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NBS TECHNICAL NOTE 961

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

stigation

of Means for Reducing the

esponse of Pressure Transducers

to Thermal Transients
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress 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 government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the

Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — Laboratory Astrophysics 2 — Cryogenics 2 — Electromagnetics" — Time and Frequency".

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

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the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units:

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Located at Boulder, Colorado 80302.
RATIONAL BUREAU

OF STANDARDS

LIBRARY

o~r

Experimental Investigation of Means

for Reducing the Response of Pressure Transducers

iw to Thermal Transients * * 4 „

John S. Hilten, Carol F. Vezzetti,

J. Franklin Mayo-Wells, and

Paul S. Lederer

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

Naval Air Systems Command

Washington, D.C. 20361

and

Transducer Committee TG/RCC

c/o Div. 9486

Sandia Laboratories Albuquerque, New Mexico 87115

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued January 1978
National Bureau of Standards Technical Note 961

Nat. Bur. Stand. (U.S.), Tech. Note 961, 64 pages (Jan. 1978) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.30 -Stock No. 003-003-01875-9
FOREWORD

The work described was performed as a task within the NBS InterAgency

Transducer Project. This is a continuing project for the development of

calibration and evaluation techniques for electromechanical transducers

and is supported by the National Bureau of Standards and a number of oth- er Government agencies. This task was assigned by the Transducer Committee, Telemetry Group, Range Commanders Council, with the concurrence of

the Naval Air Systems Command, U. S. Navy.

Paul S. Lederer, Chief

Components and Applications Section

i i i

CONTENTS

EXPERIMENTAL INVESTIGATION OF MEANS FOR REDUCING THE RESPONSE OF

PRESSURE TRANSDUCERS TO THERMAL TRANSIENTS

Page

Abstract 1 1. Introduction 2 2. Development of Method for Evaluating Coating Performance

on Metal Disks 2 2.1 Preliminary Work 3

2.2 Description of Apparatus 3 2.3 Test Procedure Considerations 4 2.4 Experiments with Various-Sized Apertures and Disk

Thicknesses 5 2.5 Experiments to Determine Method Repeatability 5 2.6 Experiments with Various Coatings 6

2.7 Test Results 6

3. Experimental Evaluation of Coatings on Pressure Transducers . . 7 3.1 Selection of Coatings 8

3.2 Selection of Transducers 8

3-3 Application of Protective Coatings 9

3.4 Description of Tests 9

3-4.1 Zero-Shift Measurements 9

3.^.2 Dynamic Response Measurements 11 3-4.3 Acceleration Sensitivity Tests 12

4. Results from Tests of Coatings on Pressure Transducers 13

4.1 Zero Shift 13

4.1.1 Unbonded Strain-Gage Transducers 13

4.1.2 Quartz-Crystal Transducers 14

4.1.3 Semiconductor Strain-Gage Transducer 15

4.1.4 Lead Metan iobate-Crystal Transducers 15

4.2 Dynamic Response 16

4.2.1 Unbonded Strain-Gage Transducers 16

4.2.2 Quartz-Crystal Transducers 17

4.2.3 Semiconductor Strain-Gage Transducer 18

4.2.4 Lead Metan iobate-Crystal Transducers 18

4.3 Acceleration Sensitivity 18

4.4 Coating Evaluations 19

5. Conclusions 20

6. Recommendations 20

7. Acknowledgment 21

8. References 21

Tables 22

Figures 32

Appendix A 53
TABLES

Page

1. Zero Shift Resulting from Thermal Radiant-Energy Transient

Applied to Protected and Unprotected Unbonded Strain-Gage

Pressure Transducers 22

2. Zero Shift Resulting from Thermal Radiant-Energy Transient

Applied to Protected and Unprotected Quartz-Crystal Pres- sure Transducers .23

3. Zero Shift Resulting from Thermal Radiant-Energy Transient

Applied to Protected and Unprotected Semiconductor StrainGage Pressure Transducer 2k

k. Zero Shift Resulting from Thermal Radiant-Energy Transient

Applied to Protected and Unprotected Piezoelectric Lead

Metaniobate-Crystal Pressure Transducers 25

5. Zero Shift Resulting from Thermal Radiant-Energy Transient

Applied to Unprotected Pressure Transducers - Summary of

Responses of Four Pairs of Transducers 26

6. Response of Two Unbonded Strain-Gage Pressure Transducers,

One Protected and One Unprotected, to a Pressure Step of

280 kPa 27

7. Response of Two Quartz Crystal Pressure Transducers, One

Protected and One Unprotected, to a Pressure Step of 280 kPa . 28

8. Response of One Semiconductor Strain-Gage Pressure Transducer, Protected, to Pressure Step of 280 kPa 29

9. Response of Two Lead Metaniobate Crystal Pressure Transducers

One Protected and One Unprotected, to Pressure Step of 280

kPa 30

10. Comparison of Coating Effectiveness 31

FIGURES

1. Sketch of thermocouple fixture with cross-section detail

of thermocouple mounted in ceramic rod 32

2. Arrangement of test-method apparatus 33

3. Sketch of disk mounting fixture 33

k. Thermocouple output as a function of time for three disk

thicknesses with 6.35~mm aperture 3^

5. Thermocouple output as a function of time for three disk

thicknesses with 9-53~mm aperture 35

6. Thermocouple output as a function of time for three disk

thicknesses with 12.7-mm aperture 36

vi
7. Results of tests using coatings on stainless steel disks ... 37

8. Results of tests using coatings on stainless steel disks ... 38

9. Responses of a pair of lead metan iobate-crystal pressure

transducers exposed to a thermal radiant-energy transient

of approximately 20 mJ/mm2 39

10. Response of an unbonded strain-gage pressure transducer,

with selected protective coatings applied to the diaphragm

outer surface, to a thermal radiant-energy transient of approximately 20 mJ/mm2

A 40

B 41

11. Response of a quartz-crystal pressure transducer, with

selected protective coatings applied to the diaphragm outer

surface, to a thermal radiant-energy transient of approximately 20 mJ/mm2

A 42

B 43

12. Response of a semiconductor strain-gage pressure transducer, with selected protective coatings applied to the

diaphragm outer surface, to a thermal radiant-energy transient of approximately 20 mJ/mm2

A 44

B 45

13. Response of a lead metaniobate-crystal pressure transducer, with selected protective coatings applied to the

diaphragm outer surface, to a thermal radiant-energy transient of approximately 20 mJ/mm 2

A 46

B 47

14. Response to a pressure step of 280 kPa of two unbonded

strain-gage pressure transducers 48

15- Response to a pressure step of 280 kPa of two unbonded

strain-gage pressure transducers 49

16. Response to a pressure step of 280 kPa of two quartzcrystal pressure transducers 50

17- Photographs of the faces forming part of the sensing assembly of a quartz-crystal and an unbonded strain-gage pressure

transducer 51

18. Response to a pressure step of 280 kPa of two quartz-crystal

pressure transducers is shown in traces 1 and 2; the re- sponse to 2070 kPa of the same transducers is shown in

traces 3 and 4 52

vi i

EXPERIMENTAL INVESTIGATION OF MEANS FOR REDUCING THE RESPONSE OF

PRESSURE TRANSDUCERS TO THERMAL TRANSIENTS

John S. Hilten, Carol F. Vezzetti, J. Franklin Mayo-Wells,

and Paul S. Lederer

Experimental efforts are described in the evaluation of protective diaphragm coatings as a means to reduce the effects produced by thermal

radiant-energy transients on pressure-transducer response.

A series of tests was carried out to investigate the effects of a variety

of protective coatings on the amount and rate of thermal energy transmission through thin metal disks (used to simulate transducer diaphragms) as revealed by measurements of the disk back-side temperature. The temperature histories of both bare and protected disks were measured with thermocouples during and after exposure of the disks to thermal radiantenergy transients (of approximately 20 mJ/mm2 at the disk) generated by

No. 22 photographic flashbulbs. Protective coatings investigated include various tapes, greases, and room-temperature-vulcanizing rubbers

(RTVs) .

Based on the results from these tests, the effectiveness of nine selected

coatings in reducing thermally-induced zero shifts of four types of pres- sure transducers was investigated. In these tests, a pair of transducers — one protected and the other unprotected, but otherwise nominally identical — were exposed to a thermal radiant-energy transient as described

above. The resulting zero shift was measured and taken as an index of

coating effectiveness.

The effect of the mass of the coatings on transducer dynamic response

was investigated by means of a shock tube in which a protected and un- protected pair were simultaneously exposed to a pressure step of approximately 280 kPa (hO psi). Also, the effect of selected coatings on transducer acceleration sensitivity was investigated by monitoring the outputs

of pairs of transducers mounted on a vibration exciter.

Test results indicate the validity of the simulated diaphragm test as a predictor of protection effectiveness. Results from both disk and transducer tests show that some coating combinations appear to be an order of

magnitude more effective than others in delaying and reducing zero shift.

Coatings of multi-layer black tape, red RTV silicone rubber, and "heat

sink" compound appear most effective, other materials less. Silicone

grease is found least effective, while single-layer black tape appears

to produce results ranging from very limited protection to actual degradation of response, based on the limited sampling carried out in this

investigation.

Key words: coatings; delayed response; dynamic; dynamic response; pressure step; pressure transducer; protective coatings; shock tube; tape;

thermal radiant-energy response; thermal protection; thermal transient

response; transducer; zero shift.
1. INTRODUCTION

A brief background for the NBS InterAgency Transducer Project, its re- cent history, and current guidelines can be found in an earlier NBS publication [1]". A previously assigned task within this Project was to develop a test method for evaluating the effects of short-duration, thermal

radiant-energy transients on pressure-transducer response [2,3,^]. Following completion of this effort, the Transducer Committee"" and NAVAIR

as cosponsors assigned as a follow-on task the evaluation of schemes in

use, or proposed, to reduce such effects. The intent was to use the

test method described in [k] for evaluating methods of protection and to

modify the method as required for generating other types of thermal inputs for testing protected transducers. The task assignment called for

investigation of methods of protecting pressure transducers from "radiated and/or convected transients of various amplitudes and durations."

The main criterion to be taken into account was the "effect of any protection scheme on transducer performance, such as increased acceleration

sensitivity, degradation in dynamic performance, etc."

Following a review of various protective schemes proposed and in field

use, the approach chosen for this task was to investigate the effects of

various coatings applied directly to the diaphragms of pressure transducers to protect them against radiant thermal transients. Plans to investigate other means of protection, such as perforated barriers, and

the effects of other types of thermal transients, such as convectedenergy transients, were initially considered but dropped because of curtailed funds

The plan of work was, first, to develop a method for evaluating coating

performance; second, to test a number of coatings using this method;

third, using the test method of [k] , to determine the zero shifts of

pairs of transducers —one with diaphragm protection and the other without; and, fourth, to compare the dynamic response of the two transducers

in each pair to pressure steps generated in a shock tube. In addition,

the acceleration response of the protected and unprotected transducers

was to be compared.

2. DEVELOPMENT OF METHOD FOR EVALUATING COATING PERFORMANCE

ON METAL DISKS

Parameters of interest for characterizing coating performance include

(1) the maximum rise in temperature measured at the back surface of a transducer diaphragm following a specified thermal input to the protected front surface and (2) the time from the onset of the thermal

transient to the peak back-side temperature. These parameters are of

Numbers in brackets indicate literature references, section 8.

"Transducer Committee, Telemetry Group, Range Commanders Council.

2
interest because results from earlier work together with knowledge of

transducer design and construction suggest that a major source of the

thermally induced zero shifts observed in a number of pressure-transducer

designs is thermal energy propagated through the diaphragm to the sensing

element. Especially for transducer designs in which little thermal energy is conducted via the path d iaphragm-to-housing-to-sensing-element

,

the temperature at the back side of the diaphragm provides a convenient measure of the energy reaching the sensing element. Most modern transducer designs minimize (mechanical) contact between housing and sensing

element. Thus, for purposes of evaluating coating performance, it is

not necessary to accept the experimental complexity that would be involved in instrumenting a transducer diaphragm in situ. Instead, the

thermal behavior of a thin metal disk mounted in a relatively massive

housing simulates the thermal behavior of the diaphragm; an additional

advantage of this approach is that nominally identical disks may be fabricated for as many tests as are required.

2.1 Preliminary Work

In order to gain familiarity with the use of thermocouples to measure

the temperature of thin disks, preliminary work was carried out with a number of thermocouple systems, wire sizes, and mounting means. Thermocouples of the system copper versus copper-45% nickel were used initially

because of their availability. Thermocouples of the system nickel-10%

chromium versus copper-45% nickel offer greater sensitivity and were

used as soon as they became available.

Thermocouples made of wire with diameters as small as 0.025 mm were

tested, but were found to pose severe practical difficulties in handling;

thermocouples of wire of twice this diameter proved to be acceptable.

Several schemes to mount the thermocouple to the disk were tried including soldering and cementing. The small size of the measuring junction

of a thermocouple made with wire as large as 0.076 mm in diameter ren- ders it nearly impossible to determine whether or not the junction is

in good contact with the disk when these mounting techniques are em- ployed. To eliminate this difficulty, a simple mechanical mount was devised in which the thermocouple junction is pressed against the disk

with a constant, but adjustable, force. The thermocouple is mounted at one end of a small ceramic rod having two axial holes for the wires.

Before the thermocouple is inserted, the junction end of the rod is tapered as shown in the detail of figure 1. The thermocouple is held in

place with a small quantity of white glue packed at the opening of each

hole. The completed thermocouple-and-rod assembly is mounted on a swinging arm in such a manner that a counterweight acts to force the thermocouple against the surface of the disk. The position of the counterweight controls the magnitude of the force. This arrangement is shown

in figure 1

.

2.2 Description of Apparatus

The apparatus of the test method for evaluating coating performance is

3
based on that for evaluating the effects of thermal radiant-energy transients on pressure-transducer response [k] . A source and the transducer

mounting block are mounted on an optical rail as shown in figure 2. The

mounting fixture for the thin metal disk, shown in figure 3, is designed

to fit into the transducer mounting block. The vertical position of the

disk, the plane of which is perpendicular to the long axis of the rail,

is adjusted until the center of the exposed portion of the disk and the

center of the source are in the same horizontal plane. The mechanism

for supporting the thermocouple is also mounted on the rail, behind the

transducer mounting block. The vertical position of the thermocouple is

adjusted so that the measuring junction is in contact with the approximate center of the exposed portion of the disk.

The thermocouple cold junction is immersed in a flask of water at nominal room temperature, the water being used as a heat sink to eliminate

small fluctuations in temperature which the junction would experience if

exposed to air currents. The thermocouple output (thermocouple emf) is

displayed on an oscilloscope whose sweep is triggered by the same switch

used to initiate the flash.

The source used is a No. 22 photographic flashbulb. The performance

characteristics of these flashbulbs have been characterized and are given in [h] . The source-to-disk distance selected is 70 mm, which re- sults in a transient with an energy density of approximately 20 mJ/mm2 at the disk.

2.3 Test Procedure Considerations

Use of the apparatus described in section 2.2 requires that certain ex- perimental precautions be taken. First, the disk should be centered in

its fixture, and the fixture tightened in a manner that does not distort

the disk. Then the thermocouple assembly (which is quite fragile) should

be mounted in position and adjusted so that the junction contacts the

center of the disk. The force exerted by the thermocouple against the

disk should be adjusted by means of the counterweight so that with the

ceramic rod parallel with the optical rail, firm pressure is felt by the

tip of a gloved finger (a thin polyethylene glove is used to prevent con- tamination of the junction by finger oils) held against the measuring

junction of the thermocouple. Care should be taken to rotate the No. 22

flashbulb so that the supporting electrodes (each in the shape of an inverted "L") for the igniter element point in a direction perpendicular

to the long axis of the optical rail. This orientation prevents the

black spot which forms on the bulb opposite the electrodes from absorbing any of the radiant energy intended to impinge on the disk.

Note: In the interests of safety, test operators should not look at the

flashbulb during ignition and should remove used bulbs with gloves,

A convenient means for recording data is to display thermocouple output

as a function of time on the screen of a storage oscilloscope and to

photograph the resulting traces. The amplitude of the trace may then
be measured, scaled, and recorded. In the tests reported, the maximum

thermocouple output was determined and recorded, as were thermocouple

output values at 50, 100, 500, and 1000 ms after flash initiation. Also

determined and recorded were the times required for the thermocouple output to rise to 10, 50, 90, and 100% of the maximum value and then to decay to 90 and 50%. The selected values are arbitrary; the intent was to measure enough data points for each test run to permit useful comparison

of various geometries and protective materials, as described in 2.4 and

2.6.

2.U Experiments with Various-Sized Apertures and Disk Thicknesses

To investigate the effect of aperture size, i.e., of the exposed area of

the disk, on the temperature history of the disk following exposure to a given thermal transient, disk mounting fixtures with three different

apertures were fabricated. The apertures were chosen to be representative of those of commercial pressure transducers and are 6.35, 9.53, and

12.7 mm in diameter (with areas of 31.7, 71.3, and 127 mm 2

, respectively)

Experiments with three disk thicknesses —0.08, 0.13, and 0.25 mm — were

also conducted. The intent of these tests was to verify the prediction

that a thin, bare disk would experience a greater temperature rise following exposure to a given thermal transient than would an otherwise

similar disk of greater thickness. A simple analysis suggests that this

hypothesis is correct because, if the paths by which thermal energy may

be removed from the disk are approximately the same in both cases, a thin disk will have to accommodate more energy per unit volume than a thicker disk.

The results of these experiments are shown in figures k, 5, and 6. The

peak temperatures reached with a particular thickness of disk are equal

(within ±10%) for the three different apertures used. The peak temperatures reached with a particular aperture are inversely proportional to

the disk thickness. This relation is confirmed by the result that the

products of thickness and peak thermocouple output, expressed in arbitrary units, range from 0.096 to 0.12 for all tests, with a coefficient

of variation of 9-2%. This degree of variation is little more than that

attributable to the range of energy output from No. 22 flashbulbs, for

which the coefficient of variation has previously been found to be 8.7%

[*].

2.5 Experiments to Determine Method Repeatability

Twelve tests were conducted, each on a new, bare, 0.08-mm-thick

stainless-steel disk, to determine the repeatability of the method.

The aperture used was 9.53 mm. The average of the thermocouple outputs

is plotted as the NP curve common to figures 7 and 8. The commercially

available thermocouple used in these tests was fabricated from 0.051-mmdiameter wires of nickel-10% chromium and copper-^5% nickel and represents the "standard" thermocouple used in the tests. This thermocouple

is supplied as meeting ANSI Type E specifications. [The manufacturer
cites a limit of error of ±1.7°C over the range of to 315°C], The co- efficient of variation calculated from the repeatability test data is

3.k%, again only slightly greater than the degree of variation attributable to the No. 22 flashbulbs.

2.6 Experiments with Various Coatings

The protection provided by 16 different coatings was evaluated using the

disk method as a preliminary screen prior to tests with transducers. The

coatings are identified in the captions of figures 7 and 8. Included are

3 two-component room-temperature-vulcanizing (RTV) rubbers, 5 singlecomponent RTV rubbers, a black plastic tape", a white plastic tape+

, hightemperature 5 fiberglass tape, a silicone "heat-sink" compound, and a silicone dielectric grease.

Each coating was applied to a stainless-steel disk 0.08 mm thick, the

aperture selected to provide a high temperature rise for a disk with a diameter representative of that of the diaphragm of a number of commercial pressure transducers. The RTV rubbers were cast in place using as a form a metal washer which was discarded after the material had set. The

thickness of the RTV layers was 0.8 mm. The tapes, in both single and

triple layers, were cut to fit the aperture by means of a cork-borer.

The single-layer thickness is approximately 0.15 mm for all tapes ex- cept the fiberglass thermal tape, which is 0.23 mm thick. The silicone

greases required peripheral support in a vertical position. The re- straint was a washer similar to that used for casting the RTV layers and was 0.8 mm thick. The hole in the washer was filled to overflowing with

the grease and the excess removed by drawing a straight-edged knife

blade across the face of the washer with the blade held at about 60 deg

from the perpendicular to the disk. The restraint was left in place as part of the coated disk. For these two coatings, clamping plates were

fabricated which were thinner than the "standard" plate by the thickness

of the restraining washer. The mass of the metal in front of the disk

itself was thus the same for all tests.

2.7 Test Results

The results of the coating-evaluation tests are plotted in figures 7 and

8. Each data point represents the average of 5 tests on each coating.

As noted previously, the no protection (NP) curves represent the average

of 12 tests. Inspection of the figures and the data shows several interesting results.

It is apparent that all but one coating provide some measure of protection in reducing the ampl i tude of the thermocouple output compared to

Meeting Federal Specification HH-l-595 B. Commercially available.

Commercially available 0. 1

_ mm- thick glass-plastic tape with maximum

continuous use temperature of 500°C and a dielectric strength of 2*10 7 V/m.

6
that from the unprotected disk. These reduced amplitudes range from

about 82% (silicone grease) to about 3% ("heat sink" silicone compound)

of the unprotected disk output. Significantly, the single layer of black

tape (figure 8) actually increases the output amplitude in these tests.

It should be noted that an informal survey of installations with measurement situations calling for protection of pressure transducers from thermal transients indicates widespread use of black tape in field measurements .

It is also seen that coatings fall into three groups from the standpoint

of timing: those for which the thermal transient response time of the

protected disk is roughly that of the unprotected disk, those for which

the response of the protected disk is delayed significantly, and those

for which that response is advanced. As measured, the peak temperature

of the unprotected disk occurs at about 250 ms after the peak output of

the thermal -trans ient source. Disks protected with silicone grease and

the white RTV rubber experience peak temperatures about 150 ms after

source peak output; these peak temperatures are lower than that of an unprotected disk. Disks protected with white tape and with a single

layer of (tan) fiberglass tape also experience an earlier and reduced

peak temperature compared to unprotected disks.

In contrast, triple. layers of black or fiberglass tape, black or red RTV

silicone rubber, and "heat sink" silicone compound produce a delay in

time of peak temperature of about 2.5 s. The remaining coatings fall between the early and delayed groups in their effect on the disk temperature,

At present, there is no explanation of why the light-colored coatings re- duce the time delay between flash and measured peak temperature, compared

to the temperature response of the unprotected disk. In this context,

it should be noted that only for silicone grease do the temperatures of

the protected disk reach higher values earlier than the temperatures of

the bare disk. The performance of the other coatings is as expected:

energy is absorbed at the front surface, some of the energy is consumed

in heating the coating, and the remainder is transmitted through the

coating material (at a slower rate than through an equivalent thickness

of air) to the disk which then experiences a delayed and attenuated rise

in temperature.

Silicone grease which is highly translucent appears to provide very

little protection from radiant-energy transients. (However, very brief

exploratory trials in which a heat gun served to generate convective

thermal transients indicated that silicone grease may provide considerable protection from such transients.)

3. EXPERIMENTAL EVALUATION OF COATINGS ON PRESSURE TRANSDUCERS

On the basis of the test results described above, nine of the coatings were selected for further tests in which the coatings were applied to

the diaphragms of four different types of pressure transducers. One
purpose of these tests was to check the results of the disk method for

evaluating coatings. A second, and more significant, purpose was to determine the effects on pressure transducer performance of the presence

of a coating on the transducer diaphragm. Performance parameters of particular interest in this regard are response and acceleration sensitivity,

3.1 Selection of Coatings

The test coatings were selected from those evaluated by means of the

metal disks (sections 2.6 and 2.7). The coatings chosen represent various degrees of thermal protection capability and several types of coating

material. Included among the selected coatings were a two-component RTV

silicone rubber, a silicone "heat-sink" compound, a silicone grease, one

and three layers of black tape, one and three layers of white tape, and one and three layers of fiberglass tape.

The various tape coatings were chosen because (1) tape is commonly used

as a protective coating by a number of transducer users (particularly

black vinyl tape), (2) tape is easy to apply and overall thickness can

be readily varied through the use of multiple layers, and (3) tapes

showed considerable variation in protective ability in the tests performed earlier. One and three layers of tape represent thicknesses of

0.15 and 0.45 mm, respectively, which are typical thicknesses reported

in use at several laboratories.

The silicone rubber was chosen because (1) silicone rubber is also com- monly used by a number of transducer users as a thermal protective means

and (2) this particular red RTV silicone rubber had performed relatively

well in. the earlier evaluation. Transducer users cite applications in

which thicknesses varying from 0.10 to 2.0 mm are used. The 0.8-mm

thickness used in these tests is considered a reasonable median value

for testing. The "heat-sink" compound was chosen because it also performed relatively well in the earlier evaluation. To provide a check on

the results of that evaluation, it was considered desirable to include one coating that had performed relatively poorly. Silicone grease was chosen because it is reported in use by a number of transducer users.

After each set of tests with a given coating, the coatings were removed

and the transducers cleaned with acetone and allowed to air dry. The

silicone materials were applied to the test transducer diaphragms after

the tapes and RTV silicone rubber applied last.

3.2 Selection of Transducers

Nine transducers of four different types were used in the coating evaluation. The four types , unbonded strain-gage (one transducer pair),

quartz-crystal (one transducer pair), semiconductor (single transducer),

and piezoelectric (two transducer pairs), are representative of those

reported in use in applications in which thermal transients are encountered .
A single model of each type was selected for the tests so that each test

pair is composed of two nominally identical transducers; all four piezoelectric transducers are nominally identical. Although four semiconductor transducers of the same model were initially available, only one was

in operating condition at the time of these tests (the diaphragm of the

selected model having proved to be more fragile than anticipated).

The strain-gage transducer model used has unbonded strain gages attached

to a structure composed of four support members; this assembly is attached to a push rod that in turn is fastened to the transducer diaphragm,

An analysis of this geometry suggested that performance should not be

affected appreciably by thermal transients of short duration. The pressure range is to 3^5 kPa . The diameter of the diaphragm is approximately 13 mm

.

The quartz-crystal transducer model used incorporates an impedancematching amplifier as part of the transducer. This model was also not

expected to have its performance appreciably affected by thermal transients. The pressure range is to 6.9 MPa. The diaphragm end of the

transducer has a diameter of approximately 10 mm.

The semiconductor transducer model used has a silicon diaphragm (approximately 3-6-mm diameter) into which a Wheatstone bridge has been diffused.

On the basis of experience with pressure transducer behavior, it was ex- pected that the performance of this model would be affected considerably

by thermal transients. The pressure range is to 3^5 kPa

.

The piezoelectric transducer model used has a lead metaniobate sensor. The body is stainless steel with a nylon pressure plate. The pressure

range is 0.7 to 3-5 MPa. This model was also expected to be affected appreciably by thermal transients. The diameter of the sensitive surface

is approximately 5.3 mm.

3.3 Application of Protective Coatings

For all tests, the protective material was applied directly to the transducer diaphragm. The exposed diaphragm surface was first cleaned with

acetone and allowed to dry; then the transducer was installed in the

mounting fixture with the diaphragm surface either flush or recessed as required by the coating.

The protective materials were applied in the same manner previously described for the disk method (section 2.6).

3. A Descriptions of Tests

3.4.1 Zero-Shift Measurements - The observed change (zero shift) in

transducer output in response to thermal radiant-energy transients with

the transducer sensing atmospheric ambient pressure was determined and
recorded according to a modification of the method of [*t]". Eleven sets

of tests were carried out using the four types of transducers. With the

exception of tests on the semiconductor strain-gage transducer, in each

test, a pair of transducers was used, one with a protective coating applied to the sensing diaphragm and the other unprotected, but otherwise

nominally identical and, where applicable, supplied with nominally identical excitation voltages. Two of the test sets were conducted for the

purposes described below with both test and control transducers unprotected; each of the remaining sets were conducted with one of the nine

selected coatings applied to the test transducer.

A brief series of tests, using the two pairs of piezoelectric transducers was carried out to provide a measure of the repeatability of the

method. Transducer output and time to peak transducer output was measured in eighteen test runs. The transducers were uncoated for these

tests

.

As the dynamic performance of each pair of transducers was measured following the zero-shift measurements, it was found convenient to mount the

fixture onto the hinged end plate of the shock tube used in the dynamic

measurements (see 3.^.2). The flashbulb source was mounted on a bracket

designed to be clamped to the end plate. This arrangement permits either

dynamic or zero-shift measurements to be carried out with a minimum of

effort required to shift from one test to the other.

Test procedure considerations are similar to those given in section 2.3

for the coating evaluations on metal disks. A detailed procedure for

these tests appears in appendix A. This procedure results in a set of

two to four photographs of the oscilloscope traces. The zero shift is

determined by scale measurement of the photographed traces. The measurements are made from whichever photographs provide the greatest time reso- lution and incorporate peak voltage values. The reference voltage levels are those levels, nominally zero, corresponding to the output of each

transducer before ignition of the flashbulb. For each transducer, the zero shift is measured as the difference between the peak output voltage

and the reference voltage level. The time, following source triggering,

at which the transducer output peaks is also determined. If positiveand negative-going peaks are present in a single trace, both peak values

and times are to be determined.

For each trace, amplitudes and times are measured at inflection points

and a sufficient number of other points to enable a reasonable facsimile

of the oscilloscope trace to be reproduced.

"in the method of [k] test transducer output is monitored and displayed

on the screen of a storage oscilloscope or otherwise recorded as the

transducer is exposed to a thermal radiant-energy transient resulting

from the ignition of a flashbulb or flashtube. As the transducer senses atmospheric ambient pressure, that is, a gage pressure nominally of magnitude zero, any observed change in output is taken as zero shift.

10
3.4.2 Dynamic Response Measurements - The effects of test coatings on the transducer dynamic performance were evaluated by comparing the simultaneous outputs of a pair of transducers —one protected, the other

not, but otherwise nominally identical — in response to pressure steps

generated in a shock tube. Transducer outputs were recorded and analyzed to determine the effects of coatings on such parameters as ringing

frequency, rise time, and amplitude response.

The shock tube employed in this work is capable of providing a calculable pressure step with rise time on the order of 10 ns and duration of

approximately A ms. (The duration is limited by the rarefaction wave

which reaches the end of the test section at approximately k ms.) Some

variations in pressure-step amplitude as seen at the end of the test

section result from the method of generating the pressure wave, which is

the rupture of a cellulose-acetate diaphragm interposed between the highpressure and low-pressure sections. This variation may be as much as 5%

and may be accompanied by a variation in waveform, especially during the

first 0.5 ms. This consideration led to a decision to expose test and

control transducers to the same pressure step and to record the outputs

simultaneously. An incidental advantage of this procedure is that any

error introduced by the calibration of the recording means (estimated to

be ±3% for the oscilloscope) may be ignored for the comparison measurement.

For these tests, the low-pressure section (i.e., the section downstream

from the diaphragm) was operated at atmospheric ambient pressure to simplify operation. With helium at about 300 kPa used in the high-pressure

section (upstream section), the amplitude of the pressure step is approximately 280 kPa when the tube is run at the laboratory ambient temperature

of 21°C.

Symmetry of the shock wave was established by a brief series of tests

using a pair of nominally identical uncoated transducers. The right and

left positions of the transducers in the shock tube were reversed after

each of six successive exposures of the transducers to a pressure step.

No differences in the transducer outputs attributable to the position of

the transducer were detected, and the shock wave is assumed to be symmetrical for the purposes of this study.

A detailed procedure for the thermal transient test method appears in

appendix A, as the information contained in that procedure may be used

as a guide for the use of whatever apparatus is available to accomplish

the same purpose. This statement is not true for the operation of the

shock tube, for which the procedure is specific to the particular device

employed, and therefore no procedure is given. The methods used to determine rise time, average amplitude, and ringing frequency depend to some extent on the transducers used in the test.

For the unbonded strain-gage transducers, the outputs of the test and

control transducers exposed to the pressure step were both displayed on the screens of two storage oscilloscopes, one operating at a sweep rate

11
of 0.1 ms/div and the other at 1.0 ms/div (total sweep times of 1 and 10 ms , respectively). The traces were photographed. Information from the

slow-sweep traces was used for monitoring the system behavior and for determining the duration of ringing to provide an estimate of the degree

of damping present. Rise times were determined from the photograph of

the fast-sweep traces as the difference between the times at which the

initial transducer pulse in response to the pressure step reaches 10%

and 90% of its peak value.

Since the output signal exhibited ringing, an averaging calculation was

used to compute the amplitude as follows: A value of "average" peak

output was determined from scale measurement of (1) two consecutive output minima at about 150 ys after the start of the response to the pressure step and (2) the included maximum. The reference level was taken

as the transducer output for zero applied pressure. The average value

of the two minima was calculated. The average of the minima average and

the included maximum was calculated to give the average amplitude. The

procedure was repeated at about 500 ys.

For determinations of ringing frequency, a two-channel transient recorder was used to record the two transducer outputs resulting from exposure

to the pressure step. The recorded signals were then supplied sequentially to a frequency analyzer-oscilloscope combination adjusted to operate

over a frequency range of to 40 kHz and with a sensitivity of k kHz/cm

of displayed trace. Each resulting trace of the frequency spectrum was photographed. Ringing frequency was then determined by inspection.

For the three other transducer types, no measurement of ringing frequency

was possible as available equipment has an upper frequency limit of 100

kHz, while the manufacturers report natural frequency values of 100 kHz

and greater for these transducers.

Average peak amplitudes and rise times were determined in a manner similar to that described for the strain-gage transducers, except that the

sweep rates were 0.01 and 1.0 ms/div, and the amplitude measurements

were carried out at 15 and 50 ys on the fast-sweep trace and at 1, 2,

and 3 ms on the slow-sweep trace.

3.4.3 Acceleration Sensitivity Tests - The effect on transducer acceleration sensitivity (i.e., the output of the transducer at zero applied

pressure resulting from sinusoidal acceleration perpendicular to the

plane of the diaphragm) of the mass added to the transducer diaphragm by

the coatings was determined using a vibration exciter. The transducer

fixture (with test and control transducers installed) was mounted on a cylindrical spacer ring attached to the vibration-exciter armature. The

surface plane of the transducer diaphragms was perpendicular to the plane

of the armature motion. The pair of transducers was then vibrated sinusoidal ly at 10 gn zero-to-peak over the frequency range 25 Hz to 3 kHz

and both transducer outputs monitored on a storage oscilloscope.

Tests were performed first without and then with two of the coatings on

12
each type of pressure transducer. "Heat-sink" compound and three layers

of black tape were selected as being representative of the types of coatings used (viscous liquid and pliable solid) and because they are among

the more massive coatings.

4. RESULTS FROM TESTS OF COATINGS ON PRESSURE TRANSDUCERS

Tests of the effects of selected coatings on pressure transducer perfor- mance were carried out as have been described. As an example, figure 9

shows the photographically recorded zero shift for a protected and unprotected transducer pair with three different coatings applied to the diaphragms .

Test data were obtained from photographs of the traces appearing on the screens of the storage oscilloscopes used in the tests. Maximum re- corded transducer zero shifts and maximum recorded time delays in transducer response are tabulated in tables 1-4, with a summary of results in

table 5. Dynamic pressure response data appear in tables 6-9- Except

for the semiconductor strain-gage, for which no control was available,

the responses of the control transducers are included to provide further

information on the repeatability of the test procedure. Times of occurrence of maximum zero shift are measured from the initiation of the thermal transient. As noted previously, the peak occurs approximately 20 ms after initiation.

4.1 Zero Shift

The results of the repeatability trials show that for 18 tests with four

nominally identical uncoated transducers the measured transducer zero- shift magnitude has a coefficient of variation of 8.4%, which is not significantly different from the variation of 8.7% ascribed to flashbulb

energy. The variation in time of peak transducer output is greater, with

a coefficient of variation of 10.4%. Time of peak output appears to be

a parameter that varies with the individual transducer, as the coefficients of variation for the four instruments are 6.1%, 8.2%, 8.0%, and

8.9%.

Table 5 summarizes the test results for both unprotected control and

protected test transducers. The coefficients of variation of transducer

zero-shift magnitude range from 7-3% to 8.4%, which result again suggests that the chief source of variation is to be found in flashbulb energy.

Detailed results are discussed in the following four sections.

4.1.1 Unbonded Strain-Gage Transducers - As shown in table 1, for the

11 tests with unbonded strain-gage transducers, the control instrument

had an average maximum zero shift of approximately 7-3% of the transducer output corresponding to full-scale applied pressure (denoted as 7.3% FS), with a sample coefficient of variation of 7-3% and an average

13
time of occurrence of 60 ms (sample coefficient of variation 8.2%). For

the two tests in which the test transducer was not protected, the average maximum zero shift was 7.6% FS with an average time for the peak

zero shifts of 65 ms ; these results show good agreement with those from

the control transducers.

While an insufficient number of tests was run to draw any statistically

significant conclusions, the data show that coatings of one layer of

white tape, three layers of white tape, one layer of fiberglass tape,

three layers of fiberglass tape, and silicone grease appear not to delay

the time of occurrence of the maximum zero shift. However, all of these

but the silicone grease appear to reduce the magnitude of the zero shift

appreciably. The use of one layer of black tape, three layers of black

tape, "heat-sink" compound, and red RTV silicone rubber tends to delay

the time of occurrence of the maximum zero shift and reduces the magnitude of the zero shift as well. Comparisons of the effectiveness of a single layer of tape with the silicone materials should take into ac- count the fact that the tape has one-fourth the thickness of these materials.

As shown in figures 1 OA and 1 OB , the zero shift of the unprotected transducers resulting from exposure to the thermal transient is relatively

small (less than 8% FS) and slow to develop. This behavior is consistent

with the construction of the transducer. Presumably, only a portion of

the thermal energy reaches the strain gage, and it takes time for that

energy to travel from the diaphragm through the push rod to the strain

gage.

4.1.2 Quartz-Crystal Transducers - Zero-shift tests with quartz-crystal

piezoelectric transducers resulted in all cases in the production of

negative shift followed by a positive shift of comparable magnitude.

For the 11 tests, the control instrument had an average negative zero shift of 5.2% FS (sample coefficient of variation 7-3%) and an average

maximum positive zero shift of 2.7% (sample coefficient of variation

8.1%). The average time at which the negative peak zero shifts occurred

was 36 ms (sample coefficient of variation 5-2%) and the average time

at which the positive peak occurred was 113 ms (sample coefficient of

variation 5.2%). The data are given in table 2.

In the tests in which the test transducer was uncoated, the average

negative zero shift was 5-3% FS at 36 ms , and the positive zero shift was 5.5% FS at 110 ms . While the positive zero shift for the test transducer was significantly larger than that for the control transducer, differences of this magnitude are not uncommon in transducers of the same model, as shown in [k]

.

As shown in figures 1 1 A and 11B, these transducers respond more rapidly

to the thermal transient than the unbonded strain-gage transducers, and

shift first in the negative direction, as the sensor is heated, and then

in the positive direction, as the sensor cools.
As in the tests with the unbonded strain-gage transducers, one layer of

white tape, one layer of fiberglass tape, three layers of white tape,

three layers of fiberglass tape, and silicone grease do not delay appreciably the time of occurrence of the maximum zero shift; all but the

silicone grease reduce the magnitude of the zero shift appreciably. The

use of one layer of black tape, three layers of black tape, "heat-sink"

compound, and red RTV silicone rubber delays the time of occurrence of

the zero shift (both positive and negative peaks) and the magnitude of

the zero shift as well.

4.1.3 Semiconductor Strain-Gage Transducer - Tests with the semiconductor strain-gage transducer showed a very rapid response to the thermal

transient input in all cases. As has been explained, since only a single

transducer was available, no control data are given in table 3- However,

the two tests with the unprotected transducer show good agreement.

In this transducer the sensing element, bridge circuit, and diaphragm

are integral, and therefore the maximum zero shift, without protective

coating, is large (almost 200% FS) and rapid (occurring almost simultaneously with the flash). Also, the transducer output increases and de- creases in a series of peaks and valleys, as shown by curve NP in figures

12A and 12B. Two possible explanations for this behavior are (1) the

transducer response may be wavelength dependent, or (2) various stresses

cancel and reinforce each other as various parts of the transducer structure heat and cool. Note in curve NP the initial negative zero shift of

about 50% FS 10 ms prior to the transient peak.

The magnitude of the zero shift is reduced substantially by the use of

protective coatings. As shown in figures 1 2A and 12B, one and three

layers of black tape, red RTV silicone rubber, and "heat-sink" compound

greatly reduce the zero shift compared to that recorded for the unprotected transducer. Three layers of white and three layers of fiberglass

tape reduce the magnitude of the shift moderately; single layers of

white and fiberglass tapes and the silicone grease appear to have little

effect on zero shift. Only the red RTV silicone rubber coating is effective in delaying the time of occurrence of the zero shift, with a small maximum positive zero shift at 500 ms.

4.1.4 Lead Metaniobate-Crystal Transducers - Tests with two of the lead

metaniobate-crystal piezoelectric pressure transducers resulted in all cases in negative zero shifts as a result of exposure to the thermal

transient input. As given in table k, these shifts range from about

50% FS for the unprotected transducers to about 5% FS for tests in which

the silicone "heat-sink" compound coated the diaphragm. For the unprotected instruments, peak zero shift occurs about 150 ms after the transient peak.

As shown in figures 1 3A and 13B, substantial reductions in zero shift

result from the use of "heat-sink" compound, one and three layers of

white tape, three layers of fiberglass tape, and the red RTV silicone

rubber. Three layers of black tape and one layer of fiberglass tape

appear to be moderately effective; neither one layer of black tape nor

15
the silicone grease appear to have any significant effect on the magnitude of the zero shift.

All coatings, except for the silicone grease, were effective in delaying

the time of occurrence of the zero shift by amounts ranging from 1.8 s to k s.

k .2 Dynamic Response

To provide a means for comparing the dynamic performance of protected

transducers, a parameter known as amplitude ratio is used to characterize

the dynamic response of a test transducer relative to that of its control,

Amplitude ratios are calculated by determining (by scale measurement of

oscilloscope trace photographs) the amplitude of the recorded test transducer output signal at 15 ys, 50 ys, 1 ms , 2 ms , and 5 ms after the start

of the pressure step, adding these five values, and dividing this sum by

the sum of the five control transducer signal amplitudes measured for

corresponding times. Use of this ratio as a performance parameter eliminates the effects of errors affecting both test and control transducers

in a given trial, such as oscilloscope amplitude calibration errors,

variations in shock-tube step pressure, and differences in transducer

sensitivity. For these tests, amplitude-ratio coefficients of variation

thus represent, largely, the effect of the coating materials, and the

degree to which accurate measurements of transducer signal amplitudes

may be made from photographs of oscilloscope traces.

The coefficient of variation is one common statistical measure of the

variation in a series of "identical" tests. Since the addition of coating materials does not appear to alter the amplitude of the transducer

dynamic" response to a pressure step, coefficients of variation are presented for each transducer type as if all tests for that type were "identical." (Quartz-crystal tape tests are not included because of airentrapment problems, and semiconductor strain-gage tests are not included because of the absence of control transducer results with which

to normalize any pressure-step variation.) The coefficients of variation calculated are 1.7%, 0.9%, and 1.7%, representing 11, 5, and 11 paired trials for unbonded strain-gage, quartz-crystal, and lead

metaniobate-crystal transducers, respectively, as shown in tables 6, 7,

and 9. These results may be interpreted as indicating that, for the

limited number of tests conducted, the coatings produce no significant

degradation of the dynamic performance of the test transducers. Detailed results are discussed in the following four sections.

4.2.1 Unbonded Strain-Gage Transducers - Figure \k shows the transducer

output response behavior of two unbonded strain-gage transducers to a pressure step of 280 kPa with both test and control instruments unprotected. Comparison of the traces representing one instrument with the

traces for the other shows slight variations in degree of damping, ringing frequency, and sensitivity. (For example, the control transducer

exhibits a ringing frequency approximately 7% higher than that of the

test transducer; on the other hand, the sensitivity of the control in16
strument is some 2% lower.) This degree of variation may be expected

with instruments of this type.

Figure 15 shows the same two transducers in a similar pressure-step

test in which the diaphragm of the test transducer is protected by three

layers of black tape; there is slight reduction in the ringing frequency

of the test transducers and a noticeable increase in damping, but rise

time and amplitude changes are too small to be significant. None of the

nine different coating materials was observed to produce a significant

change in the dynamic characteristics of the protected transducer. Table 6 summarizes the data in detail.

4.2.2 Quarts-Crystal Transducers - The results of a series of pressurestep tests with the various tapes applied as protective coatings to

quartz-crystal piezoelectric transducers show that the dynamic responses

vary widely in rise time and amplitude. The recorded output amplitudes

range from 20% above to about 90% below that of the unprotected transducer outputs. Figure 16 shows two examples of the variety of response

recorded for the tape coatings. An explanation of these results may lie

in the design of this model of transducer and in the fact that test

tapes were applied to the entire sensing face of the transducers, as at

the time of the tests the face was thought to constitute the diaphragm.

More recent information reveals that the instrument has a ridge around

the periphery of the diaphragm, as shown in figure 17. If the tape layer is applied across the entire face of the transducer covering the

ridge, there may be a tendency for air to be trapped along the inner

edge of the ridge. Such trapped air would be expected to "cushion" the

pressure step in an uncontrolled manner, causing variation in the pressure seen by the transducer diaphragm (and hence by the transducer) com- pared to the pressure amplitude of the step.

In pressure-step tests with quartz-crystal transducers protected by

silicone materials, there was no significant variation between the outputs of protected and unprotected instruments. The data are given in

table 7.

The test results discussed in the previous two paragraphs are for shocktube tests with a pressure step of 280 kPa , as has been described. This

pressure level amounts to only h% FS for the quartz-crystal transducers.

Since considerable variations in transducer output were observed in the

tests with tape coatings, as noted above now thought the result of air

entrapment, it was of interest to determine if tests with pressure steps

of greater amplitude would show similar variation. Accordingly, a series of tests was run with the diaphragm of the test transducer protected with three layers of black tape and the step-pressure amplitude

alternately at 280 kPa and 2070 kPa . The ratios of test-transducer-tocontrol -transducer output signal amplitudes for individual times of

0.5 ms, 1 ms, 2 ms , and 3 ms are 0.13, 0.15, 0.32, and 0.41, respectively, at 280 kPa and 0.73, 0.77, and 0.83 at 2070 kPa . (At 2070 kPa , data

are no longer available 3 ms after the shock wave reaches the transducers .

17
When these ratios are corrected for the slight differences in sensitivity

between the test and control instruments, the results indicate that, as expected, changes in transducer output attributable to the tape coatings

tend to decrease as the measured pressure increases.

4.2.3 Semiconductor Strain-Gage Transducer - The results of a series of

pressure-step tests with the various tapes applied as protective coatings

to a semiconductor strain-gage transducer show that the dynamic responses

vary widely in rise time, amplitude, and ringing frequency. These transducers, like the quartz-crystal instruments, have a ridge around the periphy of the diaphragm providing a site at which air may be trapped when

tape is applied to the entire sensing face, with the same results described in 4.2.2.

The silicone materials (silicone grease, "heat-sink" compound, and red

RTV silicone rubber) do not appear to alter the rise time and amplitude

response of the transducer significantly, but the ringing frequency was reduced significantly by the addition of the coatings, as shown in table

8. Because there is no control transducer to provide a basis for normalizing variations in the measurement results, and because considerable difficulty was encountered in getting the various tapes to stick to the

small diaphragm (size of a 10-32 screw, 3.8 mm in diameter), the data

presented in table 8 should be interpreted with caution.

4.2.4 Lead Metaniobate-Crystal Transducers - The results of a series of

pressure-step tests with protective coatings applied to lead metaniobatecrystal piezoelectric transducers indicate that the dynamic characteristics are not significantly altered by any of the coatings. It is not

possible to analyze the variations of transducer ringing frequency with

various coatings because the amplitude of the ringing is too small to

permit frequency measurements from the recorded oscilloscope traces and

the frequency range of the available transient recorder-spectrum analyzer combination is too limited. Table 9 summarizes the data.

k.3 Acceleration Sensitivity

Results of the acceleration sensitivity tests are not conclusive, since

for three of the transducer pairs the recorded changes in transducer output signal resulting from acceleration of the transducer are less than

the electrical noise in the system. The semiconductor strain-gage transducer did experience an approximately k0% signal increase (from 0.0016%

FS/# n without coating to 0.0023% FS/# n with coating) with three layers

of tape and a fourfold increase (from 0.0016% FS/# n to 0.066% FS/# n ) with the "heat-sink" compound at a driving frequency of 3 kHz and ajevel of 10 g n , zero to peak. However, even these increased acceleration

sensitivities are smaller than many found in the specifications for models of currently marketed pressure transducers, and the changes are probably not significant.

18
k.k Coating Evaluations

To permit ready comparison of the relative effectiveness of the various

coatings, the data obtained in the tests are summarized in table 10.

Test data from the metal -disk (simulated transducer diaphragm) experiments and from tests using the four transducer types are tabulated to

show the effects of protection on zero-shift amplitude and zero-shift delay. The data are presented in terms of normalized ratios: the maximum

observed "protected" zero shift divided by the corresponding maximum "unprotected" zero shift. A similar normalizing procedure is used for the

zero-shift delays. The protective coatings are ranked in the order of

decreasing protection (increasing zero-shift amplitude), as obtained

from the disk tests; transducer data are listed according to the protective coating applied during the respective tests.

It should be noted that, while the data from the disk tests represent

averages of five tests for each type of protective coating, transducer

results are for single tests.

The table shows generally good correspondence between disk and transducer

data on zero shift, with a few anomalies. The results of the strain-gage

transducer tests indicate three layers of black tape to be about twice as effective as suggested by the disk tests, and indicate one layer of black

tape to be slightly more effective than determined in the disk tests.

For the quartz-crystal transducers and semiconductor strain-gage transducers, black tape appears to perform much better than suggested by the

disk tests. However, these are the transducers with rimmed diaphragms

or very small diaphragms for which the dynamic response test data contain anomalies. As explained in ^.2.2, these anomalies are thought to

be the result of trapped air. Air trapped between tape coating and diaphragm would tend to reduce and delay the zero shift resulting from the

thermal transient.

Zero-shift time delays do not correspond with zero-shift amplitude

changes., however, and show considerable variations for the various types

of protective coatings, with a range of about 15 to 1 for the disk ex- periments, and of as much as 27 to 1 for the semiconductor strain-gage

transducers.

5. CONCLUSIONS

The following conclusions can be drawn from the work, although the limited number of specimens and trials suggests caution.

i 1. The metal-disk (simulated transducer diaphragm) test method for

evaluating coating materials appears to be a valid, simple, and

relatively inexpensive method for initial screening.

19
2. Data obtained from dynamic pressure tests of protected transducers indicate that the materials sampled in these tests, in

the thicknesses used, do not degrade significantly the dynamic

performance of the pressure transducers tested.

3. Anomalies observed in the results, such as the effects attributed to possible air entrapment, demonstrate that a prospective

coating should be evaluated in tests in which the coating is

applied to the transducer model it is intended to protect. Eval

uation should consist of both thermal transient and dynamic pres 1 sure tests covering the range in which the transducer is to be

used

.

k. The most desirable protection is that which eliminates com- pletely the thermal transient response of the transducers. For

most applications, next is that which reduces the amplitude of

that response. For applications in which pressure is measured

only over a short interval of time, protection which simply delays the thermal transient response well beyond that interval

should be acceptable.

In regard to the coatings tested, certain conclusions can be drawn. In general, it appears that the "heat-sink" silicone compound and the twocomponent red RTV silicone rubber offer the best all-around protection.

Multi-layer white and fiberglass tapes offer reasonable protection; single layers of these tapes offer poor protection. Silicone grease appears of little use with thermal transients similar to those used in the

tests. Although used commonly in the field, black tape appears not only

to be of dubious value as a protective coating, but may degrade dynamic

performance unpredictably, varying with transducer type and application.

6. RECOMMENDATIONS

Recommendations for future work include the following:

1. Statistically significant tests to determine the effects of

varying the thickness of protective coatings, to provide information to permit an optimum choice of thickness for a given

coating

;

2. Tests using a convective heat source;

3. Evaluation of the durability of the protective coatings; and

k. Investigation of the use of path-geometry protective schemes

(such as mechanical screens).

20
7. ACKNOWLEDGMENT

The authors wish to acknowledge the secretarial help given and patience

shown by Cindy Maria, Pat Magruder, and E. Jane Walters in the preparation of this publication.

8. REFERENCES

[1] Lederer, P. S., and Hilten, J. S., NBS InterAgency Transducer Project - Progress Report Covering Period July 1, 1974 to September 30,

197^, NBSIR 75-654 (February 1975).

[2] Lederer, P. S., Hilten, J. S., and Vezzetti, C. F., NBS InterAgency

Transducer Project - Progress Report Covering Period October 1, 1974

to December 31, 1974, NBSIR 75"532 (June 1975).

[3] Lederer, P. S., and Hilten, J. S., NBS InterAgency Transducer Project - Progress Report Number 3 Covering Period January 1, 1975 to

June 30, 1975, NBSIR 76-1 038 (March 1976).

[4] Hilten, J. S., Vezzetti, C. F., Mayo-Wells, J. F., and Lederer,

P. S., Test Method for Determining the Effect of Thermal Transients

on Pressure-Transducer Response, NBS Tech. Note 905 (March 1976).

[5] Lederer, P. S., Hilten, J. S., Vezzetti, C. F., and Mayo-Wells,

J. F., NBS InterAgency Transducer Project - Progress Report Number

4 Covering Period July 1, 1975 to December 31, 1975, NBSIR 76-1060

(June 1976)

.

[6] Lederer, P. S., Methods for Performance-Testing of Electromechanical Pressure Transducers, NBS Tech. Note 411 (February 1967).

[7] Hilten, J. S., Vezzetti, C. F., Mayo-Wells, J. F., and Lederer,

P. S., NBS InterAgency Transducer Project - Progress Report Number

5 Covering Period January 1, 1976 to June 30, 1976, NBSIR 76-1144

(November 1 , 1976) .

21
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TABLE 6

Response of Two Unbonded Strain-Gage Pressure Transducers, One Protected and One Unprotected, to a Pressure Step of 280 kPa

Test Transducer

Test No. Protective Coating on Diaphragm Ampl itude Ratio*

Rise Time 10-90%

us

Lowest Major

Resonant Frequency

kHz

1.000 14 9.8

1.026 14 10.0

.972 16 10.0

.998 16 9.8 1.008 16 9.8 1.007 16 9.6

.998 14 9.2

1.001 16 9.6

.974 16 9.9 1.024 14 8.8

.990 14 9.2

1

2

3

4

5

6

78

9 10

11

None

One Layer Black Tape Three Layers Black Tape

One Layer White Tape

Three Layers White Tape

One Layer Thermal Fiberglass Tape Three Layers Thermal Fiberglass Tape

None Silicone Grease Silicone "Heat-Sink" Compound

Two-Component Red RTV Silicone Rubber

Control Transducer

Test No. Protective Coating on Diaphragm Amplitude

Ratio*

Rise Time 10-90%

us

Lowest Major

Resonant Frequency

kHz

1

2

34

5

6

78

910

11

None

None

None

None

None

None

None

None

None

None

None

1.000

1.026

.972

.998 1.008

1.007

.998

1.001

.974 1.024

.990

914

1114

1114

1114

1116

11

10.6

10.6 10.6

10.4

10.6

10.6

10.6

10.0

10.6

10.6

10.6

Average 1.000 Coefficient of Variation, % 1.7

12

17

10.6

1.8

Average of 5 test transducer amplitudes (at 150 us, 500 us, 1 ms , 2 ms, and 3 ms) divided by 5 control transducer

amplitudes at same times. Since the sensitivity of two instruments may not be the same, the amplitude ratio may

not be 1.000.

27
TABLE 7

Response of Two Quartz Crystal Pressure

Transducers, One Protected and One Unprotected, to

a Pressure Step of 280 kPa

Test Transducer

Test No. Protective Coating on Diaphragm Amplitude

Ratio*

Rise Time

10-90%

us

None .992 1

None .985 2

Silicone Grease 1.008 1

Silicone "Heat-Sink" Compound .996 1

Two-Component Red RTV Silicone Rubber .992 4

Lowest Major

Resonant Frequency

12

19

20

21

22

NO

DATA

Control Transducer

12

19

20

21

22

None

None

None

None

None

.992

.985

1.008

.996

.992

NO

DATA

Average .995

Coefficient of Variation, % 0.9

1.2

37

Average of 5 test transducer amplitudes (at 15 ps, 50 \is, 1 ms, 2 ms, and 3 ms) divided by the average of 5 control

transducer amplitudes at the same times.

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31
Figure 1: Sketch of thermocouple fixture with cross-section detail of thermocouple mounted

in ceramic rod (A). The thermocouple measuring junction is identified by (B)

.

The thermocouple wires project back through two axial bores in the rod. White

glue (C) holds the thermocouple in place. The thermocouple-and-rod assembly is clamped in arm (D) pivotal ly mounted at (E) . The position of the counterweight

(F) may be adjusted to control the force exerted by the junction against the disk

(G).

32
0-c *S

a

v i

/ \

^ J e

Y-AXIS IN ©

TRIGGER IN O

21 2Z

Figure 2: Arrangement of the test-method apparatus. Mounted on optical rail (A), are the transducer mounting block (B) , the thermal radiant-energy source (C) , and the thermocouple bracket assembly (D) . The thermocouple cold junction is immersed

In water contained in flask (E), and the thermocouple output signal is displayed

on storage oscilloscope (F) . The ignition system (G) for the source also sup- plies a trigger signal to the oscilloscope. The disk and its fixture are hidden

within the transducer mounting block, as described in the text.

Figure 3: Sketch of disk mounting fixture. The side view at the left shows the fixture^

disassembled with the disk (A) in position ready to be clamped to the body (B/ by clamping plate (C) . As explained in the text, the diameter of the bore deter- mines the aperture, that is, the area of disk exposed to thermal radiant energy. The outer diameter of the fixture is set by the requirements of the transducer

mounting block.

33
>

E

a.

t—IDo

Q_

O

u

O

ex.

600 800

TIME (ms)

Figure h: Thermocouple output (mV) as a function of time (ms) for three disk thicknesses,

as measured by the method described In the text. The aperture used i s 6. 35 mm in diameter. The top curve represents a disk thickness of 0.08 mm; the middle

curve, 0.13 mm; and the bottom curve, 0.25 mm.

34
>

E

Q_I—O

o

u

O

200 400 600 800

TIME (ms)

1000

Figure 5: Thermocouple output (mV) as a function of time (ms) for three disk thicknesses,

as measured by the method described in the text. The aperture used is 9-53 mm in diamqter. The top curve represents a disk thickness of 0.08 mm; the middle

curve, 0.13 mm; and the bottom curve, 0.25 mm.

35
>

E

Z5

t—Z>O

=)

o

u

O

a:

400 600 800

TIME (ms)

1000 1200 1400

Figure 6: Thermocouple output (mV) as a function of time (ms) for three disk thicknesses,

as measured by the method described in the text. The aperture used is 12.7 mm in diameter. The top curve represents a disk thickness of 0.08 mm; the middle curve,

0.13; and the bottom curve, 0.25 mm.

36
10000

TIME [ms]

Figure 7': Results of tests using coatings on stainless steel disks. The plots show thermocouple output (mV) as a function of time (ms) , with time plotted on a three-cycle

logarithmic scale. As a rough guide, 1 mV corresponds to a rise of 16.6 Celsius

degrees. The peak of the source output occurs at approximately 20 ms , as indicated by the dotted line. The curve NP represents an unprotected disk for comparison. As described in the text, each data point for the coating curves is an average from five tests. Curve NP is the average of twelve tests.

Coating identification:

SRI = red RTV silicone rubber, two- component

SR2 = red RTV silicone rubber, two- component

SB = black RTV silicone rubber,

s i ng le-component

SA = aluminum RTV silicone rubber,

s i ng le-component

SW1 = white RTV silicone rubber, singlecomponent

SW2 = white RTV silicone rubber, singlecomponent

SW3 = white RTV silicone rubber, singlecomponent

SW4 = white RTV silicone rubber, two- component

37
C_3

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

~1 1 1 1 1 II

II

1 III

IB

l| III 1 1 1 1

—

—

/ nNP —

— ^-7/\!G

// j 1W^*J\.

3B V —

// /*r^*

—

/ V JP / 3W ^ ./

\\ —

/ // x~~* 3T 8 "^m^ \

^*^4VtTtr^^T:

i i i

1 1 1 +~ ^

TTYt-*

10 100 1000 10000

TIME [ms]

Figure 8: Results of tests using coatings on stainless steel disks. The plots show thermocouple output (mV) as a function of time (ms) , with time plotted on a three-cycle

logarithmic scale. As a rough guide, 1 mV corresponds to a rise of 16.6 Celsius

degrees. The peak of the source output occurs at approximately 20 ms , as indicated by the dotted line. The curve NP represents an unprotected disk for compari

son. As described in the text, each data point for the coating curves is an average from five tests. Curve NP is the average of twelve tests.

Coating identification:

1W = one layer white tape

IT = one layer fiberglass tape

IB = one layer black tape

3W = three layers white tape

3T = three layers of fiberglass tape

3B = three layers of black tape

HS = "heat sink" silicone compound

SG = si 1 icone grease

38
Figure 3: Responses of a pair of lead metan iobate-crystal pressure transducers exposed to a thermal radiant-energy transient of approximately 20 mJ/mm 2

. Photographs of os- cilloscope traces are shown. The purpose of the figure is to contrast the effect

of three coatings; top photographs, silicone grease; middle, one layer of black

tape; and bottom, red RTV silicone rubber. The upper trace in each photograph

represents the test (coated) transducer response, and the lower trace represents

the control (uncoated) response. The left-hand photographs show traces recorded

at 10 ms/division, that is, over the first 0.1s after flash initiation. The righthand traces were recorded at a rate 100 times slower, with a total sweep time of

10s. For all traces, each vertical division represents about 20% of the trans- ducer full-scale output. Note that for silicone grease, very little delay in, or reduction of, zero shift is recorded; for black tape, there is a considerable delay, but little reduction; and for red RTV silicone rubber, there is both delay

and reduction. Because of limitations imposed by conflicting requirements of trace brightness versus focus, the initial negative trace deflection is only faintly visible in the original photographs of the 10-s sweep. Initial levels for these traces

are indicated by arrows.

39
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43
-I

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I 200 -

150

OS<VOCO 100

50

CO

r«*i

50

-i—i—i i i i

j i i i i 1000

TIME [msl

Figure 12A: Response of a semiconductor strain-gage pressure transducer, with selected

protective coatings applied to the diaphragm outer surface, to a thermal

radiant-energy transient of approximately 20 mJ/mm 2

. The plots are of transducer zero shift (percent of full-scale reading) as a function of time (ms)

,

with time on a three-cycle logarithmic scale. The peak of the transient occurs at approximately 20 ms , as indicated by the vertical line. The heavyline plot (NP) represents the transducer zero shift with no protective coating,

Coating identification code:

IB = one layer of black tape

1W = one layer of white tape

IT = one layer of fiberglass tape

3B = three layers of black tape

3W = three layers of white tape

3T = three layers of fiberglass tape

44
200

i i i i i i 1

'

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i

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i SG/

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i i i i i i . . I

1

1

1

1

1

1

1

1

1 1 i i i i i 1 1 i

i ill i i i 10 100 1000

TIME Ims

Figure 12B: Response of a semiconductor strain-gage pressure transducer, with selected pro- tective coatings applied to the diaphragm outer surface, to a thermal radiantenergy transient of approximately 20 mJ/mm2

. The plots are of transducer zero shift (percent of full-scale reading) as a function of time (ms) , with time on

a three-cycle logarithmic scale. The peak of the transient occurs at approximately 20 ms , as indicated by the vertical line. The heavy-line plot (NP) rep- resents the transducer zero shift with no protective coating.

Coating identification code:

HS = "heat-sink" silicone compound

SG = s i 1 icone grease

SR = red RTV silicone grease

45
10000

TIME fms]

Figure 1 3A: Response of a lead metan iobate-crystal pressure transducer, with selected

protective coatings applied to the diaphragm outer surface, to a thermal

radiant-energy transient of approximately 20 mJ/mm2

. The plots are of transducer zero shift (percent of full-scale reading) as a function of time (ms)

,

with time on a three-cycle logarithmic scale. The peak of the transient occurs at approximately 20 ms , as indicated by the vertical line. The heavyline plot (NP) represents the transducer zero shift with no protective coating

Coating identification code:

IB = one layer of black tape

1W = one layer of white tape

IT = one layer of fiberglass tape

3B = three layers of black tape

3W = three layers of white tape

3T = three layers of fiberglass tape

46
10000

Figure 1 3B : Response of a lead metan iobate-crystal pressure transducer, with selected

protective coatings applied to the diaphragm outer surface, to a thermal

radiant-energy transient of approximately 20 mJ/mm 2. The plots are of trans- ducer zero shift (percent of full-scale reading) as a function of time (ms)

,

with time on a three-cycle logarithmic scale. The peak of the transient oc- curs at approximately 20 ms, as indicated by the vertical line. The heavyline plot (NP) represents the transducer zero shift with no protective coating

Coating identification code:

HS = "heat-sink" silicone compound

SG = silicone grease

SR = red RTV silicone rubber

47
'(jiiiujlltuuku..

r

: ._ .,,. .._ * .'

'

. i

'-

.

' .. ' .'.,.

'"

-+ r—!

i

i r

Figure 14: Response to a pressure step of 280 kPa of two unbonded strain-gage pressure trans- ducers. In these tests neither test nor control transducer had a protective coating applied to the diaphragm. Photographs 1 and 2 are of oscilloscope traces displaying the outputs of the test (upper trace) and control (lower trace) transducers. The time scale of the traces in photograph 1 is 0.1 ms/division, and in 2,

is 1.0 ms/division; for the traces in both photographs a deflection of one division in the vertical direction corresponds to 10 mV. Any variation between the

upper and lower traces in either photograph would be expected to be attributable

to the differences in performance that are commonly observed between two individual transducers of the same model. Photographs 3 and k are of traces displaying

the frequency spectrum of the traces in 2, that is, relative intensity at a given

frequency as a function of frequency. The frequency range (horizontal axis) is linear and extends from to kO kHz (4 kHz/division). ,

48
1 : ."'%

__j

+- +" L

Figure 15: Response to a pressure step of 280 kPa of two unbonded strain-gage pressure trans- ducers. In these tests the test transducer had a protective coating of three lay- ers of black tape applied to the diaphragm; the control had no protection. Photographs 1 and 2 are of oscilloscope traces displaying the outputs of the test (up- per trace) and control (lower trace) transducers. The time scale of the traces

in photograph 1 is 0.1 ms/division, and in 2, is 1.0 ms/division; for the traces

in both photographs a deflection of one division in the vertical direction cor- responds to 10 mV . Photographs 3 and k are of traces displaying the frequency

spectrum of the traces in 2. The frequency range (horizontal axis) is linear and

extends from to 40 kHz (k kHz/division).

49
Figure 16: Response to a pressure step of 280 kPa of two quartz-crystal pressure transducers,

In the tests represented by the photographs above, the test transducer had a protective coating of three layers of tape applied to the diaphragm; the control

transducer received no protective coating. The photographs are of oscilloscope

traces displaying the outputs of the test- (upper trace) and control- (lower

trace) transducers. The time scale for all traces is 1 ms/division; a vertical

deflection of one division corresponds to 100 mV . The test transducer traces in the four photographs above illustrate the wide variations in the pressure amplitude transmitted through the tape layers to the diaphragm; note that at the same

time the four control transducer traces show little variation. A possible ex- planation for the variations shown here is advanced in k.k.2.

50
Figure 17: Photographs of the faces forming part of the sensing assembly of

a quartz-crystal (left) and an unbonded strain-gage (right) pressure transducer. Part or all of these faces are the outer surfaces

of the respective instrument diaphragms. Note the presence of the annular ridge (arrow) on the face of the quartz-crystal instrument.

This ridge marks the periphery of the diaphragm; the significance

of the presence of the ridge is discussed in 4. A . 2

51
Figure 18: Response to a pressure step of 280 kPa of two quartz-crystal pressure

transducers is shown in traces 1 and 2; the response to 2070 kPa of the same transducers is shown in traces 3 and k. The diaphragm of the test transducer (response shown by traces 1 and 3) had a protective coating

of three layers of black tape. The control transducer (response shown

by traces 2 and A) had no protective coating in either set of traces. The time scale for all traces is 1 ms/division; a vertical deflection

of one division corresponds to 100 mV for the oscilloscope photograph

on the left and 500 mV for the photograph on the right.

52
APPENDIX A

The test procedure used in the evaluation of protective coatings on transducer diaphragms is given in detail below. In this procedure, the

transducer zero-shift is measured with a known thermal transient input.

1. Mount the transducer fixture (with test and control transducers

installed) onto the end plate of the shock tube.

2. Attach the mounting bracket for the flashbulb socket to the end

plate.

3. Adjust the distance from the center plane of the flashbulb to

the plane of the diaphragms to be 70 mm. Tighten clamps.

k. Install a No. 22 flashbulb in the socket position.

5. Rotate the socket so that the direction in which the supporting

(each in the shape of an inverted "L") electrodes point is approximately

perpendicular to a line between the center of the flashbulb and a point

on the face of the fixture midway between the centers of the two diaphragms and on the line joining these centers.

Note - On ignition, a black spot commonly appears on the bulb in

the area opposite to that at which the electrodes point. Step 5 prevents the spot from reducing the energy reaching the transducers

.

6. Make or check the required electrical connections, including

the flashbulb ignition-voltage source to the socket and to the trigger

of the storage oscilloscope, transducer outputs to the storage oscilloscope, and, if required, transducer excitation-voltage source to the

transducers. Turn on equipment.

7. Set the oscilloscope sweep rate to 10 ms/div and adjust triggering controls as required so that ignition of the flashbulb triggers

the sweep. Adjust beam intensity to a level that experience has shown

will result in a well -exposed photograph of the trace for a given film,

shutter speed, and aperture.

8. Load the camera with film and adjust the camera speed and aperture.

9. Fire the flashbulb. Note - The No. 22 flashbulb constitutes a very bright source; it is recommended that the test operator not look at

the bulb during ignition. The fired bulb remains hot for some time; it

is also recommended that the operator wear gloves to remove it.

10. Observe the resulting traces on the oscilloscope screen. If

they are satisfactory, proceed to step 11. If they are not satisfactory,

determine the cause. Make whatever adjustments may be necessary and re- more the spent flashbulb. Repeat steps h through 8 and fire the flash53
bulb. Repeat until satisfactory traces are obtained.

11. Photograph the traces.

12. Remove the spent flashbulb and repeat steps k, 5, and 6. Set

the oscilloscope sweep rate to 100 ms/div in step 1 . Repeat steps 9,

10, 11 , and 12.

13- Examine the photograph taken in step 12. If the traces show

that both transducer outputs have started to decay, consider the set to

be complete with two photographs. If either transducer output has not

started to decay, remove the spent flashbulb and repeat the procedure,

setting the oscilloscope sweep rate to 1 s/div or 5 s/div in step 7-

5k
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 961

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

Experimental Investigation of Means for Reducing the Response

of Pressure Transducers to Thermal Transients.

3. Recipient's Accession No.

5. Publication Date January 1978

6. Performing Organization Code

7. AUTHOR(S)

J. S. Hilten, C.F. Vezzetti , J.F. Mayo-Wells, and P. S. Lederer

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4253434

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Naval Air Systems Command , Transducer Committee TG/RCC

Washington, D. C. 20361

d c/o Division 9486

Sandia Laboratories

Albuquerque, NM 87115

13. Type of Report & Period Covered FINAL

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less {actual summary ot most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Experimental efforts are described in the evaluation

of protective diaphragm coatings as a means to reduce the effects produced by thermal

radiant-energy transients on pressure-transducer response.

A series of tests was carried out to investigate the effects of a variety of

protective coatings on the amount and rate of thermal energy transmission through

thin metal disks (used to simulate transducer diaphragms) as revealed by measurements

of the disk back-side temperature. The temperature histories of both bare and protected disks were measured with thermocouples during and after exposure of the disks to thermal radiant-energy transients (of approximately 20mJ/mm 2 at the disk) generated

by No. 22 photographic flashbulbs. Protective coatings investigated include various

tapes, greases, and room-temperature-vulcanizing rubbers ( RTVs )

.

Based on the results from these tests, the effectiveness of nine selected coatings

in reducing thermally-induced zero shifts of four types of pressure transducers was investigated. In these tests, a pair of transducers - one protected and the other

unprotected, but otherwise nominally identical - were exposed to a thermal radiantenergy transient as described above. The resulting zero shift was measured and taken

as an index of coating effectiveness.

The effect of the mass of the coatings on transducer dynamic response was investigated by means of a shock tube, in which a protected and unprotected pair were simul-

(over)

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Key Words i coatings; delayed response; dynamic; dynamic

response; pressure step; pressure transducer; protective coatings; shock tube; tape;

thermal radiant-energy response; thermal protection; thermal transient response;

transducer; zero shift.

18. AVAILABILITY |~~Xl Unlimited

I I For Official Distribution. Do Not Release to NTIS

nX] Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13. 46:961

\^ Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

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21. NO. OF PAGES

64

22. Price

$2.30

USCOMM-DC 29042-P74
16. ABSTRACT ... continuedtaneously exposed to a pressure step of approximately 280kPa (40psi). Also, the effect

of selected coatings on transducer acceleration sensitivity was investigated by

monitoring the outputs of pairs of transducers mounted on a vibration exciter.

Test results indicate the validity of the simulated diaphragm test as a predictor

of protection effectiveness. Results from both disk and transducer tests show that

some coating combinations appear to be an order of magnitude more effective than others

in delaying and reducing zero shift. Coatings of multi-layer black tape, red RTV

silicone rubber, and "heat sink" compound appear most effective, other materials less.

Silicone grease is found least effective while single-layer black tape appears to produce results ranging from very limited protection to actual degradation of response,

based on the limited sampling carried out in this investigation.
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a special service to subscribers each issue contains complete

citations to all recent NBS publications in NBS and non- NBS media. Issued six times a year. Annual subscription: domestic $17.00; foreign $21.25. Single copy, $3.00 domestic;

$3.75 foreign. Note: The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical Sciences." DEVIENSIONS/NBS

This monthly magazine is published to inform scientists, engineers, businessmen, industry, teachers, students, and

consumers of the latest advances in science and technology,

with primary emphasis on the work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution

abatement, health and safety, and consumer product per- formance. In addition, it reports the results of Bureau pro- grams in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $12.50; Foreign $15.65.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations,

and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special

publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers,

chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a world-wide program co- ordinated by NBS. Program under authority of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical Reference Data (JPCRD) published quarterly for NBS by the American Chemical Society (ACS) and the American Institute of

Physics (AIP). Subscriptions, reprints, and supplements

available from ACS, 1155 Sixteenth St. N.W., Wash., D.C.

20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research

results, test methods, and performance criteria related to the

structural and environmental functions and the durability

and safety characteristics of building elements and systems. Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The purpose

of the standards is to establish nationally recognized require- ments for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS administers this program as a supple- ment to the activities of the private sector standardizing

organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest

to the consumer. Easily understandable language and

illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, D.C.

20402. Order following NBS publications—NBSIR's and FIPS from

the National Technical Information Services, Springfield, Va. 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register.

Register serves as the official source of information in the Federal Government regarding standards issued by NBS

pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717

(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of

interim or final reports on work performed by NBS for outside sponsors (both government and non-government).

In general, initial distribution is handled by the sponsor;

public distribution is by the National Technical Information

Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau: Cryogenic Data Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

Center (275.02) Boulder, Colorado 80302.
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