ASTM E319-85(1997)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: E 319 – 85 (Reapproved 1997) An American National Standard
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Practice for the
Evaluation of Single-Pan Mechanical Balances
This standard is issued under the fixed designation E 319; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) 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
smallest weights normally used on the balance. The test procedure given herein measures the precision
with which the balance can compare the two loads, and the rates at which systematic errors may affect
the observed difference.
Cr.W 5 A 2 N (1)
1. Scope
1.1 This practice covers testing procedures for evaluating
where:
the performance of single-arm balances required by ASTM
Cr.W 5 correction for the error in adjustment to nominal
standards.
value,
1.2 This practice is intended for but not limited to sensitivity
A 5 actual value of the weight, and
ratios of 10 or better and on-scale ranges of 1000xd or more
N 5 nominal value.
where d 5 reability either directly or by estimation.
NOTE 2—In practice it is not possible to adjust weights exactly to their
1.3 This practice can also be applied to other single-pan
nominal values.
balances with mechanical weight changing of different capaci-
2.1.3 correction for error in scale indication—the correc-
ties or sensitivities with appropriate test loads and calibration
tion for the scale indication, I, is:
weights.
Cr.I 5 A 2 I (2)
NOTE 1—Mechanical balances of this type have largely been replaced
by automatic electronic balances incorporating a variety of operational
NOTE 3—The correction for the scale is taken with reference to the
principles. Nevertheless, some single-pan mechanical balances are still
measured value of a weight used as a test load during calibration of the
manufactured and many older balances will remain in service for years to
on-scale range.
come. One type of automatic electronic balance, the so-called “hybrid,”
2.1.4 index of precision—the standard deviation, computed
bears considerable similarity to single-pan mechanical balances of the null
in any acceptable manner, for a collection of measurements
type. (1)
involving a given pair of mass standards (3).
1.4 This standard does not purport to address all of the
NOTE 4—The standard deviation is computed from the data provided by
safety problems, if any, associated with its use. It is the
the instrument precision test (see Section 7) index of precision.
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
2.1.5 null-type balance—a balance which requires, as the
bility of regulatory limitations prior to use.
final step in its operation, that the observer restore the angle of
the balance beam to its original (or null) position. The least
2. Terminology
significant figures of the balance indication are obtained from
2.1 Definitions (1):
this operation.
2.1.1 accuracy—the degree of agreement of the measure-
2.1.6 optical-type balance—in this type the least significant
ments with the true value of the magnitude of the quantity
figures of the balance indication are related to the deflection
measured (2).
angle of the beam from its original (or null) position. A scale
2.1.2 correction for a weight—the correction for the error in
placed on the moving beam is optically projected onto the
adjustment is:
(stationary) balance case to provide this indication.
2.1.7 precision—the repeatability of the balance indication
This practice is under the jurisdiction of ASTM Committee E-41 on Laboratory
with the same load under essentially the same conditions.
Apparatus and is the direct responsibility of Subcommittee E41.06 on Weighing
NOTE 5—The more closely the measurements are grouped, the smaller
Devices.
Current edition approved Aug. 30, 1985. Published October 1985. Originally the index of precision will be. The precision must be measured under
e1
published as E 319 – 68. Last previous edition E 319 – 68 (1976) .
environmental conditions that represent the conditions under which the
The boldface numbers in parentheses refer to the list of references at the end of
balance is normally used.
this practice.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 319
2.1.8 readability—the value of the smallest decimal subdi- 5. Preparation of Apparatus
vision of a scale division in terms of mass units, that can be
5.1 Balance (In all cases, the balance should be used in
read, when the balance is read in the intended manner.
accordance with the manufacturer’s instructions):
5.1.1 The results obtained will depend on the environment.
NOTE 6—The readability of a particular instrument is not a measure of
Select an area which is free of excessive vibration and air
its performance as a weighing device. The relationship between the
numerical value obtained by reading devices and the ability of the operator
currents, where rapid changes in temperature and relative
to estimate the location of the reference device or index is important. It is
humidity will not be encountered, and where the floor is rigid
possible to introduce a large number of readable subdivisions of the main
enough to be free of a tilting effect on the balance indication.
scale divisions that would increase the “readability” as defined but if the
Place the balance on a sturdy bench. If the balance has been
reading device cannot be reset to the same numerical value when the beam
moved to a new location, permit it to come to thermal
is in an immovable condition, or when the load on the beam is a constant
equilibrium for at least 1 h before performing the test,
value, the readability becomes meaningless. Readability substantially less
than 1 standard deviation as determined by repeated measurement with a preferably several hours.
given test weight is usually superfluous.
5.1.2 Inspect and test the balance to make sure that it is in
2.1.9 scale division—the smallest graduated interval subdi- proper mechanical order. Arrest and release the beam to make
sure that readings are approximately repetitive. Observe the
vided either by estimation or with the aid of a vernier.
Subdivisions which appear as divisions on the vernier are not indication during arrest and release to ensure that there is no
considered to be scale divisions, but rather parts of scale “kick” that would indicate that arrestment points might be out
divisions. of adjustment. If necessary, have the balance adjusted by a
2.1.10 sensitivity weight—a small weight used to measure competent balance technician.
5.1.3 Make a few trial measurements of the interval from
the “on-scale” deflection of the balance indicator.
zero to the full-scale indication.
NOTE 7—With single-pan balances the sensitivity weight should be
5.2 Reading the Balance—The balance should be read in
equal to the value of the smallest built-in weight represented by the first
accordance with the instructions supplied by the manufacturer.
step on the dial for the smallest weights.
Optical types should include the reading of verniers or mi-
2.1.11 test load—a load chosen to represent the sample load
crometres. Null types should include the indication of the
in the test procedure.
device for restoring to null including verniers or micrometres.
2.1.12 value of the division—the change in load required to
change the balance indication by one scale division. The
6. Preliminary Testing of Single-Arm Balances
reciprocal of the sensitivity is its most useful function.
6.1 Summary of Method—With single-pan balances the
3. Summary of Practice
smallest built-in weight, indicated by the first step on the dial,
3.1 The accuracy of the direct-reading scale, the smallest is compared with a calibrated weight. The direct-reading scale
is tested for agreement with the smallest built-in weight and the
weight of the set of built-in weights, and uniformity of
sensitivity between the upper and lower halves of full-scale sensitivity is adjusted, if necessary, so that the indications of
the scale are precise in terms of the calibrated weight. A
deflections are verified by preliminary tests.
3.2 Estimates of rate of change of the zero with time, rate of “fifty-fifty” test verifies the accuracy of the midpoint at
half-full scale. This test should be performed before proceed-
change in the value of the scale division with time, and a
quantitative measure of the variability or random error are ing to other tests. After the accuracy of adjustment of the
smallest built-in weight is verified, this weight is used to test
provided by short tests for precision and bias.
3.3 An overall test of the direct-reading capability is pro- full-scale deflections. Tests are also made for the uniformity of
deflection over the lower and upper halves of the full-scale
vided by tests of the built-in weights.
deflection. The preliminary tests show either that the balance is
4. Significance and Use
operating properly, or that discrepancies indicate the presence
4.1 Monitoring Weighing Performance—This practice pro-
of sources of error. Uncertainties of perhaps one millionth of
vides results in the form of control charts which measure the
the balance capacity may be caused by dirt or foreign material
weighing capability at the time of the test. A series of tests at
in the bearings, or by unskilled handling, while larger discrep-
appropriate intervals will monitor balance performance over a
ancies may be caused by worn or damaged knife-edges or other
period of time. A marked change from expected performance
sources such as electrostatic effects. Any necessary cleaning or
may result from a variety of causes including: maladjustment,
servicing should be done at this point. If discrepancies con-
damage, dirt, foreign material, and thermal disturbances. If the
tinue, other possible sources of uncertainty should be studied.
test results are to indicate future performance, any disturbances
There is no point in proceeding with routine test procedures
that occur exterior to the balance must be brought under control
until acceptable results can be obtained with the preliminary
(2).
tests.
4.2 Acceptance Tests—This practice may also be used as
NOTE 8—With null-type balances (including the hybrid) it is possible to
acceptance tests for new balances. For this purpose, the tests
use the flexure of a segment of metal, quartz, etc. as the main pivots
should be conducted under favorable, but not necessarily ideal,
instead of knife edges. A flexure pivot is by its nature free of problems of
conditions. Since systematic error in the course of the zero and
dirt. Flexures are also generally more robust than knives. The chief
the course of the sensitivity may be caused by disturbances
problem associated with flexures is that they act like springs and thus add
external to the balance, limits on these errors are not ordinarily
a restoring force which may vary with time or temperature. This drawback
prescribed in acceptance requirements. can be minimized by careful design and all but eliminated by the use of
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
E 319
servo-control in electronic balances.
A and B should agree within 3 standard deviations (see
7.5.3). Any discrepancy smaller than 3 standard deviations may
6.2 Materials:
be ascribed to uncertainty in the preliminary measurements and
6.2.1 A watch or clock which indicates seconds,
does not necessarily indicate a real change in the value of the
6.2.2 Pencils for recording data,
scale divisions.
6.2.3 Columnar data sheets (If balance performance will be
6.4.5 Inspect the no-load readings, a, f, and k for agreement
monitored periodically, it may be useful to enter data directly
or zero drift.
into a personal computer which has been programmed for this
6.4.6 See Table 2 and Fig. 1 for examples of calculations
task.),
and observation form.
6.2.4 A calibrated weight designated S1 which has the
nominal value equal to the smallest interval on the dial-
7. Instrument Precision (4)
operated weights, and
6.2.5 Two weights of half of the nominal value of S1 7.1 Summary of Method:
1 1
designated ( ⁄2 )1 and ( ⁄2)2. (These weights need not be 7.1.1 A set of four readings is repeated four times, or more,
calibrated but they should bear distinguishing marks, prefer- to obtain pairs of readings with identical loads:
ably one, and two dots.) 7.1.1.1 A reading near zero,
6.3 Procedure—Adjust the “no-load” readings to a point 7.1.1.2 A reading near the upper end of the scale,
near the center of the vernier so that zero drift or other 7.1.1.3 A reading near the upper end of the scale with a test
deviation will not cause a negative scale reading. Perform the load plus a small weight, and
7.1.1.4 A reading near zero with the test load but with the
preliminary tests, loading the pan and changing the dial
settings according to the schedule in Table 1. Before releasing small weight removed.
the beam, record the load on the pan and the dial setting so that
7.1.2 Readings are taken at a steady pace as rapidly as
the observation will be confined to the scale reading. Release practicable, consistent with good practice, and the time is
the balance and observe the scale reading. Record the indica- observed at the start of each set of observations and at the end
tion and verify the stability of the scale reading, then arrest the of the test.
balance promptly.
7.1.3 The balance indications are plotted on a graph to
6.4 Calculations for Preliminary Tests: provide a visual presentation of errors. The zero readings are
6.4.1 Compute D1, the value of the smallest built-in weight connected to show the course of the zero with time. The
as follows: response of the balance to the small weight is plotted. The
course of the sensitivity with time is represented by a plot of
D1 5 @~a 2 b 1 f 2 e!/2# 1 S1 (3)
the interval from zero to full-scale angular motion of the beam,
where: a, b, f, and e are taken from Table 1, and
or with balances having indication to a null point by a plot of
S1 5 calibrate
...