures until acceptable results can be obtained with the preliminary tests.
NOTE 8—With null-type balances (including the hybrid) it is possible to use the flexure of a segment of metal, quartz, etc. as the main pivots instead
of knife edges.Aflexure pivot is by its nature free of problems of dirt. Flexures are also generally more robust than knives.The chief problem associated
with flexures is that they act like springs and thus add a restoring force which may vary with time or temperature. This drawback can be minimized by
careful design and all but eliminated by the use of servo-control in electronic balances.
6.2 Materials:
6.2.1 A watch or clock which indicates seconds,
6.2.2 Pencils for recording data,
6.2.3 Columnar data sheets (If balance performance will be monitored periodically, it may be useful to enter data directly into
a personal computer which has been programmed for this task.),
6.2.4 Acalibratedweightdesignated S1whichhasthenominalvalueequaltothesmallestintervalonthedial-operatedweights,
and
1 1
6.2.5 Twoweightsofhalfofthenominalvalueof S1designated( ⁄2)1and( ⁄2)2.(Theseweightsneednotbecalibratedbutthey
should bear distinguishing marks, preferably one, and two dots.)
6.3 Procedure—Adjust the “no-load” readings to a point near the center of the vernier so that zero drift or other deviation will
not cause a negative scale reading. Perform the preliminary tests, loading the pan and changing the dial settings according to the
schedule in Table 1. Before releasing the beam, record the load on the pan and the dial setting so that the observation will be
confined to the scale reading. Release the balance and observe the scale reading. Record the indication and verify the stability of
the scale reading, then arrest the balance promptly.
6.4 Calculations for Preliminary Tests:
6.4.1 Compute D1, the value of the smallest built-in weight as follows:
D1 5[~a 2 b 1 f 2 e!/2] 1 S1 (3)
where: a, b, f, and e are taken from Table 1, and S1=calibrated value of test weight.
6.4.2 Compute S1 in scale divisions to verify the full-scale value on the direct-reading scale as follows:
S1 5 c 2 b 1 d 2 e /2 (4)
~ !
where c, b, d, and e are taken from Table 1.Adjust the balance sensitivity if necessary so that the full-scale reading equals D1.
6.4.3 Compute average scale difference, A, for lower 50% of direct-reading scale as follows:
TABLE 1 Schedule for Preliminary Tests of Single-Arm Balances
Observation Time Pan Load Dial Setting Scale Reading
a Record the time zero 0 .
A
b S1 1 .
A
c S1 0 .
A
dWait30s S1 0 .
A
e S1 1 .
f Record the time 0 0 .
B
g( ⁄2) 0 .
B B
1 1 1
h Add ( ⁄2) ( ⁄2) +( ⁄2) 0 .
2 1 2
B
1 1
iWait30s ( ⁄2) +( ⁄2) 0 .
1 2
1 B 1 B
j Remove ( ⁄2) ( ⁄2) 0 .
1 2
k Record the time 0 0 .
A
S1 = calibrated weight of nominal value equal to the smallest dial-operated weight.
B
1 1 1
( ⁄2) and ( ⁄2) = weights of nominal value equal to ⁄2 S1 (not necessarily calibrated but marked for identification).
1 2
E319–85 (2008)
A 5 ~g 2 f 1 j 2 k!/2 (5)
6.4.4 Compute average scale difference, B, for upper 50% of direct-reading scale as follows:
B 5 h 2 g 1 i 2 j!/2 (6)
~
A and B should agree within 3 standard deviations (see 7.5.3). Any discrepancy smaller than 3 standard deviations may be
ascribed to uncertainty in the preliminary measurements and does not necessarily indicate a real change in the value of the scale
divisions.
6.4.5 Inspect the no-load readings, a, f, and k for agreement or zero drift.
6.4.6 See Table 2 and Fig. 1 for examples of calculations and observation form.
7. Instrument Precision (4)
7.1 Summary of Method:
7.1.1 A set of four readings is repeated fou
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