NASA Technical Memorandum 101635 , 

THERMAL ENCLOSURES FOR ELECTRONICALLY SCANNED PRESSURE 

MODULES OPERATING IN CRYOGENIC ENVIRONMENTS 

MICHAEL MITCHELL AND ERADLEY S, SEALEY 

(UASA-Tfi-tOt635) TIlBRlJILL EBCLOSUBES 908 N89-27151 

ELECTRO#LCALLY SCAIBED PBBSSUBB ISODULBS 

Langley Research Center) 16 p CSCL 14B 

OPERATING IN CEYOGBB~C ElOtf IBOEllEMTS [UASA. 

Onclas 

63/35 0224108 

AUGUST 1989 

National Aeronautics and 

Space Administration 

Langley Research Center 

Hampton, Virginia 23665 
INTRODUCTION 

Pressure measurements at the test section walls and the model surfaces in 

the National Transonic Facility (NTF) are being acquired using the 

Electronicaly Scanned Pressure (ESP) Measurement System (ref. 1). The Data 

Acquisition and Control Units (DACU) and the Pressure Calibration Units (PCU) 

for the ESP systems are located outside the tunnel while the ESP modules are 

maintained at a constant temperature in thermal enclosures mounted in the 

cryo-environment inside the tunnel and the models. This reduces pressure 

response lag due to long tubing lengths. The need to maintain the ESP modules 

at a temperature of 10 "C fl OC is dictated by the pressure module's thermal 

zero and sensitivity coefficients and the adverse effect of internally 

generated thermal energy on model boundary layers (ref. 2). 

The operating temperature range of the ESP modules is 0 "C to 79 OC. The 

thermal zero coefficient is 0.04% F.S./OC and the thermal sensitivity 

coefficient is O.O2%/OC. These factors prompted the development of the 

thermal enclosures currently being used to house ESP modules. 

are capable of maintaining a constant temperature to within fl "C at 10 "C and 

withstand the large pressures (931kPa (135 psia)) and low temperatures 

(-173 "C) associated with the tunnel operation (ref. 2). This paper describes 

the design requirements, construction, and testing procedures for these 

encl osures. 

The enclosures 

DESIGN REQUIREMENTS 

The thermal enclosures used at NTF for model and tunnel pressure 

measurements must meet certain requirements to ensure high quality 

measurements. Temperature control to within fl "C is essential because of the 

thermal zero and sensitivity coefficients of the transducers. Also the use of 

on-line calibrations to correct for temperature induced errors is expensive. 

A f207kPa (f30 psid) ESP module that experiences a 1 OC change in temperature 

also experiences a 0.04% F.S. change in zero output equal to 0.16kPa 

(0.024 psid) and a 0.02% change in sensitivity equal to 0.04kPa (0.006 psi) at 

full scale. The operating temperature (10 "C) of a package is determined by 

its effect on the boundary layer of the model or wall of the tunnel. 

The enclosure heater controller used at NTF is a proportional voltage 

type which senses the internal temperature of the enclosure using a. Resistance 

Temperature Device (KTD) mounted in the enclosure. The heater assembly used 

1 
in the models must be compatible with these heater controllers. 

temperature sensor location must be optimized and the thermal energy loss due 

to the tubing and wiring exiting the enclosure must be minimized to 

effectively maintain the internal temperature. The ESP module(s) as well as 

the tubing, associated electronics, wiring harness, and the heater assembly 

are required to fit inside the thermal enclosure. Table 1 shows the inside 

dimensions of thermal enclosures built according to these procedures. These 

dimensions plus a minimum 0.635 cm (0.25 in.) insulation wall thickness 

determine the overall dimensions of the enclosure which must fit into the 

allotted space in the tunnel or model. 

The enclosure material currently being used is a closed-cell foam 

manufactured in accordance with the specifications listed in table 2. 

material must be capable of withstanding the 931kPa (135 psia) tunnel pressure 

without deformation of shape or increase in thermal conductivity. The 

enclosure must be soundly constructed (structurally stable, snug-fitting 

parts, and overlapping joints) using a material that is equal to or better in 

thermal conductivity, strength, and performance than that of the foam 

insulation described in table 2. 

The 

The 

CONSTRUCTION TECHNIQUES 

Two methods of fabrication that have been successfully utilized are the 

milling method and the piece-wise assembly method. 

accomplished by milling the inside and outside dimensions of the enclosure 

from a block of foam (fig. 1). 

out the sides with overlapping joints and bonding them together with an 

adhesive (e.g., Crest 391 Urethane Rubber adhesive) that will withstand the 

-173 OC to 60 *C thermal environment. An example of this method is shown in 

figure 2. To avoid heat loss, the use of adhesive should be minimized since 

its accumulation at joints will increase the total heat loss of the 

enclosure. In addition, care should be taken to eliminate leaks and cracks by 

fabricating snuy-fitting mating parts and by extending the inside face of the 

tubing and wiring access wall into the enclosure cavity (fig. 1). 

The milling method is 

The assembly method is accomplished by cutting 

Provisions must be made for routing the electrical and pneumatic lines 

that are necessary for the operation of the ESP module(s) out of the 

enclosure. Table 3 shows the number and sizes of electrical wires routed 

through the enclosure wall for up to 12 modules housed in the enclosure. One 

calibration tube is used for each module pressure range in the enclosure. The 

C1, C2, and reference tubes can be shared among all modules regardless of 

their pressure range. The tubes should pass through the enclosure wall via 

individual holes, provided the enclosure is large enough.to accommodate the 

necessary holes (no more than 64, 0.15 cm (0.06 in.) O.D. holes per 

2 
square inch). 

brought through using one large exit hole. For ease in installation, it is 

recommended that the tubes and wires exit through the wall of the enclosure 

that is used to access the ESP package (fig. 3). 

If not, the tubes and wires should be grouped together and 

The enclosure size is determined by the number and type of modules and 

the allotted space in the model or tunnel. 

48-channel ESP module (48-SL) requires an inside dimension of 9.52 cm long x 

6.1 cm wide x 6.35 cm high (3.75 in. x 2.4 in. x 2.5 in.). These dimensions 

take into account the space needed for the heater assembly, control lines and 

pressure data lines, and the 6.35 cm (0.25 in.) minimum wall thickness. 

more than one module is used in an enclosure, an electrical distribution board 

is required. 

and is placed with the modules as a point of distribution for the supply 

voltages to the modules. 

6.35 cm x 0.317 cm x 2.54 cm (2.5 in. x 0.125 in. x 1 in.). A wiring 

schematic of the distribution board is shown in figure 4. These minimum 

enclosure dimensions can be increased and/or the corners may be rounded 

slightly, as shown in figure 5, to fit the allotted space within the model as 

long as the thickness does not decrease to less than 6.35 cm (0.25 in.). 

Figure 6 shows a thermal enclosure that was constructed according to the above 

procedures. 

An enclosure housing one 

If 

The distribution board is fabricated on a printed circuit board 

The minimum dimension of a distribution board is 

THERMAL CONTROL 

The enclosure heater controller used at NTF is a closed-loop, 

proportional voltage type which measures the internal temperature of the 

enclosure via a temperature sensor (RTD) mounted in the enclosure. It 

compares this temperature sensor output voltage to the user selected 

temperature set point (voltage) and outputs a proportional 0-90 Vdc to the 

heater assembly for temperature control. 

commercially available heater elements. 

flexible etched-foil heating coils laminated between two insulating layers of 

0.025 cm (0.01 in.) thick Kapton. 

(fig. 3) must be fabricated to enclose the ESP module package and to provide a 

surface to mount the heater elements. Heaters should be attached to all 

outside surfaces areas. It is required that the pressure data tubes and 

control tubes be heated before they exit the enclosure, since this is a major 

source of heat 105s. The tubes can be heated by interweaving the heater 

element(s) between the tubes or by flattening the tubing bundle and wrapping 

the bundle with the heater element(s) to insure that all the tubes are exposed 

to the heat. 

contact with the tubes (e.g., fiberglass tape between the heater and the 

tubing) to prevent melting of the pressure tubes. When all of the 

The heater assembly is a series of 

Each element consists of thin, 

A 0.08 cm (1/32 in.) aluminum shroud 

Care should be taken not to place the heater elements in direct 

3 
shroud/tub 

be between 

ng heater e 

60-75 ohms. 

ements are wired together, the total resistance shall 

TESTING PROCEDURE 

The thermal package is tested at ambient and cryogenic temperatures to 

determine its ability to maintain the set temperature. 

in an environmental chamber that is capable of obtaining cryogenic 

temperatures (-180 "C) and is then connected electrically and pneumatically to 

the Data Acquisition and Control Unit and the Pressure Calibrate Unit. 

following tests are required: 

The package is placed 

The 

1. Perform a leak check of control pressure tubes and pressure data tubes 

by applying appropriate pressures to each tubes (repair or replace 

module or tubing if leak rate is more than 0.138kPa/min 

(0.02 psi/min)). 

2. Perform ESP system calibration at ambient temperature. 

3. Apply known pressures to module(s) and acquire pressure data at 

ambi ent temperature. 

4. Analyze data for proper operation. 

5. Turn on heater controller and set temperature set point to 10 OC. 

6. Cool chamber to -180 OC. 

7. Ensure enclosure thermal stability is maintained at 10' fl "C before 

proceeding. If the temperature is not stable, the RTD must be 

repositioned to provide a slightly underdamped temperature response 

resulting in an adequate heater duty cycle. After repositioning the 

RTD, return to step 2. 

8. Perform ESP system calibration at -180 OC. 

9. Apply known pressures to module(s) and acquire pressure data at 

-180 OC. 

10. Warm chamber to ambient temperature. 

11. Perform ESP system calibration. 

4 
12. Evaluate all data for acceptable performance according to 

instrumentation speci f ications. 

The heater controller must be monitored during the cryogenic segment of 

the test. 

underdamped manner and remain steady to within f5 volts. 

temperature should then remain stable to fl OC. 

The heater voltage should seek a constant value in a slightly 

The enclosure 

CONCLUDING REMARKS 

This paper presents specific guidelines to design, construct, and test 

ESP thermal enclosures for applications at cryogenic temperatures. The 

enclosures maintain the ESP modules at a constant temperature (10 OC fl "C) to 

minimize thermal zero and sensitivity shifts, to minimize the frequency of 

expensive on-line calibrations, and to avoid adverse effects on tunnel and 

model boundary layers. The enclosures are constructed of a rigid closed-cell 

foam and are capable of withstanding the stagnation pressures to 931kPa 

(135 psia) without reduction in thermal insulation properties. This 

construction procedure has been used to construct several thermal packages 

which have been successfully used in National Transonic Facility. 

REFERENCES 

1. Mitchell, M.: Pressure Measurement System for the National Transonic 

Facility, Proceedings of the 29th International Instrumentation 

Symposium--ISA, May 1983. 

2. Macha, J. M., Pare, L. A., and Landrum, D.B.: A Theoretical Study of 

Non-Adiabatic Surface Effects for a Model in the NTF Cryogenic Wind 

Tunnel. NASA CR-3924, August 1985. 

National Transonic Facility. NASA TM 83124, July 1981. 

3. Fuller, D. E, Gloss, B. 6. and Nystrom, D.,: Guide for Users of the 

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Specification for Thermal Enclosure Material 

LAST-A-FOAM 9515 Closed-Cell Rigid Foam 

o 

o 

Density - 224.26 kg/m3 (14 lb./cu. ft.) 

Thermal Conductivity - 0.064 w/m'k (0.44 BTU in/hour/ft2/0F) 

o Liquid Nitrogen Compatibility - Material shall show no visible damage 

or degration when a room temperature material specimen is imnersed in 

liquid nitrogen for 5 fl minutes. 

o Char Yield - The specimen weight remaining at 600 OC shall be not less 

than 35% of the initial weight of the specimen when subjected to a 

temperature change of ambient to 600 OC at a rise rate of 10 "C per 

minute in a nitrogen environment. 

o Compressive Strength - The material shall have a minimum compressive 

strength of 1.79 Pa (260 psi) at the proportional limit over the full 

temperature range of -196 OC to 21 OC. 

Tab1 e 2. Specification for Thermal Enclosure Materi a1 

7 
ESP STING CABLE WIRING 

Modules 

1 

2 

3 

4 

5 

6 

7 

a 

9 

10 

11 

12 

Number of 24 AWG Wires 

Six (6) Address Lines 

One (1) Per ESP Module 

Number of 22 AWG Wires 

GND 

2 

2 

2 

3 

3 

4 

4 

5 

6 

6 

7 

7 

- +12v 

2 

2 

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3 

3 

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1 

1 

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2 

2 

2 

3 

3 

3 

4 

4 

4 

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Table 3. ESP Sting Cable Wiring 

Total #22+#24 

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21 

24 

26 

29 

33 

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38 

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~ BLACK AND WHITE PHOWWAPY 
Report Documentation Page 

1. Report No. 

NASA TM- 101635 

2. Government Accession No. 

9. Performing Organization Name and Address 

17. Key Words (Suggested by Author(s)) 

Langley Research Center 

Hampton, VA 23665-5225 

18. Distribution Statement 

12. Sponsoring Agency Name and Address 

Thermal Enclosure, Cryogenic Operations 

Electronically Scanned Pressure 

Measurements 

National Aeronautics and Space Administration 

Washington DC 20546-0001 

Unclassified--Unl imi ted 

Subject Ca tegory--35 

15. Supplementary Notes 

19. Security Classif. 20. Security Classif. (of this page) 21. No. of pages (of this report) 

3. Recipient's Catalog No. 

22. Price 

5. Report Date 

August 1989 

6. Performing Organization Code 

Unclassified 

8. Performing Organization Report No. 

15 A0 3 Uncl assi fi ed 

10. Work Unit No. 

505-61-01-05 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

16. Abstract 

This paper presents specific guidelines to design, construct, and test ESP 

thermal enclosures for applications at cryogenic temper8tures 

maintain the ESP modules at a constant temperature (10 C 21 'C) to minimize 

thermal zero and sensitivity shifts, to minimize the frequency of expensive 

on-ljne calibrations, and to avoid adverse effects on tunnel and model 

boundary layers. The enclosures are constructed of a rigid closed-cell foam 

and are capable of withstanding the stagnation pressures to 932kPa (135 psia) 

without reduction in thermal insulation properties. This construction 

procedure has been used to construct several thermal packages which have been 

successful ly used in National Transonic Faci 1 i ty. 

The enclosures