A GUARDED-HOT-PLATE APPARATUS

FOR MEASURING EFFECTIVE THERMAL

CONDUCTIVITY OF INSULATIONS

BETWEEN 80 K and 360 K

David R. Smith

Jerome G. Hust

Lambert J. Van Poolen

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

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81-1657

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January 1982

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Hat acc.-Circ

L2-C 100

r>o, A GUARDED-HOT-PLATE APPARATUS

FOR MEASURING EFFECTIVE THERMAL

CONDUCTIVITY OF INSULATIONS

BETWEEN 80 K and 360 K

David R. Smith

Jerome G. Hust

Lambert J. Van Poolen

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

January 1982

Prepared for Department of Energy

Oak Ridge Operations

Oak Ridge, Tennessee 37830

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
»

-
Contents

Page

Abbreviations and Symbols Used in This Report v

List of Figures vi

List of Tables vi

1. INTRODUCTION 1

2. DESCRIPTION AND DISCUSSION OF APPARATUS ELEMENTS 4

2.1 Introduction 4

2.2 Stack Elements 6

2.2.1 Main Heater and Inner Guard Assembly 6

2.2.2 Specimen Thickness Spacers 12

2.2.3 Top Auxiliary Heater 12

2.2.4 Bottom Auxiliary Heater 15

2.2.5 Cold Plates 16

2.3 Secondary Guarding 16

2.3.1 Outer Guard 16

2.3.2 Shroud/Insul ation Fill 17

2.3.3 Environmental Chamber 19

2.4 Refri gerati on 20

2.4.1 Fluid Transfer Lines 20

2.4.2 Cryogenic Coolant 22

2.5 Temperature Control 22

2.5.1 Top Auxiliary Heater Control 27

2.5.2 Bottom Auxiliary Heater Control 28

2.5.3 Inner Guard Heater Control . 28

2.5.4 Outer Guard Heater Control 29

2.5.5 Cabinet Temperature Control 29

2.5.6 Over-Temperature Control 29

i i i
Page

2.5.7 Assessment of Control Stability 30

2.6 Measurement 31

2.6.1 Metered Area Power 31

2.6.2 Temperature Measurement 32

2.6.3 Selector Switches 35

2.7 Vacuum/Gas Back-Fill System 38

3. DATA ACQUISITION AND CALCULATIONS 40

4. ERROR ANALYSIS 41

5. SUMMARY 48

6. REFERENCES 49

i v
Abbreviations and Symbols Used in This Report

Note: Primed abbreviations distinguish "top" from "bottom" positions of two

elements in an identical pair.

k

A

BAH

BCP

BP (BP '

BT

Cl (Cl

1 ) DTC

DVM

F (F‘)

G

GHP

HI (HI

1 )

ILN2

MH/IG

MS (MS')

ND

OD

OG

OGH

OGS

OTC

Q

R

S (S')

SR

SS (SS 1 ) Tl

t2

TAH

TC

TCA

TCP

V

AT

AX

Thermal Conductivity

Area of Metered Section

Bottom Auxiliary Heater

Bottom Cold Plate

Basepl ate

Bourdon-tube

Cold Plate Offset Insulation

Differential Thermocouple

Digital Voltmeter

Coolant Fluid Lines

Gap between Main Heater and Inner Guard

Guarded Hot Plate

Heater Offset Insulation

Current

Liquid Nitrogen

Main Heater/Inner Guard

Composite Spacers

Null Detector

Outer Diameter

Outer Guard

Outer Guard Heater

Outer Guard Supports

Over-Temperature Control

Heater Power

Resi stance

Specimen

Strain Relief

Specimen Thickness Spacers

Temperature of Specimen Cold Surface

Temperature of Specimen Hot Surface

Top Auxiliary Heater

Thermocouple

Thermal Conductivity Apparatus

Top Cold Plate

Voltage

Specimen Temperature Difference

Specimen Thickness

v
List of Figures

Page

Figure 1. Layout of the Stack, Guards, and Environmental Chamber

of the Hot Plate Apparatus Used in this Research 2

Figure 2. Details of Stack Assembly 5

Figure 3. Main Heater/Inner Guard Assembly 7

Figure 4. Rubberized Main Heater/Inner Guard Heater Elements 8

Figure 5. 20-pair Differential Thermocouple (Thermopile) 10

Figure 6. Wiring Details of Main Heater/Inner Guard/Outer Guard Assembly 13

Figure 7. Top and Bottom Auxiliary Heater Thermocouples 14

Figure 8. Outer Guard Assembly 18

Figure 9. FI uid-Transfer Lines 21

Figure 10. Temperature-Control ler Circuit for Top and Bottom

Auxiliary Heaters 25

Figure 11. Temperature-Control ler Circuit for Inner Guard 26

Figure 12. Thermocouple Tempering Blocks . . 34

Figure 13. Locations of Measuring Thermocouple Junctions in Stack .... 36

Figure 14. Vacuum/Gas Back-Fill System 39

Figure 15. Thermal Conductivity Data Form (Side A) 42

Figure 16. Thermal Conductivity Data Form (Side B) 43

List of Tables

Page

Table 1. Information obtained at selector switch positions 37

Table 2. Outline of computer program for analysis of thermal

insulation data 44

Table 3. Estimates of random variations and systematic uncertainties

in the measured quantities at room temperature 46
A Guarded-Hot-Plate Apparatus for Measuring Effective

Thermal Conductivity of Insulations Between 80 K and 360 K*

David R. Smith**, Jerome G. Hust, and Lambert J. Van Poolen***

Thermophysical Properties Division

National Bureau of Standards

Boulder, Colorado 80303

This report describes a guarded-hot-plate apparatus used to determine the effective thermal conductivity of glass fiber insulations.

Measurements can be performed at temperatures from 80 K to 360 K, from

atmospheric pressure to a vacuum of 10-4 p a (i x 10~6 torr). Various

fill gases such as air, nitrogen, argon, and helium can be utilized.

Overall uncertainties of thermal conductivities at atmospheric pressure

are 1% at the higher temperatures and 5% at the lower cryogenic temperatures. The modifications of the commercial apparatus described in

this report resulted in approximately a four-fold improvement in uncer- tainty.

Key words: guarded-hot-plate apparatus; insulation; 1 ow-temperature;

thermal conductivity.

1. INTRODUCTION

In October 1977, NBS/Boulder acquired a surplus commercial guarded-hot-plate

apparatus from NASA/Langley Research Center. This report describes the basic

operation of this apparatus and modifications performed to obtain the accuracy

and precision needed for the establishment of Standard Reference Material certification data at temperatures in the range 80 K to 360 K (-190°C to 90°C).

The layout of the central stack of this guarded-hot-plate apparatus depicted

in figure 1 is consistent with the ASTM C-177 specification. 1 This stack consists

*This work was done for the Department of Energy, Oak Ridge Operation, Oak Ridge,

TN 37830, under Interagency Agreement DOE No. DE-AI05-780R05965.

**Permanent address. South Dakota School of Mines and Technology, Rapid City, SD

57701, Dept, of Physics.

***Permanent address, Calvin College, Grand Rapids, MI 49506, Dept, of

Engineering.

^1981 Annual Book of ASTM Standards, Part 18, Thermal Insulation, American Society for Testing and Materials, Philadelphia, PA.

1
Figure 1. Layout of the Stack, Guards,

Chamber of the Hot Plate Apparatus Used

and Environmental

in this Research,

2
of a guarded-main-heater plate, two specimens, two cooling plates, and two auxiliary heater plates to provide for additional flexibility in adjusting and controlling the temperatures of the cooler faces of the specimens. The stack is

isolated from external influences by the outer guard, a shroud, and an environmental chamber. All of the assembly within the shroud is surrounded by loosefill insulation. The stack has a circular cross section perpendicular to the

vertical axis. The purpose of the stack and associated guarding is to create and

measure steady-state one-dimensional heat flow under a set of defined conditions.

From these measurements under certain conditions, as specified in the C- 177 test

method, an effective thermal conductivity may be defined (see section 3.0).

The metered section of the MH/IG plate provides the measured quantity of

heat which flows through the specimens to the lower temperature surfaces provided

by the auxiliary cold plates, or by the cold plates if the auxiliary plates are

not used. The auxiliary plates are thermally offset from the cold plates by an

appropriately chosen thermal resistance (the heater offset insulation). The temperatures of the auxiliary plates are electrically controlled to provide stable

temperature differences across the specimens. The inner guard, outer guard,

shroud, and insulation fill minimize the radial heat flow within the metered

area. The temperatures of the guard heaters are electrically controlled to match

appropriate temperatures. The cold plates intercept the heat from the heaters

and are maintained at a lower temperature by means of various coolants, including

liquid nitrogen, chilled alcohol, or water.

A wide range of choices for the specimen temperatures and temperature gradients are provided through the use of the auxiliary cold plates and the various

coolants. This flexibility is obtained through the use of the two auxiliary

heater plates mentioned above.

3
The goal of the NBS/Boulder measurement program was to establish thermal

transmission data for insulations within ±1% over the temperature range 80 to

360 K. This uncertainty is considered current state-of-the-art. The apparatus

as received was designed to measure thermal transmission within ±2% at ambient

temperatures and ±5% at liquid nitrogen temperatures in compliance with ASTM

C- 177 specifications.

Preliminary tests were conducted to see if this equipment would produce val

ues close to the ±1% uncertainty set as a goal. Results on glass fiberboard

indicated that the uncertainties were as high as ±5% at the upper end of the tern

perature range and as high as ±15% at the low end of the range. These high

uncertainties were due primarily to excessive temperature control fluctuations.

The above statement applies only to this particular surplus unit and does not

necessarily apply to new units produced by this manufacturer. Thus it was

decided that the apparatus would have to be modified to achieve our lower uncertainty requi rements . The preliminary tests also indicated that modifications

could simultaneously improve the efficiency of operation of this particular

experimental program.

A detailed description of the stack elements, guarding elements, and other

associated equipment is given in the following section. Modifications made to

the as-received apparatus are given in detail.

2. DESCRIPTION AND DISCUSSION OF APPARATUS ELEMENTS

2.1 Introduction

Figure 2 shows a more detailed picture of the guarded hot plate (GHP) assem

bly with heater leads and thermocouples omitted for clarity. Each of the plates

facing the specimens are anodized aluminum to achieve a high emissivity. The

4
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Figure

2.

Details

of

Stack

Assembly.
measured emissivity of these plates is 0.83. The components of this stack are

individually described below.

2.2 Stack Elements

2.2.1 Main Heater and Inner Guard Assembly

The MH/IG assembly is constructed of a rubberized two-element concentrically

arranged heater sandwiched between two circular, black anodized aluminum plates

also concentrically split to match the heater arrangement (fig. 3). Each twopiece plate is 9.5 mm (3/8 in.) thick and 203.2 mm (8.00 in.) in total diameter.

The anodization increases the emissivity of the surfaces and thereby promotes

radiative heat transfer to the specimens during measurements to simulate common

in-use conditions. The central disk of both the upper and lower plates, 101.6 mm

(4.00 in.) diameter, is separated from the surrounding annulus by a gap of 1.6 mm

(1/16 in.) width. Strips of plastic adhesive tape hold the disks in a centered

position within the annuli. The plates are bolted together with four flathead

screws located as shown in figure 3.

The rubberized heater element (fig. 4) is similarly constructed with a central disk of 101.6 mm diameter separated from the surrounding annulus by a gap of

1.6 mm width. The gap extends for only about 80 percent of the ci rcumference

,

however. The remaining rubber across the gap provides a bridge for the d-c

potential and current leads to the metered heater within the central disk. This

heater is powered by a d-c power supply with maximum current and voltage ratings

of 2 amps and 55 volts. It was operated in the constant current mode with a stability of better than 0.01%. The annular portion of the MH/IG heater combination

contains the Inner Guard heater element (19 ohms) which is powered by a temperature-control 1 er circuit. Details of the controller and the main heater power

supply circuit, as modified, are given in a separate section.

6
BOLT HOLE

THERMAL

TEMPERING

POST

STRAIN

RELIEF

PLATE

Figure 3. Main Heater/Inner Guard Assembly.

7
Figure 4. Rubberized Main Heater/Inner Guard Heater Elements.

8
The unbalance signal to the IG controller is produced by a Type E 20-pair

differential thermocouple (DTC), thermopile, illustrated in figure 5. The thermopile elements are constructed of smal 1 -diameter (0.20 mm) wires to minimize

heat leaks across the gap. The DTC junctions are cemented but electrically isolated, into small grooves in the MH/IG assembly plates. One half of the junctions are on the plates above the heater element and the other half are on the

lower plates. This placement is used to average the unbalance of the top and

bottom plates to zero. The configuration of this thermopile is non-inductive

(i.e., two loops are formed in opposite directions - not illustrated in figure 5)

to minimize induced signals. This Type E DTC replaces the 10-pair Type K thermopile originally supplied with the apparatus; a factor of 3 in gap unbalance sensitivity was gained at room temperature, and slightly more than a factor of 3 at

80 K, by this modification. The sensitivity of this thermopile is about 1200

mV/ K at room temperature and about 550 mV/ K at 80 K.

Originally the MH/IG plate assembly had 2 grooves 1.6 mm wide x 1.6 mm deep

machined into each aluminum plate on the side facing the specimens. One crossed

the gap and extended to the center of the metered area. The other groove stopped

just short of the gap. Type K thermocouples (TC's) in alumina tubes of 1.6 mm 0D

were fitted snugly into the grooves and were cemented in place with refractory

cement. However, at the high vacuum conditions needed occasionally in this

research the TC's in the originally supplied ceramic tubes were inadequately

anchored (for thermal tempering) to the plate surfaces. To improve the thermal

anchoring the following modifications were made: The Type K TC's were cemented

with a high conductivity refractory cement into the original grooves along their

entire lengths. The wires near the TC beads were raised to be at the upper edge

of the grooves, i.e., in the plane of the surface of the plate. Locations of

9
Figure 5. 20-pair Differential Thermocouple (Thermopile).

10
these thermocouples on the MH/IG assembly are shown in figure 3. Type E thermocouples would yield a higher sensitivity but an extensive rewiring of the entire

system would have been required to achieve this moderate improvement in uncertai nty

.

To provide further thermal tempering of the TC leads to the MH/IG surface,

cylindrical copper lugs (13 mm diam. x 13 mm long) were bolted to the edge of the

MH/IG plate at the points where the TC's leave the plate. The TC leads were

wrapped and cemented around the lugs for a length of 15 cm. Heat conduction

along the leads external to the stack should thus be diverted to the IG annulus

and not affect the metered area (see fig. 3).

The base plate (BP in fig. 2) equilibrates at a temperature cooler than room

temperature but warmer than the bottom cold plate (BCP) temperature when operating at low-temperatures. In the preliminary measurements with LN 2 coolant

there was evidence of a significant heat leak from the power terminals on the

shroud base plate to the stack via the heavy heater leads. To minimize the

effect of this heat leak the heavy leads (2-28 ga a-c leads, 2-28 ga d-c leads,

and 2-36 ga potential leads) emerging from the rubberized MH/IG heater pad were

replaced with coiled lengths of finer wire of a diameter sufficient to carry the

required currents without significant self heating. Their length was great

enough to reduce heat leaks into the MH/IG and also sufficient to permit several

turns (10 cm) to be wrapped around copper tempering posts mounted on the middle

of the 0G. The excess length (30 cm) remaining after tempering was coiled to

minimize entanglements. The junction between the original heavy leads and the

new finer wires were made on fiber-reinforced phenolic blocks mounted on the edge

of the strain relief, SR in figure 2, on the MH/IG. The other ends of the new

wires were then run to the power terminal strip on the shroud base. These modifications accomplish a diversion of heat leaks to the 0G (at points which are

11
heated anyway) and reduce heat leaks to the stack by increasing the lengths of

wires running to the stack as well as by decreasing the temperature differences

experienced by the portions of such wires immediately adjacent to the stack.

Figure 6 shows the circular MH/IG and annular OG (see section 2.3.1) and the

disposition of heater leads from the power terminal strip to the fiber-phenolic

blocks on the sides of the MH strain relief. The radial heat leaks across the

MH/IG gap and the IG/OG gap are discussed further in sections 2.5.3 and 2.5.4,

respecti vely

.

2.2.2 Specimen Thickness Spacers

Specimens involved in the present measurement program are highly compressible. Because of this, rigid spacers are placed between the inner guard and the

auxiliary heaters to fix the specimen thickness at a slightly compressed condition to assure good thermal contact between the plates and the specimens.

Matched sets of quartz or stainless steel spacer tubes have been used to

maintain a known thickness. Each set of three tubes is cut to the desired length

within a tolerance of 0.025 mm. The 7 mm diameter tubes are stuffed with glass

fiber to reduce heat transfer along the interior of the tube and are mounted at

three equally-spaced points into slots cut into the perimeter of each specimen.

After assembly the stack elements are compressed by a screw centered on the upper

base plate (see fig. 9) until all the spacers are under light compression, establishing the specimen thickness as that of the spacer tube.

2.2.3 Top Auxiliary Heater

The top auxiliary heater (TAH) assembly, figure 7, is made of two anodized

aluminum plates, each 9.5 mm thick and 203.2 mm diameter with a heater (15 ft) in

12
Figure 6. Wiring Details of Main Heater/Inner Guard/Outer Guard Assembly.

13
BOLT HOLE

THERMAL

TEMPERING

POST

(SIDE FACING SPECIMEN)

Figure 7. Top and Bottom Auxiliary Heater Thermocouples.

14
between them. The lower of the two plates had two grooves for TC's on the side

facing the specimen and one groove for a control TC on the side facing the heater.

All three grooves were filled with cement as described above in the MH/IG section

2.2.1. Two TC's were mounted, one at the center and one half way to the edge of

the side facing the specimen, and were tempered on copper posts screwed to the

side of the plate, see figure 7.

The TAH, as received, was controlled by a Type K thermocouple referenced to

an electronic ice point. To improve temperature control, a resistive temperature

sensor was installed in the TAH. This temperature control sensor is made of

30.5 m of 40 gage high resistance alloy wire (37.4 ft/m at room temperature). The

wire was doubled back on itself 200 times into a non-inductive 15 cm length,

which was then cemented into a diagonal groove machined into the internal surface

of the TAH. Copper leads were soldered to the ends of the resistance wire and

flexible cement was placed over the emerging copper leads to serve as a strainrelief. The resulting temperature-sensi ng resistor has a resistance of 1140 ft at

room temperature and a sensitivity of about 3.8 ft/K over the entire temperature

range. This resistor forms part of the temperature control circuit, described in

the section on temperature controllers.

2.2.4 Bottom Auxiliary Heater

The bottom auxiliary heater (BAH) (56 ft) assembly is similar to the TAH.

Modifications to the thermocouple grooves of the BAH assembly are similar to

those of the TAH and MH/IG assemblies. The thermocouples (Type K) were also tempered to the surface facing the specimen and to copper lugs screwed to the side

of the plate (fig. 7). In the as-received condition, the BAH was also controlled

with a TC referenced to an electronic ice point.

15
In a first modification, the BAH temperature was controlled by a DTC relative to the TAH, however, a more satisfactory result was obtained when the BAH

was controlled with an absolute controller rather than a differential controller.

The BAH temperature control sensor is of the same type as that described above

for the TAH. (Additional modifications of the controller circuitry are given in

a later section.)

2.2.5 Cold Plates

The cold plates as manufactured were spaced 5 cm from the upper and lower

baseplates and were offset from them by means of two aluminum rings. When water

was used as the coolant no problems occurred but when cooling with LN2 there

were some severe thermal link problems. The aluminum standoff rings presented a

path of low thermal resistance between the coolant and the baseplates (see fig.

2). The open spaces inside the standoff rings and between the cold plates and

baseplates would permit additional heat transfer by radiation and convection. To

improve the thermal isolation of the coolant, the aluminum standoff rings were

removed and each cold plate was mounted on 3 composite spacer rods. The offset

distance was increased to 9 cm. The space between the cold plates and baseplates

was packed with glass fiberblanket insulation (see fig. 2).

2 .3 Secondary Guarding

2.3.1 Outer Guard

The original thermal conductivity apparatus was equipped with an isothermal

shield (outer guard) surrounding the stack. Its purpose was to provide additional guarding against edge heat losses from the stack. The outer guard was

provided with a heater element and was designed to be controlled to a temperature

equal to that of the IG in the MH/IG assembly.

16
To improve the guarding of the temperature gradient of the stack a lightweight outer guard was made of copper and stainless steel which could develop a

matching temperature gradient in the portions of the guard immediately adjacent

to the specimens. The thought was that a guard with a matching temperature gradient would more effectively guard the stack than an isothermal one. Since the

new guard could also be operated i sothermal ly , tests were conducted to compare

the results from the two modes of operation. No difference was found beyond the

imprecision of these measurements ±0.4%.

Therefore, it was decided to revert to the use of a copper outer guard maintained in an isothermal condition at the temperature of the MH/IG. To simplify

changing specimens, this guard is made of half shells bolted together and is

weakly heat-sinked to the bottom cold plate (see figs. 2 and 8). The half shells

form an oval cylinder which surrounds the stack of heaters and insulation specimens with a clearance of about 20 mn from all stack surfaces, including the

strai n-rel ief blocks. This clearance is sufficient to provide space for tempering posts fastened to the outer surfaces of the heaters, and clearance for TC and

heater leads. The outer guard heater is made of resistance wire (12 ft). A tempering post is screwed to the shield to provide a mounting for a DTC. This DTC

is used in a controller circuit to hold the temperature of the outer guard close

to that of the MH/IG region.

2.3.2 Shroud/Insul at i on Fill

Originally, the bottom baseplate (BP in fig. 2) upon which thermocouple terminal strips were located, was cooled excessively by the LN2 cooled shroud

which was mounted directly on this baseplate with only a thin rubber trim for

standoff insulation. The cooling introduced unwanted thermal gradients in the

thermocouple terminal strip, hence uncertainties were introduced in temperature

17
Figure 8. Outer Guard Assembly.

18
measurement. In addition, the baseplate had fairly direct thermal contact

with room temperature through the apparatus support assembly. Thus, LN£ coolant was wasted maintaining this heat leak. To solve these problems:

1) The thermocouple terminal strips were removed from their original position on the shroud baseplate and mounted on another aluminum plate 10 cm

lower (the environmental chamber baseplate) which was directly connected

to a massive chamber support ring.

2) The shroud was remounted on a band of epoxy-fiberglass composite to provide thermal isolation between the shroud and its baseplate. The

resulting elevation of the shroud (5 cm) permitted the application of a

larger quantity of insulation fill within the shroud thereby reducing

usage of LN 2 during operation. Sliding doors were constructed in the

composite band under the shroud to permit easier removal of the insulation during disassembly.

Expanded mica insulation was used in place of the fine silica gel supplied

with the apparatus. The silica gel caused dust problems and was difficult to

keep out of the mechanical vacuum pump used to pump on the system. A copper

screen topped by 1 inch of glass fiberblanket was used to catch any stray insulation particles penetrating to the vacuum gate valve.

2.3.3 Environmental Chamber

A large metal dewar (environmental chamber) was placed over the entire stack

and guard assembly. It provided additional guarding from ambient temperatures

and conserved cryogenic coolant. This dewar replaced a glass bell jar supplied

with the apparatus. With the use of liquid nitrogen as coolant the metal dewar

was deemed safer and eliminated severe frosting which occurred with the glass

bell jar.

19
2.4 Refri gerati on

2.4.1 Fluid Transfer Lines

The original uninsulated fl uid-transfer lines within the bell jar system of

the TCA were designed principally for water as the coolant. Use of uninsulated

lines for chilled alcohol or LN 2 as the coolants wastes refri gerati on power.

Figure 9 illustrates the coolant paths after the following changes were made.

The coolant lines to the two cold plates were originally separate from the

shroud coolant lines. The shroud fluid lines were modified to be in series with

the cold plate coolant lines. The coolant leaving the stack is then efficiently

used to cool the shroud which is the largest cooled element intercepting inflowing heat.

The coolant lines serving the shroud from outside the environmental chamber

were disconnected and externally sealed. The cold-plate exit line to the outside

was re-routed to meet the old exit point for fluid leaving the shroud. The coldplate entrance line was removed and was duplicated in shape by a double-walled

vacuum-insulated replacement (see fig. 9).

Double-walled vacuum-insulated construction was continued in the fluid line

from the BCP to the TCP. Where the line originally consisted of a flexible

vacuum-tight stainless steel corrugated hose, a vacuum insulated flexible line

was incorporated with internal teflon spacers permitting flexibility and minimizing heat leak to the coolant. The exit line from the top cold plate was rerouted

to the entrance point of the shroud coolant line.

A transfer-1 i ne pump-out valve was installed in the entrance line external

to the vacuum system and another was installed in the interconnecting line

between the two cold plates, under the shroud baseplate (see fig. 9).

The external coolant entrance line contains a bayonet-type interconnection

permitting rapid change of the LN2 dewar and minimizes the heat leak into the

20
A Vacuum-insulated Line for Incoming Fluid

B Rigid Vacuum-insulated Fluid Line from BCP to TCP

CP Bottom cold plate

CP' Top cold plate

D Flexible Vacuum-insulated Line to TCP

E Rigid Fluid Line from TCP to Shroud

F Fluid Lines on Shroud

G Pump-out Ports on Vacuum-insulated Lines

Figure 9. Fluid-Transfer Lines.

21
transferred LN2 at the interconnection. The transfer line from the LN 2 storage

dewar also has a built-in pump-out valve. All three vacuum-insulated lines are

pumped out to an insulating vacuum before beginning LN 2 transfer for operation

of the TCA at cryogenic temperatures.

Another advantage of the bayonet-type interconnection is that the female

fitting will accept ordinary 9.5 mm (3/8") OD tubing with the 0-ring seal. When

water cooling is desired, a 9.5 mm (3/8") annealed copper tube joins the female

bayonet fitting to the water line. For the temperature range above 200 K a commercial circulating-fluid refrigerator can be used to cool and circulate ethanol

into the TCA system via flexible hose and another copper tube mating into the

female bayonet fitting. At present these lines from the refrigerator are insulated with a closed-cell insulation and sealed against moisture with polyvinyl

electrical tape.

2.4.2 Cryogenic Coolant

When LN 2 coolant is used it is desirable to keep the flow reasonably stable.

A gas pressure regulator on a nitrogen gas bottle permits application of a constant gas pressure to the LN 2 storage vessel supplying the coolant to the TCA.

A needle valve at the exit line from the outer Dewar baseplate ring is used to

control the flow of gas exiting from the system. These controls are used to set

the cold plate temperatures only slightly below that of the auxiliary heater

pi ates

.

2.5 Temperature Control

Preliminary checks using the four as-received temperature control circuits

for the TAH, BAH, IG, and 0G, revealed several undesirable features. First, the

22
instability of these circuits was greater than desired for high accuracy measurements and, second, these instabilities were only detectable by observing the

stack temperatures for relatively long periods of time, i.e., the controller null

indicators were too coarse to exhibit the disturbances. Generally the drifts

were about 0.2 K/h, but occasionally a 1-2 K fluctuation would occur for no obvious reason. Other undesirable features were: a) the inability to shut down each

controller separately, b) the inability to control the instantaneous power delivered to the heaters, and c) the inability to observe the average power delivered.

After a considerable amount of diagnostic testing, the excessive drift problems were traced to: a) electronic ice-point thermocouple reference devices, b)

unstable and temperature-sensi ti ve operational amplifiers in the low signal level

circuitry of the temperature controllers due partly to the lack of temperaturecontrol on the controller cabinet, c) excessive spurious emf's generated at TC

junction blocks and switches, and d) an excessively high instantaneous power

delivery to each of the heaters precluding optimum low power operation.

Extensive changes were made to eliminate the problems. Four on/off power

switches were added to provide independent operation of each controller. The

temperature control sensor circuitry for the TAH and BAH was modified so that the

need for a constant-temperature reference point for the thermocouples was eliminated. A power control circuit was added to allow control of the instantaneous

power delivered to the heaters, and a monitor circuit was installed so the operator can qualitatively gauge the average power delivered. The details of these

changes are presented below for each control circuit. For a clearer understanding of the need for these changes, the following brief explanation of the controller operation is given.

23
r

These temperature controllers employ zero-firing triacs capable of delivering up to 1000 watts of a-c line power to the heaters. Line power is delivered

for an integer number of half cycles, the frequency of the delivered half cycles

being determined by the magnitude of the error signal fed into the controller.

However, the instantaneous peak voltage during the "on" half cycles is fixed at

120 volts, i.e., line voltage. Under conditions of very low average power

demand, which occurs at low-temperature, steady-state conditions, a single cycle

may be sufficient power for a long period of time. This results in a rapid temperature rise and a slow return to the control point, i.e., the controller acts

more like an on/off controller rather than a proportional controller. Proportional controllers, generally, result in better temperature control. The solution is to reduce the peak output voltage so that the temperature rise rate

during the "on" period approximately matches the natural cooling rate of the controlled system. This simplistic solution is complicated somewhat by the following design limitations of these controllers: a) they should not be operated with

inductive devices on the output and b) the instantaneous output power during the

"on" cycle should not go below 250 watts. These limitations dictated the nature

of the design and the component sizes shown in the modified circuits, figures 10

and 11.

Figures 10 and 11 illustrate the power dissipation networks which were

installed to control the peak power delivered to the heaters while simultaneously

allowing the peak power controller output to remain above 250 watts. A high-low

power level feature was incorporated to allow large rapid changes of temperature

when needed (high power) and to achieve a good match to the system characteristics when at equilibrium (low power). In addition, figures 10 and 11 show the

addition of a neon light circuit across the output of each controller. These

lights give the operator an immediate, qualitative indication of the frequency at

24
r T xx

<<

i-m

a a

L

CM

CM ^

II cc

oCM

X X

25

Figure

10.

Temperature-Controller

Circuit

for

Top

and

Bottom

Auxiliary

Heaters.
a

8

<-J

s<

tr ? a

O

_i

co

26

Figure

11.

Temperature-Controller

Circuit

for

Inner

Guard.
which the half cycles are delivered. Experience has shown that up to about ten

pulses per second can be visually resolved.

The front end (low signal level) of the controllers contains the equivalent

of a nul 1 -detector and a bucking voltage. However, the controllers were not

designed for the degree of precision control needed to obtain highly accurate

measurements, i.e., the bucking voltage and the operational amplifiers were not

sufficiently stable. A relatively simple solution to this problem is to remove

the front end and replace it with an external nul 1 -detector amplifier with submicrovolt stability. These units are inexpensive and provided excellent control

while simultaneously introducing a sensitive meter to monitor the actual control

offset. The front end of the outer guard controller was not modified because it

was found that precise control of the outer guard was unnecessary. Specific

characteri sties of each modified control circuit are given below.

2.5.1 Top Auxiliary Heater Control

The TAH control, figure 10 operates on the basis of a temperature sensitive

resistor installed within the plate, as previously described. This resistor is

connected as one arm of a Wheatstone bridge circuit. The output (unbalance) of

the Wheatstone bridge goes to a microvolt nul 1 -detector amplifier whose output,

in turn, is the input to the temperature controller. The output of the temperature controller powers the plate heater and, in turn, returns the bridge to balance. The bridge setting determines the temperature control point.

The sensor has a temperature sensitivity of 3.8 ft/K and the sensor bridge

combination has a sensitivity of about 12 yV/mK. During equilibrium operation

the null detector range can be as low as 10 yV full scale, but generally the

30 yV range is sufficient for control stability. Experience shows that the TAH

control is usually within 1 mK/h.

27
2.5.2 Bottom Auxiliary Heater Control

This modified control circuit is also illustrated in figure 10. The circuitry is essentially the same as the TAH circuitry. The control of the BAH is

similarly within 1 mK/h.

This modified control circuit is illustrated in figure 11. The only notable

difference between this and the BAH/TAH control is that the temperature sensor

is a thermopile as previously described. The thermopile has a high output per

unit temperature unbalance, hence a higher sensitivity to deviations from the

control point. This is necessary because the heat flow across the MH/IG gap relative to heater power can be significant even for small gap temperature unbalance .

With this circuit, an average unbalance of less than 5 yV thermopile output

is readily maintained. This corresponds to an unbalance of 4 mK at room temperature and about 8 mK near liquid nitrogen temperature. For typical low density

insulations with a temperature gradient of 1 K/mm this corresponds to errors of

about 0.2% in heater power. The measured and calculated error in thermal

conductivity versus gap unbalance for glass fiberboard and fiberblanket at room

temperature is 0.04% per microvolt of thermopile output. The approximate

relative heat flow components across the MH/IG gap have been calculated and are

given below:

2.5.3 Inner Guard Heater Control

Component Percent of total gap conduction

Gas conduction 40%

Wire conduction 40%

Radi ati on 15%

Rubber bridge 5%

Gas convection Negl i gible

28
These percentages are applicable for near room temperature with air or nitrogen

as fill gas. At lower temperatures the radiation component is considerably

smaller. Under vacuum conditions the gas conduction term essentially vanishes.

2.5.4

Outer Guard Heater Control

Although the low signal level portion of this controller is unmodified, a

power dissipation and control circuit, similar to figure 11, was added. Adequate

control of peak power to the outer guard is obtained by a 20 ft resistor and 11 ft

rheostat in series with the 12 ft OGH. The effect on measured thermal conductivity as a function of outer guard unbalance was small, 0.01% change in measured

thermal conductivity per °C of outer guard offset. Therefore, accurate control

of the outer guard is not necessary and the no analysis of the various heat flow

components is given.

2.5.5

Cabinet Temperature Control

To reduce the effects of room temperature changes on the stability of the

stack controllers, a temperature controller was installed in the cabinet housing

the other four controllers. This controller maintains the temperature of the

cabinet within ±0.5 K. It has been found that this was essential with the original controllers but, after modifying the other controllers as described, cabinet

control was unnecessary.

2.5.6

Over-Temperature Control

A protective circuit to prevent excessive overheating in the stack (called

the over-temperature control, OTC) was furnished with the apparatus. A Type K

thermocouple sensing the temperature at one point in the stack is used in a

bridge circuit with an optical meter relay. If the temperature at the measured

29
point in the stack exceeds that established by a set-point indicator, the meter

relay shuts off all power to the cabinet, including that to an auxiliary power

receptacle. The electronic rack containing the d-c power supply for the MH is

connected to this receptacle, and so is also under the control of the OTC.

The OTC sensing thermocouple is at the outer edge of the MH/ I G assembly,

i.e., the hottest point in the stack. Since the TAH and BAH are colder, they are

less likely to overheat the specimens or other delicate parts, such as the rubberized heating elements. If either the MH or IG overheats, the sensing thermocouple will detect the fault; the IG is controlled to follow the temperature of

the MH and so will follow an overheating fault in the MH. If the TAH or BAH were

to overheat a temperature wave diffusing toward the MH/IG would be initiated

eventually also raising the MH/IG temperature. So the placement of the single

OTC thermocouple at the MH/IG is optimum.

2.5.7 Assessment of Control Stability

The original apparatus, as already stated, performed well below our needs

for accurate measurements. After the modifications were made tests showed marked

improvement in the temperature stability of the system. The long term (24 h)

stability of the current system as measured by the primary thermocouples is 0.1 K.

The short term (1 h) stability is 0.02 K. These instabilities include not only

the effect of the controller instabilities but also the main heater power supply

fluctuations, spurious thermocouple and switch emf's, and read-out instrumentation variation. These variations are used, in part, as the basis for the error

analysis given in a later section.

30
2.6 Measurement

2.6.1 Metered Area Power

The d-c power supply in the as-received apparatus was found to have, for the

purposes of this research, insufficient output stability. It was replaced with a

commercial high-accuracy Vol tage/Current Calibrator with a specified stability of

±0.0025% of setting or 0.0005% of range (whichever is greater) per hour after 2

minutes of constant current output, when operated in the constant current mode.

To reduce the uncertainty in power measurement, the 0.1% resistors supplied

with the apparatus for the measurement of heater current and voltage were replaced.

In the original equipment a series resistor was used to determine current and a

two-resistor voltage dividing network across the heater was used to measure

heater voltage. The present system contains a 1 Si (0.005%) standard resistor in

series with the heater for current determination while the heater voltage is measured directly with a high input resistance DVM. With this arrange ment, both

the heater voltage and the voltage across the standard resistor are high enough

that spurious thermal emf's are negligible and the need for forward and reverse

readings is eliminated.

The 1 Q. standard resistor is mounted in the main cabinet controlled to a

temperature of 37.5 ±0.5°C. The fractional change in resistance from the calibration temperature of 25°C to 37°C is 0.02%, and is calculated accurately.

Analysis indicated that the measured current supplied to the main heater is

accurate to within 0.01%, including power supply variations and all measurement

uncertainties. This is equivalent to 0.02% uncertainty in power to the main

heater.

It is noted that the d-c power supply can be controlled automatically by a

circuit which senses the difference between the actual temperature gradient and

the desired equilibrium gradient in the specimens. Power is then fed to the main

31
heater proportional to this difference resulting in a fairly rapid establishment

of equilibrium conditions. However, power data for the determination of effective thermal conductivity are taken only when the power supply is operated manually to assure accurate readings of heater current and voltage.

2.6.2 Temperature Measurement

The measurement of all critical apparatus temperatures and temperature differences is performed by spool -cal ibrated Type K thermocouples as previously mentioned.

A spool of Type KP and one of Type KN thermocouple wire were reserved for

use exclusively in this system and a Type K thermocouple made of wire from them

was calibrated using liquid He, LN 2

, dry-ice-alcohol, water-ice, room temperature water and boiling-water baths. Vapor pressure was used to determine the

temperature of the liquid helium bath, while a calibrated platinum resistance

thermometer was used to determine the temperatures of the remaining baths. The

difference between these calibration data and the standard tables* were used to

obtain the coefficients of a simple four term polynomial by least square fitting.

This calibration contains an estimated uncertainty in temperature of about 20 mK

and 50 mK at room temperature and liquid nitrogen temperature, respectively. The

correspondi ng temperature difference uncertainties for a AT = 25 K are about 0.1

and 0.2%. Differential thermocouples for the controllers were made from Type E

thermocouple wire for greater sensitivity; these DTC's do not have to be calibrated because they operate essentially at zero output. However, the DTC's must

be carefully mounted to minimize spurious emf's.

*R. L. Powell, W. J. Hall, C. H. Hyink, L. L. Sparks, G. W. Burns, M. G. Scroger,

and H. H. Plumb, Thermocouple Reference Tables Based on the IPTS - 68, National

Bureau of Standards Monograph 125, 1974.

32
A source of uncertainty in temperatures measured by the thermocouples is

spurious thermal emf's generated by the lack of isothermal conditions at terminal

blocks within the environmental chamber. The repositioning of these blocks to

minimize this problem was indicated earlier. The terminal strips were mounted on

heavy aluminum angle stock to provide an isothermal region and a vertical orientation for easy access to the terminals.

To minimize reconstructi on efforts, portions of the originally supplied TC

wire were retained. The original wire is connected to the newly calibrated wire

such that both ends of the original wires are at nearly equal temperatures minimizing the contribution of these wires to the measured emf. In addition, a second set of new wires were connected to the ends of the old wires to determine the

differences in thermopower between old and new wires. Thermocouples were mounted

on the TC blocks at both ends of the original wire to establish the magnitude of

the small temperature difference across the wire. From these data, one can correct for the small differences in calibration between the old and new wire. To

date, no such corrections have been required.

Additional aluminum blocks are used in conjunction with the TC terminal

blocks to temper the TC leads arriving at and leaving from the blocks (see fig.

12). The original TC wire is relatively large (20 gauge) and lies in grooves

milled into the opposing surfaces of two blocks. Silicone vacuum grease provides

thermal contact between the wires and the blocks. The NBS-cal ibrated wires (32

gauge) are compressed between 2 layers of electrically insulating film bonded to

the blocks with rubber cement. Silicone grease is used to hold the leads in

place during assembly as well as for thermal contact between the wires and the

bl ocks

After the original TC wires pass through the vacuum seals they are brought

to a terminal strip which is mounted on a heavy metal plate thermally insulated

33
- BOTH MOUNTED ON OUTER CHAMBER BASEPLATE -

AS = ALUMINUM

TB= NON-METALLIC TERMINAL BLOCK

Figure 12. Thermocouple Tempering Blocks.

34
inside a heavy metal enclosure called the Recorder Terminal Box. The TC wires

join the corresponding calibrated wires, which continue to an ice reference bath,

where they are joined to thermocoupl e-grade copper wires. The copper wires connect to the TC emf selector switches. Both Type K thermoelements as well as the

copper wire are 0.13 mm in diameter exclusive of electrical insulation. The

wires within the ice bath are wrapped around copper tubes over a length of 20 cm

for thermal tempering. The wires and copper tubes are divided into 3 groups and

placed in glass tubes filled with pure mineral oil. Rubber stoppers at the tops

of the tubes hold the wires from being pulled out and prevent ice from entering

the tubes during renewal of the ice bath.

Two additional thermocouples are used to gain information on conditions outside the stack. One is mounted on the fluid line between the TCP and the shroud

to indicate the coolant temperature at this point. The other is mounted to

obtain the cabinet temperature as a drift in this temperature may affect the performance of the enclosed electronic components. Figure 13 shows the location of

the measuring thermocouple junctions in the stack.

2.6.3 Selector Switches

The original 20-position selector switch was utilized for a time to read

thermocouple emf's and MH voltage/current. After about one year of use, it

became electrically noisy and was replaced by two double-pole rotary switches

specifically designed for microvolt level measurements. Experience has shown

that these switches, when mounted in an isothermal environment, are noise free to

well below 1 yV.

The switch positions and correspondi ng data are indicated in table 1. Refer

to figure 13 for thermocouple locations. Switch "A" allows the reading of data

germane to the operation of and effectiveness of guarding within the apparatus

35
36i

Figure

13.

Locations

of

Measuring

Thermocouple

Junctions

in

Stack.
Table 1. Information Obtained At Selector Switch Positions

Switch A Switch B (Switch B

=

0) (Switch A = 0)

Data Position Data

Open 0 Open

TCI (Envi ronmental Chamber 1 Shorted (Allows DVM Zeroing)

Terminal B1 ock)

TC2 (Shroud Inlet Coolant Line) 2 TC15-TC16 (Bottom Specimen Gradient)

TC4 (Cabinet Terminal Block) 3 TC18-TC17 (Top Specimen Gradient)

TC6 (KN-KN 1 ) For the calibration 4 Shorted (Allows DVM Zeroing)

of old vs. new

TC15

N

TC7 (KP-KP ') TC wires 5

TC9-TC18

N

6 TC16 ^Primary stack

temperatures

TC10-TC17 7 TC17

TC11-TC16 8

—i o

1—* <x> > Differenti al readings

TC12-TC16 9 MH Voltage

TC13-TC15 10 MH Current

TC14-TC15 11 TC3 (Temperature Controller Cabinet)
while Switch "B" yields the primary data used in computing thermal conductivity.

The control sensors are not shown in figure 13. All of the thermocouples used

for temperature measurement (not control) are referenced to an ice bath and the

differential readings are obtained by appropriate wiring at the selector switches.

2.7 Vacuum/Gas Back-Fill System

The principal change made to the vacuum system was the addition of two dial

gauges (DG and AG in fig. 14) to permit monitoring of gas pressures between 133

Pa (1 torr) and 1.33 x 10^ Pa (1000 torr) within the environmental chamber.

Two thermocouple gauges (TVG1 and TVG2) and an ion gauge (IVG), already present,

allow the measurement of pressures below 133 Pa (1 torr).

Absolute gauge AG is a precision Bourdon-tube (BT) type whose dial is calibrated to read absolute pressures up to 1.33 x 10^ Pa (1000 torr) with a resolution of 665 Pa (5 torr) over a dial scale displacement of 75 cm.

Differential gauge DG is also a BT type but whose dial is calibrated to read

up to 6650 Pa (50 torr) with a resolution of 27 Pa (0.2 torr) over a scale displacement of 75 cm. The gauge reads the differential pressure between the inside

of the BT and the space exterior to the BT but inside the sealed gauge body. By

means of valves V 6, V 7, and V 8 one can bring the gauge body to any desired pressure and then monitor departures from that pressure. Since the dial is not of

the center-zero type, however, it reads only positive changes from its initialized pressure. For this reason the gauge body is usually fully evacuated and

pressures up to 6650 Pa (50 torr) are read on it with greater precision than can

be done using AG.

The atmospheric entrance to valve V4 was somewhat inaccessible so valve V9

was added to facilitate back-filling the Environmental Chamber with any desired

gas.

38
AG = ABSOLUTE PRESSURE GAUGE

DG= DIFFERENTIAL PRESSURE GAUGE

DP = DIFFUSION PUMP

EC = ENVIRONMENTAL CHAMBER

FP = FOREPUMP

IVG = ION GAUGE

T = LIGUID-NITROGEN-COOLED TRAP

TVG2 =

! THERMOCOUPLE GAUGES

V., = ROUGHING VALVE

=

( VALVES TO ISOLATE DP V3 V4

= AIR-BLEED VALVE

V5

= AIR-BLEED VALVE

V6

= DG CASE EVACUATION

V7

= DG PRESSURE EQUALIZER

V8

= INLET TO DG

V9

= GAS BACK-FILL INLET

Figure 14. Vacuum/Gas Back-Fill System.

39
The above changes permit measurement of thermal conductivity as a function

of fill -gas species and pressure in gas-permeable specimens at any gas pressure

at and below atmospheric pressure. It is noted that the gas pressure must be

adjusted low enough to prevent condensation onto the cold plates. Condensation

causes serious errors in the measurement of effective conductivity because the

fluid migrates between the hot and cold plates.

3. DATA ACQUISITION AND CALCULATIONS

The purpose of this apparatus is to establish and measure equilibrium conditions of one-dimensional heat transmission through the metered areas of the two

specimens. Under certain conditions, specified by ASTM test method C- 177 , these

data can be utilized to define the effective thermal conductivity of the specimens.

The effective thermal conductivity, k, of the specimens in the guarded-hotplate apparatus can be determined from

. = A X =

VI_ AX

K 2A AT 2A AT ( 1 )

where Q is the rate of heat flow through the specimens found from the product of

heater voltage, V, and current, I; 2A is the combined area of the two specimens

each of area A; AX is the average thickness of the two specimens; and aT is the

average temperature difference between the hot and cold surfaces of both specimens. This value of k is usually associated with the mean temperature of the hot

and cold surfaces, T = ( T i+T 2 ) /2

The data required for these calculations are, thus, heat generated by the

MH, Q

= VI, the average thickness of the specimen, AX, the metered area, A, and

40
the temperatures of the specimen surfaces. After equilibrium has been established, as evidenced by the drift rate of the specimen surface temperatures

,

various data are recorded as indicated on the sample data sheet illustrated in

figures 15 and 16.

These figures are self explanatory in conjunction with table 1 and figure

14. The data are normally taken in triplicate approximately 30 minutes between

each set. This procedure is specified by ASTM C- 177 to assure equilibrium conditions. The data from figures 15 and 16 are entered into a computer and processed

as indicated by the steps listed in table 2. The output of this computer program

contains the variables involved in eq (1) for each equilibrium point as well as

the various conditions under which that run was conducted.

The conditions which can be varied and should be specified are: a) specimen

characterization, b) pressure and composition of the fill gas, c) plate emissivity, and d) hot and cold surface temperatures . Sets of output from this program

are further analyzed to yield dependencies of heat transmission on various parameters such as temperature, temperature difference, pressure, and density. These

final analyses are considered beyond the scope of this report and are not discussed further here.

4. ERROR ANALYSIS

The apparatus is designed for the determination of heat flow through the

specimens under a prescribed set of boundary conditions and the subsequent calculation of effective thermal conductivity, k, when applicable. The error (uncertainty) analysis will be directed toward determining the systematic uncertainty

and imprecision of k. Errors due to the inapplicability of the definition of k

and material variations (inhomogeneity) will be ignored, i.e., experimental measurement and control uncertainties will be the sole consideration.

41
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42

Figure

15.

Thermal

Conductivity

Data

Form

(Side

A)
OPERATOR

REFERENCE: Laboratory Book No. Pages:

ENVIRONMENT: Stack-to-shroud insulation: Vermiculite , Other ( ) Aux. Htr. Offset insulation: Fiberglass , Other ( ) Coolant: Water ; LN _ ; Other ( )

SPECIMENS: Conditioning: hrs at °C

SPECIMEN CODES: bottom ; top

SPECIMEN GUARD CODES: bottom ; top

Inner Grd.: Range: yV Noise BW (yV)

TAH : Wheatstone Bridge TAH

BAH: BAH

Outer Grd: Dial Setting:

ICE REF. FIXED AT IG/MH TP yV Top (+ to mid)

y V Bottom (- to mid)

TIME: REMARKS

Sl-A l-(TB-BJ)

Pos . 2-(Cool ' t)

MV 3-(TB-CAB)

(For all these TC readi ngs Sl-B must be in "0" position)

yV 4- ( KN- KN '

)

5-

( KP-KP

1 )

6- (9-18)

7

( 1 0- 17

)

8-

( 1 1- 16

)

9- (12-16

10-

(13- 15)

11-

( 14-15)

FLUKE CURR

(MA)

Figure 16. Thermal Conductivity Data Form (Side B)

43
Table 2. Outline of computer program for

analysis of thermal insulation data

1. Read experimental data

2. Compute main heater power

3. Compute temperatures of all thermocouples from calibration equation

4. Compute temperature differences

5. Compute mean temperatures

6. Correct specimen area for thermal expansion

7. Correct specimen thickness for thermal expansion

8. Compute average effective thermal conductivity from equation 1

9. Print and store results for further analysis

44
The operational definition for k, eq (1), shows that the uncertainties to

be considered are those in the control and measurement of V, I, AX, A, and aT.

Since k is usually associated with the mean temperature, T, its determination

also contributes to the total uncertainty. Probably the most subtle and difficult uncertainty to estimate is that of the heat flow through the specimen,

Q

= VI. This difficulty arises not through the inaccurate measurement of V and I

but rather mainly through the correct establishment of (a) stable and (b) unidirectional heat flow. The second most difficult aspect of this measurement is the

correct determination of the temperature difference across the specimen. Again

this is not caused primarily by calibration or read-out shortcomings, but rather,

due to the uncertainty in the effective plane of the thermocouples, the discontinuous nature of the specimen-plate interface, and deviations from planar isotherms. Estimates of random (imprecision) and systematic variations in the measured and calculated quantities for a typical run are listed in table 3. The

bottom line of the estimates in table 3 indicates an estimated imprecision, 2a,

of about ±0.4% and a systematic uncertainty of near ±0.6%. This imprecision is

approximately equal to the scatter in the data as reported in publications on

glass fiberboard and glass fiberblanket [l,2].l The overall uncertainty of ±1%

cannot be confirmed or refuted on the basis of comparison with certified data for

the glass fiberboard (1.5% disagreement), since the certified data are uncertain

by ±2%.

Table 3 contains uncertainty estimates for room temperature measurements.

To make similar estimates for liquid nitrogen temperatures, one notes that both

the measurement and control thermocouples are less sensitive by approximatel y a

factor of two. This decrease in sensitivity, combined with a reduction in heat

^-Numbers in brackets indicate literature references at the end of this paper.

45
Table 3. Estimates of random variations and

systematic uncertainties in the measured quantities at room temperature.

Vari abl

e

Absolute Variations Percent Vari ati ons random systematic random systematic

v Htr 200 yV 200 yV 0.01 0.01

! Htr 20 yA 20 yA 0.01 0.01

Q = (VI)Htr 60 yW 60 yW 0.01 0.01

Q(dri ft) a 1 mW 1 mW 0.2 0.2

Q(radial ) b 1 mW 1 mW 0.2 0.2

Q(total

)

2 mW 2 mW 0.4 0.4

VTC (calib.) 0 1 yV 0 0.1

V TC (meas.) 0.5 yV 2 yV 0.05 0.2

Vyc(radial

)

0 2 yV 0 0.2

VTC (total

)

0.5 yV 5 yV 0.05 0.5

T 0.02 K 0.2 K 0.007 0.07

AT 0.01 K 0.1 K 0.04 0.4

AX 0 0.05 mm 0 0.2

A 0 30 mm^ 0 0.2

k 0.2 mW/m*K 0.3 mW/m*K 0.4 0.6

a Effect of drift of the main heater plate as calculated from heat capacity and mass (corroborated by experiment) = 0.4% error in Q at a drift rate of 1 yV/h.

which equals a 0.016% error in Q for a drift rate of 1 mK/h.

^Effect of MH/IG gap offset as measured experimentally (corroborated by

calculation) is a 0.04% error in Q per yV of thermopile offset which also equals

a 0.04% error in Q per millikelvin of gap temperature offset.

46
flow by a factor of four (for typical glass fiber insulations), results in an

imprecision of about 2% and a systematic uncertainty of about 3%. For intermediate temperatures the imprecision and systematic uncertainty are estimated by linear interpolation of these extremes. These uncertainties in k have been calculated by first summing the absolute values of the errors in Q and V and then by

calculating the square root of the sum of the squares of the uncertainties in the

variables of eq (1). The latter calculation is justified since the errors in

these variables are experimentally uncorrel ated, i.e., they are measured by distinctly different instruments, except for Q = (VI) Htr , but that component of

uncertainty is small.

Corroborati ng experiments to support some of the estimates in table 3 have

been performed. For example, the resistance of the main heater (metered section)

has been calculated from V/I and plotted versus temperature. The scatter in the

resistances cover a range of ±0.03% as compared to the estimated value for the

scatter in VI of ±.02%. Offset runs of the inner and outer guards have been conducted to determine the sensitivity of the results to potential offsets within

detectability limits. The effect of intentionally introduced drifts in the main

heater plate has also been studied. Errors due to drift and offset are shown at

the bottom of table 3. These experiments tend to confirm the validity of the

estimates in table 3 as upper limits to our total error.

A potential systematic error due to deviations from uni -axial heat flow in

the metered region is not estimated. Flowever, it is noted that the ratio of

specimen thickness to the metered diameter is 1:4 which is usually accepted as

adequate to reduce this error to below 1%. In addition, the use of the aluminized mylar mini -shield surrounding the MH/IG assembly should further reduce this

systematic bias. For further information on this subject the reader is referred

to the work of Woodside [3].

47
5. SUMMARY

This report describes the modifications and performance of a guarded-hotplate apparatus designed and built to meet the specifications of the ASTM C- 177

test method. The purpose of these tests and modifications was to produce an

apparatus which could be used for state-of-the-art standard reference material

measurements. The major modifications performed are listed below in the order of

importance to precision and accuracy:

I. Improved the stability of the temperature controllers on the TAH and BAH.

a) Replaced unstable low signal level amplifiers with amplifiers having submicrovolt stability. These null detector amplifiers also provided the

operator with a clear indication of the system stability.

b) Replaced temperature control thermocouples with absolute resistive sensors. This eliminated the effect of drift from an ice reference bath or

an electronic ice reference as well as reduced the effect of spurious

emf's on the stability of the system.

c) Installed an adjustable power delivery network to better match the power

required by the TAH and BAH for more stable operation.

II. Improved the sensitivity and stability of the MH/IG gap controller.

a) Same as la above.

b) Replaced the original 8 pair Type K thermopile with a 20 pair Type E

thermopile. This resulted in a three fold improvement in control sensitivity of the gap offset.

c) Same as Ic above.

III. Replaced power and measurementati on instrumentation with highly stable and

accurate devices.

a) Replaced the main heater power supply with a unit having at least tenfold improved stability and accuracy.

48
b) Replaced DVM and power measuring network with units having at least tenfold improved stability and accuracy.

c) Replaced all original thermocouples with accurately calibrated thermocouples, replaced electrically noisy selector switches with high quality

sub-microvolt selector switches, and improved the thermal anchoring of

thermocouple wires to reduce spurious emf's.

IV. Provided improved isolation of the stack and coolant paths for increased

efficiency of operation.

a) Increased the spacing and insulation between the cold plates and shroud

and the warmer parts of the system.

b) Installed vacuum-insulated coolant lines.

c) Replaced the bell -jar environmental chamber with a vacuum insulated

dewar

.

The significance of the improvements described in this report can be appreciated by noting that the initial apparatus according to preliminary tests, produced results which varied by ±15%, and greater under some circumstances. The

improvements not only decreased the scatter and uncertainty to about one-fourth

their original values, but also provided the operator with an increased capability to detect when the system is not operating properly.

6. REFERENCES

[1] D. R. Smith and J. G. Hust, Measurement of Effective Thermal Conductivity of

a Glass Fiberboard Standard Reference Material, to be published in Cryogeni cs

.

[2] D. R. Smith, J. G. Hust, and L. J. Van Poolen, Measurement of Effective

Thermal Conductivity of a Glass Fiberblanket Standard Reference Material, to

be published in Cryogenics.

[3] W. Woodside, "Analysis of Errors Due to Edge Loss in Guarded Hot Plates",

ASTM STP 217, American Society for Testing and Materials, 1957, pp. 49-64.

49
N BS-1 1 4A (rev. 2 -aci U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 81-1657

2. Performing Organ. Report No. 3. Publication Date January 1 982

4. TITLE AND SUBTITLE

A Guarded-Hot-Plate Apparatus for Measuring Effective Thermal Conductivity of

Insulations Between 80 K and 360 K

5. AUTHOR(S)

D. R. Smith, J. G. Hust, and L. J. Van Poolen

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

Department of Energy

Oak Ridge Operations

Oak Ridge, Tennessee 37830

(Under Interagency Agreement DOE No. DE-AI05-780R05965

10.

SUPPLEMENTARY NOTES

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

11.

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

b i bl iography or literature survey, mention it here)

This report describes a guarded-hot-plate apparatus used to determine the

effective thermal conductivity of glass fiber insulations. Measurements can be

performed at temperatures from 80 K to 360 K, from atmospheric pressure to a vacuum of 10“^ Pa (1 x 10“^ torr). Various fill gases such as air, nitrogen,

argon, and helium can be utilized. Overall uncertainties of thermal conductivities at atmospheric pressure are 1% at the higher temperatures and 5% at the

lower cryogenic temperatures. The modifications of the commercial apparatus

described in this report resulted in approximately a four-fold improvement in

uncertainty.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

Guarded-hot-plate apparatus; insulation; low-temperature ; thermal conductivity.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[X[ Uni imited For Official Distribution. Do Not Release to NTIS 56

Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

QQ Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $8.00

USCOMM-DC 6043-P80 US GOVERNMENT PRINTING OFFICE 1982— 577 335/106 Region No 8