NBSIR 73-301

Development of Insulation Transfer-Standards

Using a Flat Plate Calorimeter

p. R. Ludtke

Cryogenics Division

institute for Basic Standards

National Bureau of Standards

Boulder. Colorado 80302

March 1973

Prepared for

NASA-Marshall Space Flight Center

Astronautics Laboratory

Materials Division

Huntsville, Alabama

NBSIR 73-301

DEVELOPMENT OF INSULATION TRANSFER-STANDARDS

USING A FLAT PLATE CALORIMETER

P. R. Ludtke

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

March 1973

Prepared for

NASA-Marshall Space Flight Center

Astronautics Laboratory ^

Materials Division

Huntsville, Alabama

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

CONTENTS

1. 0 Introduction 1

2. 0 Evaluation of the Various Insulation Materials 4

3. 0 Selection of Calorimeter 6

4. 0 Selection of Candidate Insulation Materials for PreliminaryEvaluation 7

4, 1 Basis for Selection of Candidate Materials ..... 7

4. 1. 1 Closed Cell Foams . 8

4. 1, 2 Open Cell Foams 8

4. 1. 3 Powders and Opacified Powders ...... 8

4. 1. 4 Glass Microspheres 9

4. 1. 5 Fiber Glass , 9

4. 1. 6 Multi-Layer Materials 9

4. 2 Selection of Candidate Materials for Preliminary

Testing 10

4,2.1 Closed Cell Foams 10

4. 2„ 2 Open Cell Foams 13

4. 2. 3 Powders, Opacified Powders, and Glass

Microspheres 13

4. 2. 4 Fiber Glass 15

4.2.5 Multi-Layer Materials 15

5. 0 Preparation of Samples for the MSFC Flat Plate Calorimeter 16

5. 1 Foams, Closed and Open 16

5. 2 Fiber Glass Samples 17

5. 3 Multi-Layer Transfer-Standards 19

6. 0 Evaluation of the Condition of the MSFC Calorimeter, the

Accompanying Instrumentation, and the Data Reduction

Procedures 21

iii
CONTENTS (CONT'D)

7. 0 Draft of an Interim Operating Procedure 27

7. 1 Typical Test Procedure 28

7. 1. 1 Closed Cell Foam Transfer-Standards .... 28

7. 1. 2 Open Cell Foam Samples 31

7. 1. 3 Fiber Glass and Multi-Layer Sandwiches ... 31

8. 0 Preliminary Screening Tests with the MSFC Calorimeter

on Selected Candidate Transfer -Standards 32

8. 1 Discussion of Test Results 32

8o 1. 1 Closed Cell Foams 32

8„ 1. 2 Open Cell Foams 36

8. 1. 3 Fiber Glass Transfer-Standard 37

8. 1. 4 Multi-Layer Transfer-Standard 37

9. 0 Future Work 38

iv
LIST OF FIGURES

Figure 1. Thermal conductivity of pertinent gases 12

Figure 2. Open cell foam, NBS-MSFC-100 14

Figure 3. Fiber glass sandwich 18

Figure 4. Multi-layer container 22

Figure 5. Complete multi-layer sandwich . . 23

Figure 6. Double Guarded Flat Plate Calorimeter . ..... 24

LIST OF TABLES

Table 1. Sandwich Type Multi-Layer Samples 20

Table 2. Thermal Conductivity Test Results 33

Continued 34

V

DEVELOPMENT OF INSULATION TRANSFER -STANDARDS

USING A FLAT PLATE CALORIMETER

P. R. Ludtke

This program was initiated to develop insulation

transfer - standards to be used for evaluating calorimeters

at different locations throughout the country. Various

types of insulation materials were evaluated for use as transfer -standards. Samples were prepared for preliminary evaluation from selected candidate insulation

materials. A 30, 5 cm diameter double guarded flat plate

calorimeter at MSFC was provided for testing. The

calorimeter was checked and the boiloff-gas instrumentation updated. Thermal conductivity screening tests were

conducted using liquid nitrogen on open and closed cell foam

and fiber glass samples. The mean thermal conductivity

values of the samples tested during the screening tests

are presented. Compliance with the proposed ASTM

Standard Method of Test for Heat Flux Through Evacuated

Insulations was stressed.

1. 0 Introduction

Insulation systems for cryogenic fluids require a wide range of

insulation materials of varying quality; at present, no adequate means

exist for evaluating new or improved insulation concepts or for comparing different insulation materials under the same conditions in

different laboratories at different times. The objective of this study is

to develop materials, equipment, and procedures that vAll readily

permit evaluation and comparison of insulation materials in any

competent laboratory. In essence, cryogenic insulation transferstandards will be developed along with the necessary equipment and
The general approach to the program is as follows. A

literature search for information on cryogenic insulation materials and

also on thermal insulation calorimeters was conducted. The various

insulation material properties were studied and candidate materials

were selected for evaluation. The various types of calorimeters for

measuring thermal conductivity were studied and the measuring

techniques and parameters were evaluated. The calorimeter used for

the experimental tests in this program was a double guarded

flat plate model located at the NASA-Marshall Space Flight Center. The

condition of the calorimeter, the accompanying instrumentation, and the

test procedure were evaluated. Recommendations were made for improving the calorimeter precision and an interim test procedure was

drafted for use in the preliminary screening tests. Samples of the

selected types of insulation materials were prepared for screening tests

and for subsequent repeatability tests in the calorimeter. From the

screening tests, the most promising materials were to be selected for

further testing. At present, the initial screening tests have been conducted using liquid nitrogen on all but the multi -layer samples; the

multi-layer screening tests will be conducted using liquid hydrogen.

Future work to be completed in the program is planned as

follows. Before starting the liquid hydrogen testing of the multi-layer

samples, the present interim test procedure will be modified to follow

as closely as possible, the proposed ASTM Standard Method of Test for

Heat Flux Through Evacuated Insulations Using a Guarded Flat Plate

Boiloff Calorimeter. This document is now in its fifth draft revision.

Also, the Calorimeter at MSFC will be modified and updated to

meet the requirements of the proposed Standard Method of Test.

Screening tests will then be conducted on the multi -layer samples using

2
liquid hydrogen. From the screening tests, the most promising sample

materials will be selected for further repeatability testing. After the

repeatability tests, final sample materials will be selected for the

three different thermal conductivity ranges of 0. 1 to 1, 1 to 5 0, and 5 0

to 300 uW/cm-K . Transfer-standards will then be prepared for

each of the above thermal conductivity ranges. The transfer-standard

samples will be tested, assigned a thermal conductivity value, and will

be made available for comparative testing in other flat plate calorimeters

throughout the country. The thermal conductivity of the transferstandards will be checked periodically to determine the measurement

uncertainty of the thermal conductivity value assigned to that material

and to determine long term aging effects.

After selecting the multi-layer materials that are most reproducible, they will be tested on an operational dewar. A large dewar of

practical size will be insulated with the chosen multi-layer materials,

using normal care and application techniques. The effective thermal

conductivity of the same multi-layer materials on the large dewar will

be determined by appropriate testing, for comparison with the calorimeter results.

From the information gained during the screening and

repeatability tests, design criteria for what is considered an optimum

standard calorimeter will be formulated. Design criteria will include

ease of operation, pressure loading capability, flexible operating

capability, minimum operating cost, and compliance with the ASTM

proposed standard. During the program, the temperature range of

interest will be confined to 20 to 300 K. During the screening and

repeatability tests there will be no variation of compressive force on the

samples; the effect of compressive force on the thermal conductivity of

3
the samples will not be investigated. Considerable emphasis will be

placed on the multi-layer insulation (MLI) region of thermal conductivity

and adherence to the proposed ASTM standard method of test for MLI.

2. 0 Evaluation of the Various Insulation Materials

In order to gather all of the pertinent information on thermal

insulations, a literature search was conducted. Two bibliographies

^ were compiled; one entitled, "The Properties of Thermal Insulations"

and a second entitled, "Thermal Insulation Systems. " Pertinent

documents aided in the selection of insulation materials having constancy of properties and exhibiting characteristics desirable for

transfer - standards

.

The cryogenic thermal insulation materials were grouped into

the following categories for evaluation:

1) Closed Cell Foams : These foams are made of polyurethane

or polystyrene and usually have a chlorofluorocarbon or carbon dioxide gas within the closed cells. The densities

3 3

range from 32 to 641 kg/m (2 to 40 lb/ft ) and the foam is

usually rigid. These foams will be tested in a non-evacuated

environment. The foams are available in rigid boardstock,

or they may be poured, frothed, or sprayed.

2) Open Cell Foams : These foam materials have a high percentage (60 - 95%) of open cells, the foam may be rigid,

semi-rigid, or flexible. The density is usually in the range

3 3

of 32 kg/m (2 lb/ft ). The open cell foams will be tested

under evacuated conditions. Some of the open cell foam

materials are polyurethane and polyphenyl oxide (PPO). An

opacifier material such as aluminum powder possibly could

be blended into the foam mixture before rise occurs to make

an opacified open cell foam.
3) Powders: Powders such as perlite, silica aerogel, and

pyrogenic silicates comprise another group of insulating

materials. These materials are difficult to handle, contain,

and evacuate. They may be tested with opacifiers such as copper or aluminum flakes, or aluminum powder. The

powders are usually silicon oxides and tested under evacuated conditions,

4) Fiber Glass : Fiber glass of various texture is available

(mat, , blanket, etc. ) and can be obtained with very few

impurities. The fiber glass may also be combined with

reflective shields, and will be tested under evacuated

conditions,

5) Glass Microspheres : Microspheres are very small (~ 1 5 to

150 microns diameter) hollow glass beads which may be

completely or hemispherically coated with aluminum, or

not coated at all. The thermal conductivity of the microspheres is comparable to other opacified powder systems.

The microspheres are difficult to handle, contain, and

evacuate. The microspheres are normally used under

evacuated conditions.

6) Multi- Layer Insulations : Various types of multi-layer

insulation materials are available. The shields usually

consist of polyethylene terephthalate (PETP) or polypyromelitimide (PPMI) film which is thinly coated with

aluminum, silver, or gold on one or both sides. The

base material is sometimes given a crinkled or

5
waffled texture. The PPlv4I film is more expensive and

has a higher temperature limit (400 C) than PETP film

(137 C), therefore, PPMI is normally used only when

the higher temperature capability is a necessity. When

a reflective metal coating is used only on one side, the

base material is sometimes krinkled or dimpled and used

with no spacer material. Aluminum foil is also used for

shields.

Various types of thin insulating spacer materials may

be used between the shields, e. g. , glass paper, glass

matting, silk net, nylon net, and polyurethane foam.

These spacers have varied outgassing characteristics

and moisture absorption properties.

There is one type of composite multi-layer

insulation which is different from the others. It is made

by cementing small tufts of PEPT fibers to a shield

material such as aluminized PEPT or PPMI film. The

tufts serve the same purpose as the other types of spacer

materials, and promote evacuation of the multi-layer

system. Multi-layer insulations are always used in an

evacuated environment.

3. 0 Selection of Calorimeter

Literature pertinent to the measurement of thermal conductivity

using calorimeters was compiled and reviewed, including the ASTM

Special Technical Publication No. 411 entitled "Thermal Conductivity

Measurements of Insulating Materials at Cryogenic Temperatures. " It became apparent, after review, that the type of calorimeter best suited

6
for a standard testing device for cryogenic insulation materials is the

double guarded flat plate caloriraete r. This calorimeter is undoubtedly

the most complicated thermal conductivity apparatus, but it is also the

most versatile and probably the most accurate. A double guarded flat

plate calorimeter allows one to measure thermal conductivity versus

uniform compressive force on the sample materials and to measure the

precise thickness of the sample up to 2 inches before and during the

test. The temperature of the guard shield near the edge of the sample

can be measured and controlled and thus the edge effects of multi-layer

insulation can be investigated and controlled while the thermal conductivity of the sample can be measured in various gas environments

-6

ranging in pressure from 10 to 760 torr. The cold and warm plates of

the calorimeter have a tremendous temperature range and the calorimeter

will measure thermal conductivities over various temperature intervals

of the cryogenic range. Visual capability is also available on some of

these calorimeters. This calorimeter is without a doubt the best contender for a standard thermal conductivity tester and affords the best

control of the parameters important in developing insulation transfer

standards.

4. 0 Selection of Candidate Insulation Materials

for Preliminary Evaluation

4. 1 Basis for Selection of Candidate Materials

Most of the insulation materials presently available were

considered. Some were selected for preliminary evaluation and testing

by considering the characteristics and properties deemed most important for a cryogenic transfer-standard material. These characteristics

and properties are listed below for the various groups of insulation

materials.

7
4. 1. 1 Closed Cell Foams

1) Change of thermal conductivity with time (gas equilibration

rate)

The type and thermal conductivity of the gas within the cells

Water absorption

Foam cell size

Density uniformity

Dimensional stability

Surface dusting and discoloration due to aging

Tendency to crack at cryogenic temperatures.

4. 1. 2 Open Cell Foams

Percentage of open cells

Rigid or flexible

Foam cell size

Water absorption

Density uniformity

Surface dusting and discoloration due to aging

Tendency to crack at cryogenic temperatures.

4. 1. 3 Powders and Opacified Powders

Ease of handling and testing in a flat plate calorimeter

Containment of the powder in a suitable enclosure during a calorimeter test

Moisture absorption

Maintaining uniform distribution of the opacifier within the

powder

Powder loss due to handling a confined sample in a porous

envelope over a period of time.

Compaction of the powder due to handling.

8
4. 1. 4 Glass Microspheres

1) Ease of handling and testing in a flat plate calorimeter

2) Containment of the microspheres in a suitable enclosure

during a calorimeter test

3) Loss of microspheres due to handling a contained sample in

a porous envelope over a period of time

4) Higher cost of the microspheres.

4. 1. 5 Fiber Glass

1) Packaging of the fiber glass sample in a suitable container

that would allow the sample to be handled and transferred

2) Possibly combining the fiber glass with reflecting shields to

make a composite sample

3) Uniform density of the fiber glass

4) Outgassing of the binder and lubricant normally used in

manufacturing fiber glass.

4. 1. 6 Multi -Layer Materials

[Multi-Layer Shield Materials]

1) Emissivity of the metal coating

2) Oxidation susceptibility or long term emissivity of the

material

3) Thickness of the vapor deposited coating (usually determined

by measuring the electrical resistivity of the coating)

4) Upper temperature limit of the base material

5) Long term outgassing and chemical decomposition

characteristics

6) Lateral thermal conductivity

7) Type of metal coating

9
8) Cost of the shield materials

9) Total thickness of shield

10) Texture of the material (crinkled, dimpled, waffled, tufted, or flat)

11) Metallized on one or both sides, or plain foil.

[Multi -Layer Spacer Materials]

1) Thickness and density of the spacer material

2) Water absorption characteristics

3) Long term chemical decomposition

4) Upper temperature limit

5) Cost

6) Type of material

7) Tensile strength

8) Lateral thermal conductivity

9) Type of spacer material (paper, blanket, or tufts)

10) Outgassing characteristics.

4. 2 Selection of Candidate Materials for Preliminary Testing

Candidate materials were chosen and divided into five basic

groups. The materials chosen were judged best suited for transfer

standard materials using the criteria outlined in the previous section.

The materials chosen for each group are listed below.

4. 2. 1 Closed Cell Foams

Three closed cell foams were chosen: each is described below.

Polyurethane foam NBS-MSFC-4 is a rigid foam containing

carbon dioxide gas. Foams containing CO gas usually have a higher

thermal conductivity than foams containing chlorofluorocarbon gas. The

CO gas also has the characteristic of diffusing from the foam at a

10
faster rate than the chlorofluorocarbon gases. A CO^ blown foam was

chosen because it should have a shorter equilibration time and hence can

attain a uniform thermal conductivity sooner. The thermal conductivity

of various pertinent gases at one atmosphere is shown in figure 1. This

foam also has a high degree of dimensional stability, an exceptionally

uniform density, an isotropic cell structure, and a rated mean thermal

conductivity of 360 u W/cm-K . The foam is white and has very

little tendency to dust at the surface.

The second foam chosen is NBS-MSFC-6. This is a rigid poly3 3

urethane foam of 64 kg/m (4 lb/ft ). The closed cells contain mostly

chlorofluorocarbon gas which has a relatively low thermal conductivity,

as indicated in figure 1. The foam has a rated thermal conductivity of

159 initially and 202 u W/cm-K after aging and equilibrating with air.

The water absorption characteristics of the foam are the same as the

CO^ blown NBS-MSFC-4 foam, but the dimensional stability is not nearly

as good. The foam also tends to have a dusty surface, or dusting

tendency. The chlorofluorocarbon gas diffuses from the foam considerably slower than the CO^.

The third closed cell foam chosen for evaluation is NBS-MSFC3

. This is rigid polystyrene foam, with high load bearing properties.

The closed cells are filled mostly with chlorofluorocarbon gas. The

surface is hard and has no tendency to dust. The foam has an initial

mean thermal conductivity of 202 \i W/cm-K which increases slightly

with age as the air replaces a portion of the chlorofluorocarbon gas

3 within the cells. The foam has an approximate density of 54 kg/m

3

(3. 4 lb /ft ) and is of very uniform density.

11
5000

Thermal Conductivity of Pertinent

Gases at l(one)atm and 300K

0 20 40 60 80 100 120 140

MOLECULAR WEIGHT

Figure 1. Thermal conductivity of pertinent gases.

12
4. 2. 2 Open Cell Foams

Two open cell foams were chosen as candidate materials to be

tested under evacuated conditions.

The first is an experimental rigid polyurethane foam designated

3 3 as NBS-MSFC-100. The foam has a density of 32 kg/m (2 lb /ft ) and

has a texture similar to other closed cell polyurethane foams. The

open cell content of the foam is '--60 to 70%. A photo of a 3. 81 cm

(1 1/2 inch) thick x 3. 05 cm (12 inch) diameter sample of this foam is

shown in figure 2.

The other open cell foam is polyphenyl oxide, usually called

PPO. Polyphenyl oxide foam has a fibrous cell structure which is

anisotropic and considerably different from the conventional round or

elongated cells in most other foams. The fibers are very tough and

special techniques have to be used in cutting the foam to the desired

size. The cells are elongated and parallel to the foam

fibers. The foam is slightly flexible when a force is applied perpendicular to the fibers. The foam has an average density of 40 to 48

kg/m^ (2. 5 to 3 Ib/ft^).

4. 2. 3 Powders, Opacified Powders, and Glass Microspheres

None of the powder insulations were chosen as candidate

materials because of 1) the problem of packaging and containing a powder sample over an extended period of time, 2) the high water absorption characteristics of the powders, and 3) the thermal conductivity

range of the evacuated powders is provided at the high end by the

evacuated foams and at the low end by the fiber glass. The thermal

13

conductivity span of the opacified powders and microspheres is also

provided by the fiber glass or fiber glas s -multi-laye r laminates. Hence,

opacified powders and microspheres were eliminated as candidates for

transfer-standard materials.

4. 2. 4 Fiber Glass

A special unbonded, unlubricated glass fiber material was

chosen for evaluation. The fiber glass is a flexible, resilient blanket

composed of glass fibers supplied without a binder or lubricant, having

3 3 a density of 16 kg/m (1 lb/ft ), This material was chose primarily

because it is free of impurities such as chlorides and oils which would

tend to outgas under vacuum. Fiber glass is easily evacuated, absorbs

very little water, and can be easily contained and packaged for use as a transfer-standard. Fiber glass may also be easily combined with

reflecting shields to fabricate a composite sample more resistant to

radiation heat transfer with a correspondingly lower thermal conductivity,

4. 2. 5 Multi-Layer Materials

A wide variety of multi-layer shield and spacer materials are

available. The following were chosen as candidate materials for

evaluation.

Reflecting Shield Materials:

1) PPMI film, with PETP fiber tufts, silvered on both sides

2) PETP film, waffled, silvered on both sides

3) PETP film, plain, aluminized on both sides.

Each of the above shield materials is 0. 0064 mm (0. 00025 in) thick.

15
Spacer Materials:

1) Glass mat, OJ ram (0. 004 in) thick

2) Borosilicate glass paper, nominal thickness of

3

0. 023 mm (0. 0009 in), density = 0. 180 gms/cm

3) 100% silk netting, NBS-MSFC- 55.

A shield material with gold metallizing was not included

because of its higher cost, especially when purchased in small quantities. Polyurethane foam spacer material was eliminated because of its

high outgassing characteristics.

The above multi-layer materials will be used in various combinations to prepare composite sandwich type samples for testing. The

preparation and details of construction of these sandwiches will be

discussed in the next section.

5. 0 Preparation of Samples for the MSFC Flat Plate

Calorimeter

The following samples were machined or prepared for testing

in the MSFC calorimeter.

5. 1 Foams, Closed and Open

All of the foam samples were cut or machined into 30. 5 cm

(12 in) diameter by 3. 8 cm (1. 5 in) thick discs. The flat plate

calorimeter will accommodate samples with a maximum thickness of

5. 08 cm (2 in). It was decided that the foam samples should be fairly

thick 3. 81 cm (1. 5 in) , since foam is normally used in practical

applications with a thickness of 5 to 15 cm ( 2 to 6 in). Three discs

each of the following foam materials were prepared.

1) NBSr-MSFC-3, closed cell polystyrene foam

2) NBS-MSFC-6, closed cell polyurethane foam

16
3) NBS-MSFC-4, closed cell polyurethane foam

4) Polyphenyl Oxide, open cell foam

5) NBS-MSFC-100, experimental open cell polyurethane foam.

An attempt was made to make some foam samples of the

NBS-MSFC-100 foam containing 1% by volume of powdered aluminum as an opacifier. However, this effort failed because the foam would not

rise properly.

Thus, a total of 15 foam samples were cut or machined and

stored in sealed containers for subsequent testing and evaluation in the

calorimeter at MSFC.

5. 2 Fiber Glass Samples

Two different sandwiches using fiber glass were prepared. One

sandwich is 3. 8 cm thick x 30. 5 cm diameter and is constructed in the

following manner. A 1. 3 cm wide x 3. 8 cm ring of NBS-MSFC-100 open

cell foam forms the outer ring of the sandwich. A 30. 5 cm diameter

disc of 0. 05 mm (0. 002 in) thick PETP is then cemented to one side of

the foam ring. The precut disc of untreated, unbonded, unlubricated

fiber glass of 3, 8 cm (1, 5 in) thickness is then placed inside the foam

ring, A similar 0, 05 mm thick PETP disc is then cemented onto the

top of the foam ring. The 30. 5 cm diameter PETP covers are perforated with twelve 6. 4 mm diameter holes to facilitate evacuation of the

fiber glass. A photo of this sandwich is shown in figure 3. A second sandwich using fiber glass and reflecting shields of

0. 0064 mm (0. 00025 in) thick double aluminized PETP film was also

constructed in a similar manner. For this sandwich, the foam ring was

1, 27 cm (0. 05 in) wide x 1. 9 cm (0, 75 in) thick. Similar 30. 5 cm diameter , 0. 05 mm thick PETP covers were used to make a complete

17

sandwich of 1. 9 cm thick x 30. 5 cm diameter. Inside the sandwich,

there are 19 layers of 0. 25 mm (0. 010 in) thick fiber glass spacers with

18 alternate layers of 0. 0064 mm (0. 00025 in) thick aluminized PETP film

A layer of fiber glass is next to each perforated PETP cover to facilitate evacuation of the sample interior.

Only one of each of these fiber glass samples was prepared for

preliminary evaluation. Complete specifications such as layer density,

weight density, and total weight are given in table 1.

5. 3 Multi-Layer Transfer-Standards

Five multi-layer samples were prepared using the sandwich

construction described in the previous section. The sandwich configuration was developed for the multi-layer insulations because of their

extreme susceptibility to damage with normal handling. Some containment or envelope is necessary to keep the sample clean, to allow

handling without physical damage, to allow handling without touching the

low emissivity shield surfaces, and to allow good evacuation within the

layers of the insulation. Transfer -standards will have to be shipped to

various geographical locations for calorimeter testing; it is felt that the

sandwich type of construction is well suited for protecting the insulation

samples during shipping and handling.

The sandwich construction also provides a buffer insulation

around the circumferential edge of the multi-layer insulation. The foam

will minimize edge effects on the multi-layer insulation. All of the

multi-layer samples are of the sandwich construction, and all are 1. 9

cm thick x 30. 5 cm diameter. The shields and spacer materials are cut

with templates to a diameter of 27. 3 cm. The free height (the height of

the multi-layer discs with no external compressive force) of each of the

19
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multi-layer samples is 1. 9 cm. A photo of an empty multi-layer container is shown in figure 4, and a photo of the complete sandwich is

shown in figure 5. Table 1 contains all of the information pertinent to

the multi-layer samples. Two other samples using silvered PPMI film

and silvered, waffled PETP have not yet been constructed because of

delays in material delivery.

6. 0 Evaluation of the Condition of the IVISFC Calorimeter, the

Accompanying Instrumentation, and the Data Reduction

Procedures

NBS personnel visited the Marshall Space Flight Center to

assist in determining the condition of the calorimeter at that

facility. A cross section diagram of the calorimeter is shown in figure

6. The calorimeter and accompanying instrumentation were carefully

examined. The normal test procedure and data reduction procedures

were also reviewed.

As a result of this evaluation, recommendations were submitted

to MSFC. Methods of improving the accuracy of the thermal conductivity

measurements and of updating the calorimeter instrumentation were

suggested as follows:

1) That the original measuring and guard vessel pressure

controller, which decreased atmospheric pressure variations by a factor of 36, be reconnected, or that a cartesian

diver type manostat with a controlled temperature bath be

used to control the pressure in the measuring vessel.

2) That a new wet test meter of the proper size be purchased

3 and used for boil-off rates of about 0. 08 cm /sec (0. 01

ft^/hr).

21
22
23
LIQUID NITROGEN

GUARD vessel'"

VENT LINE

VACUUM PORT

COLD PLATE OUIOE

FILL a PURGE LINES

MEASURING VESSEL

DEVICE FOR MEASURING SAMPLE THICKNESS'

SAMPLE CHAMBER

CHAbSER PRESSURE^

CCNTROL PORT

MAL INDICATOR

VACUUM .WCXET

VENT LINE

FILL a PURGE LINE

, COPPER EQUALIZERS

, VENTS

. GUARD VESSEL

VENT UNE

.FILL LINE

RADIATION SHIB.0

SAMPLE .CHAMBER SKIRT

pressure

'gage line

PURGE LINE

HYDRAULIC JACK PRESSURE

Figure 6, Double Guarded Flat Plate Calorimeter

24
3) That the boiloff gas be bubbled through a larger water bath

containing ceramic balls or marbles in order to achieve

better saturation of the boiloff gas.

4) That an accurate means of determining the warm plate

temperature of the calorimeter be implemented — new

thermocouples should be installed in the warm plate to

provide accurate temperature sensors at various locations

on the surface of the warm plate.

5) That temperature sensors be installed on the sample skirt so that the skirt temperature can be monitored and recorded.

The skirt is the cylindrical enclosure around the periphery

of the sample.

6) That a pressure controller be used on the guard vessel to

2 maintain a pressure difference of 133 to 266 n/m (1 to 2

mm Hg) between the guard and measuring vessels.

7) That the measuring vessel and all boiloff gas piping be

checked for leakage with a mass spectrometer leak detector.

After reviewing the above recommendations, MSFC personnel

agreed that every effort should be made to implement them. NBS personnel worked with technicians at the MSFC calorimeter site to make

the following modifications to the calorimeter:

a) New copper -constantan thermocouples were installed at

four locations in the warm plate of the calorimeter. At the

same time a gold (0. 07 atomic percent iron) vs Type KP

thermocouple was provided to that a thermocouple circuit

could be installed on the sample skirt at a future date.

b) Two cartesian-diver type manostats were installed; one on

the measuring vessel and another on the guard vessel. Both

25
of these pressure controllers were placed in a temperature

controlled oil bath in order to obtain raaximum pressure

control on each of these vessels.

c) A new wet test meter was purchased and installed to

measure boiloff gas.

d) Glass marbles were installed in the water saturator container and the water level was increased to 1 3 cm depth.

e) A pressure check of the measuring vessel and all piping for

boiloff gas was conducted to check for leaks.

The following recommendations were made in order to improve

the accuracy of the boiloff gas data reduction procedures.

1) That a correction term be added to the data reduction

program for boiloff gas measurements to compensate for the

gas that is vaporized without leaving the measuring vessel.

This gas occupies the space left by the vaporized liquid.

2) That a more accurate temperature of the warm plate be

used. This is obtained by using an arithmetic average of

the temperature measured by the four warm plate thermocouples.

3) That the exact temperature (to 0. 1 K) of the calorimeter

cold plate be used. This is determined from the vapor

pressure of the fluid in the measuring vessel.

4) That a mercury manometer correction term of 453 n/m

(3. 4 mm Hg) be subtracted from the actual pressure

readings. This correction term would compensate for the

temperature and gravity corrections to the manometer

readings at MSFC.

26
5) That the precise values for density and latent heat of vaporization be used for the fluid in the measuring vessel,

instead of arbitrarily using values for liquid at one

atmosphere.

6) That a more precise value of 459. 7 be used instead of 460

when correcting Fahrenheit temperature readings to the

Rankine scale.

It was agreed that each of the above recommendations would

increase the accuracy of the thermal conductivity measurements. All

of the above recommendations were incorporated in the data reduction

program.

7. 0 Draft of an Interim Operating Procedure

An interim test procedure was drawn up to be used when

testing the various candidate materials in the calorimeter using liquid

nitrogen. The purpose of this procedure was to insure that each of the

same type of samples would be tested uniformly. It was decided that

the closed cell foams should be tested in a nitrogen environment at a constant pressure of one standard atmosphere (760 mm Hg) and

fiber glass samples will be tested under evacuated conditions with a

/ 2 -5

pressure < 0. 00133 n/m (10 mm Hg). A method of preconditioning

the samples was specified in order to remove the moisture from the

samples and to define the gas remaining within the interstitial spaces or layers of the samples.

The multi-layer samples will not be tested using liquid nitrogen

because the resultant boiloff rate would be too low to establish confidence

in the test results. All of the multi-layer samples will be tested using

liquid hydrogen. A different interim test procedure will be drafted for

testing the multi-layer samples.

27
7. 1 Typical Test Procedure

A typical thermal conductivity test schedule and procedure

used during the screening tests is described below. Each thermal conductivity test requires a 1 -week period. The closed cell foam samples

were tested in a one standard atmosphere nitrogen environment with

2 2

approximately 6895 n/m (one lb /in ) compressive force on the sample.

7. 1. 1 Closed Cell Foam Transfer -Standards

Friday 1) Disassemble calorimeter and remove sample. Examine

sample for cracks or physical defects.

F 2) Place a 0. 005 mm (0. 002 in) thick stainless steel diaphragm

and an O-ring in position in order to seal the sample

chamber from the insulating vacuum space.

F 3) Place a new sample in the calorimeter and apply a com2 2 pressive force of approximately 6895 n/m (one lb/in ) to

the foam sample using the hydraulic jack.

F 4) Record initial thickness of the sample.

F 5) Assemble the calorimeter.

F 6) Evacuate the insulating vacuum space and check for leakage

past the O-ring seal into the sample chamber. Continue

pumping the insulating vacuum with the fore pump.

F 7) Through-purge the sample chamber with nitrogen gas for 30

minutes to remove most of the air from the sample chamber.

Seal the sample chamber at one standard atmosphere

pressure.

F 8) Conduct a pressure decrease vs time leak check on the

measuring vessel and all connecting piping at 68948 n/m

2

(10 lb/in ) for 15 minutes. Any pressure decrease will

28
indicate leakage in the measuring vessel system; the leak

must be sealed before continuing with the test.

F 9) Check each of the absolute manometers on the measuring

and guard vessels for leakage and consistency with the

barometer.

Saturday &

Sunday) Continue pumping the insulating vacuums during the weekend

with the fore pump.

Monday 1) Check the pressure in the sample chamber for an indication

of leakage past the O-ring.

M 2) Fill the calorimeter shield with liquid nitrogen. Monitor the

pressure levels while filling the shield for any indication of

leakage past the O-ring.

M 3) Start the diffusion pump for the insulating vacuum space.

M 4) Maintain the pressure in the sample chamber at one standard

atmosphere with nitrogen gas.

Tuesday 1) Check the pressure and vacuum in the various chambers.

Tu 2) Refill the shield with liquid nitrogen.

Tu 3) Fill the guard and measuring vessels with liquid nitrogen.

Wednesday 1) Check the pressure and vacuum in the various chambers.

2 The insulating vacuum should be at least 0. 0013 n/m

(10"^ mm Hg).

W 2) Refill the shield, the guard, and the measuring vessel with

liquid nitrogen.

/ / 2 W 3) Check the hydraulic system to maintain 6895 n/m (one

2

lb/in ) pressure on the sample.

29
W 4) Set the cartesian diver manostats to control the measuring

/ 2

vessel pressure at 109, 573 n/m (822 mm Hg), and to

2

control the guard vessel pressure at 109, 839 n/m (824 mm

2

Hg) pressure. (109, 573 n/m (822 mm Hg) will maintain the

measuring vessel temperature at exactly 78 K, considering

the temperature and gravity corrections to the manometer

readings. ) W 5) Maintain the specified pressure control on the measuring

and guard vessels for an additional 8 hours of controlledpressure test time.

W 6) During the 8 -hour period, record the following data every

30 minutes: insulating vacuum pressure, sample chamber

pressure, warm plate temperature (4 thermocouples),

measuring vessel pressure, guard vessel pressure, baroj metric pressure, wet test meter reading, wet test meter

temperature, manostat oil bath temperature, hydraulic

pressure, sample thickness, and the pressure difference

between the measuring and guard vessels (mm H O).

W 7) After the 8 -hour period, blow the liquid nitrogen out of the

measuring vessel, guard vessel, and shield.

W 8) Valve off and turn off the diffusion pump.

Thursday 1) Valve off the fore pump and pressurize the insulating

vacuum space to approximately one standard atmosphere

with nitrogen gas.

Th 2) Allow the calorimeter to warm to ambient temperature.

This completes the test cycle.

30
7. 1. 2 Open Cell Foam Samples

The test procedure for the open cell foam samples is the same

as for the closed foam samples, except:

1) There is no stainless steel diaphragm and accompanying

O-ring to contend with; the sample chamber and insulating

vacuum are common, and both are evacuated to < 0. 00133

2 -5 n/m (10 mm Hg) pressure.

2) The foam sample is initially preconditioned by evacuating

2

to approximately 1. 33 n/m (0. 01 mm Hg) pressure and

subsequently pressurizing to one standard atmosphere with

nitrogen gas. This cycle is repeated three times to help

remove moisture from the sample and remove all air from

the sample chamber, (i. e. , to insure that the residual gas

is mostly nitrogen).

7. 1. 3 Fiber Glass and Multi -Layer Sandwiches

. The test procedure for the fiber glass and multi-layer sandwiches is the same as for the open cell foam samples except the 6895

/ 2 , 2 n/m (one lb /in ) compressive force is not placed on these samples.

After installing the sandwich sample, the calorimeter sample spacing is

set and locked at either 1.9 or 3. 81 cm, whichever is the thickness of

the sandwich sample. The fiber glass and multi-layer samples are

preconditioned using nitrogen gas in the same manner as the open cell

foam samples.

After gaining experience with the calorimeter during the

screening tests using both liquid nitrogen and hydrogen, a final standard

operating procedure will be drafted for the repeatability tests. The

procedure for testing the multi-layer samples will be as consistent as

31
possible with the ASTM Standard Method of Test for Heat Fliax Through

Evacuated Insulations, presently in its fifth draft.

8. 0 Preliminary Screening Tests with the MSFC

Calorimeter on Selected Candidate Transfer -Standards

During the period from February through August 1972, twentytwo thermal conductivity tests were conducted using liquid nitrogen in

the calorimeter at NASA-Marshall Space Flight Center. The

results of these tests along with all pertinent data are given in table 2.

Some of the tests were scrapped because of difficulties in controlling the

measuring vessel pressure or guard vessel pressure, or other technical

difficulties. The test data presented is for tests with good pressure

control and consistent boiloff during the test period.

8. 1 Discussion of Test Results

8. 1. 1 Closed Cell Foams , Two different types of closed cell foams are being tested: one

type is a polystyrene or polyurethane foam containing chlorofluorocarbon gas within the cells, and the other is a polyurethane foam

containing carbon dioxide gas within the cells. A total of eight calorimeter tests have been conducted on the closed cell foam samples.

The very first test with the NBS-MSFC-3 sample produced a thermal conductivity value slightly less than the values obtained in

subsequent tests. The low thermal conductivity value was undoubtedly

due to the fact that the sample was freshly cut and the cells near the

exterior of the sample still contained a high concentration of chlorofluorocarbon gas, and very little air or nitrogen had diffused into the

foam. After conducting several additional tests on this same sample,

we verified that the changing composition of the gas within the foam

32
cells has a significant effect on the thermal conductivity of the samples.

Thus, in order to provide a closed cell foam sample of constant thermal

conductivity, the gas composition within the sample must remain

reasonably constant over an extended period of time.

One method of obtaining a nearly constant gas composition is

to shorten the time required to achieve gas equilibration between the

foam sample and the nitrogen environment that the sample experiences

during the 6 -day testing period. This technique was tried; two closed

cell samples were placed in a bell jar and evacuated to a pressure of

2

approximately 1. 33 n/m (0. 01 mm Hg). The samples were maintained

at this pressure for a period of 11 days. After the 11 -day period the

2

samples were pressurized to 83, 979 n/m (630 mm Hg) with nitrogen

gas and they were kept in this environment for 40 hours before they

were sealed in metal containers. One of the samples initially had

chlorofluorocarbon gas within the cells and the other had carbon dioxide

within the cells. These are samples numbered NBS-MSFC-3E and

NBS MSFC-4E, respectively.

The amount of parent gas pumped from the samples and

subsequently replaced with nitrogen gas is indeterminate. However, the

thermal conductivity values obtained from the four tests with these

preconditioned samples are very consistent.

Three different tests were conducted with foam containing

carbon dioxide gas within the cells.

The first test on 3-29 gave a thermal conductivity value of

260 |j, W/cm-K . The following two tests with the pre-evacuated

sample NBS-MSFC- 3E produced thermal conductivity values very close to

the value obtained with the regular sample NBS-MSFC-4 . The slight increase

33
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in thermal conductivity obtained with the test on 5-31 is insignificant.

Carbon dioxide gas diffuses from foam faster than chlorofluorocarbon

gas and consequently the foam has a faster equilibration rate. This may

be why there is no significant difference between the regular and the

pre-evacuated samples.

From the screening tests that have been conducted on the

closed cell foams, several decisions have been made. 1) No further

testing will be done on the NBS-MSFC-6 foam , This foam has a distinct tendency to dust at the surface and offers no significant advantages

over the NBS-MSFC-3 foam which has approximately the same thermal

conductivity and a much harder, non-dusting surface.

The predominate variable in trying to obtain a closed cell foam

with constant thermal conductivity is the composition of the gas within

the cells of the foam (see figure 1). It should be noted that there is

considerable difference between the thermal conductivity of nitrogen and

the chlorofluorocarbon gases usually present in the closed cell foams.

At this point in the program the pre-evacuated samples and the

NBS-MSFC-4 foam blown with carbon dioxide appear to be most promising as candidates for standard reference materials chosen from the closed cell

foams. Forthcoming repeatability tests with these materials will reveal

the magnitude of the variations in thermal conductivity due to changing

gas composition over an extended period of time.

8. 1. 2 Open Cell Foams

A total of six thermal conductivity tests have been conducted

using open cell foam samples. Four tests have been conducted on

polyphenyloxide (PPO) and another two tests have been conducted on an

experimental open cell foam NBS-MSFC-1 00 . The thermal conductivity

36
values obtained with the NBS-MSFC-100 open cell foam are very consistent but of significantly higher value. At the present time, the PPO

appears to be the better contender for use as a standard reference

material because of its more uniform density, non-dusting characteristics,

and its more open structure which allows a more thorough evacuation of

the sample. Additional tests will be conducted on both of these materials

during the repeatability phase of testing,

8. 1. 3 Fiber Glass Transfer-Standard

Only one test was conducted on the fiber glass sandwich sample,

and during this test the guard pressure was not controlled properly.

Additional testing will have to be conducted v>dth this sample in order to

check the consistency of the thermal conducti\T.ty values.

8. 1.4 Multi-Layer Transfer-Standard

One test was conducted on a 1. 9 cm thick multi-layer sandwich

sample composed of double aluminized PETP film and glass mat spacer

material. The test was conducted using liquid nitrogen. The gas boiloff

3 3 rate during the test was only 0. 114 cm /sec (0, 014 ft /hr), an extremely

low gas boiloff rate. Because of this, the multi-layer samples will all

be tested using liquid hydrogen, which will give a boiloff rate approximately 5 times the above rate.

The screening tests conducted to date have provided information

about the candidate samples (exclusive of the multi-layer region). None

of the foam samples have experienced any type of cracks or other

physical defects due to the cryogenic temperatures, and the epoxy bond

between the PETP film and open cell anniilar ring of foam shows no

sign of failure or other adverse affects. Initially, there was concern

about the integrity of this epoxy joint at cryogenic temperatures.

37
The estimated measurement uncertainty of the mean thermal

conductivity values presented in table 2 is ± 10%. This is the measurement uncertainty specified by the manufacturer of the calorimeter.

9. 0 Future Work

Additional repeatability tests using liquid nitrogen will be

conducted on the closed and open cell foam samples and the fiber glass

samples. These tests will be conducted in order to determine which

sample materials give the most consistent thermal conductivity values.

Screening tests will be conducted on the candidate multi-layer

transfer- standards using liquid hydrogen. However, before the screen

ing tests are started on the multi-layer samples, additional upgrading

of the MSFC calorimeter will be performed. Every effort will

be made to upgrade the calorimeter and operating procedure to be

consistent with the ASTM Standard Method of Test for Measuring Heat

Flux Through Evacuated Insulations.

38
FORM NBS-114A (1-71) 2. Gov't Accession

No.

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

PUBLICATION OR REPORT NO.

NBSIR 73-301

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Development of Insulation Transfer-Standards

Using a Flat Plate Calorimeter

5. Publication Date March 1973

6. Performing Organization Code

7. AUTHOR(S)

Paul R. Ludtke

8. Performing Organization

NBSIR 73-301

10. Project/Task/Work Unit No. 275.05 - 2750457

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS,

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

Boulder Labs.

11. Contract/Grant No. H-79204A

12. Sponsoring Organization Name and Address

NASA-Marshall Space Flight Center

Astronautics Laboratory

Materials Division

Huntsville, Alabama

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.) This program was initiated to develop insulation transfer-standards to be used

for evaluating calorimeters at different locations throughout the country. Various

types of insulation materials were evaluated for use as transfer-standards. Samples

were prepared for preliminary evaluation from selected candidate insulation material

A 30.5 cm diameter double guarded flat plate calorimeter at MSFC was provided for

testing. The calorimeter was checked and the boiloff-gas instrumentation updated.

Thermal conductivity screening tests were conducted using liquid nitrogen on open

and closed cell foam and fiber glass samples. The mean thermal conductivity values

of the samples tested during the screening tests are presented. Compliance with

the proposed ASTM Standard Method of Test for Heat Flux Through Evacuated

Insulations was stressed.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Calorimeters; fiber glass; foam insulation; insulation transfer-standard;

multi-layer insulation; thermal insulation; transfer standards

18. AVAILABILITY STATEMENT

I I UNLIMITED.

[x] FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Pr

USCOMM-DC 66244-P7