NBS TECHNICAL NOTE 497

National Bureau or Stanjar*

Library, E-Ol Mmin. B1*

oct n «9

The Effects of Extended High-Temperature Storage

on the Performance Characteristics

of Several Strain Gage Pressure Transducers
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NBS TECHNICAL NOTE 497

ISSUED OCTOBER 1969

Nat. Bur. Stand. (U.S.), Tech. Note 497, 22 pages (Oct. 1969)

CODEN: NBTNA

The Effects of Extended High-Temperature Storage

on the Performance Characteristics

of Several Strain Gage Pressure Transducers

Randolph Williams

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, Government Printing Office

Washington, D.C, 20402 - Price 30 cents
FOREWORD

The work described in this note was performed as part of

a project on development of methods for evaluation of electromechanical transducers. The project has been jointly supported

by a number of government agencies, including the Naval Air

Systems Command, U. S. Navy and the Office of Advanced Research

and Technology, NASA. During the period of the performance of

this work, the Project Leader was Paul S. Lederer.

Joshua Stern

Chief, Section 425.03

11
CONTENTS

Page

INTRODUCTION 1

TEST EQUIPMENT 2

TEST PROCEDURE 2

STATIC TEMPERATURE TEST RESULTS 3

TEMPERATURE -STORAGE TEST RESULTS 5

HIGH- TEMPERATURE -STORAGE EFFECTS ON TRANSDUCER

SENSITIVITY 5

HIGH- TEMPERATURE-STORAGE EFFECTS ON TRANSDUCER

ZERO PRESSURE OUTPUT 6

HIGH- TEMPERATURE -STORAGE EFFECTS ON LINEARITY

AND HYSTERESIS 7

SUMMARY OF TEST RESULTS 7

CONCLUSION 8

ACKNOWLEDGEMENT 8

REFERENCES 8

LIST OF TABLES

Table I : Transducer Specifications 10

LIST OF FIGURES

Figure 1: Extended High-Temperature-Storage Test Equipment 11

Figure 2: Constant Voltage Static Temperature Test - Metallic

Strain Gage Pressure Transducers 12

Figure 3: Constant Voltage Static Temperature Test - Semiconductor Pressure Transducers, Effect of Temperature

on Transducer Sensitivity 13

Figure 4: Constant Voltage Static Temperature Test - Semiconductor Pressure Transducers, Effect of Temperature

on Transducer Zero Value 14

iii
LIST OF FIGURES (continued)

Page

Figure 5: Typical Daily Trend During High-Temperature

Storage- -Unbonded Metallic-Strain-Gage Pressure

Transducer Al 15

Figure 6: Sensitivity Shift During High-TemperatureStorage Test 16

Figure 7: Zero Shift During High-Temperature-Storage

Test 17

Figure 8: Effect of Temperature Storage on Linearity

and Hysteresis - Transducer A3 18

IV
The Effects of Extended High-Temperature Storage

on the Performance Characteristics of Several

Strain Gage Pressure Transducers

by

Randolph Williams

Institute for Applied Technology

Electronic Technology Division

ABSTRACT

This publication reports the results of a test program to

evaluate the effects of high-temperature storage on the performance characteristics of several types of commercial strain

gage pressure transducers. The results obtained indicate shifts

in the zero pressure output and the sensitivity, some of which

are permanent. The equipment used and the procedure followed

are described.

KEY WORDS: Pressure transducer, strain gage, life testing,

temperature storage, temperature cycling, sensitivity shift,

zero shift.

INTRODUCTION

With the increasing use of transducers for measurement and control

in industrial applications, the stability of the performance characteristics of these devices becomes very important. This is particularly

important in computer-controlled industrial processes where transducers

may have to operate reliably over relatively long periods of time and

under adverse environmental conditions.

In a task recently completed as part of the NBS InterAgency Transducer Project , the effects of many thousands of pressure cycles on the

performance characteristics of strain gage pressure transducers were investigated and reported ("Life Cycling" Test on Several Strain Gage

Pressure Transducers, Technical Note No. 434). The temperature environment in which a transducer operates may also have a considerable effect on its performance. Evaluation of the temperature characteristics

over a short period of time (hours) is a well established procedure. No

publications have been found, however, which investigate the effect of

long term storage (weeks) of transducers at elevated temperatures within

the manufacturer's specified operating range. Accordingly, a test program

was initiated to investigate temperature storage effects on a number of

commercial strain gage pressure transducers, commonly used in aerospace

technology and of particular interest to our sponsors, including types

with bonded metallic wire strain gages, metallic foil strain gages, and

semiconductor strain gages. The maximum specified operating temperature
for eight of the transducers was 121°C (250°F) . The remaining two transducers had maximum specified operating temperature of 93°C (200°F) and

343°C (650°F) , respectively. Seven transducers were subjected to a temperature of about 107°C (225°F) for five weeks followed by a three week

period at laboratory ambient temperature. The remaining three transducers, including the one with the maximum operating temperature of 93°C,

were subjected to a maximum temperature of 91°C in the same procedure so

as not to exceed the 93°C limit of that one transducer.

The terms describing the calibration of transducers in this paper

conform to the definitions as given in ISA RP37.1, "Nomenclature and

Specification Terminology for Aerospace Test Transducers with Electrical

Outputs."

TEST EQUIPMENT

The test equipment used for transducer calibration in this laboratory is described in detail in NBS Technical Note No. 411, Method for

Performance-Testing of Electromechanical Pressure Transducers. Summarizing briefly, the test set-up was composed of a dead-weight piston

gage capable of generating pressures up to 500 psi with an uncertainty

of less than +0.05% and a temperature chamber with a range of -73 to

273°C and an uncertainty of +_1/2°C. For the temperature-storage evaluation, the system was modified to supply pressures of equal amplitude

simultaneously to four transducers which were mounted inside of the

temperature chamber. Thus static calibrations could be performed on as many as four transducers using one read-out device and a four-position

switch. The pressure connection from the dead-weight piston gage to the

transducers was made with copper tubing which passed through the door of

the temperature chamber. The temperature of the chamber compartment was

set and monitored by an externally adjustable thermostat. A wire thermocouple attached to the fixture holding the transducers was used to verify

the temperature of the transducer and mounting structure. A photograph

of the test set-up appears in Figure 1.

Transducer excitation voltages were supplied by commercially available semiconductor power supplied having drift characteristics specified

as less than 0.2% during an eight -hour period following warm-up. A

commercially available laboratory potentiometer capable of reading up to

11 volts was used to measure the excitation inputs. In cases where the

excitation voltage exceeded the rating of the potentiometer, a precision

voltage divider was used at the input to the potentiometer. The error

in the measurement of the excitation voltage, with or without the voltage

divider, was estimated to be less than +..02%. The same laboratory potentiometer was used to measure transducer output voltage.

TEST PROCEDURE

The temperature storage tests were run in three groups, the first

two groups being composed of three transducers and the last group, four.

2
One of three transducers of identical model number was included in each

group. 1 Each run was begun by performing three successive calibrations

at laboratory ambient conditions within an eight-hour period on each

group of transducers to obtain information on the short term repeatability of the transducers. The static temperature characteristics of

the transducers were then determined by performing eleven-point calibrations to full scale at several temperatures within operating ranges. The

results of the temperature test were plotted in terms of sensitivity

shift and zero shift versus temperature. The next portion of the calibration procedure consisted of high-temperature-storage evaluation. The

test procedure required five consecutive days and, therefore, was begun

on Monday mornings. The group of transducers was first calibrated at

laboratory ambient temperature. Following the calibration the chamber

temperature was set to a level near, but not exceeding, the maximum

operating temperature of the transducers. After the transducer temperature had reached equilibrium, the transducers were recalibrated while at

the higher temperature near the end of the first day (5-6 hours after the

initial calibration) and on each of the next three days. On the fifth

day the transducers were again calibrated at the higher temperature after

which the chamber was brought down to ambient temperature. After adequate time was allowed for temperature equilibrium, the transducers were

recalibrated. This procedure was followed for five successive weeks.

During the sixth week, the transducers were held at ambient temperature

and one static calibration performed daily. During the seventh week, the

transducers remained at ambient temperature, but no calibrations were

performed. In the eighth week, three calibrations were performed within

an eight-hour period on one day. The entire test procedure was carried

out on each of the three groups of transducers. At appropriate points

in the procedure, the data were plotted and evaluated. The voltage

excitation was continuously applied to the transducers during the entire

eight weeks of the storage test period.

STATIC TEMPERATURE TEST RESULTS

All ten transducers were designed for constant voltage operation.

Other pertinent transducer characteristics obtained from the manufacturers are given in Table I. As indicated in the table, the maximum

operating temperature for transducer F was specified as 93°C. Thus

transducer F and the two transducers tested with it, transducers A3 and

G, were tested to 93°C in the static temperature test and stored at 91°C

in the high-temperature procedure. The other transducers were calibrated

to a maximum temperature of 117°C and stored in the extended storage

Originally, the three transducers were to be used as test monitors since

these transducers were expected to have similar reaction to temperature

environments. Contrary to expectation, the three transducers, Al, A2,

and A3, exhibited dissimilar zero shifts in the static temperature test.

See Static Temperature Test Results.
procedure at about 107°C. The results of the static temperature runs are plotted in Figure 2 for bonded and unbonded metallic-strain-gage-type

transducers; in Figures 3 and 4 for the semiconductor-type transducers.

The open points in Figures 3 and 4 represent calibration points within

the operating range of the particular transducer but outside of the

range for which temperature shift specifications are given by the manufacturer. No plot is shown for the high-temperature strain gage pressure

transducer since it was not tested to its maximum operating temperature.

Six of the ten transducers were metallic-strain-gage pressure transducers designed for operation to 121°C. Transducers Al, A2, and A3 were

unbonded wire-strain-gage types carrying the same model number. Each of

the three test groups included one of these unbonded transducers. Transducer B was a bonded wire-strain-gage type, while transducers C and D were bonded metallic-foil-strain-gage types. The temperature-sensitivity

curves for the six metallic-strain-gage pressure transducers were observed

to be approximately linear with the largest thermal sensitivity being only

about 0.02%/°C over the range from about -40 to 117°C. The temperaturezero pressure output curves for six transducers showed more variety than

the temperature-sensitivity curves. Two unbonded strain-gage-type transducers carrying the same model number exhibited the largest positive and

negative thermal zero shift of the six transducers tested. Transducer

A3 had a negative thermal zero shift with a maximum change of about

-2.5% FS over the temperature range from -40 to 117°C while transducer

A2 had a thermal zero shift of +1.6% FS over the same range. The re- maining four metallic-strain-gage pressure transducers had thermal zero • shifts of less than 1% FS over the same range.

Transducer E was an unbonded high-temperature wire-strain-gage transducer designed to operate from -196 to 343°C. Static temperature tests

on this transducer covered only the range from -40 to 243°C. Over this

range the sensitivity of transducer E varied a maximum of 0.4% from its

value at room temperature. The thermal zero shift observed for transducer E was 4.9% FS over the tested range. 2

The semiconductor-type transducers showed more variation in sensitivity due to temperature change than the conventional strain-gage type

transducers. Transducers F and G had semiconductor strain elements

bonded to their diaphragms. The semiconductor element of transducer H was described as being bonded to a spring element attached to the diaphragm by a push rod. The temperature-sensitivity curves for all three

2Actually, two complete static temperature calibrations were performed on transducer E. The first test indicated a thermal zero shift of about

17%/100°C over the range from -40 to 243°C. Since this shift was far

beyond tolerances, the manufacturer was consulted as to the probable

cause. The defect was traced to a poorly fitting mounting adapter. After

the adapter was replaced transducer E was recalibrated. The results

appearing in the text represent the results of this second calibration.
transducers had similar shape but different magnitudes. Within the

region -18 to 54°C (0 to 130°F) , transducer H had the largest thermal

sensitivity shift of the three transducers. Outside of the region, the

effects of temperature compensation become apparent in transducers F and H as indicated by the bends in their curves. The temperature-zero

pressure output curves for the semiconductor-type transducers show that

in the region -18 to 54°C, transducer F had a maximum variation of less

than 1/2% FS, and transducer G had a maximum variation of about 2 1/2% FS.

The temperature- zero pressure output curve obtained for transducer H

formed a hysteresis loop. During the test, transducer H reacted to each

temperature change as if the transducer had received a thermal shock.

Because of this, the normal one-hour allowance for temperature equilibrium

was extended to assure that the transducer's temperature had equalized.

The approximate heating rate of the air inside the temperature chamber

was about 24°C per minute.

Recalibration of transducer E with an adapter designed to alleviate

case strain problems resulted in a thermal zero shift of about 4.9% FS/

100°C over the same temperature range. Static temperature tests on this

transducer did not cover its entire operating temperature range. The

sensitivity of transducer E varied a maximum of 0.4% from its value at room temperature over the tested range.

TEMPERATURE-STORAGE TEST RESULTS

Figures 5, 6, and 7 show the trends in the ambient temperature sensitivity and values resulting from the eight week temperature test. Figure

5 represents the typical daily pattern of the shifts noticed during the

extended storage procedure. Figures 6 and 7 represent the effects of the

temperature storage on the weekly ambient temperature calibrations for

all transducers. Zero pressure outputs and sensitivities obtained in the

high-temperature storage were referred to the initial ambient temperature

calibration in the first week of the extended storage test. To determine

the effects of temperature storage during a given week, the sensitivity

and zero pressure output obtained for the ambient temperature calibration

at the end of the week and the ambient temperature calibration at the

beginning of the next week were averaged. For the weeks when the transducer was stored at high temperature (the first five weeks) , this average was plotted. For the weeks when the transducer was stored at ambient

temperature, the weekly average was plotted.

HIGH-TEMPERATURE-STORAGE EFFECTS ON TRANSDUCER SENSITIVITY

The high-temperature-storage effects on transducer sensitivity are plotted in Figure 6. With the exception of transducers A2 and D, the

conventional strain-gage-type transducers underwent maximum sensitivity

shifts of less than 0.1%. Transducer A2 underwent a rapid sensitivity

shift of 0.2% during the initial week; following the rest period, the

sensitivity of transducer A2 recovered to within 0.1% of its initial
value. Transducer D underwent an initial sensitivity shift, recovered

somewhat, and then showed a gradual increase in sensitivity until the

beginning of the rest period. The sensitivity of transducer D stabilized

within 0.2% of its initial value during the rest period. The sensitivity

did not return to its original value during the remainder of the test

period.

The high- temperature strain gage pressure transducer, E, showed its

largest sensitivity shift during the second week of temperature storage.

Transducer sensitivity changed a maximum of about +0.4% from its initial

value. The sensitivity did not recover from this shift during the re- mainder of the test period.

The sensitivities of the three semiconductor-type transducers, F, G,

and H, showed a notable change following the first week of hightemperature storage. By the fourth week, the transducer sensitivities

had reached the maximum departure from their initial values. They began

to level off or recover during the fifth week. During the rest period

following the fifth week, the sensitivities drifted back toward their

initial value. Transducer F, which had a maximum shift of 0.4% from its

initial sensitivity, recovered to within 0.14% of that sensitivity by

the eighth test week. Transducer G had a maximum shift of about 0.2%

and recovered to within 0.02% by the eighth week. Transducer H had a maximum shift of about 0.7% from its initial value and recovered to

within 0.4% of that value by the eighth week.

HIGH-TEMPERATURE-STORAGE EFFECTS ON TRANSDUCER ZERO PRESSURE OUTPUT

The effects of temperature storage on zero pressure output are shown

in Figure 7. For the wire-strain-gage pressure transducers, zero pressure output shifted markedly during the first week of storage test and

more gradually afterwards. Transducers Al, A2, A3, B, and D had positive

zero shifts while transducer C had a negative zero shift. The gradual

zero shift continued through the entire high-temperature procedure until

the transducers were stored at ambient temperature (the rest period)

.

During the first week of the rest period, each zero pressure output stabilized at some fixed level and did not vary notably from that level through

the remainder of the rest period. The following shifts were observed in

the transducer zero pressure outputs following the high-temperaturestorage test: transducer Al, about +3.1% FS; transducer A2 , about

+2.3% FS; transducer A3, about +0.9% FS; transducer B, about +3.3% FS;

transducer C, about -1.4% FS; and transducer D, about +0.7% FS.

The high- temperature pressure transducer, E, also had a marked

shift in zero pressure output after the first week of high-temperature

storage. The zero pressure output continued to decrease until after the

first week of the rest period. By the eighth week of the test period

(the third week of the rest period) , the zero pressure output of transducer E had stabilized at about -3.3% FS from its initial value.
The zero pressure output of the semiconductor- type pressure transducers reacted differently to the high- temperature storage. The zero pressure output of transducer F showed little change due to hightemperature storage. Its zero pressure output had a maximum shift of

about +0.15% FS during the first five weeks of test and had recovered to

within +.05% FS at the end of the test period. Transducer G had a gradual zero output shift of about +4.3% FS during the first five weeks

and leveled off at about +4.4% FS at the end of the test period. The

zero pressure output of transducer H decreased irregularly (following a slight initial increase), reaching about -0.9% FS at the end of the fifth

week of the test period. During the first week at ambient temperature,

however, it recovered to within -0.3% FS from its initial value, where

it remained throughout the remainder of the test period.

HIGH-TEMPERATURE- STORAGE EFFECTS ON LINEARITY AND HYSTERESIS

Linearity is herein defined as the maximum deviation from the bestfit straight line as determined by least squares through all calibration

points. Hysteresis is herein defined as "the maximum different in output

at any given measurand value when that value is approached first with increasing and then decreasing measurand." (ISA RP37.1) Figure 8 shows

the linearity and hysteresis curve for three ambient temperature calibrations of transducer A3; the initial ambient, the ambient beginning the

fifth week, and the final ambient of the eighth week. This transducer

had the largest change in linearity and hysteresis during the extended

temperature storage tests. From the graphs of Figure 7 and similar

graphs for the other nine transducers, it appears that the extended high

temperature storage had no significant effect on the ambient temperature

linearity and hysteresis values.

SUMMARY OF TEST RESULTS

Briefly, the following information has been obtained from observation of the ten commercial transducers tested:

1. The five week high-temperature-storage program produced sensitivity shifts ranging from -0.7% to +0.3%, and zero shifts

ranging from -2.5% full scale to +4.5% full scale.

2. Apparent permanent changes at the end of the entire eight week

stroage and rest program ranged from -0.4% to +0.4% for sensitivity, and from -3.0% full scale to +4.5% full scale for zero output

.

3. Extended storage of a transducer at temperatures near its maximum operating temperature can result in permanent changes in the

transducer performance characteristics.

4. Conventional metallic-strain-gage pressure transducers as well

as semiconductor-strain-gage pressure transducers are

7
susceptible to high-temperature-storage effects with the zero pressure output being more susceptible to permanent change

than the sensitivity.

5. High- temperature-storage effects are apparently not prevented

nor reduced by temperature compensation. Although a transducer

may be compensated over a temperature range, extended storage

within that range can still cause performance changes.

6. The change in performance characteristics due to high temperature storage appears to be not only a function of transducer

design but also of the quality of assembly of the particular

transducer. Thus storage effects observed in one transducer

of a particular model may differ from that of another transducer of the same model.

CONCLUSION

The results of this 5-week test program indicate that changes in

the sensitivity and zero characteristics of electromechanical pressure

transducers can occur as a result of storage at temperature above

ambient laboratory conditions. In some cases these performance changes

are permanent. High- temperature soaking of some transducers before

putting them into service at elevated temperature may reduce performance

changes during their operation since some transducers were observed to

have their most drastic changes during the early portion of this

temperature-storage test.

To assure reliable performance of pressure transducers operated at

elevated temperatures for extended periods of time, the effects of the

environment on the particular transducer must be investigated, since

zero shifts up to 4.5% FS were observed in this program in which the

pressure transducers were subject to the elevated temperatures for only

5 weeks. Some applications of transducers require that the transducer

operate reliably in elevated temperature environments for much longer

periods

.

ACKNOWLEDGEMENT

The technical assistance of John H. Pinkard, Jr. is gratefully

acknowledged for his aid in compiling the data for this report.

REFERENCES

Lederer, P. S., "Methods for the Performance Testing of Electromechanical Pressure Transducers." NBS Technical Note No. 411

(February 1967) .
2. Lederer, P. S. , " 'Life Cycling' Test on Several Strain Gage

Pressure Transducers." NBS Technical Note No. 434 (October

1967).

3. Horn, Leon, "The Response of Flush-Diaphragm Pressure Transducers to Thermal Gradients." ISA Pre-print 13.3-4-65.

4. "Nomenclature and Specification Terminology for Aerospace Test

Transducers with Electrical Output." ISA RP37.1 (April 1963).
TABLE I

Transducer Specifications

(Furnished by the Manufacturers)

Identification

Type

Metallic Strain Gage

Al

A2

A3

Unbonded wire strain

gage

Unbonded wire strain

gage

Unbonded wire strain

gage

Bonded wire strain

gage

Bonded foil strain

gage

Bonded foil strain

gage

Pressure

Range

psi

Operating

Range

0-50

0-50

0-50

0-50

0-50

0-50

High - Temperature Metallic Strain Gage

Wire strain gage

Semiconductor Strain Gage

Silicon strain gage

Bonded semiconductor strain gage

Semiconductor strain

gage

0-50

0-50

0-50

0-50

-65 to 250

65 to 250

65 to 250

65 to 250

100 to 250

-100 to 250

Sensitivity

Shift

%/°F

320 to 650

-40 to 200

-65 to 250

-20 to 250

02

,02

.02

01

03

.02

01

.01

.01

Zero

Shift

% FS/°F

03

03

03

.01

,03

03

Manufacturers Error

Band % FS

02

,01

01

.5

.5

.25

.1

5

75

^Specified as "within 2% over compensated range'

10
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Figure 8: Effect of Temperature Storage on Linearity

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18
THE NATIONAL ECONOMIC GOAL

Sustained maximum growth in a free market economy, without inflation,

under conditions of full employment

and equal opportunity

THE DEPARTMENT OF COMMERCE

The historic mission of the Department

is "to foster, promote and develop the

foreign and domestic commerce" of the United States. This has evolved, as a result of legislative and administrative

additions, to encompass broadly the re- sponsibility to foster, serve and promote

the nation's economic development and

technological advancement. The Department seeks to fulfill this mission through

these activities:

^<°?<V

MISSION AND

FUNCTIONS

OF THE

DEPARTMENT OF

COMMERCE

"to foster, serve and

promote the nation's

economic development

and technological

advancement"

iflli

%$:a

Participating with

other government

agencies in the

creation of national

policy, through the President's Cabinet

and its subdivisions.

• Cabinet Committee

on Economic Policy

• Urban Affairs Council

•Environmental

Quality Council

Promoting progressive Assisting states, business policies and communities and

growth. individuals toward economic progress. • Business and Defense Services • Economic

Administration Development

Administration • Office of Field Services • Regional Planning

Commissions

• Office of Minority Business Enterprise

NOTE: This schematic is neither an organization chart nor a -?fogram outline for budget purposes. It is a general statement

of the Department's mission in relation to the national goal

of economic development.

Strengthening

the international economic position

of the United

States.

• Bureau of International Commerce

• Office of Foreign

Commercial

Services

• Office of Foreign

Direct Investments

• United States

Travel Service

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Administration

Assuring effective use and growth of the

nation's scientific and technical resources.

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Administration

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Technical Services

Acquiring, analyzing and disseminating

information concerning the nation and

the economy to help achieve increased

social and economic

benefit.

• Bureau of the Census

• Office of Business Economics

'JULY 1969

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