I


NBSIR 73-174

A Study of the Feasibility of Obtaining

a Systems Calibration of a Load

Cell and Indicator from Independent

Calibrations of the Components

D. R. Tate

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

April 1973

Summary Report

Prepared for Department of Defense

Calibration Coordination Group

Working Group on Mass and Pressure

Project No. 72-73

NBSIR 73-174

A STUDY OF THE FEASIBILITY OF OBTAINING

A SYSTEMS CALIBRATION OF A LOAD

CELL AND INDICATOR FROM INDEPENDENT

CALIBRATIONS OF THE COMPONENTS

D. R. Tate

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

April 1973

Summary Report

Prepared for Department of Defense

Calibration Coordination Group

Working Group on Mass and Pressure

Project No. 72-73

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

A STUDY OF THE FEASIBILITY OF OBTAINING A SYSTEMS

CALIBRATION OF A LOAD CELL AND INDICATOR FROM

INDEPENDENT CALIBRATIONS OF THE COMPONENTS

1. INTRODUCTION

Load cells and indicators are customarily calibrated as complete

systems by applying accurately known loads to the cell and noting the

corresponding readings on the indicator. Normally the indicator is

supplied with an internal electrical calibration check consisting of

a passive resistance circuit that can be substituted for the load cell

bridge to present a single-point bridge unbalance simulating the output of a loaded load cell. The use of the calibration check in service

gives reasonable assurance that the indicator is showing the same

electrical response as it did during the initial calibration of the

system. The calibration check does not verify the response of the load

cell output to load.

Since frequently an indicator is used with a number of load cells,

it would be advantageous to be able to calibrate a load cell independently

and match the resulting data to the indicator to obtain a systems

calibration. Such a technique would minimize the out-of-service time

for the indicator during the period needed to transmit the equipment

to the standardizing laboratory.

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2. APPROACH TO THE PROBLEM

One possible method of accomplishing the independent calibration

goal would be to calibrate the load cell by measuring the ratio of

the bridge output voltage to the bridge input voltage under a series

of precisely known loads. This technique, generally referred to as

a millivolt per volt calibration, is a well-established method and can

be performed with adequate accuracy. The load cell indicator would

have been originally calibrated in terms of its response to millivolt

per volt ratios. This technique is less well established, although

there are available several types of resistance networks which simulate

load cell bridges and afford specified ranges of voltage ratio steps.

From such a calibration, indicator readings could be computed corresponding to the voltage ratios obtained in the load cell calibration

to provide a systems calibration.

In this study, the technique was applied to a series of load cell

and indicator systems and the results compared to a direct systems

calibration at the same test loads. The load cell systems were largely

those being calibrated for DoD standards laboratories together with

some NBS systems. Each load cell and indicator combination was subjected

to the following operations.

A. A systems calibration by deadweights was performed on the

load cell and indicator at 15 or more different loads. A

second degree calibration equation was fitted to the data to

obtain a smooth curve of indicator readings.

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Immediately following step A, a voltage ratio calibration

at the same deadweight loads was performed on the load cell.

A second degree calibration curve was fitted to these data

in the same manner as outlined in step A. The voltage ratio

measurements were made with a direct current millivolt per

volt indicator reading to 0.01 microvolt per volt. This

least count is equivalent to 1 part in 200,000 of the voltage

ratio of a 2 mV/V load cell at capacity load. The reference

indicator was in turn calibrated against a four-terminal

resistance network, voltage ratio standard which had been

calibrated by the Resistance Section of the National Bureau

of Standards. The precision of the calibrated values for

the standard was given as not exceeding 0.05 microvolt per

volt. This four-terminal network was used as the reference

standard for all measurements reported in this study.

The load cell indicator was then connected to the fourterminal voltage ratio standard which had been adjusted

to have the same input and output resistance as the load

cell. Indicator readings were obtained for a series of

voltage ratios matching the load cell ratios as closely as

practicable. A second degree curve was fitted to these data

to permit the calculation of indicator readings corresponding

to any applied voltage ratio .

The voltage ratios obtained in step B were converted to

indicator readings by means of the relationship established

in Step C and the resulting values compared to the data

from step A.

3. RESULTS

Six 100,000-lbf force calibration kits were tested according

to the plan. Each kit consisted of a single 100,000-lbf compression

load cell, 120 ohms input and 2 millivolts per volt output at capacity.

The indicator was a manually balanced, alternating current bridge graduated

into 100,000 units. The following results were obtained in the

comparison of step D.

Table 1 - Direct vs. Independent Calibration for Six 100,000-lbf Load

Cell Systems

Load Nominal Difference between direct and independent

indication component calibrations.

1 2 3 4 5 6

kips units units

5 5,000 22 5 11 20 11 29

10 10,000 12 -9 2 7 -11 21

15 15,000 2 -22 -6 -7 -32 11

20 20,000 -9 -37 -16 -21 -53 0

25 25,000 -21 -51 -25 -36 -73 -11

30 30 000 -33 -67 -35 -51 -92 -24

35 35,000 -46 -82 -45 -67 -111 -37

40 40,000 -59 -99 -55 -83 -130 -52

45 45,000 -74 -115 -66 -100 -148 -67

50 50,000 -89 -133 -77 -117 -165 -83

60 60,000 -120 -169 -99 -153 -199 -119

70 70,000 -155 -207 -123 -191 -231 -159

80 80,000 -193 -247 -149 -232 -261 -202

90 90,000 -233 -290 -175 -274 -289 -250

100 100,000 -277 -335 -204 -319 -316 -301

The difference is obtained by subtracting the direct systems calibration

values from the values calculated from the independent component calibrations.

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A similar result was obtained for a 60,000-lbf load cell having

an input resistance of 350 ohms and a 3 millivolt per volt output at

capacity. The cell was made by a different manufacturer than those above

and the indicator was of different design. The indicator was manually

balanced with an alternating current carrier.

Table 2 - Direct vs. Independent Calibration for a 60,000-lbf

Load Cell System

Load Nominal

indication

Difference

kips units units

3 2,700 -7

6 5,400 -11

9 8,100 -16

15 13,500 -24

18 16,200 -28

24 21,600 -37

27 24,300 -41

30 27,000 -45

36 32,300 -53

45 40,400 -65

48 43,100 -69

54 48,500 -77

60 53,900 -85

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The results in tables 1 and 2 suggest a strong bias between the

condition of the indicator coupled to a load cell and the same indicator

coupled to the four-terminal ratio standard. The suggestion was made

that there were reactive components in the impedance of the four-terminal

ratio standard that do not appear in its primary calibration by direct

current methods. Further investigation of this hypothesis was beyond

the limited scope of the project.

As a test of the possibility that the differences arise from some

factor connected with the use of alternating current, the technique was

applied on a direct current indicator loaned by the office of the Naval

Plant Representative at Pomona, Calif. This indicator was provided with

a 12-step, multiple range, internal voltage ratio standard. The unit

read in percent of capacity load and was adjustable over a wide range

of voltage ratios. The scale was graduated to 0.001 percent of capacity

load.

Four load cells were calibrated with this unit, following the plan

outlined above. In step C where the unit was compared to the four-terminal

voltage ratio standard it was found that a linear slope correction was all

that was needed to provide satisfactory conversion from voltage ratio

values to indicator readings. The following results were obtained in the

comparison of step D.

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Table 3 - Direct vs Independent Calibration for Four Load Cells and a Direct Current Indicator

Approximate Differences in percent of capacity load

indication in - 1,000 Ibf 2,000 Ibf 5,000 Ibf 10,000 Ibf

percent of load cell load cell load cell load cell

capacity load

5 -0.003 0.002 -0.002

10 -0.012 -0.003 0.003 0.000

15 -0.012 -0.002 0.004 0.001

20 -0.012 -0.001 0.004 0.003

25 -0.012 -0.001 0.005 0.004

30 -0.012 0.000 0.005 0.005

35 -0.012 0.001 0.005 0.006

40 -0.011 0.001 0.005 0.007

45 -0.010 0.002 0.005 0.008

50 -0.009 0.003 0.005 0.009

55 -0.008 0.005 0.009

60 -0.007 0.004 0.004 0.009

65 -0.006 0.005 0.004 0.010

70 -0.004 0.005 0.003 0.010

75 -0.003 0.006 0.002 0.009

80 -0.001 0.007 0.001 0.009

85 0.001 0.007 0.001 0.009

90 0.004 0.008 -0.001 0.008

95 0.006 0.009 -0.002 0.008

100 0.008 0.009 -0.003 0.007

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The differences shown in Table 3 are somewhat greater than could

be accounted for from the precisions obtained in the calibrations of

the NBS reference millivolt per volt indicator and the Navy indicator

against the four-terminal millivolt per volt standard. Characteristic

standard deviations observed in the linear slope fits for these two

indicators were approximately 0.11 microvolts per volt (0.0055 percent

of 2000 microvolts per volt) for the NBS reference indicator and

0.012 microvolts per volt (0.0006 percent of 2000 microvolts per volt)

for the Navy indicator. The differences of Table 3 range up to

0.012 percent of capacity load, but the slight increase may very well

have been due to other factors such as variability in the load cell

outputs. It should be noted, however, that direct current indicators

are not frequently used for load cell measurements in the field, most

of the applications being the combination of a load cell and some form

of alternating current indicator.

As a part of the examination of the indicator furnished by the

Navy, a series of measurements was made of the internal voltage ratio

standardizer of the instrument set to the 2 millivolt per volt range.

The measurements were made with the Navy indicator, each run being made

with a different span adjustment and the instrument calibrated against

the four-terminal voltage ratio standard. The five runs were spaced

over a period of slightly more than two weeks. Table 4 shows the

voltage ratio values obtained for the internal standardizer and the

deviations of the individual runs from the average.

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Table 4 - Voltage Ratio Measurements of the Internal Standardizer , S/N 26279,

of a Direct Current Indicator

Standardizer step, Average Deviations from average

as percent of value

2 millivolts/volt of step Run 1 Run 2 Run 3 Run 4 Run 5 microvolts per volt

10 200.18 0.00 0.00 0.00 0.00 0.01

20 400.08 -0.01 0.00 -0.01 0.01 0.00

30 600.08 0.00 0.00 0.00 0.00 0.01

40 800.09 0.01 0.00 0.00 0.00 0.01

50 1000.10 -0.01 0.00 0.01 0.00 0.01

60 1200.12 -0.02 0.00 0.00 0.00 0.01

70 1400.16 -0.01 0.00 0.00 0.00 0.00

80 1600.20 0.01 0.00 0.00 -0.01 0.00

90 1800.25 0.00 0.00 0.00 -0.02 0.00

100 2000.25 0.00 0.00 0.00 -0.01 0.00

110 2200.31 -0.01 0.02 -0.02 0.00

120 2400.28 -0.02 0.00 0.03

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A. SUMMARY

From the results shown in Tables 1 and 2 of this report, it is

concluded that independent calibration of load cells and alternating

current type indicators may result in errors on the order of 0.3 or 0.4

percent of load and that, consequently, the method is not feasible without

further investigation. It was suspected, but not proven, that the source

of the error lies in undetected reactive (inductance) components in the

millivolt per volt ratio standard. Only one such unit was employed in

the investigation and it is possible that the fault was peculiar to this

unit. However, it is recognized that in the design of a resistance

standard where a stability and precision on the order of 20 to 30 parts

per million is required, it is exceedingly difficult to insure absolute

freedom from inductance in the windings. Alternating current indicators

commonly operate at frequencies of 1000 to 1200 Hz and only a very small

amount of inductance in the standard is required to produce a significant

error.

When an indicator operating on direct current was tested, the method

of independent calibration of load cells and indicators worked acceptably

well. As noted earlier in the report, direct current indicators are not

commonly used in load cell systems, alternating current indicators being

generally lighter and more compact as well as being somewhat faster in

operation. Direct current indicators also require special attention to

assure that observed data are not biased by thermal electromotive forces.

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A direct current indicator furnished for the investigation by the

office of the Naval Plant Representative in Pomona, California, displayed

outstandingly good performance during all tests made with it. This

indicator normally reads in percent of capacity load, but the indications

may be easily transformed into voltage ratio values by means of a linear

factor.

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