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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

I

NBS Technical Note 1211

Interlaboratory Comparison

of Force Calibrations Using

ASTM Method E74-74

Robert W. Peterson, Linwood Jenkins, and Richard A. Mitchell

NBS NBS NBS NBS NBS NBS NBS NBS NBS~NBS

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NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

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NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

iS NBS NBS NBS NBS NBS NBS NBS NBS NBS Nl

NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

iS NBS NBS NBS NBS NBS NBS NBS NBS NBS Nl

NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

iS NBS NBS NBS NBS NBS NBS NBS NBS NBS Nl

NBS NBS NBS National Bureau ofStandards NBS NBS

iS NBS NBS NBS NBS NBS NBS NBS NBS NBS Nl

IBS NBS NBS NBS NBS NBS NBS NBS NBS

NBS NBS NBS NBS NBS NBS NBS NBS Nl

WBS NBS NBS NBS NBS NBS NBS NBS NBS

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TM he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The ^ Bureau's overall goal is to strengthen and advance the nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a

basis for the nation's physical measurement system, (2) scientific and technological services for industry and

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The Bureau's technical work is performed by the National Measurement Laboratory, the National

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Conducts research and provides measurements, data, standards, reference

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Gaithersburg, MD 20899. ^Some divisions within the center arc located at Boulder, CO 80303.

at Boulder, TO, with some cicinenis ai Gaithersburg, M(;
NBS Technical Note 1211

NATIONAL BUREAU OF STANDARDS

LIBRARY

. i

Interlaboratory Comparison

of Force Calibrations Using

ASTM Method E74-74

Robert W. Peterson Linwood Jenkins Richard A. Mitchell

Mechanical Production Metrology Division Center for Manufacturing Engineering

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

April 1985

'^*^-

U.S. Department of Commerce

Malcolm Baldrige, Secretary National Bureau of Standards

Ernest Ambler, Director
National Bureau of Standards U.S. Government Printing Office For sale by the Superintendent

Technical Note 1211 Washington: 1985 of Documents,

Natl. Bur. Stand. (U.S.), U.S. Government Printing Office Tech. Note 1211 Washington, DC 20402

21 pages (Apr. 1985)

CODEN: NBTNAE
CONTENTS

Page

LIST OF FIGURES iv

ABSTRACT 1

1 . INTRODUCTION 1

2. PROGRAM DESCRIPTION 1

3. RESULTS 2

3.1 Sensor Stability 11 3.2 Relative Results 12

ij. CONCLUSION 13

5. ACKNOWLEDGEMENTS 13

6. REFERENCES 13

APPENDIX U

111
LIST OF FIGURES

Figure 1 . Calculated response at capacity load (±2.4 standard

deviations) from the compression calibration of a 0.5 klbf (2.2 kN) force sensor at NBS and at 27 other

laboratories

Figure 2. Calculated response at capacity load (±2.4 standard

deviations) from the tension calibration of a 0.5 klbf

(2.2 kN) force sensor at NBS and at 27 other

laboratories

Figure 3. Calculated response at capacity load (±2.4 standard

deviations) from the compression calibration of

a 5 klbf (22 kN) force sensor at NBS and at 27 other

laboratories

Figure 4. Calculated response at capacity load (±2.4 standard

deviations) from the tension calibration of a 5 klbf

(22 kN) force sensor at NBS and at 27 other

laboratories

Figure 5. Calculated response at capacity load (±2.4 standard

deviations) from the compression calibration of a 20 klbf (89 kN) force sensor at NBS and at 22 other

laboratories ,

Figure 6. Calculated response at capacity load (±2.4 standard

deviations) from the tension calibration of a 20 klbf

(89 kN) force sensor at NBS and at 21 other

laboratories .

Figure 7. Calculated response at capacity load (±2.4 standard

deviations) from the compression calibration of a 100 klbf (445 kN) force sensor at NBS and at 20 other

laboratories ,

Page

Figure 8. Calculated response at capacity load (±2.4 standard

deviations) from the tension calibration of a 100 klbf

(445 kN) force sensor at NBS and at 19 other

laboratories 10

IV
INTERLABORATORY COMPARISON OF FORCE CALIBRATIONS USING ASTM METHOD E74-74

R. W. Peterson, L. Jenkins, and R. A. Mitchell

A comparison of force calibrations performed by the National Bureau of

Standards and 27 other laboratories located in the United States is

reported. Force sensors of four different capacities were calibrated

in both tension and compression, repeatedly by NBS with deadweight and

one time each by the other participating laboratories. The force sensor capacities were 0.5, 5, 20, and 100 klbf (2.2, 22, 89, and 445

kN). Deadweight machines (with and without force multiplication) and

force sensor transfer standards (used in a testing machine or a loading frame) were the force standards represented in the study. The

force calibration procedure used was Method E74-74 of the American

Society for Testing and Materials.

Key Words: Force; force calibration; force sensor; interlaboratory

comparison; laboratory intercomparison; load cell.

INTRODUCTION

This report documents an intercomparison study of force calibrations performed

by the National Bureau of Standards (NBS) and 27 other laboratories located in

the United States. The participating laboratories represent load cell and

proving ring manufacturers, defense and aerospace contractors, other large and

small private industries, and federal government agencies. The intercomparison

was conducted under the joint sponsorship of NBS and the Calibration

Subcommittee of Committee E28 on Mechanical Testing of the American Society for

Testing and Materials (ASTM). Although the initial phase of this intercomparison study, involving NBS and six other laboratories, has been reported

earlier [1] , this report covers the entire study.

2. PROGRAM DESCRIPTION

Two packages of force-measuring instruments were circulated among the 27

laboratories and NBS. Package One consisted of 0.5 and 5 klbf (2.2 and 22 kN)

force sensors and one readout instrument. Package Two consisted of 20 and 100

klbf (89 and 445 kN) sensors and one readout instrument. The force sensors were commercially-produced load cells instrumented with resistance strain gages. The

readout instruments were commercially-produced dc-excitation load cell

indicators. All 27 laboratories calibrated the two lower capacity force

sensors. Only 19 of these laboratories calibrated the two higher capacity force sensors in both compression and tension; most of the remaining laboratories did

not have the capability to calibrate to the higher capacities.

1 Numbers in brackets indicate the literature references at the end of this

report.
All calibrations were performed using ASTM Method E74-74 [2]. This procedure

calls for a total of at least 30 load applications, at least 10 of which are of

different magnitude. Loads of each magnitude are applied at least twice. If

possible, every 10 percent interval of the force range is represented in the

loading program. The response of the force sensor to the applied loads is

indicated by the net output or "deflection" of the readout instrument. The 30

or more force-versus-"deflection" data points are fitted, by the method of least

squares, to the second-degree polynomial calibration equation

2 Deflection = A + BL + CL (1)

in which L represents the applied force and the coefficients A, B, and C are

determined by the fitting process. The standard deviation of the differences

between the measured "deflections" and the predicted "deflections" given by the

calibration equation is computed as a m'easure of the precision of the

calibration.

NBS performed an initial calibration on each package, and then recalibrated the

package after it had been calibrated by not more than three of the other

laboratories. NBS also analyzed the calibration data from all of the

laboratories. A report was sent to each laboratory giving an analysis of their

data in comparison with the results of the corresponding before and after NBS

calibrations that bracketed their calibration. An example of such a report is

given in the Appendix. These reports, issued soon after the recalibration by

NBS, enabled each laboratory to evaluate its own calibration process. In a few

cases this self -evaluation led to a modification in the laboratory calibration

process, to correct an apparent problem, followed by a recalibration by the

laboratory. In such cases of recalibration by a laboratory, only the latter

calibration is included in the results presented here.

3. RESULTS

Figures 1 through 8 are plots of results that are representative of the entire

intercomparison program. Each vertical bar in these plots represents a single

calibration at one laboratory. The coordinates of the midpoint of each calibration bar correspond to the date of calibration and to the net sensor output or "deflection" at capacity load as computed by the calibration equation, eq. 1.

The units of the vertical scale correspond to the arbitrary units of the readout

instrument used. The length of each calibration bar is ±2.4 times the standard

deviation (i.e. total length is 4,8 standard deviations) of the calibration data

relative to the fitted calibration equation, a quantity used in

2 "Deflection" is defined in ASTM Method E74-74 [2] as the difference between the

reading of an instrument under load and the reading at zero or tare load.
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10
the ASTM force calibration procedure [2] to compute the loading ranges over

which the force sensor may be used in calibrating either testing machines or other force sensors. The letter symbol above each calibration bar identifies

the type of machine used to apply the calibration loads, as follows:

N - NBS deadweight

D - Deadweight (other than NBS)

M - Deadweight with lever or hydraulic multiplication

L - Loading frame (with force sensor standard)

T - Testing machine (with force sensor standard)

The ends of the calibration bars representing the repeated NBS deadweight

calibrations are connected with straight lines forming a segmented band. This

NBS band is a useful reference since it represents a large number of calibrations by the same laboratory over the duration of the program. Gross

discontinuities in the NBS band are labeled on the plots and explained in the

text below. The calendar year horizontal-coordinate labels are centered on January 1 of the year. Each participating laboratory can identify its results

by its calibration date. The 0.1% reference on the vertical scale of each plot

represents a percentage of sensor output at capacity load and is included to

facilitate comparison of plots having different scales. Calibration results

that are outside the range of a plot are indicated in the lower right corner by

machine symbol, date, sensor output, and calibration bar length.

3.1 Sensor Stability

The segmented NBS bands in figs. 1 through 8 indicate the stability of the force

sensors over the duration of the program. There is some evidence of long-term

drift in each of the plots. In most cases the amount of drift is not serious,

although it does demonstrate the importance of periodic recalibration of the

force sensors by one laboratory in a long-term intercomparison such as this.

Three of the force sensors exhibited more serious instability as indicated by

the gross discontinuities evident in the NBS band (figs. 1, 2, 3. ^, 7, and 8).

An explanation of these problems with each of these three sensors follows,

0.5 klbf (2.2 kN) Sensor - The force sensor used at the beginning of the program

was destroyed by overloading in February 1975. The replacement sensor had a different response curve, which explains the first discontinuity (figs. 1 and

2). In the Spring of 1977, this replacement sensor was repaired to correct a condition which allowed its internal overload-protection mechanism to interfere

with its normal response at a point slightly below capacity load. The repair

resulted in a change in the sensor response curve, which explains the 1977

discontinuity.

5 klbf (22 kN) Sensor - This force sensor (figs. 3 and ^) , underwent a significant change in response sometime between the November 1977 and July 1978 NBS

calibrations. Judging from the calibration results from the other laboratories,

it appears that the change occurred between the December 1977 and February 1978

calibrations. The cause of the change is not known.

11
TOO klbf (445 kN) Sensor - This force sensor (figs. 7 and 8) was first repaired

in 1976 to correct what was thought to be excessive drift during the first two

years of the program. As a result of this repair the response of the sensor, at

capacity load, increased by about 0.3 percent. About two years later, the

sensor again required repair as indicated by a shift of about one percent in the

NBS calibration results (data out of bounds in figs. 7 and 8). The cause of

this change in response is not known. Sometime after the November 1980 NBS

calibration, the response of this sensor again indicated a significant

instability. During the December 1981 NBS calibrations, sensor response at

capacity load varied by as much as 0.13 percent between loading runs of the

same calibration. It was decided that a third repair of the sensor was not

justified at that stage of the intercompar ison program. Laboratories participating in the program during that period were notified of the problem, and

it was suggested that if they chose to calibrate this sensor they should view

the results with much skepticism.

3.2 Relative Results

The segmented NBS bands (figs. 1 through 8) serve as a useful reference for

comparison of the results from all the participating laboratories. Most of the

results from the other laboratories appear to be distributed about the NBS band

and about 69 percent of their calibration bars overlap the NBS band. In some

cases, however, the calibrations by the other laboratories are not bracketed by

before and after NBS calibration of an apparently stable force sensor, for

example the three calibrations of the 5 klbf (22kN) sensor in 1977-1978. Table

1 summarizes the conditions of overlap (OL) for all calibrations by the other

laboratories that are clearly bracketed (BR) by before and after NBS calibrations of an apparently stable sensor. Totals and percentages are given for the

different types of loading machines used in the calibrations.

Table 1 - Summary of Results

0.5

(2.2

klbf

kN)

5 klbf

(22 kN)

^0 klbf

(89 kN)

100

(445

COMP

BR OL

klbf

kN)

TENS

BR OL *BR

GAL

MACH

COMP

BR OL

TENS

BR OL

COMP

BR OL

TENS

BR OL

COMP

BR OL

TENS

BR OL

^OTALS

*0L POT

D 15 14 15 12 5 4 5 4 2 1 2 2 1 1 1 1 46 39 85

L 5 3 5 3 14 9 14 8 14 10 14 11 12 10 1 1 8 89 62 70

M 4 3 4 2 1 1 3 1 2 2 15 8 53

T 3 1 3 1 4 2 4 3 3 3 2 2 1 2 1 24 11 46

TOTALS 27 21 27 18 24 15 24 15 22 12 21 17 15 12 14 10 174 120 69

*BR = Number of non-NBS calibrations that were "BRACKETED" by NBS calibration.

*0L = Number of non-NBS calibrations that were "BRACKETED" by NBS calibration

and that "OVERLAPPED", or extended into, the NBS band.

It is not surprising that the most frequent overlap with NBS resulted from the

deadweight calibrations (D) and that the least frequent overlap resulted from

the calibrations done in testing machines (T). Figures 1 through 8, however,

12
Illustrate clearly an important limitation of the overlap criterion. These

plots show many examples of overlap resulting from a combination of a large

offset of capacity output (bar midpoint) relative to NBS and a large standard

deviation (bar length). Conversely, the plots also show many cases of a small

offset in combination with a small standard deviation resulting in no overlap.

4. CONCLUSION

The results reported here give an indication of the range of calibration perfor- mance that might be obtained with the force sensors, loading machines, and

procedures used. The results also give each participating laboratory a basis

for evaluating and perhaps improving calibration performance. Of course, each

laboratory is in the best position to evaluate its performance in relation to

its own requirements and available resources.

5. ACKNOWLEDGEMENTS

Much credit for the initiation and success of this work should go to the late

Roscoe L. Bloss who was, for several years, both the NBS manager responsible for

force calibration and the chairman of the ASTM subcommittee having jurisdiction

in this area. Charles P. Reeve of the NBS Statistical Engineering Division

developed the computer software used to generate the plots of results.

6. REFERENCES

[1] Peterson, R. W.; Bloss, R. L., Interlaboratory comparison of force calibrations using ASTM Method E74-74, Phase 1. Nat. Bur. Stand. (U.S.) NBSIR76-

1145; 1976.

[2] American Society for Testing and Materials, Standard methods of calibration

of force-measuring instruments for verifying the load indication of testing

machines , E74-74 . Annual Book of ASTM Standards, Part 10. 194-201.

Philadelphia, PA: American Society for Testing and Materials; 1974.

13
APPENDIX

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16
NBS-114A (REV. 2-6C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS/TN-1211

2. Performing Organ. Report No. 3. Publ Ication Date

April 1985

4. TITLE AND SUBTITLE

Interlaboratory Comparison of Force Calibrations Using ASTM Method E74-74

5, AUTHOR(S)

Robert W. Peterson, Linwood Jenkins, and Richard A. Mitchell

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

GAITHERSBUR6, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS-fStreet. City. State, ZIP)

Same as in item 6 above.

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here)

A comparison of force calibrations performed by the National Bureau of Standards

and 27 other laboratories located in the United States is reported. Force sensors

of four different capacities were calibrated in both tension and compression, re- peatedly by NBS with deadweight and one time each by the other participating

laboratories. The force sensor capacities were 0.5, 5, 20, and 100 klbf (2.2, 22, 89,

and 445 kN). Deadweight machines (with and without force multiplication) and force sensor transfer standards (used in a testing machine or a loading frame) were the

force standards represented in the study. The force calibration procedure used was Method E74-74 of the American Society for Testing and Materials.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

force; force calibration; force sensor; interlaboratory comparison; laboratory

intercomparison; load cell.

13. AVAILABILITY

[XI Unlimited

Q For Official Distribution. Do Not Release to NTIS

[J[\ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

\2Z\ Order From National Technical Information Service (NTIS), Springfield, VA. 2216 I

14. NO. OF

PRINTED PAGES

21

15. Price

USCOMM-DC 6043-P80
J


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