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NATL INST OF STANDARDS & TECH R.I.C.

A1 1102616742

Mitchell, Richard A/Force calibration at QC100 .U5753 N0.1227 1986 V198 C.I NBS-P

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

NBS

PUBLICATIONS

NBS Technical Note 1227

Force Calibration at the

National Bureau of Standards

Richard A. Mitchell

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#1227

1986
T'M he National Bureau of Standards' was established by an act of Congress on March 3, 1901. The

f 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

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

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the Federal Information Processing Standards Program, developing Federal ADP standards guidelines, and managing Federal participation in ADP

voluntary standardization activities; provides scientific and technological advisory services and assistamce to Federal agencies; and provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers:

• Programming Science and

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Conducts research and provides measurements, data, standards, reference

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'Hcadquaricrs and laboratories at Gaithcrsburg, MD, unless otherwise noted; nnailing address Gaiihcrsburg, MI) 20899 ^Somc divisions within the center are located at Boulder, CO 80303. 'Located at Boulder, CO, with some elements at Gailhersburg, MD.
NBS

RESEARCH

INFORMATION CENTER

NBS Technical Note 1227 ^''^^ Force Calibration at the

National Bureau of Standards

Richard A. Mitchell

Automated Production Technology Division Center for Manufacturing Engineering

National Engineering Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

August 1986

^

U^. Departmait of Commeroe

Maloolm 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 1227 Washington: 1986 of Documents,

Natl. Bur. Stand. (U.S.), U.S. Government Printing Office,

Tech. Note 1227, Washington DC 20402

26 pages (Aug. 1986)

CODEN: NBTNAE
CONTENTS

Page

LIST OF FIGURES iv

ABSTRACT 1

1

INTRODUCTION 1

2. DEADWEIGHT CALIBRATION 2

2.1 Operational Characteristics of NBS Deadweight Machines 2 2.2 Uncertainty of Applied Deadweight Forces 6

2.3 Force Calibration Procedures 8

2.4 Data Analysis 9

2.5 Systematic Errors 12

3. COMPARISON FORCE CALIBRATION 12

4. FORCE SENSOR CHARACTERIZATION TESTS 14

4.1 Temperature Sensitivity Test 14

4.2 Pressure Sensitivity Test 15

4.3 Creep Test 15

4.4 Eccentric Load Sensitivity Test 15

4.5 Pattern Evaluation Tests 19

5. CONCLUSION 19

6. REFERENCES 21

ili
LIST OF FIGURES

Page

Figure 1. One-mill ion-lbf (4.4-MN) deadweight machine 3

Figure 2. 300-klbf (1.3-MN) deadweight machine 4

Figure 3. The two weight stacks of the 112-klbf (i<98-kN)

deadweight machine 5

Figure 4. Schematic drawing of the 25.3-klbf (113-kN) and

6.1-klbf (27-kN) deadweight machines 7

Figure 5. Deviation from the fitted calibration curve versus applied force, from the compression calibration of

a 100 klbf (Mi45 kN) proving ring force sensor 10

Figure 6. Deviation from the fitted calibration curve,

relative to capacity output, versus applied force,

from the compression calibration of a 6-klbf (27-kN)

strain gage force sensor. Curves indicate the

pointwise 95 percent confidence intervals 11

Figure 7. Setup for the comparison calibration of a 3-million-lbf (13-MN) strain gage force sensor .... 13

Figure 8. Temperature sensitivity test setup. The instruments

are identified in the text 16

Figure 9. Pressure sensitivity test system. The instruments are identified in the text 17

Figure 10. Creep response of a 50-klbf (222-kN) strain gage force

sensor loaded in compression at three different

temperatures 18

Figure 11. Eccentric load sensitivity of the two measurement

bridges of a 5-klbf (22-kN) strain gage force sensor. Deviation from full scale output, in percent of full

scale output per millimeter of load eccentricity, is

plotted versus angular orientation of the eccentric

loading fixture 20

iv
FORCE CALIBRATION AT THE NATIONAL BUREAU OF STANDARDS

Richard A. Mitchell

Force calibration and force measurement services available at the

National Bureau of Standards (NBS) are described. Direct deadweight

calibrations of force sensors are performed in both compression and

tension up to one million Ibf (4.4 MN) . Comparison calibrations

relative to force sensor transfer standards are performed in compression up to 12 million Ibf (53 MN). In addition to force calibrations,

the following tests are performed to further characterize force

sensors: temperature sensitivity, pressure sensitivity, creep, and

eccentric load sensitivity. Tests prescribed by the International

Organization of Legal Metrology (OIML) for the classification of load

cells used in electronic weighing systems are performed in compression

for load capacities between about 26 klbf (116 kN) and 109 klbf (485

kN). The capabilities and limitations of the calibration and test

facilities are briefly described.

Key Words: Comparison force calibration; creep; deadweight force

calibration; eccentric load sensitivity; force calibration; force

measurement; force sensor; NBS force calibration; OIML load cell test;

pressure sensitivity; temperature sensitivity.

1 . INTRODUCTION

This document describes the force calibration and test services that are available at the National Bureau of Standards (NBS). Although it is written for the

general technical community, it is intended particularly as a guide to users and

prospective users of the NBS services. The capabilities and limitations of the

force calibration and test facilities are described. Inquiries are invited in cases where additional information is needed.

In this document the generic term "force sensor" is used to represent a wide

variety of force measuring instruments and systems. This includes, but is not

necessarily limited to, instruments and systems that are variously referred to

by names such as load cell, proving ring, force gage, force link, force

transducer, load ring, ring dynamometer, compression dynamometer, tension

dynamometer, and crane scale. The important defining characteristic is that

these instruments or systems produce an output that is approximately proportional to an applied force;

A very important distinction about this NBS calibration service is that it

provides a force calibration. If the calibrated force sensor is used as a load

cell in a weighing application to measure mass, the force sensor functions as the spring element of the weighing system. It is incumbent upon the user to

make the appropriate corrections for the local gravity field and for air

buoyancy.
2. DEADWEIGHT CALIBRATION

Tension and compression force calibrations over the range from 10 Ibf (44 N) to

1000 klbf (4.4 MN) are performed in the six NBS deadweight machines described in

Table 1

.

Table 1 - NBS deadweight machines

Capacity, klbf

(kN)

1000

(4448)

300

(1334)

112

(498)

25.3

(113)

6.1

(27)

0.5

(2.2)

Minimum load, klbf

(kN)

50

(222)

10

(44)

3

(13)

0.2

(0.89)

0.1

(0.44)

0.01

(0.044)

Minimum increment, klbf

(kN)

50

(222)

10

(44)

1 (4.4)

0.05

(0.44)

0.05

(0.22)

0.005

(0.22)

Compression setup space:

Vertical, m Horizontal, m1.98

0.86

1 .65

0.91

1 .02

0.71

0.76

0.50

0.61

0.47

0.25

0.29

Tension setup space:

Vertical, m Horizontal, m 4.45

1.17

2.49

0.91

2.16

0.71

0.91

0.66

0.76

0.64

0.56

0.29

2.1 Operational Characteristics of NBS Deadweight Machines

1000-klbf (4.45-MN ) Machi ne - Although this is the largest NBS deadweight

machine, it is also the simplest in design and operation (Fig. 1). The machine

applies twenty equal force increments of 50 klbf (222 kN). The force increments

are applied one-at-a-time as the lifting frame is raised by the hydraulic jack.

Consequently, the weights are always applied and removed sequentially. For

example, to apply and remove 350 klbf (1.55 MN) the necessary sequence is 0,

50, 100, 150, 200, 250, 300, 350, 300, 250, 200, 150, 100, 50, klbf.

300-klbf (1.33~MN) Ma ch ine - This machine (Fig. 2) is similar in design and

operation to the million-pound-force machine except for the fact that it has

weights of three different sizes. There are three 30-klbf (133-kN) weights,

four 20-klbf (89-kN) weights, and thirteen 10-klbf (44-kN) weights. The weights

are arranged to enable one to apply sequentially ten equal- increment test forces

over the three ranges: 100, 200, and 300 klbf (0.44, 0.89, and 1.33 MN) . Some

of the larger increments necessarily consist of two or three weights applied

sequentially.

112-klbf (498-kN) Machine - This machine (Fig. 3) has one stack of ten 10-klbf

(44-kN) weights and a second stack of nine 1-klbf (4.4-kN) weights. The larger

weights are applied as in the two larger machines, one-at-a-time in a fixed

sequence, as the lifting frame is raised by a hydraulic jack. The smaller

weights are applied, one-at-a-time in a fixed sequence, as they are lowered onto

the loading frame by screw jacks. The weight applied to the force sensor being

calibrated is the sum of the weight applied from the two stacks plus the 3~klbf

(13-kN) weight of the loading frame. This arrangement enables one to apply any

multiple of 1 klbf (4.4 kN) between 3 klbf (13 kN) and 112 klbf (498 kN).

However, a particular loading sequence may require that the previous load be

partially removed before the next larger load is applied. For example, to

increase the load from 7 klbf (31 kN) to 14 klbf (62 kN), the load must first be

reduced to 4 klbf (18 kN).
Jack Room Floor

Measurement

Room Floor

Weight

Room Floor

Hydraulic Jack

Tension Platen

Lifting Frame

Loading Platen

Compression

Platen

Support Sockets

Figure 1. One-million- Ibf (4.4-MN) deadweight machine.
Jack Room Floor

Measurement

Room Floor

Weight

Room Floor

Hydraulic Jack

Tension Platen

Lifting Frame

Loading Platen

Compression

^ Platen

Support Sockets

Figure 2. 300-klbf (1.3-MN) deadweight machine.
Figure 3. The two weight stacks of the 112-klbf (498-kN) deadweight machine,

5
25.3~klbf (113~kN) Machine - Figure 4 illustrates schematically the operating

principles of this machine. Each weight, when not being used to generate an applied force, is supported by c6il springs. When a weight is selected for a force application the supporting springs are compressed by the hydraulic

cylinders, thus lowering the weight to a preset position. When the lifting

frame is raised by the screw jacks, all of the weights that have been selected

and lowered are picked up by the loading frame. The weight applied to the force

sensor is the sum of the weights selected plus the weight of the loading frame.

An operational limitation of this machine is that the lifting frame must be

lowered sufficiently to return the applied force to zero before a different

combination of weights is applied. Therefore, tests to determine the hysteresis

characteristics of a force sensor cannot be performed with this machine.

A set of small supplemental weights is available that can be added manually to

the loading frame to give nominal kilogram-force values for the summation of the

pound-force weights and the supplemental weights.

6.1-klbf (27-kN) Machine - Figure 4 also illustrates schematically the operating

principles of this machine. The method of weight selection and force application, as well as the operational limitation that the applied force must be

returned to zero before the application of a different force, are the same as those of the 25.3~klbf (113~kN) machine. This machine has one set of weights

that generates nominal pound-force loads and a second set of smaller weights

that, when added to the larger weights, results in nominal kilogram-force

values. The' kilogram-force option can be switch-selected at the machine

console.

500-lbf (2.2-kN) Machine - Although this machine is functionally similar to the

next two larger machines (Fig. 4), it is fully manually operated. Each weight,

when not being used to apply a force, is supported on two steel bars. Before

applying a load, the loading frame and all weights are first raised by a manually operated worm-gear mechanism. The support bars are then manually

removed from beneath the selected weights. When the loading frame is then

lowered, the selected weights remain on the loading frame and are applied to the

force sensor while the other weights are rested on their support bars. This

machine has the operational limitation that the applied force must be returned

to zero before the application of a different force.

2.2 Uncertainty of Applied Deadweight Forces

The masses of the weights of the six NBS deadweight machines have been adjusted

to apply vertical forces in nominal pound-force or kilogram-force increments.

The vertical force exerted by a weight is given by the formula

^°^^^ = 9.80665 ^^ " dJ

in which M = mass of the weight,

g = local acceleration due to gravity, m/s , d = air density, and

D = density of the weight.

If the mass of the weight is in pounds, the force is in pounds-force; if the mass is in kilograms, the force is in kilograms-force.

The weights of the three larger machines are made of AISI 410 stainless steel

having a density of 7.72 g/cm ; those of the three smaller machines are made
SCREW JACKS

LIFTING FRAME

CALIBRATION

DEVICE

LOADING FRAME

DEADWEIGHT STACK

HYDRAUUC

CYLINDERS

Figure 4. Schematic drawing of the 25.3-klbf (113-kN) and 6.1-klbf (27-kN)

deadweight machines.
3 AISI 302 stainless steel having a density of 7.89 g/cm . The mass^ of the

weights were adjusted for an assumed air density of 1.17 x 10 g/cm . It is

estimated that the maximum difference between the actual air density in the

laboratory and the assumed air density would result in a difference in applied

force of no greater than 0.0005 percent.

The weights were adjusted for the local acceleration due to gravity at a point

near the center of gravity of each weight stack. These local gravity values

were based on a direct determination of gravity at one location in the Force

Laboratory (Ref. 1), with corrections made for the differences in elevation

between the weight stacks and the site of the gravity determination.

Each weight was adjusted to apply a force that differed from its nominal force

value by less than 0.0005 percent. The mass of each weight, so adjusted, was

then determined by substitution methods, working up from the NBS standard

kilogram.

The estimated total uncertainty of the vertical component of force applied by

any of the six NBS deadweight machines is 0.002 percent. If corrections were

made for the actual air density during a calibration and for the actual adjusted

mass of each weight, the uncertainty of the applied force could be reduced to

0.001 percent.

2.3 Force Calibration Procedures

An NBS deadweight force calibration usually includes at least 30 force applications, with the applied loads selected to represent approximately every 10

percent of the calibration range. The forces are applied in two or more

calibration runs, with the force sensor rotated to a. different orientation in

the machine between runs. At least some of the applied loads are repeated in

different runs of the same calibration. Within these general guidelines, a broad range of standard procedures prescribed by standards writing organizations, as well as special procedures to satisfy user requirements, can be

accommodated. The particular calibration procedure that is appropriate depends

on the requirements of the application and the characteristics of the force

sensor and sensor readout.

The standard force calibration procedures most frequently used by NBS are those

included in the American Society for Testing and Materials (ASTM) Standard

Practice E7^ [2]. These procedures are prescribed by ASTM for the calibration

of either (1) force sensors that are used to calibrate testing machines or (2)

force sensors that are used to calibrate other force sensors that are then used

to calibrate testing machines. The same procedures ar.e also adequate for many

other applications.

Special force calibration procedures are ordinarily consistent with the minimum

requirements of the general guidelines given above, but they may also include

supplemental force applications or controls that are based on the requirements

of the particular application of the force sensor. For e"xample, particular

loading sequences, loading rates, and waiting times may be used to more

thoroughly characterize the hysteresis, time dependence, and r epeatibil ity of

the force sensor. Special tests to determine other characteristics of force sensors are discussed in Sec. 4.
As known forces are applied to the force sensor, the response indicated by the sensor readout is recorded. Force sensor readouts can be functionally classified into three different types: (1) integral, (2) system, and (3)

interchangeable. All three readout types are used extensively at NBS. An

integral readout i s an integral part of the force sensor, for example the dial

micrometer of a proving ring. A system readout is a particular electrical or hydraulic instrument that is both calibrated and used in combination with a particular force sensor; the combination is referred to as a force measurement

system and the calibration of the combination is referred to as a system

calibration. An interchangeable readout is an instrument that is first

calibrated independent of a force sensor and then used to measure the response

of the force sensor, for example a dc voltage-ratio readout instrument that can

be calibrated by a four-terminal network of stable electrical resistors.

2.4 Data Analysis

A polynomial curve is usually fitted to the f orce-versus-response calibration

data. The curve most frequently used is of the form

2

Response = A + B (Force) + C (Force)

in which A, B, and C are constant coefficients that are estimated by the method

of least squares. An estimate of the standard deviation of the pooled calibration data relative to the fitted curve is computed by the formula

s =

2 2 2 2

d, d^ d^ d

1 + 2 + 3 + . . . + n

(n - m)

1/2

in which d , d^, etc. = differences between the curve and the n observed response values, and

m = number of coefficients of the fitted curve.

Figure 5 is a plot of the results from such a curve fit for the compression

calibration of a 100 klbf (445 kN) proving ring force sensor. The fitted

calibration curve is represented by the horizontal zero axis and the data points

indicate deviations from the fitted curve, in percent of capacity-load output.

The dashed horizontal lines indicate the resolution of the output. Although

this analysis is adequate for data that are distributed as randomly as those

shown in Fig. 5, a more general analysis is needed to represent the non-random

components of the response of many force sensors.

A new statistical analysis of force calibration data is being developed at NBS

[3], but it has not yet been implemented for routine use in calibration reports.

It makes use of statistical tests for significance to determine the degree of

polynomial that best fits the data. Confidence intervals are computed for the

sensor response predicted by the fitted polynomial. Figure 6 shows computed 95

percent confidence intervals, relative to a second degree polynomial fit, for

the compression calibration of a 6-klbf (27-kN) strain gage force sensor.

Between-run (zero offset and load proportional) and model (serpentine) error

components are evident in the plot. The analysis estimates the separate con- tributions of these systematic error components as well as the random error.
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An alternative data analysis, described in Ref . 2, is used in the calibration of

so called "limited load devices," force sensors whose response characteristics

do not justify interpolation of measurements at force magnitudes other than

those applied in the calibration. In this analysis, the response data for each

of the repeated calibration forces are averaged to give the predicted response

of the force sensor.

2.5 Systematic Errors

Systematic errors evident in deadweight calibrations are ordinarily much greater

than the estimated 0.002 percent uncertainty of the applied vertical deadweight

force given above (Sec. 2.2). Most deadweight calibrations performed at NBS

have an estimated standard deviation of the pooled data relative to a fitted

calibration curve in the range 0.002 to 0.02 percent of full scale output. The

apparent systematic error, indicated by the non-random structure of the data

(Fig. 6), is often greater than the apparent random error. The two major

sources of systematic error are usually mechanical misalignment and load-time

effects. Complex mechanical interactions between the force sensor and the

deadweight machine, due to small mechanical misalignment of the sensor and the

machine, can cause significant bending, shear, and torsional loads to act in

combination with the precisely known vertical force. Also, the response of the

force sensor is inherently dependent on the loading sequence, the loading rate,

and the duration and stability of the load. These sources of calibration error

are discussed in Refs. 4, 5, and 6. Of course, the same types of error are

present, to some degree, when the calibrated force sensor is used to measure

force in a different setup. Reference 7 reports a round robin study of the

calibration of a set of four force sensors at NBS and at 27 other laboratories.

The results give an indication of the range of performance that might be obtained with the force sensors, loading machines, and procedures used.

3. COMPARISON FORCE CALIBRATION

Compression force calibrations up to a capacity of 12 million Ibf (53 MN) are

done by comparison with NBS transfer standard strain gage force sensors. These

comparison calibrations are done in the compression section of the NBS 12-

million-lbf (53~MN) capacity universal testing machine described in Ref. 8.

Figure 7 shows the setup for the calibration of a 3-million-lbf (13-MN) strain

gage force sensor by comparison with three NBS 1 -million-lbf (4.4-MN) transfer

standards loaded in parallel. Each of the 1 -mill ion-lbf (^.^-MN) transfer

standards has been calibrated in compression in the NBS 1 -mill ion-lbf (^.4-MN)

deadweight machine. NBS has a set of four 3-million-lbf (13-MN) transfer standard strain gage force sensors that are each calibrated in compression by

comparison with three NBS 1 -mill ion-lbf (4.4-MN) transfer standards. To

calibrate in compression above 3 million Ibf (13 MN) either three or four of the

NBS 3-million-lbf (13-MN) transfer standards are loaded in parallel in a setup

similar to that shown in Fig. 7.

The 1 2-million-lbf (53-MN) universal testing machine applies the calibration

force with a hydraulic ram and senses the applied force with an annular-ring

hydraulic capsule equipped with a pressure sensor. The applied force is con- trolled by manually operated hydraulic flow-control valves. With this method of

force control, it is not possible to apply and hold constant a precise force as is done in a deadweight calibration. The usual procedure is to approximate a

12
Figure 7. Setup for the comparison calibration of a 3-inillion-lbf (13-MN)

strain gage force sensor.

13
target force by making the comparison calibration measurements as the hydraulically applied force slowly increases through the target force value as indicated

by the dial of the universal testing machine. The readouts from all of the

force sensors in the setup (Fig. 7) are recorded simultaneously. To facilitate

this, the readout instruments used with the NBS transfer standards are electrically interconnected so that simultaneous readings can be obtained by operating

a single freeze switch.

The applied calibration force is assumed to be the sum of the forces indicated

by the NBS transfer standards that are loaded in parallel (Fig. 7). To investigate the validity of this assumption, a setup equivalent to that shown in Fig.

7 was made in the compression section of the NBS 1 -million-lbf (4.4-MN) deadweight machine. Repeated calibration runs up to 1 million Ibf (4.4 MN) were

then performed with one or more of the smaller transfer standards rotated and/or

moved slightly in a radial direction between runs to simulate the normal variations that might be expected in multiple-standard setups. The average of the

differences between the applied deadweight force and the sum of the forces

indicated by the three parallel 1-million-lbf (4.4-MN) transfer standards was

84 Ibf (374 N).

Estimates of the uncertainties in the forces applied in comparison calibrations

are computed by combining in quadrature the estimated uncertainties contributed

by each of the three or four transfer standards that are loaded in parallel.

The uncertainty of each of the 1-million-lbf (4.4-MN) transfer standards is

estimated to be 2.4 times the standard deviation of the pooled data from the

deadweight calibration of that transfer standard, as prescribed by ASTM E74

(Ref. 2). The uncertainty of each of the 3-million-lbf (13-MN) transfer standards Is estimated to be 2.4 times the standard deviation of the pooled data

from its comparison calibration plus the estimated uncertainty of the loads

applied during its calibration. Based on repeated calibrations of the transfer

standards over several years, the estimated uncertainties in the forces applied

in comparison calibrations are:

To 3000 klbf (13 MN) using three of the smaller standards — 0.3 klbf (1.3 kN)

To 9000 klbf (40 MN) using three of the larger standards ~ 1.3 klbf (5.8 kN)

To 12000 klbf (53 MN) using four of the larger standards — 1.6 klbf (7.1 kN)

4. FORCE SENSOR CHARACTERIZATION TESTS

Determinations of force sensor characteristics other than the calibration curve

are needed for many applications. The NBS Force Laboratory can perform temperature sensitivity, pressure sensitivity, creep, eccentric load sensitivity, and

pattern evaluation tests over certain ranges as described below.

4.1 Temperature Sensitivity Test

Two methods are available at NBS for controlling the test temperature in order

to determine the temperature sensitivity of a force sensor: the constanttemperature room and the constant-temperature chamber. In the first method the

temperature of the entire laboratory room is varied. In the second method the

force sensor is placed inside a temperature chamber that is installed in a deadweight machine.

The measurement rooms of all six NBS deadweight machines can be adjusted up or down several degrees Celsius and held at a constant temperature. Thus, the

14
temperature sensitivity over a moderate temperature range can be determined, for

tensile or compressive applied force, over the force range from 10 Ibf (^^ N) to

1000 klbf (4.45 MN).

The NBS temperature chamber has a range of -15 °C to 50 °C. However, it can

.only be used in compression and, as it is presently equipped, it can only be

used over the force range from 100 Ibf (445 N) to 112 klbf (498 kN) . The inside

dimensions of the chamber are 36 cm diameter by 43 cm height. Within these

limitations, the constant-temperature chamber method is much more efficient than

the constant-temperature room method. Figure 8 shows the insulated temperature

chamber installed for a test in the 112-klbf (498-kN) deadweight machine. Other

equipment used in the test, shown left- to-right in Fig. 8, are a constanttemperature liquid bath that controls the chamber temperature, an ac power line

conditioner for removing electrical transients (underneath the bath), a fourterminal resistance network used to calibrate the dc readout instrument for a strain gage force sensor, the readout instrument for a strain gage force sensor,

the deadweight machine control console, a desktop computer used to monitor the

readout instrument and thermocouples located inside the chamber, and a temperature data scanner. During this test the maximum temperature difference between

the top and bottom of the force sensor was less than 0.5 °C over a test temperature range of -10 °C to 40 °C.

4.2 Pressure Sensitivity Test

Force sensors that are hermetically sealed can have a significant sensitivity to

changes in ambient air pressure. Intuitively, one would expect that the pressure sensitivity would be approximately constant throughout the loading range of

the force sensor. The system shown in Fig. 9 is used to determine the pressure

sensitivity of unloaded or lightly loaded force sensors over the atmospheric

range, from 710 mm Hg (95 kPa) to 785 mm Hg (105 kPa). The pressure chamber is

38 cm in diameter by 71 cm in height which allows enough room to apply small

tare weights to the force sensor being tested. The vacuum pump and gas supply

provide a continuous range of ambient pressure above and below laboratory am- bient pressure. Pressure is regulated by the manual momentary valve (visible in

Fig. 9 attached to the side of the pressure chamber) and read by the dial gage

absolute pressure sensor.

4.3 Creep Test

Drift in the output signal of a force sensor while it is subjected to a constant

applied force, commonly called creep, can be a critical characteristic in many

applications. The creep characteristics of force sensors over the force range

from 10 Ibf (44 N) to 1000 klbf (4.4 MN) can be determined in the NBS deadweight

machines (Ref. 9). Where it is practical the force can be applied in one step.

For example, a 50 klbf (222 kN) creep test can be performed by applying in one

step the initial force increment of the 1000-klbf (4.4-MN) deadweight machine.

Creep tests can be performed at laboratory room temperature or in the temperature chamber (Sec. 4.1). Figure 10 illustrates the importance of ambient

temperature in a creep test.

4.4 Eccentric Load Sensitivity Test

Force sensors are ordinarily designed with a symmetry that is intended to make

them relatively insensitive to load eccentricity. In general, however, there is

a residual eccentric load sensitivity due to imperfections in machining.

15
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sensitivity of universal (tension and compression) force sensors that are loaded

in compression can be measured in the NBS deadweight machines by the use of

special eccentric loading fixtures. The test determines this sensitivity, as a percentage of full scale output, per millimeter of load eccentricity. Figure 11 shows the results of a test of a 5-klbf (22-kN) capacity sensor having two

different force sensing electrical bridges. In this test a vertical compressive

load was applied at a 2-mm eccentricity and the eccentric loading fixture was rotated in 60-deg. angular steps. These results indicate that one bridge has

about four times the eccentric load sensitivity of the other bridge and that the axes of maximum sensitivity differ by about 90 deg. In some cases the eccentric

load sensitivity test can also be used to calibrate the transverse bending

moment bridges of multi-axis force-moment sensors. Reference 10 describes the

test in greater detail and gives the results of tests on five different sensors.

4.5 Pattern Evaluation Tests

The International Organization of Legal Metrology (OIML) has adopted a standard

(Ref. 11) for the pattern evaluation testing of load cells (force sensors) used

in electronic weighing systems. The (U.S.) National Conference of Weights and

Measures has revised its standard (Ref. 12) to be in harmony with the OIML

requirements. The OIML standard prescribes deadweight loading tests for

linearity, repeatability, hysteresis, and creep over the temperature range -10

to 40 °C and a test for pressure sensitivity at room temperature. The loading

tests require the application of a sequence of five or more deadweight load

increments following the application of an initial minimum deadweight (tare)

load. These loading tests can be performed, in compression, in the 112-klbf

(498-kN) deadweight machine. However, because of the operational characteristic

of this machine (Sec. 2.1) it is suitable for performing these tests only on

load cells of capacity between about 26 klbf (116 kN) and 109 klbf (485 kN) . Plans are being developed to extend the NBS test capability to cover a greater

range of load capacities, in tension as well as compression.

5. CONCLUSION

Force calibration and force measurement services available at NBS have been

described. Detailed procedures for obtaining these services are given in Ref.

13. The fee schedule for deadweight force calibration is published in Ref. 14.

The fees for comparison force calibrations and for force sensor characterization

tests are computed for each job.

Inquires about the available services, either by mail or telephone, are

welcomed. Personal visits to the laboratory can also be arranged, by scheduling

in advance. Prior consultation, either by mail, telephone, or visit, are particularly encouraged for first-time users of these services.

19
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20
6. REFERENCES

1. Tate, D. R. Acceleration due to gravity at the National Bureau of

Standards, Nat. Bur. Stand. (U.S.), Monogr . 107, 1968.

2. Calibration of force-measuring instruments for verifying the load indications of testing machines, ASTM E?^, in Annual Book of ASTM Standards, Vol.

03.01. Philadelphia, PA: American Society for Testing and Materials; 1983.

226-234.

3. Reeve, C. P. A new statistical model for the calibration of force measuring

devices. Nat. Bur. Stand. (U.S.) NBSIR (in preparation).

4. Mitchell, R. A.; Pontius, P. E. Force sensor-machine interaction. ISA

Instrumentation in the Aerospace Industry 27: 225-232; 1981.

5. Pontius, P. E. ; Mitchell, R. A. Inherent problems in force measurements.

Exper. Mech. 22 (3): 81-88, 1982.

6. Mitchell, R. A. Progress in force measurement at NBS. Proc. 10th Conf.

IMEKO TC-3; 1984; Kobe, Japan. 67-70.

7. Peterson, R. W. ; Jenkins, L. ; Mitchell, R. A. Interlaboratory comparison of

force calibrations using ASTM method E74-74. Nat. Bur. Stand. (U.S.) Tech.

Note 1211 , 1985.

8. Kirstein, A. F. Universal testing machine of 12-million-lbf capacity at the

National Bureau of Standards. Nat. Bur. Stand. (U.S.) Spec. Publ . 355,

1971 .

9. Mitchell, R. A.; Baker, S. M. Characterizing the creep response of load

cells. VDI-Berichte Nr. 312: 43-48; 1978.

10. Mitchell, R. A.; Seifarth, R. L. ; Reeve, C. P. Eccentric load sensitivity

of force sensors. Proc. 11th Conf. IMEKO TC-3; 1986; Amsterdam, the

Netherlands.

11. Metrological regulations for load cells, OIML International Recommendation

No. 60, 1985.

12. Specifications, tolerances, and other technical requirements for weighing

and measuring devices. Nat. Bur. Stand. (U.S.) Handbook 44, 1985.

13. Uriano, G. A.; Garner, E. ; Kirby , R. K.; Reed, W. P.; editors. National

Bureau of Standards Calibration Service User Guide, Nat. Bur. Stand. (U.S.)

Spec. Publ. 250, 1986-1988 edition.

14. Calibration and related services — fee schedule. Nat. Bur. Stand. (U.S.)

Spec. Publ. 250 Appendix, June 1986. (This publication is usually revised

and reissued twice yearly.)

21
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

r©S/lN-1227

2. Performing Organ. Report No. 3. Publication Date August 1986

4. TITLE AND SUBTITLE

Force Calibration at the National Bureau of Standards

5. AUTHOR(S)

Richard A. Mitchell

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

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

GaitiieraDurg, J''^ 20399

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

C3I-G a.i iteri 6

10. SUPPLEMENTARY NOTES

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

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

bibliography or literature survey, mention it here)

Force calibration and force measurement services available at the National Bureau of

Standards (NBS) are described. Direct deadweight calibration of force sensors are performed in both compression and tension up to one million Ibf (4.4 MN) . Comparison calbrations relative to force sensor transfer standards are performed in compression up to

12 million Ibf (53 MN) . In addition to force calibrations, the following tests are per'

formed to further characterize force sensors: temperature sensitivity, pressure sensitivity, creep, and eccentric load sensitivity. Tests prescribed by the International Organization of Legal Metrology (OIML) for the classification of load cells used in electronic weighing systems are performed in compression for load capacities between about

26 klbf (116 kN) and 109 klbf (485 kN) . The capabilities and limitations of the calibration and test facilities are briefly described.

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

Comparison force calibration; creep; deadweight force calibration; eccentric load

sensitivity; force calibration; force measurement; force sensor; NBS force calibration;

OIML load cell test; pressure sensitiA/itv: temperature sensitivity.

13. AVAILABILITY

ly-j Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X~| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Q^] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

26

15. Price

USCOMM-DC 6043-P80


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