1、Designation: E74 12Standard Practice ofCalibration of Force-Measuring Instruments for Verifying theForce Indication of Testing Machines1This standard is issued under the fixed designation E74; the number immediately following the designation indicates the year of originaladoption or, in the case of
2、revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 The purpose of this
3、 practice is to specify procedures forthe calibration of force-measuring instruments. Procedures areincluded for the following types of instruments:1.1.1 Elastic force-measuring instruments, and1.1.2 Force-multiplying systems, such as balances and smallplatform scales.NOTE 1Verification by deadweigh
4、t loading is also an acceptablemethod of verifying the force indication of a testing machine. Tolerancesfor weights for this purpose are given in Practices E4; methods forcalibration of the weights are given in NIST Technical Note 577, Methodsof Calibrating Weights for Piston Gages.21.2 The values s
5、tated in SI units are to be regarded as thestandard. Other metric and inch-pound values are regarded asequivalent when required.1.3 This practice is intended for the calibration of staticforce measuring instruments. It is not applicable for dynamicor high speed force calibrations, nor can the result
6、s ofcalibrations performed in accordance with this practice beassumed valid for dynamic or high speed force measurements.1.4 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 establish appro-p
7、riate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE29 Practice for Using Significant Digits in Test Data toDetermine Conformance with Specificatio
8、ns2.2 American National Standard:B46.1 Surface Texture4ELASTIC FORCE-MEASURING INSTRUMENTS3. Terminology3.1 Definitions:3.1.1 elastic force-measuring devicea device or systemconsisting of an elastic member combined with a device forindicating the magnitude (or a quantity proportional to themagnitude
9、) of deformation of the member under an appliedforce.3.1.2 primary force standarda deadweight force applieddirectly without intervening mechanisms such as levers, hy-draulic multipliers, or the like, whose mass has been deter-mined by comparison with reference standards traceable tonational standard
10、s of mass.3.1.3 secondary force standardan instrument ormechanism, the calibration of which has been established bycomparison with primary force standards.3.2 Definitions of Terms Specific to This Standard:3.2.1 calibration equationa mathematical relationship be-tween deflection and force establishe
11、d from the calibration datafor use with the instrument in service, sometimes called thecalibration curve.3.2.2 continuous-reading devicea class of instrumentswhose characteristics permit interpolation of forces betweencalibrated forces.3.2.2.1 DiscussionSuch instruments usually have force-to-deflect
12、ion relationships that can be fitted to polynominalequations. Departures from the fitted curve are reflected in theuncertainty (8.4).3.2.3 deflectionthe difference between the reading of aninstrument under applied force and the reading with no appliedforce.1This practice is under the jurisdiction of
13、ASTM Committee E28 on MechanicalTestingand is the direct responsibility of Subcommittee E28.01 on Calibration ofMechanical Testing Machines and Apparatus.Current edition approved Dec. 1, 2012. Published May 2013. Originallyapproved in 1947. Last previous edition approved in 2006 as E74 06. DOI:10.15
14、20/E0074-12.2Available from National Institute for Standards and Technology, Gaithersburg,MD 20899.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Doc
15、ument Summary page onthe ASTM website.4Available from American National Standards Institute, 25 W. 43rd St., 4thFloor, New York, NY 10036.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States13.2.3.1 DiscussionThis definition applies to ins
16、trumentsthat have electrical outputs as well as those with mechanicaldeflections.3.2.4 loading rangea range of forces within which theuncertainty is less than the limits of error specified for theinstrument application.3.2.5 readinga numerical value indicated on the scale,dial, or digital display of
17、 a force-measuring instrument under agiven force.3.2.6 resolutionthe smallest reading or indication appro-priate to the scale, dial, or display of the force measuringinstrument.3.2.7 specific force devicean alternative class of instru-ments not amenable to the use of a calibration equation.3.2.7.1 D
18、iscussionSuch instruments, usually those inwhich the reading is taken from a dial indicator, are used onlyat the calibrated forces. These instruments are also calledlimited-load devices.3.2.8 uncertaintya statistical estimate of the limits of errorin forces computed from the calibration equation of
19、a force-measuring instrument when the instrument is calibrated inaccordance with this practice.4. Significance and Use4.1 Testing machines that apply and indicate force are ingeneral use in many industries. Practices E4 has been written toprovide a practice for the force verification of these machin
20、es.A necessary element in Practices E4 is the use of deviceswhose force characteristics are known to be traceable tonational standards. Practice E74 describes how these devicesare to be calibrated. The procedures are useful to users oftesting machines, manufacturers and providers of force mea-suring
21、 instruments, calibration laboratories that provide thecalibration of the instruments and the documents oftraceability, and service organizations that use the devices toverify testing machines.5. Reference Standards5.1 Force-measuring instruments used for the verification ofthe force indication syst
22、ems of testing machines may becalibrated by either primary or secondary standards.5.2 Force-measuring instruments used as secondary stan-dards for the calibration of other force-measuring instrumentsshall be calibrated by primary standards. An exception to thisrule is made for instruments having cap
23、acities exceeding therange of available primary standards. Currently the maximumprimary force-standard facility in the United States is1 000 000-lbf (4.4-MN) deadweight calibration machine at theNational Institute of Standards and Technology.6. Requirements for Force Standards6.1 Primary StandardsWe
24、ights used as primary forcestandards shall be made of rolled, forged, or cast metal.Adjustment cavities shall be closed by threaded plugs orsuitable seals. External surfaces of weights shall have a finishof 125 or less as specified in ANSI B46.1.6.1.1 The force exerted by a weight in air is calculat
25、ed asfollows:Force 5Mg9.80665S1 2dDD(1)where:M = mass of the weight,g = local acceleration due to gravity, m/s2,d = air density (approximately 0.0012 Mg/m3),D = density of the weight in the same units as d, and9.80665 = the factor converting SI units of force into thecustomary units of force. For SI
26、 units, this factoris not used.6.1.2 The masses of the weights shall be determined within0.005 % of their values by comparison with reference stan-dards traceable to the national standards of mass. The localvalue of the acceleration due to gravity, calculated within0.0001 m/s2(10 milligals), may be
27、obtained from the NationalGeodotic Information Center, National Oceanic and Atmo-spheric Administration.5NOTE 2If M, the mass of the weight, is in pounds, the force will bein pound-force units (lbf). If M is in kilograms, the force will be inkilogram-force units (kgf). These customary force units ar
28、e related to thenewton (N), the SI unit of force, by the following relationships:1 lbf 5 4.44822N (2)1 kgf 5 9.80665N exact!The Newton is defined as that force which, applied to a 1-kg mass,would produce an acceleration of 1 m/s/s.The pound-force (lbf) is defined as that force which, applied to a 1-
29、lbmass, would produce an acceleration of 9.80665 m/s/s.The kilogram-force (kgf) is defined as that force which, applied to a1-kg mass, would produce an acceleration of 9.80665 m/s/s.6.2 Secondary StandardsSecondary force standards maybe either elastic force-measuring instruments used in conjunc-tion
30、 with a machine or mechanism for applying force, or someform of mechanical or hydraulic mechanism to multiply arelatively small deadweight force. Examples of the latter forminclude single- and multiple-lever systems or systems in whicha force acting on a small piston transmits hydraulic pressure toa
31、 larger piston.6.2.1 Elastic force-measuring instruments used as second-ary standards shall be calibrated by primary standards and usedonly over the Class AA loading range (see 8.5.2.1). Secondarystandards having capacities exceeding 1 000 000 lbf (4.4 MN)are not required to be calibrated by primary
32、 standards. Severalsecondary standards of equal compliance may be combinedand loaded in parallel to meet special needs for highercapacities. The uncertainty (see 8.4) of such a combinationshall be calculated by adding in quadrature using the followingequation:Uc5 = Uo21U121U221.Un2(3)where:Uc= uncer
33、tainty of the combination, and5Available from National Oceanic and Atmospheric Administration (NOAA),14th St. and Constitution Ave., NW, Room 6217, Washington, DC 20230.E74122Uo, 1,2.n= uncertainty of the individual instruments.6.2.2 The multiplying ratio of a force multiplying systemused as a secon
34、dary standard shall be measured at not less thanthree points over its range with an accuracy of 0.05 % of ratioor better. Some systems may show a systematic change in ratiowith increasing force. In such cases the ratio at intermediatepoints may be obtained by linear interpolation between mea-sured v
35、alues. Deadweights used with multiplying-type second-ary standards shall meet the requirements of 6.1 and 6.1.2. Theforce exerted on the system shall be calculated from therelationships given in 6.1.1. The force multiplying system shallbe checked annually by elastic force measuring instrumentsused w
36、ithin their class AA loading ranges to ascertain whetherthe forces applied by the system are within acceptable rangesas defined by this standard. Changes exceeding 0.05 % ofapplied force shall be cause for reverification of the forcemultiplying system.7. Calibration7.1 Basic PrinciplesThe relationsh
37、ip between the appliedforce and the deflection of an elastic force-measuring instru-ment is, in general, not linear. As force is applied, the shape ofthe elastic element changes, progressively altering its resis-tance to deformation. The result is that the slope of theforce-deflection curve changes
38、gradually and uniformly overthe entire range of the instrument. This characteristic full-scalenonlinearity is a stable property of the instrument that ischanged only by a severe overload or other similar cause.7.1.1 Localized NonlinearitiesSuperposed on this curveare localized nonlinearities introdu
39、ced by the imperfections inthe force indicating system of the instrument. Examples ofimperfections include: non-uniform scale or dial graduations,irregular wear between the contacting surfaces of the vibratingreed and button in a proving ring, and voltage and sensinginstabilities in a load cell syst
40、em. Some of these imperfectionsare less stable than the full-scale nonlinearity and may changesignificantly from one calibration to another.7.1.2 Curve FittingIn the treatment of the calibration data,a second degree polynomial fitted to the observed data usingthe method of least squares has been fou
41、nd to predict within thelimit of the uncertainty (8.4) deflection values for applied forcethroughout the loading range of the elastic force measuringinstrument. Such an equation compensates effectively for thefull-scale nonlinearity, allowing the localized nonlinearities toappear as deviations. A st
42、atistical estimate, called theuncertainty, is made of the width of the band of thesedeviations about the basic curve. The uncertainty is, therefore,an estimate of the limits of error contributed by the instrumentwhen forces measured in service are calculated by means of thecalibration equation. Actu
43、al errors in service are likely to begreater if forces are applied under loading and environmentalconditions differing from those of the calibration.7.1.3 Curve Fitting for High Resolution DevicesThe useof calibration equations of the 3rd, 4th, or 5th degree isrestricted to devices having a resoluti
44、on of 1 increment ofcount per 50000 or greater active counts at the maximumcalibration load. Annex A1 recommends a procedure forobtaining the degree of the best fit calibration curve for thesedevices.NOTE 3Experimental work by several force calibration laboratories infitting higher than second degre
45、e polynomials to the observed dataindicates that, for some devices, use of a higher degree equation mayresult in a lower uncertainty than that derived from the second degree fit.(ASTM RR:E28-1009) Overfitting should be avoided. Equations ofgreater than 5th degree cannot be justified due to the limit
46、ed number offorce increments in the calibration protocol. Errors caused by round-offmay occur if calculations are performed with insufficient precision.A force measuring device not subjected to repair, overloading,modifications, or other significant influence factors which alter its elasticpropertie
47、s or its sensing characteristics will likely exhibit the same degreeof best fit on each succeeding calibration as was determined during itsinitial calibration using this procedure. A device not subjected to theinfluence factors outlined above which exhibits continued change ofdegree of best fit with
48、 several successive calibrations may not havesufficient performance stability to allow application of the curve fittingprocedure of Annex A1.7.2 Selection of Calibration Forces A careful selection ofthe different forces to be applied in a calibration is essential toprovide an adequate and unbiased s
49、ample of the full range ofthe deviations discussed in 7.1 and 7.1.1. For this reason, theselection of the calibration forces is made by the standardizinglaboratory.An exception to this, and to the recommendations of7.2.1 and 7.2.4, is made for specific force devices, where theselection of the forces is dictated by the needs of the user.7.2.1 Distribution of Calibration Forces Distribute thecalibration forces over the full range of the instrument,providing, if possible, at least one calibration force for every10 % interval throughout the range. It is not nec
