AlllOb EbDSSl

REFEFIENCE

NBS

Publications

NBSIR 84-2862

Development of a Polymer

Pressure Gage With Temperature

Compensation

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymers Division

Washington, DC 20234

December 1983

Interim Report

Contract No. DLYV 82-64

James R. Holder, Contract Officer

Issued April 1384

190

.056

34-2362

1984

Prepared for: Air Force Armament Laborator/

Vulnerability Assessment Branch

Eglin AFB, Florida 32542

UATIOMAL OF ST.' LIDPJiKY

NBSIR 84-2862

DEVELOPMENT OF A POLYMER

PRESSURE GAGE WITH TEMPERATURE

COMPENSATION

Anthony J. Bur and Steven C. Roth

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

Polymers Division

Washington, DC 20234

December 1983

Interim Report

Contract No. DLYV 82-64

James R. Holder, Contract Officer

Issued April 1984

Prepared for: Air Force Armament Laboratory

Vulnerability Assessment Branch

Eglin AFB, Florida 32542

U.S. DEPARTMENT OF COMMERCE, Malcolm Baidrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
=!PFM'

i^'

& it>

f/ ^

. 'M&i

Ffln

“ '> '^.

T*i

•V,/

01

>> I ip'« m

.. 4'i , A. yom\

r If ,

"Km'.

/^s '4

h'

v/*»V *uv,.-K 3>'-^

r

*

'

vJV i^esas

Mk

'* a

:!;/ I

...;v ‘^•.•y«v

wet

SB WJO .<^

.vititoM ,'l^‘ I4fr h-kiA i>«v^ '*'

1 -

; 7‘<

lipni

' 'u'> - - i-f>^

'

, •<T

.

^ ^

'jff'i

’ 'i, i 1^ V

Ife:

?(?>/: 1 •ts.- *

' ': It?,' I

^

:^;iW'tW|l>#

^ottJvjrjeJ tni»mei*wiA

t/^*mi«#caA wiuV i|

>f4£t .a^A ftttgt r

,'ir

,^y- f

., •M' «• >.

¥1

N.'mta i'ArClTAW

t'M\

»F(7 J'-m

* n t
PREFACE

The development of a temperature compensated pressure transducer is

described. The pressure sensing element of this transducer is a thin film

of polyvinyl idene fluoride which is both piezoelectrical ly and pyroelectrically

active. In order to measure accurately a pressure pulse which is also accompanied

by a temperature pulse due to adiabatic compressional heating, it is necessary

to correct for the pyroelectric signal. The temperature compensation technique

which we use is to measure the temperature with a fast response thermocouple, to

amplify the thermocouple signal in accordance to the pyroelectric response of the

transducer and to combine the transducer and amplified thermocouple signals

to produce an output voltage proportional to pressure only. A compensation

circuit with a frequency range of 1 Hz to lo'' Hz was constructed and tested.

The transducer was calibrated and tested using pressure pulses whose peak

value was 2.1 x 10^ Pa (3000 psi) and whose pulse width was approximately

5 to 10 ms. For these measurements, the transducer was placed in an oil

pressure chamber at room temperature and the pressure pulse was initiated

by dropping a 16 kg mass onto a plunger in the chamber. Signals from the

PVDF transducer, from a reference pressure transducer and from a thermocouple

were captured by a transient recorder. The pressure measured with the

calibrated and compensated PVDF transducer was found to agree satisfactorial ly

v/ith the signal from the reference pressure gage.

SECTION I

INTRODUCTION (U)^

The piezoelectric material which forms the active element in this

pressure gage is polyvinyl idene fluoride (PVDF). It is available as thin

sheets or films which are normally inactive and becomes piezoelectrically

and pyroelectrically active by poling a region of the material with a large

electric field. Because of its transducer behavior, PVDF has been studied

and characterized in many materials experiments and transducer applications in

12 3 4. our laboratory and elsewhere ’ ’ The designs of a variety of gages, hydrophones, pyroelectric detectors and acoustic drivers have incorporated an active

5 6 7

PVDF element ’ ’

. Although most of these applications involve small pressure

signals, there have been some published data on the use of PVDF to measure

pressure in shock tests. In laboratory shock tube experiments, Bauer has

9 2 shown that PVDF can be used as a detector up to pressures of 2.5 x 10 newtons/m

C Q

(Pa) (3.6 X 10 psi) . There appears to be no inherent materials shortcoming

which would inhibit the use of PVDF at high pressures.

One of the thermodynamic consequences of pulses of high pressure imposed

on a PVDF pressure gage and its surroundings is adiabatic heating which will

produce a pyroelectric response along with the piezoelectric response. This

effect was discussed by DeReggi et al who considered the relative time effects

of pressure induced temperature changes in PVDF and temperature changes which

5 occur in the surrounding medium . Adiabatic heating of the PVDF will occur in

coincidence with its pressure change, i.e. there will be no time delay of a therm

energy pulse with respect to the pressure pulse. When adiabatic heating of the

®(U) indicates that this material is unclassified.
Ts ‘(u)

c

''j

y

f ’ ti 4 ;t^‘«^ irf4- 4prrtw H ^ ai?sn<r--

,

' _ ' ,. ' .? ?D ' H

uf%9: '^:ik*yes9 ^t4n oHf^f9

^ ^

'",.>' V- ffl m

- *ii' ', 1^

,l.s.

^svtJsa i»a s»«4«rj%'JG^4r :>Hl'yitecrrxa ^jonod^^

5^yi:^ais ' risiie g uilRar K<{» s^tifcy H o^miiU . •''*^-^^r?r»

#3 beH'gMdwq SJS.OC n»9d javcrt »i!WtJ ,f£ftng>4 _ . » ', ' ^

''’'

- 4$

fmi . Zjij»f9!«f ^HSi mn f?o>arf^4f •*''' ' (9

fl A , *^'- '

. 81 j(jk 'w\2fi44*wir if«'tk?te«hfl#3 ifv a g« tw>iij $d nsa WS

i trttfltoaJiGiJ# iJ Jiw- i*flRt w i|d rr*s>«qs srtfKT * >~

iiJ <.... .'"(Uq ‘or X ^*41 {S4J :*% '58 8»u 9iU j^dTf'f'tor

V ''fe

to «Ml|«i^Viri'gs?K»a^aooa at«85Y,b^ tc *t»0

;® ^a

.^, ,"m T^- TTIfi Ofifc aW rj*w 8<r«| 4. Vto

^'

' ^ "

<

'-''

'- *

. ''^"%

’?>rtT' .i««iWRi»n IBIJ I a^uto^q

*«

'!P' sF" Vr-*, sv^^r*l»'« qfl5 od^ |« IfO x<J i>sfiwelO 2 CH *$i3F>t%

' ' “

'"’ \' fv

“

pf 'fuaao fi'fw rr. ?ntbftaotT:v2 Yaa.a#®

-O "'"

' ^ *'

' 8,,;1 o,_^ a#l ort, adlltm-rtfidi *4^* ,'*0fl 8<u s^,t.?4^q 5if dahj^aan^.s^aflifa?"'

odJ to 2'f#6tl8 fb« *xyrfW . 9«f«4&a^*^tt»ia 5d5 a? JjoqgiH rt^tw stfyq tsr*»M

. 4''#w.-*f- '
surrounding medium occurs, the thermal time constant for diffusion of heat

into the gage will determine its pyroelectric response as a function of time.

If the temperature changes In PVDF and its surroundings are identical, then

there will be no heat transfer and the pyroelectric response will be due to

adiabatic heating of the PVDF only. Calculations for this special case show

that the pyroelectric charge signal is approximately 8% of the piezoelectric

c charge . In general, adiabatic heating in both PVDF and its surroundings

along with the time scale of each effect must be considered. For the PVDF

gages used in this investigation, the time constant, x, for heat to diffuse

into the gage from the surroundings is approximately 30 milliseconds (ms).

For pressure measurements at t « x temperature compensation can be achieved

by applying the 8% correction to the transducer signals. For longer times

the conduction of heat from the surroundings must be measured in order to

apply the appropriate correction to the electrical signal from the PVDF gage.

The measurement system, which we describe here, has been developed for

the purpose of obtaining accurate dynamic pressure data in the presence of a

changing thermal environment. Compensation is achieved by using a thermocouple

with a fast response time to measure temperature changes in the PVDF gage. The

dynamic range of the compensated measuremient can be as broad as 0.1 Hz to 10^ Hz.

6 8

Pressure measurements at higher frequencies, 10 - 10 Hz, can also be made

as long as the 8% temperature compensation is applied.

SECTION II

METHOD OF COMPENSATION (U)

The method of temperature compensation we have chosen is: (a) to measure th

temperature change of the PVDF transducer using a thermocouple; (b) to

amplify the thermocouple voltage to equal that generated by the pyroelectric
I 1
response of the gage; and (c) to add the transducer voltage to the amplified

thermocouple voltage yielding a corrected gage voltage. This corrected output

is proportional to the pressure which is applied to the active area of the

transducer.

Figure 1 illustrates the compensation method. Since the PVDF transducer

is a charge generating device, its signal is converted to a voltage via a feedback

capacitor C^, in a charge amplifier. When the transducer responds to both

temperature and pressure simultaneously, the transducer charge, = Pp -

where

qp

is the charge proportional to pressure and q^^ is the charge proportional

to temperature. These charges combine out of phase because a positive pressure

and a positive temperature will generate charges of opposite sign. The feedback

capacitor converts these charges to corresponding voltages,

e^ = qp/C^ - ~ ^p

' ^th ®th voltage proportional to the

temperature change AT, ep is a voltage proportional to the pressure change AP,

and e^ is the output (transducer) voltage from the charge amplifier. In terms

of pyroelectric and hydrostatic piezoelectric coefficients X and d^ we have

‘’th "

where fs the area of the electrodes. Equations (1) and (2) define the

coefficients X and d, . h

The thermocouple junction, which is positioned close to the active area

of the transducer, provides a voltage, = KAT where K is the thermocouple

constant. is amplified to equal the quantity e^^ by an amplification factor,

A^, so that When e^^ is set equal to e'^ by adjusting A^ and whtn

e^ and e^^ are added, we have

®t ®th

= e. - e th + e th

= e (3)

and the compensation is achieved.
\
4

can be obtained experiimntally by subjecting the transducer to a temperature

change with no accompanying pressure change. In this case, the gage output

should be zero. In experiments to be described below, the transducer is

irmersed in hot or cold water and the value is adjusted until the addition,

Vth ^

A calculation of A^ can be carried out by noting that

XA^AT

e

Substituting AT = V^^/K into equation (4) we have

Our objective is to achieve the equality, e^^ = e^^, where

e th Mth*

(4)

(5)

( 6 )

Thus, a comparison between (5) and (6) yields

(7)

which is a dimiensionless quantity.

4

A^ is proportional to a and inversely proportional to C^. Using = 10

picofarads (pf) and considering typical values of X = 4 nanocoulombs per square

2 2 centimeter per degree Centigrade (nC/cm °C), A^ = 1 square centimeter (cm ) and

K = 41 microvolts per degree Centigrade (viV/°C) (for copper-constantan )

,

XA

C^K ( 3 )

5

SECTION III

THE MEASUREMENT SYSTEM (U)

The temperature compensated pressure gage contains three essential elements:

(a) the active PVDF transducer; (b) a thermocouple having a short rise time;

and (c) compensation amplifiers which add the transducer and thermocouple

signals.

1. (U) DESIGN OF THE TRANSDUCER

The PVDF transducer with thermocouple is shown in Figure 2. It is made

from four sheets of PVDF which have been laminated together using epoxy. The

inner two sheets (12 micrometer (ym) thickness) contain active areas on which

aluminum electrodes have been deposited. The active area is 1 cm in diameter.

The outer two layers (25 micrometer (ym) thickness) serve as protection for the

inner two so that the gage can be used in environments requiring mechanical

ruggedness. A copper-constantan thermocouple junction, made with 75 ym

(3 mil) wire, is placed between the inner two sheets and within 2 millimeters

(mm) of the active transducer area. The overall thickness of the transducer

is approximately 0.13 mm.

Prior to lamination the electroded regions are made piezoelectrical ly

active by poling them at room temperature with an electric field of 2 megavolts/

centimeter (MV/cm). The active areas are then laminated face-to-face so

that the polarization vectors in each element point in opposite directions.

In this bilaminate pattern, the ground electrodes are on the exterior surface

and the inner electrodes carry the signal potential. One of the advantages of

I this design is that signals generated in the two elements by bending are opposite

in polarity and add to zero. Other details regarding gage construction can be

q IQ obtained from published reports ’

.

6

We have chosen the symmetric geometrical configuration of Figure 2

because this arrangement will yield nearly coincident thermal response times

for both the transducer charge, q(t),and the thermocouple voltage, V^^(t). If

we consider thermal energy associated with a pressure pulse traversing across

the thickness of the PVDF film from one side to the other, the conduction of

heat can be described approximately by a solution to the heat flow equation

in one dimension'*'*. The responses q(t) and V^^(t) will not be exactly the same,

however, because V^^(t) changes directly with T(t), the temperature change, but

12 13

q(t) follows the integral of temperature over the thickness of the film ’

.

Solutions to the heat equation are that both T(t) (or V^^(t)) and q(t) are expressed

as Fourier series both of which contain the same dominant thermal time constant,

where I is the thickness of the transducer and k is its thermal conductivity.

Since the time constant varies with it is important that thickness of the

transducer be the same at the region of piezo-sensitivity and at the location

of the thermocouple.

The differences in V^|^(t) and q(t) appear in the coefficients of the Fourier

series and in the constant term, but these differences are minimized in the

symmetric configuration. For a transducer thickness of 2 mil (50 micrometers)

with thermocouple and piezo-sensitive region symmetrically positioned in the

center, calculations show that V^j^(t) lags q(t) by 0.4 milliseconds (ms) and that

the overall rise time is approximately 9 ms. (It should be emphasized that

this response time corresponds to the thermal response of the gage and is not

indicative of its response to pressure. Response to pressure as fast as 50

O nanoseconds (ns) has been observed in shock tests by Bauer .

)

The inherent rise time of the thenr.ocouple, independent of its surroundings,

was measured using a light flash to deposit thermal energy at the thermocouple

junction. In this experiment the thermocouple signal is not determined by the

thermal conductivity of the medium surrounding the junction, but rather by the

geometry and electrical characteristics of the junction itself. Using a light

pulse with a 20 microsecond (ys) rise time, we observed that the rise time of

the thermocouple was 50 ys. Thus, the thermocouple can be used to detect adiabati

heating in the PVDF for times longer than 50 ys and can detect satisfactorially

the diffusion of heat from the surroundings into the PVDF §age, for which the

time constant is 30 ms. For times shorter than 50 ys, an 8% correction due

to the adiabatic heating of PVDF should be used.

2. (U) DESIGN OF THE COMPENSATION CIRCUIT

In Figure 3 we show the amplifiers to which the signals from the transducer

and the thermocouple are connected. The PVDF transducer is grounded on the

side of negative polarization so that the output voltage of the charge amplifier

is negative when a positive pressure is applied, i.e. e^ = -AQ/C^ where AQ is

the charge generated by a positive pressure change AP. The time constants,

R^C- and R

2 C

2

, determine the high and low frequency 3 db points and are set at

220 ms and 32 ys respectively; this corresponds to a frequency range from

4

1 Hz to 10 Hz. The amplifier ^ voltage follower with a gain of 1 which

serves as a buffer.

Amplifiers A^ and A^ determine the amplication factor A^ for the thermocouple

signal. The exact value of A^ which is needed for a particular transducer can

be obtained by adjusting the gain of A^. The value of the input amplification,

A,, is set at 400. High and low frequency 3db points are established using

time constants R^C^ and R^C^ where R^C^ = R

2 C2

and R^C^ = R^C^ so that the

phase shifts for the transducer and thermocouple amplifiers are the sam,e.

8

Since positive AP will contract the volume of the transducer and positive

at will expand the volume, these two energy excitations will produce opposite

electrical charge responses in the transducer. Therefore the corrected signal

ep is obtained by adding e^ and e^^. This is done in the adder A^. The output

of Ac is the reverse polarity of the input and is a positive voltage for a

positive AP applied to the transducer.

In the compensating circuit, low noise operational amplifiers are used.

This is particularly necessary at the high gain input amplifier A^ where an

0P16E operational amplifier is used. The resultant noise level permits a

sensitivity of AT = 0.2 ®C. Values of circuit elements are listed in the

Appendix.

The compensation circuit also contains output connections so that the

thermocouple signal and the uncorrected transducer signal can be monitored.

By inspecting the uncorrected signal, it is possible to obtain information

4

about the pressure pulse at frequencies higher than 10 Hz.

SECTION IV

TESTING THE COMPENSATION CIRCUIT (U)

The operating characteristics of the compensating circuit were examined

by measuring the outputs, e^ and e^^, when a step function charge signal and

a step function voltage signal were applied to the transducer and thermocouple

amplifiers. The input charge signal consisted of a 5V step in series with

a 3300 pf capacitor having a rise time of 5 ys. The input voltage to the

thermocouple amplifier, A^, was a 4 mv step with a rise time of 5 ys. The

outputs e^ and e^^, display a characteristic roll-off with a 220 ms time constant

which is identical in both amplifiers and therefore the resultant phase shift for

each signal is the same.

In order to determine the amount of temperature compensation attainable

using this circuit, two experimental tests were designed. In one test, a

transducer with encapsulated thermocouple was immersed into a cold water bath

by slapping the broadside surface of tne transducer against the surface of

the water. In the other test, the surface of the transducer was sprayed with

a dull black paint which serves as a light absorber. A light pulse was then

directed at the surface of the transducer and a pulse of thermal energy was

absorbed. In both experiments, we attempt to simulate the onset of a thermal

pulse at the site of the transducer.

The physics of the two experiments are somewhat different, however.

For the water immersion experiment, the surface of the transducer in contact

with the water is held at the bath temperature and heat diffuses in such a

way that the transducer reaches an equilibrium temperature equal to that of

the bath. For the light flash experiment, a pulse of thermal energy is

imparted to the surface of the transducer and subsequently diffuses into the

gage; at longer times the transducer has an average temperature which is

higher than the surroundings. The water immersion test approximates

the physical conditions present when the transducer is buried in soil, i.e.

the transducer maintains contact with the soil which acts as a thermal

reservoir. The thermal flash approximates conditions relevant to the transducer in air.

Although we have obtained valuable information from both tests, v;e

have relied on the water immersion test to set the gain level of amplifier

A^. Since this experiment is carried out at zero pressure change, the

desired output of the compensation circuit is zero. In Figure 4, v/e

show the response curves from two immersion tests: Each experiment yields

three curves: (a) the output from the transducer, e^; (b) the signal, e ; C L II

10

and (c) the corrected (temperature compensated) output, 6p. In Figure 4a, the

gain level has been set too high. In Figure 4b the gain has been set at the

optimum for maximum cancellation of the thermal or pyroelectric signal.

For the data of Figure 4, the peak temperature change is approximately 3 °C

and the peak value of e^ is approximately IV.

When the initial response of the transducer and thermocouple is

coincident and the rise time of each signal is the same, maximum compensation

will be achieved. In Figure 4b, we note that small differences in the two

signals lead to small excursions from zero in the output, Op. Typically,

95% or more of the pyroelectric signal can be compensated by this method.

SECTION V

CALIBRATION OF GAGE IN AN OIL PRESSURE CELL (U)

The objective of the calibration is to obtain a value of d^, the

hydrostatic piezoelectric constant, for dynamic pressure pulses having a

pulse width of approximately 10 ms and peak pressures between 100 and 3000

5 7

psi (7 X 10 and 2.1 x 10 Pa). For short pressure pulses, a temperature

change usually accompanies the pressure change, so that

‘'t "'’P - \h ='‘h ''e'’ - • (1°)

When T, P and are functions of time,

qjt) + T{t)

‘‘h " AgPtt)

( 11 )

Since this equation will be the relationship used to determine d^,

we must make an independent measurement of X. This is done by subjecting

the transducer to a temperature change at zero pressure change. The water
' i

w

r?

fiiq

v'fi- . /,

:
11

immersion experiment, by which we adjust the value of A^, can also be used

to determine X. From the data of Figure 4, X can be determined by taking

the ratio of charge to change in temperature and dividing by the area of the

electrodes. Values of X for several transducers are shown in Table I.

For the pressure experiment, the PVDF transducer is placed in an oil

pressure cell at room temperature and the pressure pulse is initiated by

dropping a 16 kg mass through a distance of approximately 30 cm onto a plunger

in the cell as shown in Figure 5. Signals from an accelerometer (attached to

the drop weight), the transducer, the reference pressure transducer and the

thermocouple are captured by a transient recorder signal processor.

The pressure cell consists of a stainless steel block, 7.5 cm wide x 16 cm

deep X 15.2 cm high (3 in. x 6^ in. x 6 in.), with a 1.90 cm bore traversing

the 16 cm dimension. At one end of the bore, the reference transducer, which is

a resistive device with a response time of 60 ys, is placed; at the other end,

feedthrough connectors carry signals from the PVDF transducer and thermocouple.

The plunger fits as a piston into a 2.54 cm bore with vertical orientation which

intersects with the horizontal bore. In preparation for the measurements, the

cell is filled with either a vacuum pump oil or an alkyl benzene dielectric oil.

Care was taken to remove all visible bubbles from the oil, but we did not degas

the oil under vacuum. During the test, the cell is placed at the bottom of a

vertical column which guides the drop weight as it falls onto the plunger.

Signals from the accelerometer, PVDF, thermocouple and reference pressure

gage, respectively a(t), q^(t), T(t) and P (t), are captured individually by

a signal processing system. The compensation circuit is not employed here,

but rather, these four signals are used to verify its operation and to

independently ascertain the degree of compensation which is attainable.

Examples of the signals observed during a drop test are shown in Figures 6 and

12

7. Here, we note that T(t) contains a significant amount of unwanted noise. It

is the result of low signal-to-noise ratio at the input amplifier of the signal

processing system. The total temperature change is about 0.5 °C or a thermocouple

signal of 20 microvolts (yV) and the overriding noise is approximately 2.5 uV

peak-to-peak. The correction to the transducer signal due to the pyroelectric

effect will be about 15%.

The data of Figure 6 are captured digitally, each trace containing several

hundred points. Using equation (11), we make a point-by-point calculation of d^

across the trace of the pressure pulse. The result is shown in Figure 7 where

d^ for transducer AF-31 is plotted versus time along with a(t). Near zero time,

oscillations occur in the result because the signals are small and contain

electrical and ringing noise. When P(t) (or a(t)) is close to zero the

calculation involves dividing by a small number and obtaining an abnormally

large value for Over the central region of the pulse, however, d^ is

seen to be constant with a value of 13.6 ^

SECTION VI

MEASUREMENT OF PRESSURE USING THE COMPENSATION CIRCUIT (U)

The calibrated transducer AF-31 and the compensator were employed

together for the measurement of pressure pulses in the same oil cell. A

comparison was made to a reference pressure pulse obtained from the

accelerometer signal, a(t). Here,

'^These numbers are called gage constants and are calculated using the area of

the face of the transducer A . The material's d constant is this vino

since the bilaminate configuration would require dividing t..e chaige by

instead of A^.

where m is the mass of the drop weight and A is the area of the plunger.

Equation (12) was checked in turn with the resistive pressure gage in the

chamber. The estimated uncertainty in the measurement of P(t) using the

accelerometer signal is ± 1^.

The two pressure signals are shown in Figure 8. It is seen that the PVDF

pressure pulse faithfully follows the reference signal. There is, however,

noise on the PVDF signal which is attributed entirely to the thermocouple input

amplifier. If translated into pressure, the uncertainty in the PVDF pressure

5

measurement due this noise is ± 3.50 x 10 Pa (± 50 psi).

SECTION VII

DISCUSSION AND CONCLUSIONS (U)

The adiabatic heating under pressure of the PVDF and oil was just large

enough that compensation was appropriate. In soil, which usually contains a

small fraction of gas, or in air, we expect that adiabatic heating will be larger

and the need for compensation greater. It should be emphasized that compensation

achieved for times greater than 50 ps which is the rise time of the thermocouple.

At shorter times the adiabatic heating of the PVDF will produce a pyroelectric

signal which decreases the transducer output by approximately 8%. Both long

and short time data can be acquired using the compensating circuit because the

uncorrected signal, e^, from the charge amplifier will contain short-time

information. If e^ is sent thru a high pass RC filter, whose time constant

is 32 ps, and if this signal is then increased by Q% and added to e (the compensa

P signal), a full spectrum of pressure data for long and short times will be

obtained.

A constant value of d^, which we observe up to a pressure of 2.1 x 10^ Pa

(3000 psi), indicates that we are operating the transducer in the linear range.

At much higher pressures (5 x 10^ psi). non-linear effects can be expected since
!L '; • I »r :,
14

g

the compressibility of the PVDF decreases with pressure . This would be reflected

as a decrease in output charge per unit applied pressure or, in other words, an

effective decrease in d^. A close inspection of the data of Figure 7 shows a

slight dip in the center of the curve corresponding to a minimum in d^^ at peak

pressure. We observed this effect routinely in the calibration experiment when

the peak pressure surpassed 7 x

10'^ Pa (1000 psi). The effect is small at these

relatively low pressures, only 3 or 4%, i. e. several times less than the overall

accuracy of our measurement of pressure which we estimate to be ± 10% with a

sensitivity of 3.5 x 10^ Pa (50 psi).

A needed improvement in the compensation circuit is to lower the electrical

noise in amplifier A^, the input amplifier for the thermocouple signal. At present,

A^ is an 0P16E, an operational amplifier with 5 yV peak-to-peak noise and wide

bandwidth capacity. At the time of construction of this compensating amplifier,

the 0P16E represented the optimum for low cost, wide bandwidth amplifiers. As

the market and the state-of-the-art improve in this area, other units will be

available to replace the 0P16E.
'w, 'ia

ttfi 'Hi; 'f^iiQi^trm *m

Q»4v,^n^ditX3 5/i^ aiT .,^:^istnq'.*

*'"f* 0^ "

. ffr'

-'

.

' «?

'

' ,' - ;r^;,'®. '^:i<../ .flH

''

' ^ '"'4'^.'

\.:’,.M J':*,,. B , B .

" _ , 1 »" 'll

» -^t#* iir33»-:t)e»ss |i.-» >o^3»tj^^

,.ffls- s'? '‘{M X i ,e*'T0 Vi3»>3>i«»f

'WS

9^ ll^

n*39iq 3A ,^.(»«e+»' 9f«j«>00lrt9(to; 443 '‘^ -<^mrm^^ ««1- >5* -«>»*«W^I M^|nj“

, »fch» bw'^Wofl, ? 'Ill* 4«j??^^3Bo:»avi4«(. '» . WW n» *^'

, im ,(H' iautJ'Efwl tq a(n-^4J JA rtlbh^fled'"

f|^^9(lTi,?W ejftnt T3«9 *(l*|n*. i»lBe<£,sAj(

JT^ — ‘'— tm

m

“t-i*,.-

, 'H

..!?* y* *4 i« •• jf ‘TS .««

','i E',;^

r<? *

. .

V

.'>'" ii 3

„ :% 1^'

Wi.

r;.i^

i ‘it a::\ rrljy

iVii

> at?ft

'
SECTION VII

APPENDIX (U)

The following is a list of the components used in the temperature

compensation circuit shown in Figure 3.

f Amplifiers A^ , ^3^* ^4 ^5*

operational amplifier 0P16E

0 Resistors and capacitors:

R. - 5 ohms

in

R^ - 22 X 10^ ohms

R2 - 4.7 X 10^ ohms

R^ - 560 ohms

R^ - 2.2 X 10^ ohms

Rg - 22 X 10^ ohms

Rg - 1 X 10^ ohm variable

Ry - 5 X 10^ ohm

Ro - 5 X 10^ ohm

8

3

Rg - 5 X 10 ohms variable

C^- - 10^ pF

C2 - 6800 pF

- 150 pF

C5 - 10 yF

0 Thermocouple:

Copper-Constantan

d Operational amplifier, 0P27, has lower noise characteristics than the

0P16E and could be used for amplifier A^.

References

!•) "Piezoelectricity and Pyroelectricity in Polyvinyl idene Fluoride - A

Model" by M. G. Broadhurst, G. T. Davis, J. E. McKinney and R. E. Collins,

0. Appl. Phys. 49, 4992 (1978).

2.

) "Piezoelectricity and Pyroelectricity in Polyvinyl idene Fluoride" by

P* G. Kepler and R. A. Anderson, J. Appl. Phys. 4490 (1978).

3.

) "Characterization and Piezoelectric Activity of Stretched and Poled

Polyvinyl idene Fluoride. Part I: Effect of Draw Ratio and Poling

Conditions" by R. j. Shuford, A. F. Wilde, J. J. Ricca, and G. R. Thomas,

Polym. Eng. Sci. ]6_, 25 (1976).

^•) 'Ferroelectric Reorientation of Crystallites in Polyvinyl idene Fluoride"

by N. Takahashi and A. Odajima, Ferroelectrics 49 (1981).

5.) Piezoelectric Polymer Transducers for Dynamic Pressure Measurements"

by A. S. DeReggi , s. Edelman, S. C. Roth, National Bureau of Standards

Internal Report, NBSIR 76-1078, June 1976-

5.) Ferroelectric Polymers and Their Applications" by Michael Marcus,

Ferroelectrics 29 (1982).

^* ) Piezoelectricity in Polyvinyl idene Fluoride" by G. M. Sessler, J.

Acoust. Soc. Am. 70, 1596 (1981 ).

3.) Piezoelectric and Pyroelectric Polymers" by F. Bauer, Polymers as Synthetic Metals Conf . , London, May, 1983.

3.) Room Temperature Poling of Polyvinyl idene Fluoride with Deposited Metal

Electrodes" by J. Kenney and S. C. Roth, J. Res. Nat-'l. Bur. Stds.

447 (1979).

10.

) "Piezoelectricity and Pyroelectrici ty in Polymer Electrets" by

M. G. Broadburst, W. P. Harris, F. I. Mopsik, and C. G. Malmberg,

in Electrets, Charge Storage and Transport in Dielectrics , M.

Perlman, Ed., Electrochemical Soc., Princeton, NJ, p. 492 (1973).

11.

) Conduction of Heat in Solids , by H. S. Carslaw and J. C. Jaeger,

Oxford University Press, Amen House, London, 1948, p. 84.

12.

) "Determination of Charge or Polarization Distribution Across Polymer

Electrets by the Thermal Pulse Method and Fourier Analysis" by

A. S. DeReggi, C. M. Guttman, F. I. Mopsik, G. T, Davis and

M. G. Broadhurst, Phys. Rev. Letters 413 (1978).

13.

) "Numerical Evaluation of the Dielectric Polarization Distribution From

Thermal Pulse Data" by F. I. Mopsik and A. S. DeReggi, J. Appl . Phys.

4333 (1982).

FIGURE CAPTIONS

Figure 1.. The technique of temperature compensation is described.

Figure 2. A diagram of the PVDF transducer with thermocouple is shewn.

Figure 3. A schematic diagram of the temperature compensation circuit

is shown.

Figure 4. The response curves, e^, e^^ and e^, are shown for two water

immersion experiments. Temperature change was approximately

3 ®C. (a) Amplification factor, A^, set too high; (b) A^

set for optimum compensation.

Figure 5. A diagram of the drop test and pressure cell is shown.

Figure 6. Signals, T(t), q(t) and P^(t),.from a drop test experiment

are shown. Peak pressure is approximately 7 x 10^ Pa (1000 psi

Maximum temperature change is approximately 0.5 ®C. The peak

charge is approximately 7.4 x 10’^ C.

Figure 7. dj^ and a(t) are plotted versus time for PVDF transducer AF-31

.

Figure 8. P(t) is plotted versus time for the reference pressure gage

(the accelerometer) and for the calibrated PVDF transducer

AF-31. The signal with the larger noise component is from the

PVDF transducer.
^ a:? V,,'. :--'K .. *i;

.-, S

!?3 E»

* ,' Xi-’ll— >t

T V’^ :*.»

•»‘^‘»* -V

. •. * 1 l^ii^ i :'si4 !« , .:_

w»«^ as ^ ^ F %.

3<a«^

')' 0-«Hl’t?#^l3 A^ ^ ^C; nUl

Ji2^0'^ , vj', ' ^^'•Jyjf ''

, ’'|i'-i • -^ '^M3

^ If . \> . , jImVI ^;< _. '1^ — -- A ^ i A .

., _ ra,. "

'«, ‘,'# '. ®

' »ii

^\;iA9q 'I^L "

It' ."

' D

' ^ '^' itet ®"^,-vx V.T if »«''fwi3 g^;.''^^ ,3

T!tCi!tAnw3^ b«!ii«^rq -m U)§ Nt ^6

^ bU i- PQ9

fc«'’.. ,./.J9;,u!5£„8#1®'*i#»i^^^^ j, ^^_ ,. ' -«-. w ®fe, '^^^tui

* I

%fit Itifl^n 9rfT JE-'IA

^

f •’®' ^ . s - . iai;beflfi*f 3 10 '/^'

'ly/.'j';!-

' -V . . :V/: yiUI

' " '* , i fe-rt k'te

sr.8

^ HI
TABLE I

Pyroelectric Coefficient, X

Transducer

AF-16 4,4

nC

cm^°C

AF-18 4.0

AF-19 3.0

AF-31 3.4

These numbers are called gage constants and are calculated using the area of the face of the transducer, A . The material's X constant is

only half this value since

dividing the charge by 2A^

the bilaminate configuration

instead of A . e

would require
r m it

' fi.L.

r<

P'Sy

U

:'

• /ff '< 4>;-

jj

-*VA^,

F4, 'J

5i r

a.. a 'M.'

ff-^A

t-F

Tl > i

-v'"\

YaJ.*p^

•i5*.I^^ rV

•3i

.

' I i

ryl®' 'iC^. at "

-fS> ru ItsdfnSfr asisilT ” ' '^

,^A' »snJ -aHi' 'to »r 0uT«v ?t‘H^ t W'

''V({, 1^

4'»P ., ;, J^'A

K' 5-4^

>* i/'.' is;

»^'j

m

if^

Ilf'?

hW< • JS

jiSi

:< '

‘sn

ca

..-t
Compensation

Cf

_ ^

Cf C,

where = AAqAT

Qp = dhAeAP

Figure 1

PVDF Transducer

Figure 2

IJncortecled

Output

I’lcuRi';

TIME ms

TIME m s

CD

<t)

Drop Test

Plunger

Steel Block

Reference Gage

PVDF Gage

FIGURE

FIGi RE

a(t) m/sec^

O lO

CNJ TLO o

N/Od "p

QJ

f3

C75

TIME

ms

00

(Od

OJ

i_CO

Ll_

S-01.X isd

TIME

ms

NSS-1 14A R£ V.

'J.5. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET I See instructions)

1, PUBLICATION OR

REPORT NO. NBSIR 84-2862

2. Performing Organ, Report No. 3. Publication Date

April 1984

4. TITLE AND SUBTITLE

Development of a Polymer Pressure Gage With Temperature Compensation

5. AUTHOR(S)

1 A. J. Bur, S. C. Roth 1

6. PERFORMING ORGANIZATION (If )oint or other than N BS. see in struction s) 7. Concract/Gcant No. i

national bureau of standards

i DEPARTMENT OF COMMERCE 3. Type of Report a Period Ccveree WASHINGTON, D.C. 20234

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

I

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bl iography or literature survey, mention it here)

The development of a temperature compensated pressure transducer is described. The

pressure sensing element of this transducer is a thin film of polyvinylidene fluoride

which is both piezoelectrically and pyroelectrically active. In order to measure a pressure pulse which is also accompanied by a temperature pulse due to adiabatic heating, it is necessary to correct for the pyroelectric signal. The temperature compensation technique which we use is to measure the temperature with a fast response

thermocouple, to amplify the thermocouple signal in accordance to the pyroelectric re- sponse of the transducer and to combine the transducer and amplified thermocouple signals to produce an output voltage proportional to pressure only. A compensation circui : with a frequency range of 1 Hz to 10^ Hz was constructed and tested. The transducer war calibrated and tested using pressure pulses v/hose peak value was 2.1 x lO’^ Pa (3000 psi

and whose pulse width was approximately 5 to 10 ms. For these measurements, the

transducer was placed in an oil pressure chamber at room temperature and the pressure

pulse was initiated by dropping a 16 kg mass onto a plunger in the cha.mber. Signals

from the PVDF transducer, from a reference pressure transducer and from a thermoc' U" ,

.• were captured by a transient recorder. The pressure measured with the calibrat'^d ; compensated PVDF transducer was found'to agree satisfactorially with the signal fr -m '..f reference pressure gage. ^

'

12.

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

•

] Adiabatic heating; piezoelectricity; polyvinylidene fluoride; pressure pulse;

j pyroelectricity; temperature compensation, transducer I

I 13. AVAILABILITY 14. NO. OF 1 PRINTED PAr^r S I [X Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

1

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington. D.C. - 4 20402. 15. Pncr 1

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

1 d . ')