N O T I C E 

THIS DOCUMENT HAS BEEN REPRODUCED FROM 

MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 

CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 

IN THE INTEREST OF MAKING AVAILABLE AS MUCH 

INFORMATION AS POSSIBLE 
DEVELOPMENT OF 

...... . INA"""· t.t .. '24Otm 

-- - -

OPTICAL DIAPHRAGM DEFLECTION SENSORS 

FINAL REPORT 

by 

W. L Ghering, D. Varsh~'V8, 

L A. Jeff .... , R. T. Bailey, and J. W. Berthold 

f hA SA-Ch-175JCEj VEVEICFME~~ LF CPT' DIAPHrAG'" ,,_ ' .. l\..AL ~ LJ ~EclE(Tl(h ~EN5C~~ fl' 

t r;· b'" . - ~ ~na Lteport La coer. dUd illlccx lc) 4"- - • :J r he. AJ3/Mf A:Jl 

L.JCL 20f 

G3/74 

THE BABCOCK Sa WILCOX COMPANY 

Research and Development Division 

Prepared for 

~8o-I::1 13 

Uncld~ 

J4 t ')5 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

NASA Lewis Research Center 

Cleveland, Ohio 44135 

Contract NAS3-23712 , \ 

. -
Section 

1 

2 

3 

t 

. -

4 

5 

6 

7 

CONTENTS 

SUMMARY •••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 

I NTRODUCT ION .••..•..••.••.••.....•••••••...•••.•••••••.•••.•••••• 

DEVELOPMENT OF MICROBEND SENSOR •.••.•...••.•..•••••••.••...•.•••• 

3.1 Sensor Des; gn .............................................. . 

3 .1 . 1 Des; gn 1 ......•..•..............•..••..•............. 

3.1 .2 Des i gn 2 .................................................. . 

3.2 Sensor Evaluation •...••..•..•••••.•••...•••.•.••••.•...•.••. 

3.2.1 Operational and High-Temperature Tests for D~sign 1 .• 

3.2.2 Vibration Test for Design 1 ••••••..••.•.•.•.••••.•••• 

3.2.3 Overpressure T~sts for Design 1 •••.•••••••••••••••••• 

3.2.4 Operational and High-Temperature Tests for Design 2 •. 

DEVELOPMENT OF PHASE SENSITIVE SENSOR •..•••••••••.••.••••••..•••• 

4.1 Sensor De'" i gn ................................................... . 

4.2 Sensor Evaluation ...................... ,. ................... ·· ... ·· ...... ·· ... · ... · ... · ... · ... · ... · ... · ... · 

Operational and High-Temperature Evaluation •..••..•.• 4.2.1 

4.2.2 

4.2.3 

Vibration Tests .....••.•••.•.••...••••••....••••••... 

Overpressure Tests ............................................................. 

OTHER SENSORS INVESTIGATED •.••.....••••••••..•••••...•.••...•••.. 

5.1 Two-Fiber Transmission Sensor ...•...••.••..••••.••.•...•.•.. 

5.1.1 Sensor Design ................ ························ 

5.1.2 Operational and High-Temperature Evaluation ..••.•...• 

1 

3 

5 

5 

5 

9 

11 

11 

15 

18 

19 

25 

25 

28 

28 

31 

31 

35 

35 

35 

37 

5.2 Cantilever Slope Sensor ....•.....••..••..•••.••...•.•••..•.. 38 

5.2.1 Sensor Design ..••.....••••..••.••..•••••.....•••••... 38 

5.?2 Operational and High-Temperature Evaluation .•.•..••.• 39 

5.3 Photoelastic Sensor .••.••..••.........•.•....•.•..••...•.... 40 

5.3.1 Sensor Design .•.••..•..•....•••.•.••••••.....•••••••• 40 

5.3.2 Operational and High-Temperature Evaluation ••.••.•... 41 

DISCUSSION OF RESULTS ....•..••.•••..•.••...•...•..•......••.•..•. 43 

SUMMARY OF RESULTS AND CONCLUSIONS .••••...•....•......•.•..•..•.. 47 

PRECEDING PAGE BLANK NOT FILMED iii 
LIST OF ILLUSTRATIONS 

Figure Page 

1 Mi crobend sensor Design 1 ..................................... 6 

t. Mi crobend sensor Design 1 exploded view ....................... 8 

3 Microbend sensor diaphragm ....................................... 8 

4 Microbend base plate with fused silica block .•...•••....••.....•. 9 

5 Microbend sensor Design 2 10 

6 Microbend sensor Design 2 exploded view •.••••••••..•.•••..•••• 10 

7 Microbend sensor system block diagram 12 

8 Test setup schematic diagram ....................•.•...••••.....•. 13 

9 Typical microbend sensor response data for Design 1 •....•••....•• 15 

10 Electronic circuit for microbend sersor vibration test ••.•..•...• 16 '1 

11 Instrumentation for vibration results .......................... .. 17 ~ 

12 Typical microbend sensor vibration results ..................... .. 17 

13 Microbend sensor response during overpressure test sequence 18 

14 Analysis of microbend sensor response during overpressure 

t- tes t sequence .••............................................•.... 19 

~ . 15 l Design 2 microbend sensor with pressure chamber ..•.••.••..••...•• 20 

f 16 Typical microbend sensor response data -- Design 2 ...•...•.....•• 21 

17 Phase sensitive sensor 26 

18 Phase sensitive sensor -- exploded view ........................ .. 27 

19 Phase sensitive sensor system block diagram .................... .. 29 

20 Phase sensitive sensor -- Run 3 response 30 

21 Typical phase sensitive 5em,cr vibration results .......... , ..... . 31 

22 Analysis of phase sensitive sensor response during overpressure 

tes t sequence .............•.......•...•..•....................... 33 

;':~~CEDJNG PAGE BLANK NOT FILMED v 

.'-".-... -,...~ '®
Figure 

23 

24 

25 

26 

27 

28 

Table 

1 

2 

3 

4 

5 

vi 

LIST OF ILLUSTRATIONS (Continued) 

Two-fiber transmission sensor .................................... 

Two-fiber transmission sensor system block diagram ••••.•••.•..•.• 

Cantilever slope sensor .......................................... 

Cantilever slope sensor system block diagram •..•.••.............. 

Photoelastic sensor ................... ., ......................... . 

Photoelastic sensor system block diagram •••..••...•••••.••.•...•. 

LIST OF TABLES 

Microbend Sensor Response -- Design 1 

Microbend Sensor Response -- Design 2 

(Results of pressure curve fits only) 

............................ 

............................ 

Mi crobend Sensor Response -- Des i gn 2 ••••.•••.••.....•..•..••...• 

(Results of combined temperature and pressure curve fits) 

Phase Sensitive Sensor Phase 1 Calibration Data 

Phase Sensitive Sensor Phase 2 Calibration Data 

36 

37 

38 

39 

40 

41 

14 

21 

22 

30 

32 
1. SUMMARY 

The objective of this project was to develop high-temperature pressure sensors 

using non-metallic components and optical sensing methods. The sensors were to 

operate over a temperature range from room temperature (~20°C) to 540°C, to 

respond to internal pressure up to 690 kPa, to respond to external pressure up to 

690 kPa, and to withstand external overpressure of 2070 kPa. 

Fused silica was selected as the materia~ for the sensor modull vll tne basis of 

its high temperature and low thermal expansion characteristics. Optical sensing 

methods evaluated included the following: microbend, phase sensitive, tWO-fiber 

transmission, cantilever slope, and photoelastic. These methods were tested 

either in the pressure module or under conditions similar to that of the pressure 

module configuration. Of these sensing methods, the microbend and the phase 

sensitive sensor were selected for further development and evaluation. These 

sensors were tested as a function of temperature, pressure, overpressure, and 

vibration. The microbend sensor (an absolute sensor) exhibited required sensitivity to measure pressure, temperature effects that could be compensated, and 

insensitivity to vibration; but exhibited a small offset upon overpressurizatior.. 

The phase sensitive sensor (a relative sensor) exhibited required sensitivity to 

measure pressure, temperature effects that could be compensated, and insensitivity 

to vibration and overpressure. 

The project results showed that high-temperature pressure sensors based on glass 

components and optical sensing methods are feasible. The mic;obend sensor exhibited the required sensitivity and stability to be used as the basis for a compensatable absolute sensor. For the microbend sensor, the 95% confidenc~ level 

deviation of input pressure from the pressure calculated from the overall 

temperature-compensated calibration equation was 3.7% of full scale. The limitations of the sensors evaluated were primarily due to the restricted temperature 

range of suitable commercial available optical fibers and the problems associated 

with glass-to-metal pressure sealing over the entire testing temperature range. 

. " ... _ .. _-_ .... ® 
,-

.... 

~ 

~ 

f 

> 

• 

,. 

l 

2 

Glass/ceramic diaphragms and/or optical sensing methods for detecting diaphragm 

deflection in high-temperature pressure sensors should also be considered for 

applications such as advanced energy conversion systems, in addition to highperformance aircraft. Such applications with reduced or different design 

constraints may be directly amenable to solution by state-of-the-art components 

and sensing methods such as those investigated in this project. 

'-
,. 

J 

\ .\ 

2. INTRODUCTION 

Instrumentation for in-flight monitoring of inlet and engine conditions is ne~ded 

for high-performance aircraft to improve fuel efficiency, engine performance, and 

overall reliability. This instrumentation must withstand the hostile engine 

environment which includes the high-temperature operating conditions and vibrations. One of the needed measurements is that of gas path pressure in inlets of 

engines. In the pasL, the performance of high-temperature pressure transducers 

with metallic diaphragms has been limited by creep and hysteresis. One method of 

eliminating these mechanical and temperature related problems is to employ a nonmetallic diaphragm. A further step in this direction would be to employ an optical diaphragm deflection sensing method using non-metallic components such as 

optical fibers. 

The use of optical fibers and optical sensing methods have recently been applied 

to a number of measurements in hostile environments; these measurements include 

displacement, velocity, strain, flow, temperature, particle size distribution, gas 

composition, and fluorescence. These optical sensors can be designed to operate 

at high temperatures, in regions of high electro-magnetic fields, and other 

unusual environments. 

The scope of this project for high-temperature pressure measurements includes 

evaluating sensing techniques and sensor systems. These efforts include materials 

selection. sensing methods, sensor design, sensor fabrication, and sensor testing. 

The results of this work are applicable to not only the aircraft requirements, but 

also to similar uses in hostile environments such as advanced energy conversion 

systems. The information presented is timely in that it reveals the advantages, 

limitations, and the need for further development of optical sensors and optical 

components for use in hostile environments. 

The purpose of this project was to investigate and develop optical methods to 

measure diaphragm deflection under hostile conditions. The ultimate objective of 

-:i-= ... ~-;1 
", \ 

NASAls overall program is to develop pressure transducers capable of operating in 

, \ a reliable, repeatable fashion to 540°C. The specific objective of this project 

was to study, design, develop, fabricate, assemble, test, evaluate, and document 

optical diaphragm deflection sensors that: 

4 

• Accurately measure the strain, position, or some other property of 

diaphragm deflected by a pressure difference across its surface 

• Meet the operational and environmental requirements of pressure 

measurement systems for a high-performance aircraft 

• Take full and complete advantage of pertinent current technology in 

optics, electronics, and materials science 

This report describes the pressure sensors that were designed, constructed, and 

tested in this projeLt. The primary sensors identified were the microbend and the 

phase sensitive. Two configurations of the microbend are described; the second 

confi~uration was to reduce the temperature-induced extraneous sensor response. 

Three other sensors that were tested in Phase 1 of the project were the two-fiber 

transmission, cantilever slope, and photoelastic. These sensors are briefly 

described. 

The description of the sensors is followed by a discussion of the tfSt results. 

The final section is a summary of results and conclusions. 

.z:.. .............. •. ___________ _ 
3. DEVELOPMENT OF MICROBEND SENSOR 

The development of the Microbend Optical Diaphragm Deflection Sensor (ODDS) 

included the sensor module design, the original sens~r design, a redesign of the 

pressure sealing, and the sensor evaluation by a series of specified tests. 

3.1 SENSOR DESIGN 

The sensor design was based on the objectives of using a non-metallic diaphragm to 

eliminate or reduce creep and hysteresis, and to meet the requirements and temperature, pressure, and vibrational response. Design constraints included operation to 540°C, response to differential pressure of ±690 kPa full scale, ability 

to withstand 3X full-scale overpressure, and minimal vibration response from 50 to 

2500 Hz. 

3.1.1 Desigr:l. 

The original s?n.sor design included the basic module design used for all of the 

sensors. the specific design for the microbend sensing of diaphragm displacement, 

and the selection of the best available optical fiber. 

The basic module design consisted of a fused silica dia~hragm optically contacted 

to a fused silica base plate (see Figure 1). Such a modular construction incorporated the high temperature and low thermal expansion characteristics of fused 

silica and was inherently vibration resistant. The module design optimized the 

diaphragm deflection under the constraints of the material tensile strength and 

the sensor operational specifications. These considerations led to a diaphragm 

thickness of 5l1li1, a radius of 20lllll, and a deflection of about 2.5 11m at full-scale 

pressure of 690 kPa. For this diaphragm deSign. the first normal vibrational mode 

was >100 kHz. 

5 

, 

• 

-"--_r __ . __ . __ _ .. __ ____________ 

\ 

I 

-------se __ .. ®
STAINLESS 

lTEEL FLANGE -

FUSED SILICA 

DIAI'HRAOM 

OPTICAL 

CONTACT 

FUSED SILICA 

IAIE PLATE 

STAINLESS 

STEEL FLANGE 

/" 

GRAI'HITE /' 

I.S. TUIES -

-- , 

---, ~t===~==~~~=~,~~ - ~-t 

---r 

__ J 

__ --I 

FIBERS 

INTERNAL PRESSURE 

CROSS-SECTIONAL VIEW 

NOTE ALL DIMENSIONS IN MILLIMETERS 

Figure 1. Microbend sensor Desi gn 1. 

80DIA 

o 

o 

TOP VIEW 

BASE PLAU 

o 

/ 

The microbend sensor is based on the attenuation of light transmitted through an 

optical fiber, the attenuation being caused by a periodic bending of the fiber. 

Tne microbend sensor is diagrammed in Figure 1. An optical fiber is s~ueezed 

between two sets of corrugations. One set of corrugations is ground along a fused 

silica block, which is optically contacted to a fused silica base plate. A second 

set of corrugations is ground along the end of a pedestal located at the center of 

the diaphragm. The diaphragm is optically contacted to the fused silica base 

plate around an outer annulus. The optical fiber leads and thermocouples to 

measure sensor temperature are inserted through holes in the fused silica base 

plate. The pressure seal between the fused silica module and the positive 

(external) or negative (internal) pressure is accomplished using graphite gaskets, 

stainless steel flanges, and flat (A/10) optical surfaces. This sealing arrangement replaced the original gla5s tube seals that were prone to fracture at 

elevated temperatures. The stainless steel flanges, however, produced an 

additional thermally-induced response of the sensor; therefore, the flanges were 

removed in Design 2. 

6 

.¥ . 

-. . 

- -----------

t 

1 
• j 

, I 

: 

~ , I 

t 

The corru~ations on the pedestal are displaced with pressure-induced deflection of 

the diaphragm. The deflection causes a change in the amplitude of the periodic 

distortion of th~ fiber between the corrugations. Optical power in the fiber core 

is attenuated in proportion to the distortion amplitude via coupling from propagating to radiation modes. The power lost from the core to radiation modes is 

optimum when the fiber spatial bend frequency equals the difference in propagation 

constants, ~~, between propagating and radiation modes; shown as: 

2IT = +-

- A 

where A is the corrugation spacing. 

The bend spacing was optimized for an aluminum-coated mu1timode optical fiber with 

a core radius of 60 ~m and a normalized core clad refractive index difference of 

0.003. Both theory and experiment gave an optimum corrugation spacing of-3mm. 

The aluminum-coated fused silica mu1timode optical fiber was the best available 

high-temperature fiber for the sensor design and evaluation. This fiber was 

rugged, and resistant to vibration-induced loss. Also. the fiber exhibited good 

119 · ht t i it· L.. '0· .. ~~n·,.. H L • 1 ransm ss on proper~,es oe~ween ~ ana ~~¥ ¥. • cwever, mecnan1ca degradation of the fiber was noted upon cooling and heat cycling above 450·C. 

Compensation for light source changes and changes in the fiber transmission as a 

function of temperature were accump1ished by using a reference fiber (which 

bypassed the microbend section of the sensor) and a sensor output in terms of the 

ratio of the difference and sum of the two fiber transmission signals. Compensation for temperature-induced change of the fiber preload would need to be accomplished for a practical pressure transducer using a post processor and the curve 

fits obtained in the pressure-temperature calibrations. 

An exploded view of the microbend sensor with the flanges for pressurization is 

shown in Figure 2. Details of the microbend diaphragm and the fused silica block 

optically contacted to the base plate are shown in Figures 3 and 4 (in the second 

desigr. configuration without bolt holes). Assembly of the sensor provided the 

required preload on the active fiber fvr response to the differential pressure of 

either positive or negative (external or internal) values. 

7 

-- ., " P ........... -,; -----------.~-.. -~~
R 

ORIGINAL PAGE IS 

OF POOR QUALITY 

Fi gure 2. Mic robe nd senso r -- Design 1 exploded view. 

Fi gur 3. i~ ob nd sensor d;aphragm . 
ORIGINl\l PAGE IS 

Of POOP. OUAlITY 

Figure 4. Mic robend base plate with fu~ d silica block. 

3. 1. 2 Desi n 2 

Tne ~ec ond mic robcnd s nso r desig n stil l used t he ba si c mo dul e componen t s bu 

inco rpo rat d d n w r ss ure scal i ng rr dng~ln\:n t .:IS shown i n Fio'! r 5. Thi s 

pr~ ssu c s~aling arrang ment r educ ed the seal area and t hus th e thermally -induc ed 

stress in to t he sensur mod ul This sign eliminated most of the sealing- i duced 

diaphragm str ss and t 'mp erature-induc ed extran JUS se ns or respo ns e observed 

during t st i y 0 t h· or ig inal d.:si gn. 

An Lxplod d view 0 th microbend sensor with the modi fi d pressure seal i s sh own 

in Fi ur 6. :;s\:lnbly of ~ Senso r provi des th e scali ng pr ssu re on th e graphit e 

gask t s, and th op t ical co nta c t be twee n th e didphr gm and th fus d silica bas e 

pla t . provides t h preload on the op tical fib e r. his assembl y is sealed i nt o a 

pr cssur ch amber at U' _ ub e extensio ns or pressu re and t Inpe r atur t sting . Fo r 

th e Design '2 sensor, in t rnal press ure was pt at a nosph r ; t ern al pr ssur 

was n vc r allow d 0 go below atmospheric . 

- - - - -- ---- ---

®
10 

,---.......... - ----t-- . 

OPTICAL 

CONTACT -----)----- r' 

I 

25 FUSED SILICA ----

BASE PLATE ___ 

GRAPHITE --------

GASKET 

a:::r------- l ~ 

BellEVillE ~_---- I 

WA:; ----~ . '--TC 

:~~'NLESS STEEL "8ERsJ\ It".'RS OR KOVAR INTERNAL PRESSURE 

THREADED TUBE 

CROSS· SECTIONAL VIEW 

NOTE: All DIMENSIONS IN MILLIMETERS 

Figure 5. Microbend sensor -- Design 2. 

Figure 6. Microbend sensor -- Design 2 exploded view. 
3.2 SENSOR EVALUATION 

The microbend ODDS was evaluated by a series of specified tests to determine the 

pressure, temperature, overpressure, and vibration response of the sensor. The 

sensor evaluation can be summarized as follows: 

• Response was approximately linear with pressure from -690 kPa to 

+690 kPa. 

• The sensor exhibited high sensitivity to diaphragm deflection. 

• Rapi~ response to pressure changes. 

• The sensor was insensitive to vibration for frequencies of 50 Hz to 

2500 Hz. 

• The sensor withstood 3X full-scale pressure. 

• The sensor module was capable of 540°C operation. 

• The best available high-temperature optical fiber produced repeatable 

response up to about 450°C. 

• A small zero offset with temperature was observed for the sensor in 

the Design 2 configuration. 

• The slope of the sensor response with pressure (sensitivity) 

decreased with an increase in temperature. 

• The pressure sealing method for the sensor (Design 1) response 

characterization produced an additional zero offset with temperature. 

• The pressure sealing method for the sensor (Design 1) response 

characterization produced additional data shifts and scatter. 

• The experimental problems exr~ .. ienced with the microoend sensor 

(Design 1) necessitated the rcdfsign (Design 2) for the subsequent 

operational and high-temperature tests. 

• The microbend sensor (Desi~n 2) test data exhibited the sensitivity 

and stability as a function of temperature and pressure to be used as 

the basis for a compensatable sensor. 

3.2.1 Operational and High-Temperature Tests for Design 1 

The purpose of operational and high-temperature tests were to characterize the 

sensor response for a sequence of temperature~ to 540°C at a sequence of differential pressures from -690 kPa (internal) to +690 kPa (external). 

-- t~~ ....... ; 
12 

The microbend sensor setup for testing is shown in Figure 7. The output from a 

LED is split into two parts by a 3 dB coupler and coupled into the aluminumcoated, high-temperature, multimode optical fibers. The fiber output signals are 

generated by a dual photodetector and associated output circuits. The signals 

(A, B) are digitized and converted to the form (A - B) / (A + B) for the 

compensated sensor signal. 

lE0I--__ ~3cdB==co=u=p=l=ER)::::o:::PT§'~c:s::Pl='c::E::s=:~~~ri 

r A B A-8 

A/O CONVERTERS 

rigure 7. Microbend sensor system block diagram. 

A schematic diagram of the test setup is shown in Figure 8 for the microbend 

sensor. This setup used end plates for pressure sealing. Compressed air was used 

for the internal and external pressurization. The sensor temperature was controlled by resistance heaters in the laboratory electric furnace. The sensor 

signals, the calibrated pressure signal, and the calibrated temperature signal 

were processed by a conventional digital data acquisition system where the 

experimental results were recorded on floppy disc and also printed after each run. 

The data was subsequently fitted to a third order polynomial equation of the form: 

p(T 0) 

1 

where: 

P(Ti ) is pressure at temperature (Ti

) 

x is sensor output signal 

ao' aI' a2, and a3 are constants for a given temperature 

.. ' 

'- ~ ________ ~._:....... __ 0 __ - f ....... ~ ..... ; __ ... ~~ ______ _ 
. , 

I 

:,-

, 

~ . 

SIGNAL' 

LINES 

PRESSURE 

TRANSDUCER P)--~ 

REFERENCE 

PRESSURE 

TUBE 

---- PRESSURIZING 

TUBE 

FURNACE ENCLOSURE r-----------, 

I 

L ___ _ 

NEEDLE CONTROL VALVE 

PRESSURE REGULATOR 

INPUT PRESSURE 

Figure 8. Test setup schematic diagram. 

The initial operational and high-temperature test data showed an unexpected zero 

pressure response shift with temperature and excessive data scatter. This 

abnormal zero pressure shift was subsequently traced to the pressure sealing 

method with the stainless steel end plates and graphite gaskets. Although the 

pressure sealing method of Design 1 introduced spurious sensor response with 

temperature, the end plates facilitated the testing and avoided the necessity for 

an optical contact or bonding of the fused silica module. Further, once the 

spurious effects were characterized, the experimental data could D~ corrected for 

the additional zero shift. However, the Design 1 data could not ~e corrected for 

the additional data scatter. 

, 

" 

13 " 

----~ '®
I 

. \ 

14 

Typical microbend response data for the Design 1 configuration is shown in Table 1 

and Figure 9. The sensor response at a given temperature was approximately linear 

with pressure from -690 kPa to +690 kPa. The sensor electronics sensitivity was 

about 0.56 kPa per bit giving a pressure sensitivity of about 0.08% of full scale. 

The deflectlull sensitivity, based on the design deflection of 2.5 J..I.m for fullscale pressure, was about 2 nm. The change in slope of the sensor response to 

pressure (sensitivity) with a change in temperature was noted for the microbend 

Design 1 data. Part of this change was attributed to the pressure sealing method 

which changed the preload on the sensor; an additional part was attributed to the 

change in the diaphragm stiffness as a function of temperature. The change in the 

sensor zero response as a function of temperature due to the pressure sealing 

method can be corrected by using the observed zero shift of about 0.1 full-scale 

response per 1000e temperature change for the unclamped module. The corrected 

zero response is shown in Figure 9. The 95% confidence level deviation of the 

input pressure from the pressure calculated from the individual calibration 

equations is shown in Table 1. This data shows an increasing error as a function 

of temperature. The zero shifts and data scatter in the higher temperature runs 

were partly attributed to the pressure sealing method; this observation was 

confirmed by additional characterization tests. The experimental problems 

experienced with the microbend sensor (Design 1) necessitated the redesign (Design 

2) for the subsequent operational and high-temperature sensor system tests. 

Table 1 

MICROBEND SENSOR RESPONSE DESIGN 1 

Nominal Full-Scale 

Run No. Temperature Error 

Tl 24°C 1.8% 

T2 130°C 8.8% 

T3 230°C 14.4% 

T4 330°C 19.6% 

T5 450°C 34.5% 

. -. 
, 

. 

'. 

0'2~---------------------------------------------------------. 

0.11 

III 

~ 0.10 ..... 

Z 

o 0.09 

in 

ffi 0.08 

~ 

e. 0.07 

w 

~ 0.06 

o 

A. 

~ 0.05 

a: 

~ 0.04 

III ffi 0.03 

III 

~ 0.02 

~ 

~ 0.01 w 

a: 

o 

·700 -600 -500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 

PRESSURE (kPa) 

Figure 9. Typical microbend sensor response data for Design 1. 

3.2.2 Vibration Test fc~ u~~ign 1 

The vibration test objective was to observe and record any mechanical resonances 

associated with the sensing device. The vibration tests were conducted at room 

temperature and at zero differential pressure. The sensing device was mounted to 

a shaker with a suitable fixture with clamping rings and subjected to a sweep 

frequency sine wave vibration. The sensing device was tested with res~ect to 

three mutual iy perpendicular planes, one of which was the plane of the diaphragm. 

The vibration amplitude was adjusted such that the mean value of the sensor peak 

velocity was maintained above a value of 12.7 x10-3 m/second (0.5 inch/second) for 

the frequency range of 50 to 2500 Hz. The duration of the frequency sweep was 

sufficiently slow to observe and record any mechanical resonance~ of the sensing 

device. The sensor was activated during the test with the sensor output signal, 

Vout ' recorded as a function of frequency. As shown in Figure 10, Vout is determined using the input signals A, B, and A - B from the photodetector output 

circuit (see Figure 7). The sum signal A + B is obtained from LF356, and AD535 is 

used to calculate the real-time ratio 10(A - B / A + B) which is recorded as V t' 

ou 

15 

.. --_._------....... _--. ® 

. ~ 

.f~"'~""" ; .- 
10k 

10k 

VOUT : 10(Z2 - Z1) : 10(~ ) X1 - X2 -X2 

10k 

B '\-----'lcA/'v\A_--' 

( A-B) = 10 A-B 

-(A+B) .... ______ --,.-______ ----.J 

OUT 

+ 

+ 

x2 x1 'OOk ± 

:r SOk 

(A-B~ Z2 100 _ 

AD535 

- + 

100k '1 

Y1 Z, to. 

100 

Figure 10. Electronic circuit for microbend sensor vibration test. 

A block diagram of the test setup and instrumentation is shuwn in Figure 11. The 

test results were recorded on an x-y-y recorder. Typical test results are shown 

in Figure 12 where the sensor response versus frequency was plotted along with the 

sensor acceleration level. Fer all the tests, the sensor (sensing device with 

activated optical fibers) response was less than 1/41 full-scale pressure 

response. Therefore, the sensor was found to have no sigllificant resonances and 

was considered to be essentially immune to vibration over the frequency range of 

the tests. 

16 
. , 

r 

I 

-- -._._------ ------

TRANSDUCER Elr:CTRONIC XVY CONDITIONER VOlTMETER RECORDER 

V, X 

-----.~.-.-- - f--· 

I 

TRANSDUCER CHARGE ElECTRONIC 

AMPLIFIER VOLTMETER 

L 

[ ";~~~~:~J 

------p DC OUTPUT 

SHAKER POWER SWEEP 

AMPLIFIER OSCillATOR 

. ____ --.J 

DESIRED SIGNAL fOR MONITORING OSCILLOSCOPE 

Figure 11. Instrumentation for vibration results. 

(!I 20 SENSOR PEAK 

Z ACCElERA TlON, 

--- 0 

i= 

<[ 10 a: 

III 

-' 

III 

U 

U 0 <[ 

TRANSDUCER 

POSITION 112 

~ 0. f ~ENSOR RESPONSE \ t IJ]::: 

0.0 I I ~ ___ 'C":""""'t'"=~ ___ l _:-__ L:~ __ !u _____ l __ ~~~_ r' 

o 200 400 ~OO 800 1000 1200 1400 1600 1800 2000 2200 2400 

FREQUENCY . 1Hz) 

Figure 12. Typic3l microbend sensor vibration results. 

17 .®

J»-~ ~4 .. ·' , 
r, 

. • \ 

I 

, 

t 

, . 

I' , 

3.2.3 Overpressure l~~ts fo-'=---.Qesign 1 

The overpressure tests were to verify that the microbend sensor would withstand 

three times the full-scale pressure without failure and to determine any changes 

in the sensor response with respect to a baseline calibration equation. 

The overpressure tests were co~ducted at room temperature. The microbend sensing 

device was subjected to a series of calibration runs alternating with overpressure 

excursions. The calibration runs after each overpressure excursion were from 

minus to plus full-scale pressur~. The test setup, shown previously in Figure H, 

was modified to provide the overpressure excursion as well as to provide control 

of the calibration runs. 

For each static calibration, the 95% confidence level deviation of the input 

pressure from the pressure calculated from the calibration equation from a baseline static calibration was obtained as a percentage rf full-scale pressure. The 

results of the overpressure test sequence are shown in Figures 13 and 14. Figure 

13 shows the raw sensor data versus pressure as a function of the calibration run 

number. A shift in the sensor response was noted as a function of the run number. 

~ :~: ~---.----- ---~-.--------------- ----0 0 RUN .~i 

Vl 0--0 RUN 112 z 008 w - ~ 

o 0.07 

0- -0 RUN 116 

~~~~,~- --..::::: --:- ... ~ 0 06 1 ~~::: -:- "" -. 

~ 006l ~::::~@-. ~ J --.:::::. :::: -:-.-. H .... er 004 -""'::::ti::::::-:-"'::-A_ 

o Vl 0 03 0""",--___ _tj_:-_§ '. _ -. __ 

~ o--~ ::::'''::=-Aw Vl OO? t· 0""",--~ -...tj--- -...'-... 

~ 

> 001 o~ 

:3 I 

6- - -6 RUN 1111 

UJ 0 l_-L. __ ~L--_--L. --1 .. _._~_~~1--._~1 _~ __ L-__ -"--______ --, 

700 600 500 ,400 300 200 100 0 100 200 300 400 500 600 700 

PRESSURE (kPa) 

Figure 13. Microbend sensor response during overpressure test sequence. 

.

\
2 3 4 5 6 7 B 9 10 11 12 13 

Figure 14. Analysis of microbend sensor response during overpressure 

test sequence. 

This effect is also shown in Figure 14 as the error (in percent deviation of full 

scale) in predicting the pressure from the sensor signal using the third order 

curve fit from a calibration run. The comparison is shown using three calibration 

equations for the prediction. These figures show that each overpressure excursion 

causes an offset in the sensor response. The first overpressure caused about a 5% 

offset, each additional overpressure excursion caused ab0ut a 11 offset. 

3.2.4 Q2~ra~ional~~~~Temperature Tests for Design ~ 

The operational and Iligh-temperature tests for the Design 2 microbend sensor were 

to chardcterize the sensor response for a limited sequence of temperatures to 

19 
42U OC and xt~rnal pressures to 690 kPa. The De sign 2 microbeno sensor is shown 

in Figure 5. The sensor system and test se tup were similar to the Design 1 t ests 

shown earlier in Figures 7 and 8 : the major differenc e heing that the pressure 

sealing area had bee n substanti ally reduceo. The sensor WdS placed in a pressure 

chamb er (shown in Figure 15) which wa s located in the furnac e enclosure for 

testing at high t emp e ratures . 

j 

__ iiiiiiiiiiiiiI==~1 

j[ 

Figure 15. De sign 2 microbend sensor with pressure chamber. 

Th 2 senso r Si!] ndls , th e calihra t ed pre ss ure si 0na l , un d the cali br a t e.1 t empe ra ture 

si gnal we r e pr ocE ss ed by d con venti onal di gital data acqu i sitio n syste1n. Th e d:3 td 

WdS subs equ ently fitt ed t o third- or de r po lynom i dl equa ti ons to di t pr('ssu r.! 

from th e sensor si gnal a t a Cl iv en t mp e r a ture (r ef e r t o Section 3.2.11. The da t a 

was al so fi tter! to an ove r all t Clnpe rdt ur e cOlll penSrl t ed cal i br:lti on c'1u a ti on giv en 

by : 

P T + T2 T3 T '2 J c

1 + c2 c3 + c t- c r. X t- c

b + c

7 to T + c

8 x :.J 

2 2 T 2 ~ x2TJ + 3 3 

+ clJ + c10 x + C 11 x T + c i l CD x + c 14 

J T2 J 3 

+ CIS x + c 16 x T 

whL'rl.' x ~ s nso r output si gnd l anct T i s t tun . 

An exalll plt' of l1esi !]n 2 mi c r obe n<i S0 11 S0 r r espons da t a for t he oper a ti onal nd 

hijh-temp eratufl! t est s i s s hown i l) Fi ure 16 . In t hi s f i gur . . t 1e dat a for th 

dec r casin9 t l!mp <: rature Sl'qut' nc ' run s was not pl ot t ed for pre s~ c.'s oth r t han l r o 

for c la ri ty . Th e il es i C) n ? s enso r ponS ~ xhi bited a much sma ll er l r o pr ess ur e 

20 
shift wi th temperature than that of Design 1. The Design 2 response data was 

similar to the corrected Design 1 data as previously shown in Fi gure 9. 

04 

NOMINAL 

RUN. ~VM80~ TEMP 

ill 36 

7 §---O 1 T 20 C 

III 2 100 ... 8 ---O~ I ... 3 180 Z 0 0 ~ ____ 0 4 

t 

260 

ih 

9::::::'0 0 ____ ~ Ii 340 Z 

... 03 6 420 

:I 

C " 8 o~ 0_____... 0 . 7 380 

w 110'--...0... o_-........o~ 8 300 

III 9 220 Z 

12 9'--... ~O--.........o~ 0 Ii 

0 0 026 10 1 140 Q. 

III 11 60 ... 

Ir o~o O~O. 12 20 

Ir 

0 o ~ 03 III 

z 02 

O--.........O~02 ... 

III 

... 

> 

;: 

Ir 

.., 

5 

016 

O~Ol 

01 L. I 1 .1_._._ L _______ _ 

o 100 200 300 400 500 600 700 

PRESSURE IkPol 

Figure 16. Typical microbend sensor response data -- Design 2. 

The results of the Design 2 data curve fits are shown in Tables 2 and 3. Table 2 

lis!~ the 95% confidence level deviation of input pressure from the pressure calculated from the individual calibration equations for each nominal temperature for 

full-scale response. 

Table 2 

MICROBEND SENSOR RESPONSE -- DESIGN 2 

(Results of prescure curve fits only) 

Run No. Full-Scale Error Run No. Full-Scale Error -.--- -- --

n 3.1 T7 5.3 

T2 4.8 T8 2.0 

T3 3.3 T9 2.0 

T4 4.8 no 2.3 

T5 5.4 Tl1 0.3 

T6 9.1 TI2 1.1 

21 
Variable 

Intercept 

T 

x 

T2 

T3 

xT 

Table 3 

MICROBEND SENSOR RESPONSE -- DESIGN 2 

(Results of combined temperature and pressure curve fits) 

Number of 

Parameters Used 

8 

10 

15 

Overa 11 

Full-Seal e Error 

3.73% 

3.64'1, 

3.73'1, 

REGRESSION DATA FOR 10-PARA~£TER CURVl FIT 

-2.33286 102 

-4.24E35 

2.U075 105 

3.43232 10- 3 

-4.4917 10- 5 

5.11445102 

Variable 

-----

xT3 

x2 

x2T 

x3 

x3T2* 

~ression Coefficie~j 

1.66907 1(1-3 

-1.177046 10 7 

-9.03524 103 

1.62064 108 

-2.67319 102 

* For example, where x3T2 = x3r2 

In Tdble 2, the relative magnitude of the full-scale error is increased due to the 

limited number of data points used for the curve fits. However, most of the 

increase in error with temperatul'e, noted in the Design 1 data, lid' been 

el iminated. 

Tab~~ j lists the 95t confidence level deviation of input press~re from the 

pressure calculated frorn the overall temperature compensdted ca~ibrdtion equation 

for several curV0 fits. Not all of the terms in the calibration equation were 

significant: the stepwise regression analYSis eliminated ttrms of low tolerance. 
The average of the individual curve fit error was about 3.6% which is about equal 

to the overall curve fit error. This shows that the sensor temperature response 

is predictable and. thus compensatable. 
4. DEVELOPMENT OF PHASE SENSITIVE SENSOR 

The development of the Phase Sensitive Optical Diaphragm Deflection Sensor (phase 

sensitive sen~or) included the sensor design, the sensor module design, and the 

sen~or eva1~uv;on by a series of specified tests. 

4.1 SENSOR" ~IGN 

The sensor G2sign was based on the objectives of using a non-metallic diaphragm to 

eliminate or reduce creep and hysteresis, and to meet the requirements of temperature, pressure, and vibration response. Design constraints include operation to 

540°C, response to differential pressure of ~690 kPa full scale, ability to 

~;~~stand 3X full-scale overpressure, and minimal vibrational response from 50 to 

2500 Hz. 

Tre basic module design was similar to that used for the microbend sensor discussed eariler in ~ection 3.1.1. For the phase sensitive sensor, the module 

consisted of a fused silica base used to secure the end of a bundle of optical 

fibers, and a fused silica diaphragm to act as a reflector. 

The phase sensitive sensor is illustrated in Figure 17. A bundle of single-mode 

optical fibers was cemented into a central hole in the base plate. Both the fiber 

leads and a thermocouple to measure sensor temperature were inserted through the 

stainless steel tubes of the base flange. Multiple single-mode fibers were used 

to provide redundance in case of failure of anyone fiber, and to provide a choice 

of that fiber with optimum response to diaphragm deflection. 

This sensing method can be explained as follows: Light propagating in one of the 

single-mode fibers is partially reflected at the fiber ~nd surface-air interface 

(reference light signal). The transmitted beam is partially reflected from the 

air-diaphragm surface interface. A portion of this reflected bedm couples back 

into the fiber (deflection light signal) and both reflected components propagate 

iJRSCEDING PAGE BLANK NOT FlL~mD 

25 

@

___ .•. 
back to the fiber input end where they are mixed on a photodiode. This sensor 

acts a~ ~ two-beam interferometer if the surfaces are uncoated. To avoid mUltiple 

fringe ambiguity at full-scale pressure, the diaphragm deflection should be less 

than A/8. Based on the model of a clamped diaphragm, the required diaphragm 

thickness was determined to be 14 mm. 

EXHRNAl 

PRESSURE 

I 

r------- -r-:~ ::: L--=-l- STAINLESS ' 

STE El FLANGE ,,,I 11---+' --1 ....... -1-1--..: 

FUSED SILICA 

DIAPHRAGM~ 

OPTICAL 

CONTACT 

FUSED SILICA -

CEMENT 

FUSED SILICA 

BASE PLATE-

---. 10 STAINLESS 

STEel FLANGE 

~----~~~~---~ 

GRAPHITE 

S S TUBES 

I 

I 

t_ FIBER 

INTERNAL PRESSURE 

CROSS SECTIONAL VIEW 

NOTE All DIMENSIONS IN MILLIMETERS 

Figure 17. Phase sensitive sensor. 

The photodiode output signal is proportiona', to the optical phase difference 

between the two reflected components. Since environmental perturbations of the 

fiber affect both reflected components identically, phase shifts from these 

perturbations cancel. Phase shifts are imposed on the light that exits the fiber; 

this phase difference is given by: 

26 
where: 

ORIGINAL PAGE IS 

OF POOR QUALITY 

n = Refractive index of air (or other gas) in the gap between the 

didphragm and base plate 

h = Gap height of di aphragm froln base plate 

w2 = Phase shift on reflection from diaphragm 

wI = Phas ~ shift on reflection from fiber surface 

A = Wavelength of source in vacuum 

The primary sensor respo'1se is generated from th e change in h wi t'1 pressure . 

Other lJossible sourc es of sensor response are changrs in n, 1P2' and wI' 

The pressure seals for both internal and external ~ressu e were pr~vi~~d by 

graphite gaske ts between the steel flanges and th e fused silica componen ts. 

Sealing pressure was provided by d suitabl e means such as bo lts b2 twe en the end 

flanges. An exploded view of t he sensor is shown in Figure 18 for tIle st8el 

flange t est configuration. 

Figure 18. Phase sensitive sensor -- exploded view . 

7. 1 

@
4.2 SENSOR EVALUATION 

The phase sensitive sensor was evaluated by a series of specified tests to 

determine the pressure, temperatur~, overpressure, and vibration response of the 

sensor. Sensor evaluation can be summarized as follows: 

• The sensor resronse was ~pproximately a cosine-squared function of 

pressure. 

• The senS0r exhibited high sensitivity to diaphragm deflection. 

• Response to pressure changes was rapid. 

• The sensor was insensitive to vibration for 50 Hz to 2500 Hz. 

• The sensor withstood 3X full-scale pressure. 

• The s~nsor module was capahle of 540°C operation. 

• The sing12-mod~ optical fiD~rs were capable of 54UoC operation. 

• Th= sensor response, as tested, passed throug" multiple fringes upon 

full-scale external or internal pressurization. 

• The sensor response was different for internal pressurization than 

for external pressurization. 

4.2.1 Operationala~d nigh-Temperature Evaluation 

The purpose of the operational and 11igh-t~mperature t~sts was to characterize the 

sensor response for a sequence of differential pressures from -690 kPa (internal) 

to +690 kPa (external). The phase sensitive sensor setup for testing is shown in 

Figure 19. Tne out;:>ut ()f the hser dio,je is splicel1 to a 3 dB single-mode coupler. One of th~ coupler outputs is precisely positioned to feed through index 

matching fluid to the single-mode fiber in the sensor. The other coupler output 

was not used and led to an opticdl sink consisting of a drop of index matching 

fluid. The r2turn opti,al fiber frolo the coupler goes to a single silicon 

detector. The d~tcctor s~nse5 the interference of light refl~cted from the 

op~osite end of the optical fiber in the sensor and the light reflected from the 

s~nsor diaphrag~. The output of the el~ctronics goes to tne AID converter for 

ddta processing and storage. 

28 

"~

~.;; SW··.""'" .; ; 

", ______________________________ -~,~a_
,. 

" 

, 

" 

~iE.DllrDE-o~'~~~~E---r====~s:I:NG:L:E:M:o:D:E~==~~X~yB~Z-----EJ COUPLER POSITIONERS ODDS 

SILICON 

DETECTOR 

AID CONVERTER 

Figure 19. Phase sensitive sensor system block diagram. 

The test setup was similar to that shown previously in Figure 8; however, the 

sensor was in a pressure chamber located in the furnace. Multiple fringes were 

observed upon both positive and negative pressurization. The sensor characterization data runs were then curve-fit to the expression: 

where: 

y = Predicted sensor response 

B(I) = Scale factor 

B(2) = Fringe period factor 

B(3) = Fringe phase factor 

8(4) = Signal offset 

x = Pressure 

The change in the index of refraction upon internal pressurization required 

separate curve fits for internal and exernal pressure response. Figure 20 shows 

the curve fit for positive pressure at about 120°C. The results of curve-fit data 

is shown in Table 4. The % error of individual runs was not meaningful due to the 

multiple fringe ambiguity. Also, the overall t error was not meaningful due to 

multiple fringe ambiguity for a given temperature, and phase ambiguity between 

runs at different temperature. The assumption of a clamped diaphragm was 

29 

----------------.-.-------"-------~ 

I 

; 

I 

1 

I 

,j 

. f# • .. .. ,/'t..:<Jr • .. -.... _~;.:..., __ "'__ -_' ______ --.:...( ::.>~.:...~......ioI!. m_r ®

•
r 

r 

,. 

incorrect to predict the desired deflection; thus, a thicker diaphragm would be 

~ required to provide a full-scal~ deflection less than one-half a wavelength. 

.. ,. 

" 

~ 

30 

Ilol 

II) 

z 

2 

II) 

0'200

1 

I 

0_ .Q , .. 

, I "', I 0 

0,1110 , I \ 01 

\ I \ I 

\ ~ \ I 

\ I \ I 

~ 0,1100 

\ I \ I ~ 101 

c 

o 

II) 

Z 

Ilol 

II) 

\ I \ I 

\ I \ I 

\ I \ I 

0.1010 

\ I \ I 

\ I \ ,0 \ t \0 I 

_I '_I 

O. 1 000L-~_~_...&..---l_--,-_....L-_.L..---1._--'-_....I......_.L..-~_-'---= 

o 100 200 300 400 700 

' .. EllUl'! III'" 

Figure 20. Phase sensitive sensor -- Run 3 response. 

Table 4 

PHASE SENSITIVE SENSOR 

PHASE 1 CALIBRATION DATA 

Cal1b. NOIIfnal Pressure Sensor Span No. Fringes 

Run TeMp ·c kPI ESS SO Per Fringe Per 690 kPa 

24 External 0.8168 10-5 1.668 10-3 0.1765 10.1 3.96 

2 24 Internal 0.7809 10-5 1.141 10.3 0.1577 10.1 5.15 

3 120 External 0.3944 10-5 0.811 10.3 0.1434 10-1 4.13 

4 220 External 0.3883 10-5 0.804 10.3 0.8967 10.2 4.48 

ESS = Est.mated Sums of Squares; SO • Standard Deviation 
4.2.2 Vibration Tests 

The vibration test objectives were to observe and record any mechanical resonances 

associated with the sensing device. These tests were similar to those previously 

reported in Section 3.2.2 for the microbend sensor. An example of test data is 

shown in Figure 21 where 1% full-scale response corresponds to 0.3 millivolt. 

Essentially no response to vibration was observed over the frequency range of the 

tests. 

24----------------------- ----_____________________ ~ 

20 

§ l' z 

o 

i= 

: 12 III 

~ 

III 

U 

IJ 

C • 

o 

+0.11 

SENSORRMS 

ACCElERATION ~ 

SENSOR 

RESPONSE, 

SENSOR 

ORIENTATION ~ 

~ 

> I--------------------'~'~-------------------------I ! 00t-

·0.6 

o 250 1100 750 1000 12110 1500 1750 2000 2250 2500 

FREQUENCY IH.) 

Figure 21. Typical phase sensitive sensor vibration results. 

4.2.3 Overpressure Tests 

The overpressure tests were to verify that the phase sensitive sensor would 

withstand three times full-scale pressure without failure and to note any changes 

in the sensor response with respect to a baseline calibration equation. 

The overpressure tests were similar to those reported earlier in Section 3.2.3 for 

the microbend sensor. A series of additional calibration runs were conducted 

'. 

• I 

~ . 
along with the overpressure and standard static calibration runs. Calibration run 

data is shown in Table 5. For each static calibration. an estimate of the 95% 

confidence level deviation of the input pressure from the pressure calculated from 

the calibration equation from a baseline static calibration was obtained as a 

percentage of full-scale pressure. 

Table 5 

PHASE SENSITIVE SENSOR 

PHASE 2 CALIBRATION DATA 

~ 

Cllib. Na.fnll Pressure Sensor Spin No. Frfng@s TeIIf! ·C kPI ESS SO Per Frfnge Per 690 kPI 

24 Externll 0.4938 10-3 8.399 10-3 0.2649 2.19 

2 24 Internal 0.1350 10-2 13.887 10-3 0.2553 3.78 

3 24 External 0.7429 10-3 10.302 10-3 0.2705 2.22 

4 24 Internal 0.1211 10-2 13.153 10-3 0.2535 3.77 

5 24 External 0.8809 10- 3 11.218 10-3 0.2708 2.25 

6 24 Internal 0.9475 10-3 11.634 10-3 0.2534 3.74 

ESS • Estimated Sums of Squares; SO z Standard Deviation 

The results of the overpressure test sequence are shown in Figure 22. The 

overpressure static calibration data was compared to three baseline calibrations. 

The change in the predicted error was attributed to an uncompensated phase shift 

with time during the tests. This unco~~ensated phase shift was in turn attributed 

to micromotion of the fiber optic bundle with respect to the fused silica reference platform due to differential expansion effects. With an improved method of 

securing the fiber optic bundle and pressure sealing the module, the sensor should 

be insensitive to overpressure excursions. 

Although the phase sensitive sensor performed well, it had the following problems 

which made the overall performance inferior to the microbend sensor: securing 

fiber ends, time dependent drift. different response for internal and external 

pressure. and diaphragm deflection greater than ~ne wavelength which led to 

multiple fringe ambiguity problems. 

32 

...... 

. ~ h .... I\:. .... ;. :,'~~. 

-=-------------
-

3~---------------------------------------------------------, 

~ ., Z 

.., 

u 

a:: 

r ·2 ERROR IN ftREDICTING PRESSURE 

FROM THE SENSOR SIGNAL USING 

CURVE FITS FROM CALlIRATION RUNS 

o3L-__ ~ ____ -L ____ ~ ____ L-__ ~ ____ ~ ____ ~ ____ ~ __ ~ ____ ~ 

2 3 4 I • 7 I • 10 11 

RUN NUMIER FOR OVERPRESSURE TEST SEQUENCE 

Figure 22. Analysis of phase sensitive sensor response during overpressure 

test sequence. 

.-

-

33 
:j 

, , 

, 

, 

~ I 

. ! 

5. OTHER SENSORS INVESTIGATED 

Nine sensors were investiga~ed in this project. Of these, five were recommended 

for initial development and testing. The microbend sensor and the phase sensitive 

sensor were previously discussed in Sections 3 and 4. In this section (Section 5) 

the two-fiber transmission sensor, the cantilever slope sensor, and the photoelastic sensor will be briefly discussed. 

5.1 TWO-FIBER TRANSMISSION SENSOR 

The two-fiber transmission sensor sensed diaphragm deflection via a light intensity change. Light coupled into receive optical fibers changed with translation 

of a transmit optical fiber attached to the diaphragm, when both transmit and 

receive fibers were nearly aligned. Thus, the change in the light intensity in 

the receive fibers, as a result of the change in coupling, was used as a measure 

of translation caused by diaphragm deflection. 

5.1.1 Sensor Design 

The two-fiber transmission sensor and associated components are shown in Figure 

23. Multimode optical fibers were cemented into grooves ground along the ends of 

two pedestals. One pedestal was located in the center of the diaphragm; the 

second pedestal was optically contacted to the fused silica base plate. The 

diaphragm was optically contacted to the base plate around an outer annulus. The 

optical fiber leads and thermocouple to measure sensor temperature were inserted 

through metal tubes on the sensor metal end plate. The modified test setup was 

used for this test; end plates were used with graphite gaskets to provide the 

pressure seals. 

I)RECEDING PAGE BLANK NOT FaMED 

t 

I 

1 

! 

~ 

j 

{ 

1 

l 

t 

, 

I 

, 

I 

35 

~ - -~1 §.-:£.-........ 695 ..;. ,. 

... , .. . __ ___ u_u .----. 

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.TEEL FLANGE 

FU.ID SILICA 

DIA'""AGM 

OI'TICAL 

EXTE"NAL '''U'U''E 

IUNDLE .E'A"ATION - ZlO .. m 

CONTACT --)--I:§t-...... _-_L....JIot 

FU.ICI "LlCA 

IA.E 'LATE 

ITAINLES • 

• TIIL FLANGE 

O"A'"ITE 

•.•. TUIES 

V·G"OOVEI TO LINE U, 

INTE"NAL '''n'U''E 

NOTE: ALL DIMENSIONS IN MILLIMETE'" 

C"OIS·SECTIONAL VIEW 

Figure 23. Two-fiber transmission sensor. 

Multiple fibers were cemented into the grooves on each pedestal using fused silica 

cement. Th;s approach provided: 

1) Redundancy in case of failure of any fiber 

2) Non-critical alignment requirements for the two pedestals during assembly 

3) Multiple transmit and receive fibers from which to choose those 

which provided optimum response to concave and/or convex diaphragm deflection 

With the light-transmitting fiber mounted on the diaphragm pedestal, deflection of 

the diaphragm caused translation of the fiber a~is along the y-direction. The 

motionless receive fibers on the reference platform pedestal were axially aligned 

with the transmit fiber. When the transmit fiber moved with diaphragm deflection, 

the amount of optical power coupled into the receive fibers changed. 

36 

, 

" ; 
. 

• 

, 

f 

. '. ", 

. \ For this sensor, a multimode, high-temperature optical fiber with a numerical 

aperture of about 0.65 and a core-to-clad area ratio of about 25:1 was required to 

achieve the best sensitivity possible. The best available optical fiber meeting 

those requirements was limited in operating temperature to about 400·C. As a 

result, the sensor was not repeatable at or above about 400·C. 

5.1.2 Operational and High-Temperature Evaluation 

The two-fiber transmission sensor setup for testing is shown in Figure 24. The 

laser diode source was coupled to a se1ecte( transmit fiber. The two selectfd 

receive fibers were coupled to the dual photodetector and associated electronics. 

The test sequence was similar to that described for the microbend sensor in 

Section 3. 

L.4lf" 

DIODE OIl'TIC FilE" ODDS 

c=Jf---~ ~~-'----+--. 

DUAl FitOTODETECTO" • OUT'UT CI"CUITS 

A I 

Figure 24. Two-fiber transmission sensor system block diagraM. 

The sensor response was approximately linear with pressure for both internal and 

external pressurization. The sensor response was relatively weak however, being 

about a factor of 5 less sensitive than the microbend sensor. This relative 

sensitivity was determined by comparing the slope of pressure response for both 

the two-fiber and microbend sensors. The major problems with t~is sensor, as 

tested. were the limitations imposed by the best available optical fiber. The 

light transmission characteristi~s of the fiber in the test configuration were a 

function of temperature and were limited to about 400·C. Further. the fiber differential expansion, with respect to the fused silica module material, caused 

degradation of the cemented fiber bundle ends with repeated temperature cycles to 

540·C. Thus. a large numerical aperture optical fiber with improved hightemperature performance. and an improved method of securing the fiber ends to 

fused silica are required to make this sensor rugged, reliable. and repeatable • 

. ••• M ... -----------------~ _~ _ ~ _ _ _ _ ____ ~ 37 • _ }

~. 

---~ .. 

J'iI .. ".. -..... ; .: ~ , . 
5.2 CANTILEVER SLOPE SENSOR 

The Cantilever Slope Optical Dia~hragm Deflection Sensor (cantilever slope sensor) 

was based on the principle of differential collection of light from a reflector 

that was subject to tilting. A change in the reflector tilt angle selectively 

increased the light coupled into some fibers and decreased the light coupled into 

other fibers. This differential light coupling was used as a measure of reflector 

tilt corresponding to diaphragm deflection. 

5.2.1 Sensor Design 

The cantilever slope sensor and its components are shown in Figure 25. In this 

configuration. the diaphragm deflects a cantilever beam that acts as the rot"lecthe surface. 

di aphragm by a 

the cantilever. 

The slope of the cantilever was greater than the slope on the 

factor equal to the ratio of the diaphragm radius to the length of 

The cantilever was therefore made as short as practicable. It 

was anticipated that a mechanical gain of at least 5 could be achieved. 

38 

ITAINLESS 

InlL FLANGI 

FUIID IIUCA 

DIA'HIIAGM -_" 

O"ICAl 

CONTACT --

.UIID IILlCA -t----I ....... ----1H----I;U 

CEMINT 

.UIED IILlCA 

IA&I 'LATE -

ITAINUII 

InlL FLANGE 

GIIA,"ITE 

I I TUIES 

L ... IIIS 

INTEIINAl "'IIIUllf 

CIIOII IICTlONAl VIEW 

Non ALL DIMENIIONI IN MILLIMETEIII 

Figure 25. Cantilever slope sensor. 

'- .,... q FW:.- .: 
t 

• 

A bundle of fibers was cemented into the base plate. From this bundle, one fiber 

was used as the transmitter; two or more fibers were used as the receive ffbers. 

5.2.2 Operational and High-Temperature Evaluation 

The cantilever slope sensor setup for testing is shown in Figure 26. A laser 

diode was coupled to the transmit fiber. The receive fibers were coupled to a 

dual photodiode and associated electronics. To achieve the best possible 

sensitivity, high-temperature fibers satisfying the require~nts of numerical 

aperture of about 0.65 and a core-to-clad area ratio of abou~ 25:1 were u~ed for 

the tests. However, these optical fibers limited the testing to an upper temperature of about 400·C; the fibers also were difficult to cement or secure in a 

bundle due to differential expansion effects with respect to fused silica over the 

temperature range of operation. These problems were similar to those encountered 

with the two-fiber transmission sensor described earilier in Section 5.1.1. 

LASE.. XYl 

DIODE II'OSITIONE .. S ~ 

D--+O--+ t-O-t----~~U 

0005 

Figure 26. Cantilever slope s~nsor system block diagram. 

The cantilever slope sensor response was approximately linear with pressure. 

However, the signal change with cantilever deflection was greate,· than that due to 

slope change. Thus, the differential sensitivity was unsatisfactory. The sensor 

response a I so exhi bHed excessive scatter, which appeared to be due to nonuniform 

relative motion or slippage between the diaphragi. "tooth" and the clr!tilever (see 

Figure 25). Thus, for this sen~or, a large numerical aperture fiber with improved 

high-temperature performance, an improved method of securing the fiber ends, and 

an improved cantilever mechanical design are required to make this sensor reliabl~. repeatable, and rugged. 

39 
'" \ , 

I 

. \ 

~ 

, 

5.3 PHOTOELASTIC SENSOR 

The Photoelastic Optical Diaphragm Deflection Sensor (photoelastic sensor) was 

based on measurement of the change in stress-induced birefringence in a single 

mode optical fiber clamped between ~ nedestal and the diaphragm. The relative 

change in birefringence was caused by changes in pressure on the diaphragm which 

caused changes in stress on the clamped fib~r. 

5.3.1 Sensor Design 

The photoelastic sensor and associat~d compont·("s ar0 shown in Figure 27. The 

fused silica sensor mOdule was similar to that u5ed for the other sensors. 

STAINLESS 

STEEL FLANGE 

FUSED SIUCA 

DIAPHIIAGM 

OPTICAL 

CONTACT --"'---t~--, ~""""--"'''''''' 

FUSED SILICA 

lASE 'LATESTAINLESS 

STEEL FLANGE 

GRAPHITE 

S.S. TUIES -

FilERS 

INTERNAL 'RESSURE 

CROSS·SECTIONAL VIEW 

NOTE ALL DIMENSIONS IN MILLIMETERS 

Figure 27. Photoelastic sensor. 

A single mode, polarization preserving optical fiber was squeezed between a rigid 

pedestal and the diaphragm. Both the pedestal (at its base) and the diaphragm 

were optically contacted to the fused silica base plate. The optical fiber leads 

and thermocouple to measure sensor temperature were inserted through the metal 

tubes on the sensor metal end plate containing the graphite pressure seal gasket. 

4f) 

'- r;6"'~- ,; 

----------------
.-

, 

. , , 

. .f 

. , 

, 

, . 

i 

~ 

f 

i 

-#'.~ *: '!. ' 

.-

5.3.2 Operational and High-Temperature Evaluation 

The photoelastic sensor was tested using the associated components shown in Figure 

28. Separate tests as a function of temperature, and pressure at elevated 

temperature showed that the desired birefringence response could be obtained. 

However, the sensor was not tested in the fused silica pressure module configuration due to the lack of an optic fiber that would withstand pressurization without 

fracture or deformation of the fiber coating. For proper operation of this 

sensor, a single mode. polarization preserving optical fiber is needed with. 

coating that has hardness comparable to fused silica to prevent deformatic··. "~~ 

an optic fiber was not available for the ~esting of the assembled photoel~. 

sensor module. 

DIODE POLARIZER 00 

D----Q LASER I ~DS Xyzl----------

POSITIONER. POSITIONERS 

GLAN-THOMPSON PRISM 

+ DETECTORS 

OUTPUT CIRCUITS 

AID CONVERTERS 

Figure 28. Photoelastic sensor system block diagram • 

<:. J 

L.... ·_:~ ... -_ •• -. ______ 

------- ~~~_~_~~ __ A ..... _ _________ - ~ 41:._:.~~--:!

;..-::-~ #" 
.-

, 

6. DISCUSSION OF RESULTS 

The results of this project and the degree to which the o~jectives were met were a 

$ dir~ct function of the project constraints. Thus, the project will be discussed 

in terms of the effect of the constraints on the results. 

The primary constraints for the sensor module design were the use of a 

non-metallic diaphragm, operation with eit~cr internal or external pressure to 

690 kPa, able to withstand 3X full-scale overpressure, and operation in the 

temperature range of-20° to 540°C. As most high-temperature, non-metallic 

materials are brittle and must be limited in tension, the design result was a 

thick diaphragm with little deflection upon pressurization. The fused silica 

modules used in the sensors satisfied the pressure and temperature requirements. 

However, they presented problems in pressure sealing, attachment to other components was difficult, residual stress problems were difficult to avoid, and the 

fused silica was prone to fracture at points of inadvertent high stress during 

testing. These problems required the redesign of the module for the Phase 2 

sensor systems tests. The redesigned sensor module, however, retained the thick 

diaphragm with its small deflection with applied pressure. 

The constraints for the sensing system included the use of an optical method to 

sense diaphragm deflectio~, the sensing system was to be non-metallic, and practical considerations required the use of commercially available optical fibers and 

high-temperature cements. As the diaphragm deflections were small, the optical 

sensing methods were required to measure -10-8 

m deflections. The microbend, 

phase sensitive, and photoelastic sensing methods were sufficiently sensitive for 

these small deflections; the two-fiber transmission and the cantilever slope 

sensing methods possessed only marginal sensitivity for these small deflections. 

It should be noted that a relaxation of the overpressure requiremEnts would allow 

a two- to five-fold increase in the diaphragm deflection and would allow the use 

of less sensitive sensing methods. 

PRECEDING PAGE BI::-ANK NOT FII,MF.D 

43 

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_________________ -011.-.-". " '- ~ .. 1:1 p~ .: 

I 

i 

~ 1 
t 

I 

. \ 

Using commercially available optical fibers and high-temperature cements restricted the useful temperature range for several of the sensors. Although fibers that 

will operate at 540°C are available, the optical sensing methods required fibers 

with specific optical parameters such as non-standard numerical aperture. Optical 

fibers meeting the sensing method requirements were often limited in their temperature range. This was the case for thE fibers used in the microbend, two-fiber 

transmission, and cantilever slope sensors. 

Sensors with a continuous optical fiber, such as the microbend and the photoelastic sensors, were advantageous in that the fiber needed only to be constrained 

to t~e sensing region. Sensors such as the phase sensitive, two-fiber transmission, and cantilever slope required that the ends of fibers or fiber bundles be 

secured in the sensor. Instability resulted when commercially available fi~~rs 

and cements were used because these materials invariably exhibited a mismai.:!~ 'iii 

thermal expansion between the optical fibers and the fused silica module. 

Repeated temperature cycles to 540°C tended to loosen the fibers or fiber bundles 

and eventually allow micro-movement of the fibers with respect to the module. Due 

to the small diaphragm deflection and the high sensitivity of the optical sensing 

methods, any relative movement of the fibers or the sens~r module components 

caused a change (gain, offset, or both) in the sensor signal. Thus, second-order 

effects were often important. For example, this was seen in the zero shift due to 

changes in the sealing pressure for the Design 1 module. 

The constraints of operation at peak velocity of 12.7 x 10-3 m/second (0.5 inch/ 

second) and frequency response to 2 kHz required support of the fibers to eliminate resonant modt and sufficiently fast electronics. Due to the high sensitivity 

of the optical sensing methods and the limited signal-to-noise ratio, a longer 

time constant on the sensor electronics would be advantageous to reduce data 

scatter. 

The microbend optical diaphragm deflection sensor met most of the requirements and 

objectives of the project. The microbend sensor exhibited high sensitivity to 

diaphragm deflection; the response was approximately linear with pressure from 

-690 kPa (internal pressure) to +690 kPa (external pressure). The sensor response 

to pressure change was rapid, it was insensitive to vibration from 50 to 2500 Hz, 

and it withstood 3X full-scale pressure. The sensor module ~as capable of 540°C 

44 

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, 

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r 

I.; 

r 

~ , 

, 

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i 

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~., J 

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.' 

, 

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operation; the best available high-temperature fiber produced repeatable response 

to about 450°C. The Design 2 module reduced the zero offset with temperature. 

The Design 2 test data showed that many of the problems associated with Design 1 

had been eliminated. The Design 2 microbend sensor data as a function of pressure 

and temperature was self-consistent and allowed an overall curve fit with a deviation that was close to the project objective of acceptable level of-3%. The 

Design 2 data still showed hysteresis that is believed to be associated with the 

interaction of the sensor microbend "teeth" and the aluminum coating on the optic 

fiber. It is believed that this hysteresis could be reduced or eliminated by a 

combination of larger diaphragm deflection, smoother finish on the microbend 

"teeth", and an improved coating on the optical fiber. 

45 

...... -. __ .. _---." '.".,..,"""!=~---' .. ,.... - •.. -.. --~,~ .... - .' .... ''''.b~ ... ______ ... ; _. . 
7. SUMMARY OF RESULTS AND CONCLUSIONS 

The objective of this project was to study. design. develop. fabricate. assemble. 

test. evaluate, and document optical diaphragm deflection sensors which: 

• Accurately measure diaphragm deflection due to a pressure difference 

across its surface 

• Meet the operational and environmental requirements of pressure 

measurement systems for high-performance aircraft 

• Take advantage of current technology in optics. electronics. and 

material science 

The project results showed that a high-temperature pressure sensor based on a 

• non-metallic (ceramic or glass diaphragm) is feasible. Glass diaphragms. such as 

fused silica, are limited in their allowable deflection and thus require sensitive 

sensing methods. The microbend sensor, which was based on the attenuation of 

light transmitted through the core of an optical fiber caused by periodic bending 

of the fiber. was sufficiently sensitive to meet the project objectives. The 

Design 2 microbend sensor exhibited the desired sensitivity and stability as a 

function of temperature and pressure to be used as the basis for a compensatable 

sensor. 

The present limitations on the microbend sensor performance are due to the temperature limits on suitable commercially available optical fibers. the available 

coatings on such fibers, and pressure sealing of the sensor module for differential pressures. 

Ceramic/glass diaphragms and/or optical methods for detecting diaphragm deflection 

in a high-temperature pressure sensor should also be considered for applications 

such as advanced energy conversion systems. in addition to high-performance 

aircraft. Such applications with reduced or different design constraints may be 

directly amenable to solution by state-of-the-art components and sensing methods 

such as those investigated in this project. 

PRECEDING PAGE BLANK NOT rn.MED 47 

i 

,~ 

i ' 

... _ ...____ ._.t .. ®

," 

. . 

--
. \ 

Improvements in commercially available optical fiber characteristics such as 

maximum operating temperature and surface coating are required. Optical fiber 

manufacturers should be encouraged to develop fibers with improved hightemperature characteristics. 

48 

~
. 

. \ 

NASA CR-175008 

1. Ripon No. r Governmlnt AccI •• lon No. 

4. Titil Ind Subtitle 

DEVELOPMENT OF OPTICAL DIAPHRAGM DEFLECTION SENSORS, 

FINAL REPORT 

7. Author(l) 

W. L. Ghering, D. Varshneya, L. A. Jeffers, 

R. T. Bailey, and J. W. Berthold 

8. Plrformlng Organization Name and Addre .. 

THE BABCOCK & WILCOX COMPANY 

Research and De~elopment Division 

1562 Beeson Street, Alliance, Ohio 44601 

12. Sponsoring Agency Name and Address 

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 

Washington, [C. 20546 

15. Supplementar)' Notes 

John P. Barranger, Project Manager 

NASA Lewis Research Center, Cleveland, Ohio 44135 

16. Abatract 

3. Reclpllnt'. Cltllog No. 

5. Ripon Dltl 

JUNE 1985 

II. Plrformlng OrglnlZltion Code 

505-40-14 

II. Plrformlng Organization Ripon No. 

RDD:85:4101-12-01:01 

10. Work Unit No. 

11. Contract or Grlnt No. 

NAS3-23712 

13. TyPi 01 Ripon and Period Covered 

CONTRACTOR REPORT 

14. Sponsoring Agency Code 

The objective of this project was to develop high-temperature pressure sensors using 

non-metallic components and optical sensing methods. The sensors were to operate 

over a temperature range from room temperature (-20°C) to 540°C, to respond to 

internal pressure up to 690 kPa, to respond to external pressure up to 690 kPa, and 

to withstand external overpressure of 2070 kPa. Project tasks included evaluating 

senSing techniques and sensor systems. These efforts included materials and sensing 

method selection, sensor design, sensor fabrication, and sensor testing. Sensors 

were tested as a function of temperature, pressure, overpressure, and vibration. 

The ~roject results showed that high-temperature pressure sensors based on glass 

components and optical sensing methods are feasible. The microbend optical 

diaphragm deflection sensor exnibited the required sensitivity and stability for use 

as a pressure sensor with temperature compensation. For the microbend sensor, the 

95% confidence level deviation of input pressure from the pressure calculated from 

the overall temperature-compensated calibration equation was 3.7% of full scale. 

The limitations of the sensors evaluated were primarily due to the restricted 

temperature ranye of suitable commercially available optical fibers and the problems 

associated with glass-to-metal pressure sealing over the entire testing tel~perature 

range. 

17. Key Worde (Suggl.tld by Author(I)) 18. Dlllribution Stltement 

Pressure Measurement, High Temperature, 

Fiber Optic Sensors, Fused Silicon Devices, FOR GENERAL RELEASE 

Diaphragm Pressure Transducers, Aircraft 

E.nyine Sensors 

19. Security CIIIIII. (01 thle repon) 20. Security Ctlllil. (01 thla plgI) 21. No. 01 page. 22. Price' 

UNCLASSIFIED UNCLASSIFIED 62 

• 'For sale by the National Technicallnlormation ServIce. Springfield. VirginIa 22161 

-