ASTM E319-1985(2003) Standard Practice for the Evaluation of Single-Pan Mechanical Balances《单盘机械天平的评定》.pdf

1、Designation: E 319 85 (Reapproved 2003)Standard Practice for theEvaluation of Single-Pan Mechanical Balances1This standard is issued under the fixed designation E 319; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of las

2、t revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRODUCTIONThe balance performs two basic functions: (1) it compares an unknown load with one or moreweights, and (2) it indicates

3、the difference between the two loads for differences smaller than thesmallest weights normally used on the balance. The test procedure given herein measures the precisionwith which the balance can compare the two loads, and the rates at which systematic errors may affectthe observed difference.1. Sc

4、ope1.1 This practice covers testing procedures for evaluatingthe performance of single-arm balances required by ASTMstandards.1.2 This practice is intended for but not limited to sensitivityratios of 106or better and on-scale ranges of 1000xd or morewhere d = reability either directly or by estimati

5、on.1.3 This practice can also be applied to other single-panbalances with mechanical weight changing of different capaci-ties or sensitivities with appropriate test loads and calibrationweights.NOTE 1Mechanical balances of this type have largely been replacedby automatic electronic balances incorpor

6、ating a variety of operationalprinciples. Nevertheless, some single-pan mechanical balances are stillmanufactured and many older balances will remain in service for years tocome. One type of automatic electronic balance, the so-called “hybrid,”bears considerable similarity to single-pan mechanical b

7、alances of the nulltype. (1)21.4 This standard does not purport to address all of thesafety problems, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations

8、 prior to use.2. Terminology2.1 Definitions (1):2.1.1 accuracythe degree of agreement of the measure-ments with the true value of the magnitude of the quantitymeasured (2).2.1.2 correction for a weightthe correction for the error inadjustment is:Cr.W 5 A 2 N (1)where:Cr.W = correction for the error

9、in adjustment to nominalvalue,A = actual value of the weight, andN = nominal value.NOTE 2In practice it is not possible to adjust weights exactly to theirnominal values.2.1.3 correction for error in scale indicationthe correc-tion for the scale indication, I, is:Cr.I 5 A 2 I (2)NOTE 3The correction

10、for the scale is taken with reference to themeasured value of a weight used as a test load during calibration of theon-scale range.2.1.4 index of precisionthe standard deviation, computedin any acceptable manner, for a collection of measurementsinvolving a given pair of mass standards (3).NOTE 4The

11、standard deviation is computed from the data provided bythe instrument precision test (see Section 7) index of precision.2.1.5 null-type balancea balance which requires, as thefinal step in its operation, that the observer restore the angle ofthe balance beam to its original (or null) position. The

12、leastsignificant figures of the balance indication are obtained fromthis operation.1This practice is under the jurisdiction of ASTM Committee E41 on LaboratoryApparatus and is the direct responsibility of Subcommittee E41.06 on WeighingDevices.Current edition approved Aug. 30, 1985. Published Octobe

13、r 1985. Originallypublished as E 319 68. Last previous edition E 319 68 (1976)e1.2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.2.1.6 op

14、tical-type balancein this type the least significantfigures of the balance indication are related to the deflectionangle of the beam from its original (or null) position. A scaleplaced on the moving beam is optically projected onto the(stationary) balance case to provide this indication.2.1.7 precis

15、ionthe repeatability of the balance indicationwith the same load under essentially the same conditions.NOTE 5The more closely the measurements are grouped, the smallerthe index of precision will be. The precision must be measured underenvironmental conditions that represent the conditions under whic

16、h thebalance is normally used.2.1.8 readabilitythe value of the smallest decimal subdi-vision of a scale division in terms of mass units, that can beread, when the balance is read in the intended manner.NOTE 6The readability of a particular instrument is not a measure ofits performance as a weighing

17、 device. The relationship between thenumerical value obtained by reading devices and the ability of the operatorto estimate the location of the reference device or index is important. It ispossible to introduce a large number of readable subdivisions of the mainscale divisions that would increase th

18、e “readability” as defined but if thereading device cannot be reset to the same numerical value when the beamis in an immovable condition, or when the load on the beam is a constantvalue, the readability becomes meaningless. Readability substantially lessthan 1 standard deviation as determined by re

19、peated measurement with agiven test weight is usually superfluous.2.1.9 scale divisionthe smallest graduated interval subdi-vided either by estimation or with the aid of a vernier.Subdivisions which appear as divisions on the vernier are notconsidered to be scale divisions, but rather parts of scale

20、divisions.2.1.10 sensitivity weighta small weight used to measurethe “on-scale” deflection of the balance indicator.NOTE 7With single-pan balances the sensitivity weight should beequal to the value of the smallest built-in weight represented by the firststep on the dial for the smallest weights.2.1.

21、11 test loada load chosen to represent the sample loadin the test procedure.2.1.12 value of the divisionthe change in load required tochange the balance indication by one scale division. Thereciprocal of the sensitivity is its most useful function.3. Summary of Practice3.1 The accuracy of the direct

22、-reading scale, the smallestweight of the set of built-in weights, and uniformity ofsensitivity between the upper and lower halves of full-scaledeflections are verified by preliminary tests.3.2 Estimates of rate of change of the zero with time, rate ofchange in the value of the scale division with t

23、ime, and aquantitative measure of the variability or random error areprovided by short tests for precision and bias.3.3 An overall test of the direct-reading capability is pro-vided by tests of the built-in weights.4. Significance and Use4.1 Monitoring Weighing PerformanceThis practice pro-vides res

24、ults in the form of control charts which measure theweighing capability at the time of the test. A series of tests atappropriate intervals will monitor balance performance over aperiod of time. A marked change from expected performancemay result from a variety of causes including: maladjustment,dama

25、ge, dirt, foreign material, and thermal disturbances. If thetest results are to indicate future performance, any disturbancesthat occur exterior to the balance must be brought under control(2).4.2 Acceptance TestsThis practice may also be used asacceptance tests for new balances. For this purpose, t

26、he testsshould be conducted under favorable, but not necessarily ideal,conditions. Since systematic error in the course of the zero andthe course of the sensitivity may be caused by disturbancesexternal to the balance, limits on these errors are not ordinarilyprescribed in acceptance requirements.5.

27、 Preparation of Apparatus5.1 Balance (In all cases, the balance should be used inaccordance with the manufacturers instructions):5.1.1 The results obtained will depend on the environment.Select an area which is free of excessive vibration and aircurrents, where rapid changes in temperature and relat

28、ivehumidity will not be encountered, and where the floor is rigidenough to be free of a tilting effect on the balance indication.Place the balance on a sturdy bench. If the balance has beenmoved to a new location, permit it to come to thermalequilibrium for at least 1 h before performing the test,pr

29、eferably several hours.5.1.2 Inspect and test the balance to make sure that it is inproper mechanical order. Arrest and release the beam to makesure that readings are approximately repetitive. Observe theindication during arrest and release to ensure that there is no“kick” that would indicate that a

30、rrestment points might be outof adjustment. If necessary, have the balance adjusted by acompetent balance technician.5.1.3 Make a few trial measurements of the interval fromzero to the full-scale indication.5.2 Reading the BalanceThe balance should be read inaccordance with the instructions supplied

31、 by the manufacturer.Optical types should include the reading of verniers or mi-crometres. Null types should include the indication of thedevice for restoring to null including verniers or micrometres.6. Preliminary Testing of Single-Arm Balances6.1 Summary of MethodWith single-pan balances thesmall

32、est built-in weight, indicated by the first step on the dial,is compared with a calibrated weight. The direct-reading scaleis tested for agreement with the smallest built-in weight and thesensitivity is adjusted, if necessary, so that the indications ofthe scale are precise in terms of the calibrate

33、d weight. A“fifty-fifty” test verifies the accuracy of the midpoint athalf-full scale. This test should be performed before proceed-ing to other tests. After the accuracy of adjustment of thesmallest built-in weight is verified, this weight is used to testfull-scale deflections. Tests are also made

34、for the uniformity ofdeflection over the lower and upper halves of the full-scaledeflection. The preliminary tests show either that the balance isoperating properly, or that discrepancies indicate the presenceof sources of error. Uncertainties of perhaps one millionth ofthe balance capacity may be c

35、aused by dirt or foreign materialE 319 85 (2003)2in the bearings, or by unskilled handling, while larger discrep-ancies may be caused by worn or damaged knife-edges or othersources such as electrostatic effects. Any necessary cleaning orservicing should be done at this point. If discrepancies con-ti

36、nue, other possible sources of uncertainty should be studied.There is no point in proceeding with routine test proceduresuntil acceptable results can be obtained with the preliminarytests.NOTE 8With null-type balances (including the hybrid) it is possible touse the flexure of a segment of metal, qua

37、rtz, etc. as the main pivotsinstead of knife edges. A flexure pivot is by its nature free of problems ofdirt. Flexures are also generally more robust than knives. The chiefproblem associated with flexures is that they act like springs and thus adda restoring force which may vary with time or tempera

38、ture. This drawbackcan be minimized by careful design and all but eliminated by the use ofservo-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 bemonitored periodicall

39、y, it may be useful to enter data directlyinto a personal computer which has been programmed for thistask.),6.2.4 A calibrated weight designated S1 which has thenominal value equal to the smallest interval on the dial-operated weights, and6.2.5 Two weights of half of the nominal value of S1designate

40、d (12)1 and (12)2. (These weights need not becalibrated but they should bear distinguishing marks, prefer-ably one, and two dots.)6.3 ProcedureAdjust the “no-load” readings to a pointnear the center of the vernier so that zero drift or otherdeviation will not cause a negative scale reading. Perform

41、thepreliminary tests, loading the pan and changing the dialsettings according to the schedule in Table 1. Before releasingthe beam, record the load on the pan and the dial setting so thatthe observation will be confined to the scale reading. Releasethe balance and observe the scale reading. Record t

42、he indica-tion and verify the stability of the scale reading, then arrest thebalance promptly.6.4 Calculations for Preliminary Tests:6.4.1 Compute D1, the value of the smallest built-in weightas 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, andS1 = calibrated va

43、lue of test weight.6.4.2 Compute S1 in scale divisions to verify the full-scalevalue 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 thebalance sensitivity if necessary so that the full-scale readingequals D1.6.4.3 Compute average s

44、cale difference, A, for lower 50 %of direct-reading scale as follows: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 (see7.5.3).Any discrepancy smaller than 3

45、 standard deviations maybe ascribed to uncertainty in the preliminary measurements anddoes not necessarily indicate a real change in the value of thescale divisions.6.4.5 Inspect the no-load readings, a, f, and k for agreementor zero drift.6.4.6 See Table 2 and Fig. 1 for examples of calculationsand

46、 observation form.7. Instrument Precision (4)7.1 Summary of Method:7.1.1 A set of four readings is repeated four times, or more,to obtain pairs of readings with identical loads:7.1.1.1 A reading near zero,7.1.1.2 A reading near the upper end of the scale,7.1.1.3 A reading near the upper end of the s

47、cale with a testload plus a small weight, and7.1.1.4 A reading near zero with the test load but with thesmall weight removed.7.1.2 Readings are taken at a steady pace as rapidly aspracticable, consistent with good practice, and the time isobserved at the start of each set of observations and at the

48、endof the test.TABLE 1 Schedule for Preliminary Tests of Single-Arm BalancesObservation Time Pan Load Dial Setting Scale Readinga Record the time zero 0 .b S1A1 .c S1A0 .dWait30sS1A0 .e S1A1 .f Record the time 0 0 .g(12)1B0 .h Add (12)2B(12)1+(12)2B0 .iWait30s12)1+(12)2B0 .j Remove (12)1B(12)2B0 .k

49、Record the time 0 0 .AS1 = calibrated weight of nominal value equal to the smallest dial-operated weight.B(12)1and (12)2= weights of nominal value equal to12 S1 (not necessarily calibrated but marked for identification).E 319 85 (2003)37.1.3 The balance indications are plotted on a graph toprovide a visual presentation of errors. The zero readings areconnected to show the course of the zero with time. Theresponse of the balance to the small weight is plotted. Thecourse of the sensitivity with time is represented by a plot ofthe interval