ASTM E74-18e1

Standard Practices for Calibration and Verification for Force-Measuring Instruments

SIGNIFICANCE AND USE
4.1 Testing machines that apply and indicate force are in general use in many industries. Practices E4 has been written to provide a practice for the force verification of these machines. A necessary element in Practices E4 is the use of force-measuring instruments whose force characteristics are known to be traceable to the SI. Practices E74 describes how these force-measuring instruments are to be calibrated. The procedures are useful to users of testing machines, manufacturers and providers of force-measuring instruments, calibration laboratories that provide the calibration of the instruments and the documents of traceability, service organizations that use the force-measuring instruments to verify testing machines, and testing laboratories performing general structural test measurements.
SCOPE
1.1 The purpose of these practices is to specify procedures for the calibration of force-measuring instruments. Procedures are included for the following types of instruments:  
1.1.1 Elastic force-measuring instruments, and  
1.1.2 Force-multiplying systems, such as balances and small platform scales.  
Note 1: Verification by deadweight loading is also an acceptable method of verifying the force indication of a testing machine. Tolerances for weights for this purpose are given in Practices E4; methods for calibration of the weights are given in NIST Technical Note 577(1)2, Methods of Calibrating Weights for Piston Gages.  
1.2 The values stated in SI units are to be regarded as the standard. Other metric and inch-pound values are regarded as equivalent when required.  
1.3 These practices are intended for the calibration of static force measuring instruments. It is not applicable for dynamic or high speed force calibrations, nor can the results of calibrations performed in accordance with these practices be assumed valid for dynamic or high speed force measurements.  
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Jan-2018
• ICS: 17.100 - Measurement of force, weight and pressure
• Technical Committee: E28 - Mechanical Testing
• Drafting Committee: E28.01 - Calibration of Mechanical Testing Machines and Apparatus

Relations

• Refers: ASTM E4-14 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2014
• Refers: ASTM E1012-12e1 - Standard Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• Effective Date: 01-Jun-2012
• Refers: ASTM E1012-12 - Standard Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• Effective Date: 01-Jun-2012
• Refers: ASTM E4-10 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2010
• Refers: ASTM E4-09a - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Nov-2009
• Refers: ASTM E4-09 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Apr-2009
• Refers: ASTM E4-08 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Dec-2008
• Refers: ASTM E29-08 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Oct-2008
• Refers: ASTM E4-07 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jan-2007
• Refers: ASTM E29-06b - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Nov-2006
• Refers: ASTM E29-06a - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Sep-2006
• Refers: ASTM E29-06 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-May-2006
• Refers: ASTM E1012-05 - Standard Practice for Verification of Test Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• Effective Date: 01-Jun-2005
• Refers: ASTM E29-04 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Dec-2004
• Refers: ASTM E4-03 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 10-Aug-2003

Overview

ASTM E74-18e1: Standard Practices for Calibration and Verification for Force-Measuring Instruments specifies standardized procedures for the calibration of devices that measure force. These practices ensure that force-measuring instruments provide accurate and traceable force measurements, critical for the verification of testing machines across various industries. The standard is developed by ASTM International and adheres to globally recognized principles of standardization as outlined by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

Key Topics

• Calibration Procedures: Outlines methods for calibrating elastic force-measuring instruments and force-multiplying systems (such as balances and platform scales), with reference to SI traceability.
• Force Standards: Differentiates between primary force standards (deadweights) and secondary force standards (instruments calibrated against primary standards), with clear criteria for their application and calibration.
• Force Verification: Includes essential practices for ensuring the accuracy of force-indicating systems, referencing ASTM E4 for force verification of testing machines.
• Measurement Uncertainty: Introduces statistical concepts such as the lower limit factor (LLF), which represents the estimate of measurement uncertainty contributed by an instrument during calibration.
• Instrumentation Terminology: Defines terms relevant to force measurement such as deflection, resolution, verified range of forces, and creep recovery, standardizing communication among users and calibration labs.
• Safety and Compliance: Notes the responsibilities of users to address safety, health, and environmental practices and to ensure compliance with applicable regulations.

Applications

The ASTM E74-18e1 standard is essential for organizations and professionals involved in:

• Material Testing Laboratories: Ensures test results are reliable by using properly calibrated force-measuring equipment.
• Calibration Laboratories: Provides a framework for issuing internationally traceable calibration certificates for force-measuring instruments.
• Manufacturers and Suppliers: Guarantees their force-measuring products meet industry and international standards for accuracy and traceability.
• Quality Assurance in Industrial Settings: Supports compliance by maintaining measurement systems traceable to the International System of Units (SI).
• Verification of Testing Machines: Used by service organizations to validate the performance of universal testing machines and other force-application devices in the field.

Typical force-measuring instruments falling under this standard’s scope include:

• Load cells and force transducers
• Proving rings and other elastic instruments
• Mechanical and digital platform scales
• Force-multiplying hydraulic or mechanical systems

Related Standards

ASTM E74-18e1 is closely linked with several other standards and technical documents, complementing a comprehensive approach to mechanical testing and calibration:

• ASTM E4: Practices for Force Verification of Testing Machines
• ASTM E29: Practice for Using Significant Digits in Test Data
• ASTM E1012: Practice for Verification of Testing Frame and Specimen Alignment Under Tensile and Compressive Axial Force Application
• ASME B46.1: Standard on Surface Texture, Roughness, Waviness, and Lay
• NIST Technical Note 577: Methods of Calibrating Weights for Piston Gages

Practical Value

Implementing ASTM E74-18e1 helps organizations:

• Achieve test result consistency and compliance with international quality systems
• Reduce measurement uncertainty through traceable calibrations
• Enhance customer confidence and meet audit requirements for traceability and documentation
• Optimize the performance of testing machines by ensuring accurate force application and measurement
• Safeguard product integrity by verifying equipment used in critical quality testing

Keywords: ASTM E74, force-measuring instrument calibration, force verification, load cell calibration, traceability to SI, mechanical testing standards, primary force standard, calibration laboratory, measurement uncertainty, compliance, international standard.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
ϵ1
Designation: E74 −18
Standard Practices for
Calibration and Verification for Force-Measuring
Instruments
ThisstandardisissuedunderthefixeddesignationE74;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
ε NOTE—Editorial corrections were made to 3.2.4.1 and 3.2.9.1 in April 2019.
1. Scope 2. Referenced Documents
1.1 The purpose of these practices is to specify procedures
2.1 ASTM Standards:
for the calibration of force-measuring instruments. Procedures E4Practices for Force Verification of Testing Machines
are included for the following types of instruments:
E29Practice for Using Significant Digits in Test Data to
1.1.1 Elastic force-measuring instruments, and Determine Conformance with Specifications
1.1.2 Force-multiplyingsystems,suchasbalancesandsmall
E1012Practice for Verification of Testing Frame and Speci-
platform scales. men Alignment Under Tensile and Compressive Axial
Force Application
NOTE 1—Verification by deadweight loading is also an acceptable
method of verifying the force indication of a testing machine. Tolerances
2.2 ASME Standard:
for weights for this purpose are given in Practices E4; methods for
B46.1Surface Texture, Surface Roughness, Waviness and
calibration of the weights are given in NIST Technical Note 577(1) ,
Lay
Methods of Calibrating Weights for Piston Gages.
1.2 The values stated in SI units are to be regarded as the FORCE-MEASURING INSTRUMENTS
standard. Other metric and inch-pound values are regarded as
equivalent when required.
3. Terminology
1.3 These practices are intended for the calibration of static
3.1 Definitions:
force-measuring instruments. It is not applicable for dynamic
3.1.1 force-measuring instrument—a system consisting of
or high speed force calibrations, nor can the results of
anelasticmembercombinedwithanappropriateinstrumentfor
calibrations performed in accordance with these practices be
indicating the magnitude (or a quantity proportional to the
assumed valid for dynamic or high speed force measurements.
magnitude) of deformation of the member under an applied
force.
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3.1.2 primary force standard—a deadweight force applied
responsibility of the user of this standard to establish appro-
directly without intervening mechanisms such as levers, hy-
priate safety, health, and environmental practices and deter-
draulic multipliers, or the like, whose mass has been deter-
mine the applicability of regulatory limitations prior to use.
mined by comparison with reference standards traceable to the
1.5 This international standard was developed in accor-
International System of Units (SI) (2) of mass.
dance with internationally recognized principles on standard-
3.1.3 secondary force standard—an instrument or
ization established in the Decision on Principles for the
mechanism, the calibration of which has been established by
Development of International Standards, Guides and Recom-
comparison with primary force standards.
mendations issued by the World Trade Organization Technical
3.2 Definitions of Terms Specific to This Standard:
Barriers to Trade (TBT) Committee.
These practices are under the jurisdiction of ASTM Committee E28 on
Mechanical Testing and is the direct responsibility of Subcommittee E28.01 on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Calibration of Mechanical Testing Machines and Apparatus. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Current edition approved Feb. 1, 2018. Published April 2018. Originally Standards volume information, refer to the standard’s Document Summary page on
approved in 1947. Last previous edition approved in 2013 as E74–13a. DOI: the ASTM website.
10.1520/E0074-18E01. Available from American Society of Mechanical Engineers (ASME), ASME
The boldface numbers in parentheses refer to a list of references at the end of International Headquarters, Two Park Ave., New York, NY 10016-5990, http://
this standard. www.asme.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
ϵ1
E74−18
3.2.1 calibration equation—a mathematical relationship be- which the reading is taken from a dial indicator, are used only
tweendeflectionandforceestablishedfromthecalibrationdata at the calibrated forces. These instruments are also called
for use with the instrument in service, sometimes called the limited force-measuring instruments.
calibration curve.
3.2.10 lower limit factor, LLF—a statistical estimate of the
error in forces computed from the calibration equation of a
3.2.2 continuous-reading instument—a class of instruments
whose characteristics permit interpolation of forces between force–measuring instrument when the instrument is calibrated
in accordance with these practices.
calibrated forces.
3.2.2.1 Discussion—Such instruments usually have force-
3.2.10.1 Discussion—The lower limit factor was termed
to-deflection relationships that can be fitted to polynominal
“Uncertainty” in previous editions of E74. The lower limit
equations.
factorisusedtocalculatethelowerendoftheverifiedrangeof
3.2.3 creep—The change in deflection of the force-
forces, see 8.5. Other factors evaluated in establishing the
measuring instrument under constant applied force.
lower limit of the verified range of forces are the resolution of
3.2.3.1 Discussion—Creep is expressed as a percentage of the instrument and the lowest non-zero force applied in the
the output change at a constant applied force from an initial calibration force sequence, The lower limit factor is one
time following the achievement of mechanical and electrical
component of the measurement uncertainty. Other uncertainty
stabilityandthetimeatwhichthetestisconcluded.Validcreep components should be included in a comprehensive measure-
testsmayrequiretheuseofprimaryforcestandardstomaintain
mentuncertaintyanalysis.SeeAppendixX1foranexampleof
adequate stability of the applied force during the test time measurement uncertainty analysis.
interval. Creep results from a time dependent, elastic deforma-
4. Significance and Use
tionoftheinstrumentmechanicalelement.Inthecaseofstrain
gage based force-measuring instruments, creep is adjusted by 4.1 Testing machines that apply and indicate force are in
generaluseinmanyindustries.PracticesE4hasbeenwrittento
strain gage design and process modifications to reduce the
strain gage response to the inherent time-dependent elastic provide a practice for the force verification of these machines.
A necessary element in Practices E4 is the use of force-
deflection.
measuring instruments whose force characteristics are known
3.2.4 creep recovery—Thechangeindeflectionoftheforce-
to be traceable to the SI. Practices E74 describes how these
measuring instrument after the removal of force following a
force-measuring instruments are to be calibrated. The proce-
creep test.
dures are useful to users of testing machines, manufacturers
3.2.4.1 Discussion—Creep recovery is expressed as a per-
andprovidersofforce-measuringinstruments,calibrationlabo-
centagedifferenceoftheoutputchangeatzeroforcefollowing
ratories that provide the calibration of the instruments and the
a creep test and the initial zero force output at the initiation of
documents of traceability, service organizations that use the
the creep test divided by the output during the creep test. The
force-measuring instruments to verify testing machines, and
zero force measurement is taken at a time following the
testinglaboratoriesperforminggeneralstructuraltestmeasure-
achievement of mechanical and electrical stability and a time
ments.
equal to the creep test time. For many force-measuring
instruments, the creep characteristic and the creep recovery
5. Reference Standards
characteristic are approximate mirror images.
5.1 Force-measuringinstrumentsusedfortheverificationof
3.2.5 deflection—the difference between the reading of an
the force indication systems of testing machines may be
instrumentunderappliedforceandthereadingwithnoapplied
calibrated by either primary or secondary force standards.
force.
5.2 Force-measuring instruments used as secondary force
3.2.5.1 Discussion—This definition applies to instruments
standards for the calibration of other force-measuring instru-
that have electrical outputs as well as those with mechanical
ments shall be calibrated by primary force standards. An
deflections.
exceptiontothisruleismadeforinstrumentshavingcapacities
3.2.6 verified range of forces—in the case of force-
exceeding the range of available primary force standards.
measuring instruments, the range of indicated forces for which
Currently the maximum primary force-standard facility in the
the force-measuring instrument gives results within the per-
United States is 1000000-lbf (4.4-MN) deadweight calibra-
missible variations specified.
tion machine at the National Institute of Standards and Tech-
nology.
3.2.7 reading—a numerical value indicated on the scale,
dial,ordigitaldisplayofaforce-measuringinstrumentundera
6. Requirements for Force Standards
given force.
6.1 Primary Force Standards—Weights used as primary
3.2.8 resolution—the smallest reading or indication appro-
force standards shall be made of rolled, forged, or cast metal.
priate to the scale, dial, or display of the force-measuring
Adjustment cavities shall be closed by threaded plugs or
instrument.
suitable seals. External surfaces of weights shall have a finish
3.2.9 specific force-measuring instrument—an alternative
(Roughness Average or R ) of 3.2µm (125µin.) or less as
a
class of instruments not amenable to the use of a calibration
specified in ASME B46.1.
equation.
6.1.1 The force exerted by a weight in air is calculated as
3.2.9.1 Discussion—Such instruments, usually those in follows:
ϵ1
E74−18
Mg d 6.2.2 The multiplying ratio of a force-multiplying system
Force 5 1 2 (1)
S D
9.80665 D usedasasecondaryforcestandardshallbemeasuredatnotless
than three points over its range with an accuracy of 0.05% of
where:
ratioorbetter.Somesystemsmayshowasystematicchangein
M = mass of the weight,
ratio with increasing force. In such cases the ratio at interme-
g = local acceleration due to gravity, m/s ,
diate points may be obtained by linear interpolation between
d = air density (approximately 0.0012 Mg/m ),
measured values. Deadweights used with multiplying-type
D = density of the weight in the same units as d, and
secondary force standards shall meet the requirements of 6.1
and 6.1.2. The force exerted on the system shall be calculated
9.80665 = the factor converting SI units of force into the
from the relationships given in 6.1.1. The force-multiplying
customary units of force. For SI units, this factor
system shall be checked annually by elastic force-measuring
is not used.
instruments used within their classAAverified range of forces
6.1.2 The masses of the weights shall be determined within
toascertainwhethertheforcesappliedbythesystemarewithin
0.005% of their values by comparison with reference stan-
acceptablerangesasdefinedbythisstandard.Changesexceed-
dardstraceabletotheInternationalSystemofUnits(SI) (2)for
ing 0.05% of applied force shall be cause for reverification of
mass. The local value of the acceleration due to gravity,
the force multiplying system.
calculated within 0.0001 m/s (10 milligals), may be obtained
from the National Geodetic Information Center, National
7. Calibration
Oceanic and Atmospheric Administration.
7.1 Basic Principles—The relationship between the applied
NOTE 2—If M, the mass of the weight, is in pounds, the force will be
force and the deflection of an elastic force-measuring instru-
in pound-force units (lbf). If M is in kilograms, the force will be in
ment is, in general, not linear.As force is applied, the shape of
kilogram-force units (kgf). These customary force units are related to the
the elastic element changes, progressively altering its resis-
newton (N), the SI unit of force, by the following relationships:
tance to deformation. The result is that the slope of the
1kgf 59.80665N exact (2)
~ !
force-deflection curve changes gradually and continuously
over the entire range of the instrument. This characteristic
1 lbf 54.44822N
curveisastablepropertyoftheinstrumentthatischangedonly
The Newton is defined as that force which, applied to a 1-kg mass,
would produce an acceleration of 1 m/s/s.
by a severe overload or other similar cause.
The pound-force (lbf) is defined as that force which, applied to a 1-lb
7.1.1 Superposed on this curve are local variations of
mass, would produce an acceleration of 9.80665 m/s/s.
instrument readings introduced by imperfections in the force-
The kilogram-force (kgf) is defined as that force which, applied to a
indicatingsystemoftheinstrument.Examplesofimperfections
1-kg mass, would produce an acceleration of 9.80665 m/s/s.
include: non-uniform scale or dial graduations, irregular wear
6.2 Secondary Force Standards—Secondaryforcestandards
between the contacting surfaces of the vibrating reed and
maybeeitherforce-measuringinstrumentsusedinconjunction
button in a proving ring, and instabilities in excitation voltage,
withamachineormechanismforapplyingforce,orsomeform
voltagemeasurement,orratio-metricvoltagemeasurementina
of mechanical or hydraulic mechanism to multiply a relatively
load cell system. Some of these imperfections are less stable
small deadweight force. Examples of the latter form include
thanthecharacteristiccurveandmaychangesignificantlyfrom
single- and multiple-lever systems or systems in which a force
one calibration to another.
actingonasmallpistontransmitshydraulicpressuretoalarger
7.1.2 Curve Fitting—To determine the force-deflection
piston.
curve of the force-measuring instrument, known forces are
6.2.1 Force-measuring instruments used as secondary force
applied and the resulting deflections are measured throughout
standards shall be calibrated by primary force standards and
the range of the force-measuring instrument. A polynomial
used only over the Class AA verified range of forces (see
equation is fitted to the calibration data by the least squares
8.6.3.1). Secondary force standards having capacities exceed-
method to predict deflection values throughout the verified
ing1000000lbf(4.4MN)arenotrequiredtobecalibratedby
range of force. Such an equation compensates effectively for
primary force standards. Several secondary force standards of
the nonlinearity of the calibration curve. The standard devia-
equal compliance may be combined and loaded in parallel to
tion determined from the difference of each measured deflec-
meetspecialneedsforhighercapacities.Thelowerlimitfactor
tionvaluefromthevaluederivedfromthepolynomialcurveat
(see 8.5) of such a combination shall be calculated by adding
that force provides a measure of the error of the data to the
in quadrature using the following equation:
curve fit equation.Astatistical estimate, called the lower limit
2 2 2 2
LLF 5 LLF 1LLF 1LLF 1. LLF (3)
=
c 0 1 2 n factor, LLF, is derived from the calculated standard deviation
and represents the width of the band of these deviations about
where:
the basic curve with a probability of approximately 99%. The
LLF = lower limit factor of the combination, and
c
LLF is, therefore, an estimate of one source of measurement
LLF = lower limit factor of the individual instru-
0,1,2. n
uncertainty contributed by the force-measuring instrument
ments.
whenforcesmeasuredinservicearecalculatedbymeansofthe
calibration equation. Actual measurement uncertainty in ser-
vice is likely to be different if forces are applied under
Available from National Oceanic and Atmospheric Administration (NOAA),
14th St. and Constitution Ave., NW, Room 6217, Washington, DC 20230. mechanical and environmental conditions differing from those
ϵ1
E74−18
of calibration. Other sources of measurement uncertainty such 400 3resolutionforClassA verifiedrangeofforces (4)
asthoselistedinAppendixX1couldincreasethemeasurement
2000 3resolutionforClassAA verifiedrangeofforces
uncertainty of measurement of the force-measuring instrument
in service. While it is the responsibility of the calibration
An example of a situation to be avoided is the calibration at
laboratory to calibrate the force-measuring instrument in ac-
ten equally spaced force increments of a proving ring having a
cordance with the requirements of these practices, it is the
capacity deflection of 2000 divisions, where the program will
responsibility of the user to determine the measurement uncer-
failtosamplethewearpatternatthecontactingsurfacesofthe
tainty of the instrument in service.
micrometer screw tip and vibrating reed because the orienta-
nd
7.1.3 Curve Fitting using polynomials of greater than 2
tionofthetwosurfaceswillbenearlythesameatalltenforces
degree—Theuseofcalibrationequationsofthe3rd,4th,or5th
asatzeroforce.Inforce-measuringinstrumentscellcalibration
degree is restricted to force-measuring instruments having a
with electrical instruments capable of linearizing the output
resolution of 1 increment of count per 50000 or greater active
signal, whenever possible, select calibration forces other than
counts at the maximum calibration force. Annex A1 specifies
those at which the linearity corrections were made.
theprocedureforobtainingthedegreeofthebestfitcalibration
7.2.2 The resolution of an analog type force-measuring
curve for these force-measuring instruments. Equations of
instrument is determined by the ratio between the width of the
greater than 5th degree shall not be used.
pointer or index and the center to center distance between two
NOTE3—Experimentalworkbyseveralforcecalibrationlaboratoriesin
adjacent scale graduation marks. Recommended ratios are ⁄2,
fitting higher than second degree polynomials to the observed data
1 1
⁄5,or ⁄10 . A center to center graduation spacing of at least
indicates that, for some force-measuring instruments, use of a higher
1.25mm is required for the estimation of ⁄10 of a scale
degree equation can result in a lower LLF than that derived from the
second degree fit. (ASTM RR:E28-1009) Overfitting should be avoided.
division. To express the resolution in force units, multiply the
Equations of greater than 5th degree cannot be justified due to the limited
ratio by the number of force units per scale graduation. A
number of force increments in the calibration protocol. Errors caused by
vernierscaleofdimensionsappropriatetotheanalogscalemay
round-off can occur if calculations are performed with insufficient
be used to allow direct fractional reading of the least main
precision.
A force-measuring instrument not subjected to repair, overloading,
instrument scale division. The vernier scale may allow a main
modifications, or other significant influence factors which alter its elastic
scale division to be read to a ratio smaller than that obtained
propertiesoritssensingcharacteristicswilllikelyexhibitthesamedegree
without its use.
of best fit on each succeeding calibration as was determined during its
initial calibration using this procedure.Aforce-measuring instrument not 7.2.3 The resolution of a digital instrument is considered to
subjectedtotheinfluencefactorsoutlinedabovewhichexhibitscontinued
be one increment of the last active number on the numerical
change of degree of best fit with several successive calibrations may not
indicator, provided that the reading does not fluctuate by more
have sufficient performance stability to allow application of the curve
than plus or minus one increment when no force is applied to
fitting procedure of Annex A1.
the instrument. If the readings fluctuate by more than plus or
7.2 Selection of Calibration Forces—Acareful selection of
minus one increment, the resolution will be equal to half the
the different forces to be applied in a calibration is essential to
range of fluctuation.
provide an adequate and unbiased sample of the full range of
7.2.4 Number of Calibration Forces—A total of at least 30
the deviations discussed in 7.1 and 7.1.1. For this reason, the
force applications is required for a calibration and, of these, at
selection of the calibration forces shall be made by the
least 10 must be at different forces. Apply each force at least
calibration laboratory.An exception to this, and to the recom-
twice during the calibration.
mendations of 7.2.1 and 7.2.4, is made for specific force-
7.2.5 Specific Force-Measuring Instruments (Limited
measuring instruments, where the selection of the forces is
Force-Measuring Instruments)—Because these force-
dictated by the needs of the user.
measuring instruments are used only at the calibrated forces,
7.2.1 Distribution of Calibration Forces—Distribute the
calibration forces over the full range of the force-measuring select those forces which would be most useful in the service
function of the instrument. Coordinate the selection of the
instrument,providing,ifpossible,atleastonecalibrationforce
for every 10% interval throughout the range. It is not calibrationforceswiththesubmittingorganization.Applyeach
calibration force at least three times in order to provide
necessary, however that these forces be equally spaced. Cali-
brationforcesatlessthanonetenthofcapacityarepermissible sufficient data for the calculation of the standard deviation of
the observed deflections about their average values.
andtendtogiveaddedassurancetothefittingofthecalibration
equation.Ifthelowerforcelimitoftheverifiedrangeofforces
7.3 Temperature Equalization During Calibration:
of the force-measuring instrument (see 8.6.1) is anticipated to
7.3.1 Allow the force-measuring instrument sufficient time
be less than one tenth of the maximum force applied during
to adjust to the ambient temperature in the calibration machine
calibration,thenforcesshouldbeappliedatorbelowthislower
prior to calibration in order to ensure stable instrument
force limit. In no case should the smallest force applied be
response.
below the lower force limit of the force-measuring instrument
7.3.2 The recommended value for room temperature cali-
as defined by the values:
brationsis23°C(73.4°F)butothertemperaturesmaybeused.
7.3.3 During calibration, monitor and record the tempera-
Supporting data have been filed atASTM International Headquarters and may
ture as close to the force-measuring instrument as possible. It
beobtainedbyrequestingResearchReportRR:E28-1009.ContactASTMCustomer
Service at [e-mail]. isrecommendedthatthetesttemperaturenotchangemorethan
ϵ1
E74−18
60.5 °C (1 °F) during calibration. In no case shall the ambient considered in making this decision. It is pointed out, however,
temperature change by more than 61.0 °C (2 °F) during that a lengthy series of incremental forces applied without
calibration. return to zero reduces the amount of sampling of instrument
performance. The operation of removing all force from the
7.3.4 Deflections of non-temperature compensated force-
instrument permits small readjustments at the load contacting
measuring instruments may be normalized in accordance with
surfaces, increasing the amount of random sampling and thus
Section 9 to a temperature other than that existing during
producing a better appraisal of the performance of the instru-
calibration.
ment. It is recommended that not more than five incremental
7.3.5 Deflections of non-temperature compensated force-
forces be applied without return to zero. This is not necessary
measuring instruments shall be corrected in accordance with
when the instrument is calibrated with decreasing forces;
Section 9 to a nominal calibration temperature if the tempera-
however, any return to zero prior to application of all the
ture changes more than 60.2 °C during calibration.
individualforceincrementsmustbefollowedbyapplicationof
7.4 Procedural Order in Calibration—Immediately before
the maximum force before continuing the sequence.
starting the calibration, slowly and smoothly apply the maxi-
7.5 Randomization of Force Application Conditions—
mum force in the calibration sequence to the force-measuring
During the calibration sequence, maintain the force measure-
instrument at least two times. This procedure is referred to as
ment axis of the force-measuring instrument coincident with
exercising the force-measuring instrument. Exercising is nec-
the force axis of the machine. Shift the position of the
essary to reestablish the hysteresis pattern that tends to
force-measuring instrument in the calibration machine before
disappear during periods of disuse, and is particularly neces-
repeatinganyseriesofforces.Introducevariationsinanyother
sary following a change in the mode of force application, as
factors that normally are encountered in service, as for
from compression to tension. Some force-measuring instru-
example, disconnecting and reconnecting electrical cables.
ments may require more than two exercise cycles to achieve
Allow sufficient warm up time if electrical disconnections are
stability in zero-force indication.
made.
NOTE 4—Overload or proof load tests are not required by these
7.5.1 In a compression calibration, position the force-
practices. It must be emphasized that an essential part of the manufactur-
measuringinstrumenttoa0degreereferenceposition,andthen
ing process for a force-measuring instrument is the application of a series
rotate to positions of approximately 120 degrees and 240
ofoverloadstoatleast10%inexcessofratedcapacity.Thismustbedone
degrees.Anexceptionismadeforforce-measuringinstruments
by the manufacturer before the instrument is released for calibration or
service.
that cannot be rotated by 120 degrees such as some proving
rings, force dynamometers, and Brinell HardnessTest Calibra-
7.4.1 After exercising, apply the calibration forces, ap-
tors. For these types of force-measuring instruments, position
proaching each force from a lesser force. Forces shall be
the force-measuring instrument at 0 degrees, and then rotate to
applied and removed slowly and smoothly, without inducing
positions of approximately 60 degrees and 300 degrees, keep-
shockorvibrationtotheforce-measuringinstrument.Thetime
ing its force axis on the center force axis of the machine. This
intervalbetweensuccessiveapplicationsorremovalsofforces,
exception is made to minimize parallax error.
andinobtainingreadingsfromtheforce-measuringinstrument,
7.5.2 In a tension calibration, position the force-measuring
shall be as uniform as possible. If a calibration force is to be
instrument to a 0 degree reference position, and then rotate to
followed by another calibration force of lesser magnitude,
positions of approximately 120 degrees and 240 degrees. An
reduce the applied force on the force-measuring instrument to
exception is made for force-measuring instruments that cannot
zero before applying the second calibration force. Whenever
berotatedby120degreessuchassomeprovingringsandforce
possible, plan the force application schedule so that repetitions
dynamometers. For these types of force-measuring
of the same calibration force do not follow in immediate
instruments, position the force-measuring instrument at 0
succession. For any force-measuring instrument, the errors
degrees, and then rotate to positions of approximately 60
observed at corresponding forces taken first by increasing the
degrees and 300 degrees, keeping its force axis on the center
force to any given test force and then by decreasing the force
force axis of the machine. Shift and realign any flexible
to that test force may not agree. Force-measuring instruments
connectors between positions. This exception is made to
are usually used under increasing forces, but if a force-
minimize parallax error.
measuring instrument is to be used under decreasing force, it
7.5.3 Inatwo-modecalibration(compressionandtension),
shall be calibrated under decreasing forces as well as under
perform a part of the calibration in one mode, switch modes
increasing force. Use the procedures for calibration and analy-
and continue the calibration, then finish the calibration in the
sis of data given in Sections 7 and 8 except where otherwise
initial calibration mode. It is acceptable practice to change
noted. When a force-measuring instrument is calibrated with
modes at each rotational position
both increasing and decreasing forces, the same force values
should be applied for the increasing and decreasing directions
NOTE 5—Force-measuring instruments have sensitivity in varying
of force application, but separate calibration equations should
degrees depending on design to mounting conditions and parasitic forces
be developed. and moments due to misalignment. A measure of this sensitivity may be
made by imposing conditions to simulate these factors such as using
7.4.2 The calibration laboratory shall decide whether or not
fixtures with contact surfaces that are slightly convex or concave, or of
a zero force reading is to be taken after each calibration force.
varying stiffness or hardness, or with angular or eccentric misalignment,
Factors such as the stability of the zero force reading and the
and so forth. Such factors can sometimes be significant contributors to
presence of noticeable creep under applied force are to be measurement uncertainty and should be reflected in comprehensive
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measurment uncertainty analyses.
8.2.1 Exercise the force-measuring instrument to the maxi-
mum applied force in calibration at least two times.Allow the
8. Calculation and Analysis of Data
zero reading to stabilize and record the value. Apply the
maximum applied force used in calibration of the force-
8.1 Deflection—Calculate the deflection values for the
measuring instrument and hold as constant as possible for 5
force-measuring instrument as the differences between the
minutes. Remove the applied force smoothly, but as quickly as
readings of an instrument under applied force and the reading
possible and record output at 30 seconds and 5 minutes. Creep
with no applied force. The method selected for treatment of
recovery error is calculated as follows:
zero should reflect anticipated usage of the force-measuring
8.2.1.1 Creep Recovery Error, % of Output at Maximum
instrument. The deflection calculation shall (a) utilize the
Applied Force = 100 × (Output 30 seconds after zero force is
initial zero value only or (b) a value derived from readings
achieved – Initial zero reading) /Output at Maximum Applied
taken before and after the application of a force or series of
Force
forces. For method (a), the deflection is calculated as the
8.2.2 A zero return error shall be calculated as follows:
difference between the deflection at the applied force and the
8.2.2.1 Zero Return Error, % of Output at Applied Force =
initial deflection at zero force. For method (b), when it is
100 × (Initial zero reading – Final zero reading 5 minutes after
electedtoreturntozeroaftereachappliedforce,theaverageof
the applied force is removed) /Output at Applied Force. The
the two zero values shall be used to determine the deflection.
creeptestshallberepeatedifthezeroreturnerrorexceeds50%
For method (b) when a series of applied forces are applied
of the creep recovery error limits.
before return to zero force, a series of interpolated zero force
8.2.3 For force-measuring instruments calibrated for use
readings may be used for the calculations. In calculating the
overthefollowingverifiedrangesofforces,thecreeprecovery
average zero force readings and deflections, express the values
error limits of the output at the applied force are:
tothenearestunitinthesamenumberofplacesasestimatedin
Class AA: 6 0.020%
reading the instrument scale. Follow the instructions for the
Class A: 6 0.050%.
rounding method given in Practice E29. If method (a)is
8.3 Calibration Equation—Fit a polynomial equation of the
elected, a creep recovery test is required per the criteria of 8.2
following form to the force and deflection values obtained in
to ensure that the zero return characteristic of the force-
the calibration using the method of least squares:
measuring instrument does not result in excessive error.
2 5
Deflection 5 A 1A F1A F 1. A F (5)
0 1 2 5
8.2 Determination of Creep Recovery—Creep affects the
where:
deflection calculation. Excessive creep is indicated if large
non-return to zero is observed following force application F = force, and
A through A = coefficients.
during calibration. A creep recovery test is required to ensure
0 5
that the creep characteristic of the device does not have a
A2nddegreeequationisrecommendedwithcoefficients A ,
significant effect on calculated deflections when method (a) is
A , and A equal to zero. Other degree equations may be used.
4 5
usedtodeterminedeflections.Thecreeptestistobeperformed
For example the coefficients A through A would be set equal
2 5
for new force-measuring instruments, and for force-measuring
to zero for a linearized force-measuring instrument.
instruments that have had major repairs, force-measuring
8.3.1 For high resolution force-measuring instruments (see
instruments suspected of having been overloaded, or force-
7.1.3), the procedure of Annex A1 shall be used to obtain the
measuring instruments that show excessive non-return to zero
maximum degree of the best fit polynomial equation statisti-
following calibration. Creep and creep recovery are generally
cally supported by the calibration data set. This calculation is
stable properties of a force-measuring instrument unless the
performedwithapolynomialequationfittedtotheaveragedata
force-measuring instrument is overloaded, has experienced
ateachappliedforcefollowingthemethodofAnnexA1.After
moisture or other contaminant incursion, or is experiencing determinationofthedegreeofthebestfitpolynomialequation,
fatigue failure. If method (b) is used to determine deflections fit a polynomial equation of that degree, or a lower degree, to
on a force-measuring instrument both during calibration and the entire data set (not the averaged data set) in accordance
subsequent use, the creep recovery test is not required. The with 8.3, and proceed to analyze the data in accordance with
creep recovery test is performed as follows: 8.4 – 8.6.3.2.
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of the force-measuring instrument.
8.4 Standard Deviation—Calculate a standard deviation
from the differences between the individual values observed in
8.6.3.2 Class A—For force-measuring instruments used to
the calibration and the corresponding values taken from the
verify testing machines in accordance with Practices E4,or
calibrationequation.Calculateastandarddeviationasfollows:
similar applications, the LLF of the force-measuring instru-
ment shall not exceed 0.25 % of force. The lower force limit
2 2 2
d 1d 1.1d
1 2 n
s 5Π(6) for use over the Class A verified range of forces is therefore
m
n 2 m 21
400timestheLLF,inforceunits,obtainedfromthecalibration
where:
data.
d ,d , etc. = differences between the fitted curve and the n
1 2
NOTE 9—In the example of Note 8, the lower force limit for force-
observed values from the calibration data,
measuring instruments calibrated for use over the Class A verified range
n = number of deflection values, and offorcesis16N×400=6400N(3.7lbf×400=1480lbf).TheLLFwill
m = the degree of polynomial fit. be less than 0.25% of force for forces greater than this lower force limit
up the maximum force applied during calibration of the instrument.
NOTE6—Itisrecognizedthatthedeparturesoftheobserveddeflections
NOTE 10—The term “verified range of forces” used in these practices
from the calibration equation values are not purely random, as they arise
are parallel in meaning to the same term in Practice E4. It is the range of
partly from the localized variation in instrument readings discussed in
forces over which it is permissible to use the force-measuring instrument
7.1.1. As a consequence, the distributions of the residuals from the least
in verifying a testing machine or other similar device. When a verified
squares fit may not follow the normal curve of error and the customary
range of forces other than the two standard ranges given in 8.6.3 is
estimates based on the statistics of random variables may not be strictly
desirable, the appropriate limit of error should be specified in the
applicable.
applicable method of test.
8.5 Determination of Lower Limit Factor, LLF—LLF is
8.7 Specific Force-Measuring Instruments—Any force-
calculated as 2.4 times the standard deviation. If the calculated
measuring instrument may be calibrated as a specific force-
LLF is less than the instrument resolution, the LLF is then
measuring instrument. Elastic rings, loops, and columns with
defined as that value equal to the resolution. Express the LLF
dialindicatorsasameansofsensingdeformationaregenerally
in force units, using the average ratio of force to deflection
classed as specific force-measuring instruments because the
from the calibration data.
relativelylargelocalizednonlinearitiesintroducedbyindicator
NOTE 7—Of historical interest, the limit of 2.4 standard deviations was
gearing produce an LLF too large for an adequate verified
originally determined empirically from an analysis of a large number of
range of forces. These instruments are, therefore, used only at
force-measuringinstrumentcalibrationsandcontainsapproximately99%
the calibrated forces and the curve-fitting and analytical
of the residuals from least-squares fits of that sample of data.
procedures of 8.3 – 8.5 are replaced by the following proce-
8.6 Verified Range of Forces—Calculate the verified range
dures:
of forces of the force-measuring instrument as follows:
8.7.1 Calculation of Nominal Force Deflection—From the
8.6.1 Lower Force Limit of the Verified Range of Forces—
calibration data, calculate the average value of the deflections
Calculate the lower force limit to the verified range of forces
corresponding to the nominal force. If the calibration forces
for a specified percentage limit of error, P, as follows:
applied differ from the nominal value of the force, as may
100 3LLF
occurinthecaseofacalibrationbysecondaryforcestandards,
lowerforcelimit 5 (7)
P
adjust the observed deflections to values corresponding to the
nominal force by linear interpolation provided that the force
8.6.2 The verified range of forces shall not include forces
differencesdonotexceed 61%ofcapacityforce.Theaverage
outsidetherangeofforcesappliedduringthecalibration.Ifthe
valueofthenominalforcedeflectionisthecalibratedvaluefor
lower force limit is less than the lowest non-zero calibration
that force.
forceapplied,thenthelowerforcelimitoftheverifiedrangeof
8.7.2 Standard Deviation for a Specific Force-Measuring
forces is equal to the lowest calibration force applied.
Instrument—Calculate the range of the nominal force deflec-
8.6.3 Standard Verified Ranges of Forces—Two standard
tions for each calibration force as the difference between the
verified ranges of forces are listed as follows, but others may
largest and smallest deflections for the force. Multiply the
be used where special needs exist:
average value of the ranges for all the calibration forces by the
Class AA: 6 0.050 %
appropriate factor from Table 1 to obtain the estimated stan-
Class A: 6 0.25 %.
dard deviation of an individual deflection about the mean
8.6.3.1 Class AA—For force-measuring instruments used as
value.
secondary force standards, the LLF of the instrument shall not
exceed 0.05 % of force. The lower force limit of the force-
measuring instrument as expressed by Equation (7) over the
Class AA verified range of forces is therefore 2000 times the
TABLE 1 Estimates of Standard Deviation from the Range of
LLF, in force units, obtained from the calibration data.
Small Samples
NOTE 8—For example, a force-measuring instrument calibrated using
Number of Observations Multiplying Factor
primaryforcestandardshadacalculatedLLFof16N(3.7lbf).Thelower
at Each Force for Range
forcelimitfortheClassAAverifiedrangeofforcesistherefore16×2000
3 0.591
= 32 000 N (3.7 × 2000 = 7400 lbf).The LLF will be less than 0.05 % of
4 0.486
force for forces greater than this lower force limit to the maximum force
5 0.430
applied during calibration of the force-measuring instrument. It is recom- 6 0.395
mendedthatthelowerforcelimitbenotlessthan2%( ⁄50)ofthecapacity
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8.7.3 Lower Limit Factor for Specific Force-Measuring 9.4.2 If a force-measuring instrument is used at tempera-
Instrument—The LLF for a specific force-measuring instru- turesotherthanthetemperatureatwhichitwascalibrated,itis
ment is defined as 2.0 times the standard deviation, plus the the user’s responsibility to ensure that the performance of the
resolution. Convert the LLF into force units by means of a force-measuring instrument does not exceed the limits of
suitable factor and round to the number of significant figures paragraphs 9.4.1, or if such limits would be exceeded, that the
appropriatetotheresolution.TheLLFisexpressedasfollows: force-measuring instrument is calibrated at the expected tem-
peratureofuse,oroverarangeoftheexpectedtemperaturesof
LLF 5 ~2s1r!f (8)
use and corrected accordingly.
where:
NOTE 11—There is a negligible effect on the maximum values for
s = standard deviation,
force-measuringinstrumentscalibratedforuseovertheClassAA,verified
r = resolution
range of forces, LLF (0.05% of applied force) and ClassA, LLF (0.25%
f = average ratio of force to deflection from the calibration
of applied force) when these values are added as root-sum-squares with
the values for temperature error given in 9.4.1. Such a combination of
data.
errorsourcesisvalidinthecaseofindependenterrorsources.Itshouldbe
8.7.4 Precision Force—A specific force-measuring instru-
noted the temperature differences between conditions of calibration and
ment does not have a verified range of forces as specified in use may result in significant errors. This error source should be evaluated
by users to assure compliance with these requirements, when such usage
8.6, since it can be used only at the forces for which it was
occurs. Adequate stabilization times are required to ensure that thermal
calibrated. The use is restricted, however, to those calibrated
gradientsortransientsintheforce-measuringinstrumenthaveequilibrated
forces that would be included in a verified range of forces are
with the environment in which testing is to be performed. Otherwise,
calculated in 8.6 – 8.6.3.2.
thermal gradients may cause significant errors in both temperature
compensated and uncompensated force-measuring instruments.
It is recommended that the effect of temperature on the sensitivity of
9. Temperature Corrections for Force-Measuring
force-measuring instruments calibrated for use over the ClassAAverified
Instruments During Use
range of forces not exceed 0.0030% ⁄°C (0.0017% ⁄°F) and for force-
measuring instruments calibrated for use over the Class A verified range
9.1 Referenced Temperature of Calibration—It is recom-
of forces, that the effect of temperature on the sensitivity not exceed
mended that the temperature to which the calibration is
0.010% ⁄°C (0.0056% ⁄°F).
referenced be 23 °C (73 °F), although other temperatures may
As an example, for the case of force-measuring instruments that have
be referenced (see 7.3.2).
temperature coefficients equal to the maximum recommended values, the
error due to the temperature is negligible within 63 °C for force-
9.2 Temperature Corrections—Nearlyallmechanicalelastic
measuringinstrumentscalibratedforuseovertheClassAAverifiedrange
force-measuring instruments require correction when used at a
of forces and 65 °C for force-measuring instruments calibrated for use
temperatureotherthanthetemperaturetowhichthecalibration
over the ClassAverified range of forces referenced to the temperature at
which those force-measuring instruments were calibrated.
is referenced. This category includes proving rings, Amsler
boxes, and rings, loops, and columns equipped with dial
10. Force-Multiplying Systems
indicators. Uncompensated instruments in which the elastic
element is made of steel with not
...