1、Designation: E898 88 (Reapproved 2013)Standard Test Method of TestingTop-Loading, Direct-Reading Laboratory Scales andBalances1This standard is issued under the fixed designation E898; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision
2、, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.INTRODUCTIONThis method is designed to test commonly used laboratory scales that read the entire range of weightup t
3、o the capacity without manual operation. In essence, the entire reading range is on-scale and nomanipulation of weights, riders, or dials is required; except some scales with optical reading devicesmay require the operation of a micrometer dial to interpolate the final one or two significant figures
4、.1. Scope1.1 This test method covers the determination of character-istics of top-loading, direct-reading laboratory scales andbalances. Laboratory scales of the top-loading type may havecapacities from a few grams up to several kilograms. Resolu-tion may be from 1/1000 of capacity to 1/1 000 000 or
5、 more.This method can be used for any of these instruments and willserve to measure the most important characteristics that are ofinterest to the user. The characteristics to be measured includethe following:1.1.1 warm-up,1.1.2 off center errors,1.1.3 repeatability, reproducibility, and precision,1.
6、1.4 accuracy and linearity,1.1.5 hysteresis,1.1.6 settling time,1.1.7 temperature effects,1.1.8 vernier or micrometer calibration, and1.1.9 resistance to external disturbances.1.2 The types of scales that can be tested by this method areof stabilized pan design wherein the sample pan does not tiltou
7、t of a horizontal plane when the sample is placed anywhereon the pan surface. The pan is located generally above themeasuring mechanism with no vertical obstruction, except fordraft shields. Readings of weight may be obtained from anoptical scale, from a digital display, or from a mechanical dial.We
8、ighing mechanisms may be of the deflecting type, usinggravity or a spring as the transducer, or may be a force-balancesystem wherein an electromagnetic, pneumatic, hydraulic, orother force is used to counterbalance the weight of the sample.Other force-measuring devices may be tested by this method a
9、slong as a sample placed on a receiving platform produces anindication that is substantially a linear function of the weight ofthe sample.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to
10、establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Method2.1 Throughout this method, the instrument is used in themanner for which it is intended. One or more weights are usedto test each of the characteristics, an
11、d the results are expressedin terms of the least count or ultimate readability of the display.3. Terminology3.1 Definitions of Terms Specific to This Standard:23.1.1 accuracythe degree of agreement of the measure-ment with the true value of the quantity measured.3.1.2 capacitythe maximum weight load
12、 specified by themanufacturer. In most instruments, the maximum possiblereading will exceed the capacity by a small amount.3.1.3 full-scale calibrationthe indicated reading when astandard weight equal to the full scale indication of the scale isplaced on the sample pan after the device has been corr
13、ectlyzeroed. Usually some means is provided by the manufacturerto adjust the full scale indication to match the weight of thestandard.1This test method is under the jurisdiction of ASTM Committee E41 onLaboratory Apparatus and is the direct responsibility of Subcommittee E41.06 onWeighing Devices.Cu
14、rrent edition approved Dec. 1, 2013. Published December 2013. Originallyapproved in 1982. Last previous edition approved in 2005 as E898 88(2005). DOI:10.1520/E0898-88R13.2ANSI/ISA S51.1 “Process Instrumentation Technology”. Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
15、 4th Floor, NewYork,NY 10036.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.1.4 linearitythe degree to which a graph of weightvalues indicated by a scale vs. the true values of the respectivetest weights approximates a straight lin
16、e. For a quantitativestatement of linearity errors, the concept of terminal-basednon-linearity is recommended, such as, the maximum deviationof the calibration curve (average of the readings at increasingand decreasing test load, respectively) from a straight linedrawn through the upper and lower en
17、dpoints of the calibrationcurve.3.1.5 off-center errorsdifferences in indicated weightwhen a sample weight is shifted to various positions on theweighing area of the sample pan.3.1.6 hysteresisdifference in weight values indicated at agiven test load depending on whether the test load was arrivedat
18、by an increase or a decrease from the previous load on thescale.3.1.7 repeatabilitycloseness of agreement of the indicatedvalues for successive weighings of the same load, underessentially the same conditions, approaching from the samedirection (such as, disregarding hysteresis).3.1.8 reproducibilit
19、ycloseness of agreement of the indi-cated values when weighings of the same load are made overa period of time under essentially the same conditions but notlimited to the same direction of approach (such as, hysteresiserrors are included).3.1.9 precisionthe smallest amount of weight differencebetwee
20、n closely similar loads that a balance is capable ofdetecting. The limiting factor is either the size of the digitalstep of the indicator readout or the repeatability of the indicatedvalues.3.1.10 standard deviationused as a quantitative figure ofmerit when making statements on the repeatability, re
21、produc-ibility or precision of a balance.3.1.11 readabilitythe value of the smallest unit of weightthat can be read without estimation. In the case of digitalinstruments, the readability is the smallest increment of theleast significant digit (for example, 1, 2 or 5). Optical scalesmay have a vernie
22、r or micrometer for subdividing the smallestscale division. In that case, the smallest graduation of thevernier or micrometer represents the readability.3.1.12 standard weightany weight whose mass is given.Since weights are not always available with documentedcorrections, weights defined by class ma
23、y be used if the classchosen has sufficiently small limits and there is an understand-ing that errors perceived as being instrumental in nature couldbe attributed to incorrectly adjusted weights.4. Significance and Use4.1 This method will enable the user to develop informationconcerning the precisio
24、n and accuracy of weighing instru-ments. In addition, results obtained using this method willpermit the most advantageous use of the instrument. Weak-nesses as well as strengths of the instrument should becomeapparent. It is not the intent of this method to compare similarinstruments of different ma
25、nufacture, but to enable the user tochoose a suitable instrument.5. Apparatus5.1 Manufacturers Manual.5.2 Standard WeightsA set of weights up to the capacityof the scale with sufficient subdivisions of weight so thatincrements of about 10 % of the capacity up to the capacity canbe tested.5.3 Thermom
26、eter, room temperature, with a resolution of atleast 1 C.5.4 Stop-Watch, reading to15 s.6. Preparation6.1 Make sure that the scale and weights are clean.6.2 Place the standard weights near the instrument.6.3 Place the thermometer on the bench in such a positionthat it can be read without being touch
27、ed.6.4 Allow the instrument and the weights to sit undisturbedfor at least 2 h with the balance turned off. Monitor thetemperature during this time to make sure that there is no morethan approximately 2C variation over the last hour beforebeginning the test.6.5 Read the manufacturers instructions ca
28、refully. Duringeach step of the test procedure, the instrument should be usedin the manner recommended by the manufacturer. Know thelocation of any switches, dials, or buttons as well as theirfunctions.7. Test Procedure7.1 Warm-up Test:7.1.1 If it is required in the normal operation of the scale tot
29、urn it “on” as an operation separate from weighing, performthat operation simultaneously with the starting of the stop-watch.7.1.2 If a zeroing operation is required, do it promptly.Record the temperature.7.1.3 At the end of 1 min, read and record the indicationwith the pan empty.7.1.4 At the center
30、 of the sample pan place a standardweight nearly equal to but not exceeding 98 % of the capacityof the scale. If the scale allows no weight readings above thestated nominal capacity, then this test should be performedwith standard weights equal to 90 % of capacity. When theindication is steady, reco
31、rd the indication and remove theweight from the pan.7.1.5 At the end of 5 min, repeat steps 7.1.3 and 7.1.4without rezeroing.7.1.6 At the end of 30 min, repeat again.7.1.7 At the end of 1 h, repeat again. Record the tempera-ture.7.1.8 Compute for each measurement as follows:kt5 W/Iw2 Io! (1)where:Iw
32、= indication with the standard weight on the pan,Io= indication with pan empty,W = known or assumed value of the standard weight, andE898 88 (2013)2kt= calibration factor for time t.7.1.9 Plot the values of ktagainst the time (1 min, 5 min, 30min, and 60 min). The time at which ktapparently no longe
33、rdrifts in one direction can be assumed to be the warm-up timerequired.7.1.10 If there is a user-adjustable full-scale calibrationprocedure recommended by the manufacturer, this adjustmentshould be made after the warm-up time determined in 7.1.9.7.1.11 If the calibration cannot be adjusted by the us
34、er, thefactor ktcan be used as a multiplier for an indicated weight tocorrect to true weight.7.1.12 Plot Ioas a function of time to determine the zerodrift. For individual measurements of weight, the zero can bemonitored or corrected prior to a weighing. However, if thechange in weight of a sample a
35、s a function of time is ofimportance, and if the sample cannot be removed for zeroing,it is also important to know the course of the zero as a functionof time.7.2 Off-Center ErrorsThe geometry of the stabilizingmechanism for the sample pan determines whether or not thescale is sensitive to the posit
36、ion of the load on the pan. Thiseffect is measured by placing the load in various positions onthe pan and observing any difference in indication. Place thestandard weight (100 % or 90 % of capacity, as per 7.1.4)in5positions on the pan, noting the indication for each position:center-front-backright-
37、left; or center and corners. The differ-ence between the lowest and the highest indication is themaximum off-center error.7.3 RepeatabilityA computation of the standard deviation() of a series of observations at the same load apapproachedfrom the same direction provides a measure of precision. Theco
38、mputation of 3 will indicate with a high degree of assurancethat any single measurement will fall within that limit of error,providing hysteresis is negligible. A control chart can begenerated by periodically remeasuring the standard deviationand plotting it as a function of time (perhaps by date).A
39、ny timethat the standard deviation falls outside of a pattern of values(control limits) there may be a reason to investigate theinstrument or the measuring technique to determine whetheradjustments may be required.7.4 HystersisBalances do not usually have problems withhysteresis. Nevertheless the te
40、st for hysteresis is simple andshould be performed on newly-acquired balances. Perform thetest as follows:7.4.1 Zero the balance,7.4.2 Place a weight or weights equal to about one-half thebalance capacity on the pan and record the reading once it isstable,7.4.3 Add more weights to the balance pan un
41、til 90 % to100 % of full capacity is reached. Wait for a stable reading,although the actual value need not be recorded.7.4.4 Remove the weights which were added in 7.4.3 andrecord the balance reading once it is stable.7.4.5 Remove the rest of the weights from the balance andrecord the reading as soo
42、n as it is stable. The five operationscan be shown in tabular form:Operation Weight on Pan Balance Reading1 nil 0212 capacity W13 full capacity W2412 capacity W15 nil ZIf the quantity W1 W1 + Z/2 differs significantly fromzero, the difference can be attributed to hysteresis. The test maybe repeated
43、several times and the results averaged to reducemeasurement scatter.7.5 PrecisionTo calculate the balance precision, combinethe uncertainties due to lack of repeatability and to hysteresis.7.6 Accuracy and LinearityThese tests are made togetherbecause they represent the same thing. Since accuracy re
44、pre-sents the proximity to true value, the nonlinearity is a point-by-point measure of accuracy if the zero point and thefull-scale calibration point have been set true. Set the zero andfull-scale indications as described in 7.1.10 if possible. Placeweights on the pan in increasing increments of abo
45、ut 10 % ofthe capacity and observe the indications. Plot the indicatedvalues against the known or assumed value of the weight. Thedifference at any point is the inaccuracy. Keep in mind that theaccuracy cannot be better than the precision and that everyobservation includes an uncertainty of as much
46、as 3 so thatspecifying a higher accuracy may be misleading. However, aprocedure that includes multiple observations at each point andwhich minimizes any hysteresis effects and off-center errorscan improve the precision, and therefore produce an accuracymeasurement which is more significant.7.7 Settl
47、ing TimeThe time for an indication to reach astable value after the application of a load is a measure of howsoon an indication may be read. This time is controlled byseveral factors including the moment of inertia of the system,the degree of damping or, in the case of digital instruments, thetime-c
48、onstant of the digital conversion rate. In addition, somedigital designs may permit a flicker between two or more digitsbecause of hunting in a servo loop. A knowledge of the timerequired may prevent a reading in error. Zero the scale inaccordance with the manufacturers instructions. Place a stan-da
49、rd weight equal to the capacity of the scale on the pansimultaneously starting the stop-watch. Stop the watch when itappears that the indication is steady. Record the elapsed time.Repeat several times to ensure that there is reasonable corre-lation between measurements.7.8 Temperature EffectsThe ambient temperature mayhave an effect on the zero as well as the full-scale calibration.If means are available to test the instrument at varioustemperatures, such a test can be valuable, especially if thelocation in which the instrument is used is subject t
