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NBS TECHNICAL NOTE 933
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and

technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the

Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — Laboratory Astrophysics 2 — Cryogenics 2 — Electromagnetics 2 — Time and Frequency'.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor Radiation — Physical Chemistry.

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THE INSTITUTE FOR COMPUTER SCffiNCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus wthin the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer

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THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process

to facilitate technological change in the private sector by examining and experimenting with Government policies and practices in order to identify and remove Government-related barriers and to correct inherent market imperfections that impede

the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units:

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Office of International Standards — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Located at Boulder, Colorado 80302.
-. M<JU,

An Infrared Technique

for Heat-Loss Measurement

=ett: ; j^

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D.M. Burch, T. Kusuda,

and D.G. Blum

Building Environment Division

National Bureau of Standards

Washington, D.C. 20234

Prepared for

Federal Energy Administration Washington, D.C. 20461

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U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

VARIATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1977
National Bureau of Standards Technical Note 933

Nat. Bur. Stand. (U.S.), Tech. Note 933, 52 pages (April 1977) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $1.10

Stock No. 003-O03-O1746-9
CONTENTS

Page

iv Nomenclature

Conversion Factors to Metric (S.I.) Units vi

1. Introduction 2

2. Description of Infrared Television System J

3. Description of Heat-Flow Reference Pad 3

4. Laboratory Heat-Loss Measurements n

5. Field Heat -Loss Measurements 21

6. Conclusions j_8

References 2Q

Appendix - Technical Considerations for the Design of a Heat-Flow

Reference Pad ^"^-

lli
NOMENCLATURE

A Surface area

C

f Fraction of energy emitted from front side

C Constant-pressure specific heat

P d Spacing between heating elements

E Total thermal radiation leaving a surface

Fo Fourier modulus, Fo = t/(p*L # C /h)

P

h Heat-transfer coefficient

2 H Biot number, H = h *d /k«L

I Electric current or isotherm level

k Thermal conductivity

L Thickness

m Heat generation rate per unit length

p Rate of internal heat generation per unit volume

q Heat-flux rate

r Object distance

s Edge length of square

t temperature

T Absolute temperature, T = t + 459.7

V Voltage

x distance

Z Transfer function for IR camera

iv
Greek Symbols

a Angle subtended by edge of square

9 Dimensionless temperature, = (t - t )/(p*L/h f

)

3.

e Emissivity

r\ Dimensionless distance, rt = x/d

T Time

p Density

a Stefan-Boltzmann constant

U Dimensionless temperature, U = (t - t )/(m»d/L»k)

3.

SUBSCRIPTS

a Ambient surroundings

b Back side

f Front side

h Hemispherical property

i Initial

n Normal property

R Heat-flow reference pad

s Building surface
Conversion Factors to Metric (S.I.) Units

Physical

quantity Symbol

To Convert

from To Multiply

by

Length X ft m .305

Area A ft

2 m

2 9.29 x 10~2

Volume f t

3 m

3 2.83 x

10~ 2

Temperature t Fahrenheit Celsius t

c=(t f -32)/l

Absolute

temperature T Rankine Kelvin 0.556

Temperature

difference At Fahrenheit AK 0.556

Mass lb kg 0.454 • 1.602 x 101 Density P lb/ft 3 kg/m3

Thermal

conductivity k Btu/h'ffF W/m-k 1.73

Heat-flow rate Btu/h W .293

Heat-flux rate q Btu/h.ft 2 W/m2 3.153

(or self-emitted

radiation) E

4.187 x 103 SDecific heat Cn Btu/lb-F J/kg-k

VI
AN INFRARED TECHNIQUE FOR HEAT-LOSS MEASUREMENT

By

D. M. Burch, T. Kusuda, and D. G. Blum

This paper describes a newly developed technique for estimating

heat-loss rate utilizing an infrared television system. A device called

a heat-flow reference pad was developed that makes it possible to estimate

quantitatively the heat-loss rate at the surface of a building without

the need for a conventional heat-flow meter to be mounted on the surface.

Technical considerations for the design of a heat-flow reference pad

are presented. The infrared measurement technique predicted heat-loss

rates in the laboratory and field within approximately 12%.

Key words: Heat-flow measurements; infrared heat-loss measurement

technique; measurement technology; thermography.

1 . INTRODUCTION

With the increasing scarcity of domestic energy resources and increasing cost of fuel for heating and cooling buildings, better insulation

of walls and roofs has become both a national goal and a matter of financial

concern to home and building owners. Since the presence of insulation and

the quality of workmanship used in its installation are generally not

visible, compliance with specifications and code provisions is not easily

ascertainable by conventional means. Therefore, at the request and under

sponsorship of the Federal Energy Administration, NBS has conducted research

to develop an infrared television technique for evaluating the heat-loss

characteristics of completed buildings, walls, and roofs.

An infrared television system senses the total thermal radiation

emitted and reflected from a surface. A thermal picture is produced on

a television screen in which the level of gray tones approximately

corresponds to surface temperature. Since such a system essentially

produces a temperature profile map of a surface, it is an effective tool

for locating and identifying major heat leaks of buildings [1,2,3]*. In

a recent study [k] conducted at the National Bureau of Standards, it was

shown that infrared (IR) thermography could be used as a survey technique

to distinguish between insulated and non-insulated wood-frame cavity walls.

This research was supported by the Federal Energy Administration.

* Figures in brackets indicate the literature references at the end

of this paper.

1
Recently there has "been much interest in developing an IR measurement technique that would permit the measurement of heat-loss rates from

the surfaces of "buildings. The development of such a technique would

make it possible to conduct surveys of the side walls of residences in

order to evaluate their heat-loss characteristics. Another application

for the IR heat-loss measurement technique would "be to obtain a heat-loss

profile map of a built-up roofing system, thereby permitting regions

having wet insulation to be located. These regions would have higher

heat-loss rates than regions having dry insulation.

Previous attempts to quantitatively measure heat-loss rates using

IR thermography have been frustrated by the difficulty of converting

surface temperatures into corresponding heat-loss rates. To perform this

conversion, a heat-transfer coefficient which depends on the convective

air movement at the surface as well as the radiation heat transfer with

surrounding surfaces must be accurately specified. The radiation exchange

is in turn a complex function of the emissivity of the surface, geometric

view factor, and the temperature of the surrounding surfaces. The inability to know all of these factors with certainty makes it extremely

difficult to predict accurately the heat-transfer coefficient.

The purpose of the present paper is to present preliminary findings

of a study to develop an IR measurement technique. A device called a heat-flow reference pad (HFRP) , was developed that makes it possible to

estimate the heat-loss rate through the exterior surface of a building.

The procedure for performing a heat-loss measurement at the surface

of a building is as follows: Both the building surface and the HFRP are included in the thermal picture of an IR television system. The frontside heat flux of the HFRP is adjusted until it appears the same as the

building surface in the thermal picture. The front-side heat flux is

adjusted by varying the electric power supplied to the device. At this

condition, the heat-loss rate from the HFRP will be very nearly equal to

the heat-loss rate from the building surface, providing the emissivity

and surface air velocity of the HFRP and the building surface do not

differ significantly.

The IR heat-loss measurement technique has several advantages over conventional heat-flow transducers for measuring surface heat-loss rates.

When using the IR heat-loss measurement technique it is not necessary

for a measuring transducer to touch the surface; therefore, the original

heat-flow rate is not disturbed. This feature becomes a significant

advantage when measuring the heat-loss rates through a building surface

having a low thermal resistance. Another advantage of the IR heat-loss

measurement technique is that it can provide a heat-flow profile map of

a building surface, providing that the emissivities of the components

that comprize the building surface are essentially the same. When a surface of a building has a wide variation of heat-loss rate, many

conventional heat-flow transducers are required to determine the heat

flow pattern.
2. DESCRIPTION OF INFRARED TELEVISION SYSTEM

A photograph of the IR television system used with the HFRP to

measure surface heat-flow rates is shown in figure 1. The equipment

from right to left consists of IR television camera, the black and white

television monitor, the color television display, and the temperature

profile adapter. The thermal radiation that is emitted and reflected

from a surface is sensed by the IR camera and converted into a video

signal. The IR camera consists of an optical scanning system with a liquid-nitrogen-cooled indium-ant imonide photovoltaic detector having

high sensitivity in the spectral range 2.0 to 5.6 micrometers.

The video signal from the IR camera is processed in the black and

white television monitor, where it is converted into a thermal picture

in which the gray tones of the picture directly relate to surface

temperatures. This thermal image is displayed on a television screen which may be photographed with a conventional camera. The video

signal is also fed into the color television display where the temperature range has been subdivided into ten regions, each color-coded with

a separate color. The temperature profile adapter selects a single

horizontal trace line* of the thermal picture of the black and white

television monitor and displays this line such that the temperature

change throughout the line is approximately represented as a vertical

displacement.

3. DESCRIPTION OF HEAT-FLOW REFERENCE PAD

In designing a heat-flow reference pad it is necessary to consider

the following features and factors:

° Uniform front-side heat flux with small edge effects.

° Fast response time.

° Small back-side losses.

° Sufficient size to permit its surface temperature to be

resolved by IR television camera.

° Measurement error that occurs when the emissivity of the

HFRP is different from that of the building surface.

° Measurement error that occurs when the convective heat

transfer at the surface of the HFRP is different from

that at the building surface.

° Measurement error due to the minimum detectable temperature

difference of the IR camera.

A major source of uncertainty in the IR technique for heat-loss measure- ment is the minimum detectable temperature difference of the IR camera,

which determines the precision to which the radiance level of the HFRP

can be adjusted to the level of a building surface. It is shown in the

appendix that, when emittance and rate of convective heat transfer for

both the HFRP and the building surface are identical, the measurement

* The thermal picture consists of a series of consecutive horizontal

trace lines displayed on a cathode-ray tube.

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error is given by ATm/ (Ts - Ta), where ATm is the minimum detectable

temperature difference and (Ts - Ta ) is the surface-to-air temperature

difference at the building surface. When the rate of convective heat

transfer is large or the heat-loss rate at the building surface is small,

the surface-to-air temperature difference becomes small and the uncertainty

in the heat-loss measurement becomes large. For instance, it was shown

that for a 15-mph wind condition the measurement error could be as high

as 48 percent for an insulated wood-frame wall. However, under still-air

conditions, the measurement error due to minimum detectable temperature

difference was shown to be less than 12 percent.

Other factors which cause measurement error are departures of the

rate of convective heat transfer and emittance for the HFRP from that of

the building surface. It is shown in the appendix that under still air

conditions, when the convective air movement at the surfaces of the heatflow reference pad and building surface are very likely to be approximately

the same, the error due to differences in emissivity will usually be less

than 5 percent. On the other hand, when measurements are performed under

high wind conditions, precautions need to be taken to insure that both the

building surface and the HFRP are exposed to the same convective conditions,

in order to obtain accurate measurements. It is shown in the appendix that

exposure of the HFRP to a 30-percent greater convective heat-transfer con- dition could result in a measurement error of approximately 19 percent.

A prototype heat-flow reference pad was constructed. The device,

shown in figure 2, consists of a thin-foil heater, having a thickness of

72 mils, spot-glued to a 3.875-inch thick polystyrene insulating board

(2x2 ft). The heater (10.87 x 10.59 in) contains electric heating

elements spaced 0.147 inch on center (see figure 3). The front-side

surface of the heater was spray-painted with two layers of matt-black

paint having an emissivity of approximately 0.96.

A schematic of the control and measurement system used to adjust

and measure the electric power delivered to the HFRP is shown in figure

4. A variable-voltage D.C. power supply delivers electric power to the

HFRP. When the switch is in the left position, the D.C. voltmeter

measures the voltage drop across the thin-foil heater. When the switch

is in the right position, the D.C. voltmeter measures the voltage drop across the 80-ohm resistor, permitting the current to the thin-foil

heater to be calculated from Ohm's law.

Two methods were used to determine the front-side heat flux for the

HFRP. The first method, voltage (V) and current (I) supplied to the

HFRP were measured and the front-side heat flux (q) was determined from

the relation:

q = C

f .Vl/A (1)

where C^ is the fraction of the electric power input to the HFRP that

appears as front-side heat flux, after a steady-state condition has been
Figure 2. Experimental prototype heat-flow

reference pad.
reached. The factor Cf is determined by subtracting the back-side

losses from the total power input and dividing by the total power

input. The surface area of the thin-foil heater is denoted by A. One

problem encountered in using this method was that the back-side losses

varied as a function of time with a rather long time constant, requiring

a long time to complete the successive adjustments of the HFRP.

The second method of measuring the front-side heat flux was to use three heat-flow meters that were spot-glued in a horizontal line across

the thin-foil heater. Each heat-flow meter consisted of a 2.0-inch

diameter, 0.13-inch thick, circular disk made of polyvinylchloride

filler material, having an embedded thermopile. The millivolt signal

generated by the thermopile is proportional to the heat flow through

the circular disk. The heat-flow meters were calibrated in an 8-inch

guarded-hot-plate apparatus conforming with the requirements of

ASTM Standard Method of Test C 177-71 [5]. These three heat-flow meters

were connected in series. The presence of these heat-flow meters has

a very small effect on the original heat-flow pattern of the HFRP

owing to the large thermal resistance of the back-side insulating

board. Heat-flow rates measured with these heat-flow meters were not

corrected for the thermal resistance of heat-flow meters.

Figure 3. Thin-foil heater,
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4. LABORATORY HEAT-LOSS MEASUREMENTS

Known heat-loss rates were produced in the laboratory and were

measured by the IR heat-loss measurement technique. In creating laboratory heat-loss rates, values were selected that were representative of

heat-loss rates through the walls of actual buildings.

A schematic of the apparatus for producing known heat-loss rates is

shown in figure 5. The device consisted of a small wooden rectangular

enclosure (1.51 x 1.51 x 2 ft). One face of the enclosure contained

a wooden frame that supported a piece of polystyrene insulating board of

known thickness. An 8-junction copper-const ant an thermopile was installed

across the insulating board as shown in figure 5. The exterior surface

of the insulating board was spray-painted with two layers of matt-black

paint having emissivity of approximately 0.96. An electric heater and

fan were installed inside the box as shown in figure 5. Various heatflow rates through the- insulating board were created in the laboratory

by varying the electric power supplied to the heater. Edge losses

from the insulating board were assumed to be negligibly small, since

the thermopile was located a distance from the edge equivalent to four

thicknesses of the insulating board.

The thermal conductivity of the insulation board was measured with

the guarded-hot-plate apparatus in accordance with ASTM Standard Method

of Test C 177-71 [5] and found to be 0.270 Btu* in/h-f

t

2 «F. The thick- ness of the board was measured and found to be 0.703 inch. The heatflow rate is related to the temperature difference across the insulating

board by the relation:

q/A = k-At/L (2)

where

q/A = heat-transfer rate, Btu/h-ft 2 (W/m2

) k = thermal conductivity, Btu/h-ffF (W/m« K)

At = temperature difference, F (K)

L = thickness, ft (m)

Substituting the above values into equation (2), the heat flux is given

by the relation, q/A = 0.384* AT. The foregoing equation assumes that

the heat transfer in the vicinity of the thermopile is one-dimensional.

The heat-loss rate of the insulating board was also measured with a calibrated heat-flow meter similar to the one used on the HFRP. This

heat-flow meter was spot-glued at the center of the exterior surface of

the insulating board. Signals recorded from this heat-flow meter were

observed to contain fluctuations due to convective air movements.

Therefore, this heat-flow meter was only used as a back-up measurement

for the thermopile heat-loss measurement. The heat-flow meter itself
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had a small effect on disturbing the original heat-flow pattern,

since its thermal resistance was approximately 0.03 times that of the

heat-flow path through the insulating board. Heat-flow rates measured

with the heat-flow meter were not corrected for the thermal resistance

of the heat-flow meter.

It was found that the greatest sensitivity (ability of the IR television system to detect temperature difference) could be attained when

the temperature-profile adapter of the IR television system was used. In

conducting an IR heat-loss measurement, the procedure used was to pass

the single trace line displayed on the temperature-profile adapter

through the image of both the HFRP and the apparatus for producing known

heat-flow rates. The front-side heat-flux rate of the HFRP was adjusted

until the vertical displacement of the trace line was the same for both

the reference pad and the apparatus. A photograph of the cathode-ray

tube of the temperature profile adapter at the conclusion of an IR heatloss measurement is shown in figure 6.

Comparisons of actual heat-flow rates created by the apparatus of

figure 5 and corresponding measured values using the IR heac-loss measure- ment technique are given in tables 1 and 2. For the measurements of

table 1 the front-side heat flux of the HFRP was predicted from its

electric power input rate and the calculated reverse-side heat loss,

whereas for the measurements of table 2 the front-side heat flux was determined by using three heat-flow meters attached to the HFRP.

When the front-side heat flux of the HFRP was determined by measuring the electric power delivered to the HFRP (table 1) , the accuracy of

the IR heat-loss measurement technique was found to be within approximately 10%. The table indicates a tendency for the heat-loss values

determined from the IR heat-loss measurement technique to be higher than

corresponding measured values, due to the fact that insufficient time

was allowed to attain a steady-state condition. Initially a significant

portion of the electric power input to the HFRP appears as back-side

losses. The transient back-side loss requires a long time interval to

decay to a small steady-state value.

When the front-side heat-flux rates of the HFRP were determined by

the three heat-flow meters (table 2), predicted heat-flow rates determined by the IR heat-loss measurement technique were within approximately

6 percent of measured values.

5. FIELD HEAT-LOSS MEASUREMENTS

The IR heat-loss measurement technique was used to measure the

heat-loss rate at the exterior wall surface of Building 226 at NBS. The

wall was constructed of fire-clay brick having an emissivity of approximately 0.94 (see table A-l in the appendix). It should be pointed out

that the particular wall studied was the rear wall of a recessed loading

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platform, so that the surface being investigated was partly protected

from wind gusts. A photograph showing the experimental set-up for the

field heat-loss measurement is shown in figure 7.

The thick heat-flow meter that was attached to the HFRP for the

present study had a thermal time constant of approximately 15 minutes

and required considerable time to reach a steady-state condition. For

the final prototype HFRP, it is recommended that this heat-flow meter be

very thin and thereby have a small time constant.

Upon moving into the field, it was observed that the measured

heat-loss rate through the building wall and the HFRP contained rather

large fluctuations due to convective air movements at the exterior surface.

It was, therefore, necessary to feed the signals from both the heat-flow

meter mounted on the building surface and the three series-connected heatflow meters attached to the HFRP into strip-chart recorders. Figure 8

shows variation in signal observed from the heat-flow meters of the HFRP.

It is recommended that the final prototype HFRP be used with an analog

integrator for averaging the signal from the heat-flow meters of the

HFRP. Similar variations in heat-flow rate were observed by use of the heatflow meter attached to the building wall.

For the field heat-loss measurements, the front-side heat flux of

the HFRP was determined by using the heat-flow meters. A comparison of

measured wall heat-loss rates and corresponding values determined by the

IR heat-loss measurement technique is given in table 3.

TABLE 3

Field Heat-Flow Measurements*

Test

No.

Heat-Flow Rate, Btu/h«ft2 Error

Wall HFRP %

1

2

3

5.1

5.5

6.3

5.7

6.1

6.2

+11.6

+10.9

- 1.6

*using heat-flow meters attached to HFRP

At the time of the field and laboratory measurements, the authors

were unaware of the very long period required for the heat-flow

meters of the HFRP to reach a steady-state condition. Much of the error

for test numbers 1 and 2 of table 3 was believed to be due to the

existence of non-steady conditions. During the initial successive

adjustments of the HFRP, the front-side heat flux of the HFRP was increased in a stepwise fashion. The temperature difference across the

heat-flow meters is initially large and decays slowly to a reduced

steady-state value. For test number 3, it was believed that sufficient

time had elapsed for a near steady-state condition to be attained.

15
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30 40

TIME, SEC.

Variations in signal observed from the

heat-flow meters of the HFRP.

17
For the laboratory measurements presented in the previous section,

still-air conditions were present. Errors due to surface convective

heat transfer were probably small. However, for the field measurements,

some differences in convective heat transfer at the surfaces of the HFRP

and the building surface may have existed, thereby giving rise to measure- ment error. Air velocities at these surfaces were not measured, so it was not possible to investigate errors due to differences in surface

convective heat transfer.

Of particular importance is the time required to perform a heatloss measurement using the IK. technique. For a final prototype HFRP

(one utilizing a fast heat-flow meter), the total measurement time would

include the following:

o successive adjustments of HFRP % 5 minutes

o conditioning period for heat-flow meter to reach a steady-state

condition ^ 1 minute

o sufficient period to integrate heat-flow signal ^ 5 minutes

Note that the total time to perform a heat-loss measurement using the

IR technique could be reduced to 11 minutes, provided a fast heat-flow

meter is utilized.

6. CONCLUSIONS

A technique for estimating heat-loss was presented that utilizes

an IR television system. A device, called a heat-flow reference pad

(HFRP), was described which makes it possible to estimate the heat-loss

rate at the surface of a building without the need for a conventional

heat-flow meter to be mounted on the surface. Technical considerations

for the design of a HFRP were presented.

The IR heat-loss measurement technique predicted surface heat-loss

rates in the laboratory within approximately 10%. When the front-side

heat flux of the HFRP was determined using heat-flow meters as opposed

to measuring its electrical power input, the accuracy of the IR heatloss measurement was improved to approximately 6%. For a final prototype HFRP, it is recommended that the front-side heat flux of the HFRP

be measured with a thin heat-flow meter having a small time constant.

This would significantly shorten the time required to perform successive

adjustments of the HFRP necessary in conducting an IR heat-loss measurement,

18
In the field the IR heat-loss measurement technique predicted

surface heat-loss rates within approximately 12 percent. Outdoor con- vective air movements caused large fluctuations in the front-side heatflux rate of the HFRP. it is recommended that the final prototype HFRP

use an analog integrator for averaging the signal produced by the heatflow meter mounted on the HFRP.

A major source of uncertainty in the IR technique for heat-loss measure- ment is the minimum detectable temperature difference of the IR camera.

It was shown that for a 15-mph wind condition, the error due to minimum

detectable temperature differences could be as high as 48 percent for an insulated wood-frame wall. However, under still-air conditions, the

measurement uncertainty would be less than 12 percent.

An analysis of the error due to differences in the rates of connective heat transfer and surface emissivity at the surfaces of the HFRP

and the building surface was performed. It was shown that for the range

of emissivities of most common building materials, under still-air

conditions, the measurement error would probably be less than 5 percent.

However, under windy conditions, a HFRP placed in front of a wall may be

more exposed to wind than the wall. It was shown that exposure of the

HFRP to a 30-percent greater convective heat-transfer condition under

high-wind conditions could result in a measurement error of 19 percent.

The error analysis showed that the most accurate measurements could

be attained under still-air conditions. Under such conditions, the net

error due to the combination of minimum detectable temperature difference

and emissivity would be less than 13 percent.

7 . REFERENCES

1. Ivar Paljak and Bertil Pettersson, "Thermography of Buildings,"

Swedish Council of Building Research, Lund Institute of Technology,

Lund, Sweden, 1972.

2. Claus G. Bankvall, "Investigation of Temperature and Thermal

Insulation by Radiation Measurements," Report 11, Division of Building

Technology, Lund Institute of Technology, Lund, Sweden, 1969.

3. R. H. Munis, R. H. Berger, S. J. Marshall, and M. A. Bush,

"Detecting Structural Heat-Losses with Mobile Infrared ThermographyPart I Description of Technique - Part II Survey of Pease Air Force

Base, Portsmouth, New Hampshire - Part III Survey of USACRREL,"

Research Reports 326, 338, and 348, Cold Regions Reserach and

Engineering Laboratory, Hanover, New Hampshire, 1975.

19
4. D. M. Burch and C. M. Hunt, "Retrofitting an Existing Wood-Frame

Residence for Energy Conservation—An Experimental Study," NBS Report

in review.

5. ASTM C 177-71, "Standard Method of Test for Thermal Conductivity of

Materials by Means of the Guarded Hot Plate," 1974 Annual Book of

ASTM Standards, Part 18, American Society for Testing and Materials.

20
APPENDIX

Technical Considerations for the Design

of a Heat -Flow Reference Pad

Technical Considerations for the Design

of a Heat-Flow Reference Pad

1

.

Introduction

The purpose of this section is to provide an analysis of various

features which must he considered in the design of a heat-flow -reference

pad (HFRP).

2. Back-Side Losses

An important feature of a heat-flow reference pad is that its

hack-side heat losses he small, so that most of the power released "by

the electric heater should appear as front-side heat flux. Under steadystate conditions the fraction of the electric input power (c f

) that

appears as front-side heat flux is equal to the thermal conductance

of the front-side heat-transfer path divided by the sum of the

conductances of the front-side and hack-side heat -transfer paths, or

Cf

= (A-l)

h~ + 1

f l/hb

+ L/k

Here hf and h. are the front-side and hack-side heat-transfer coefficients;

L and k are the thickness and thermal conductivity of the insulating

board,' respectively. This equation assumes one-dimensional steady-state

heat transfer. The thermal resistance of the insulating board (L/k)

should be. selected to be as large as possible, so that the front-side

factor (Cj is very nearly unity.

3. Uniformity of Front-Side Heat Flux

Another important feature of a HFRP is that it should produce a very

uniform front-side heat flux with small edge effects. To analyze the non- uniformity of the front-side heat flux, the heater of the HFRP was re- placed with the simplified mathematical model shown in figure A-l. The

separate heating elements are replaced with line-source heating elements

separated by a distance (d) . Heat conduction from the heater into the

insulating board i,s neglected; thus, the back-side surface is treated

as adiabatic. Since the heater is very thin, temperature gradients

across the heater (perpendicular to the surface of the heater) can be

neglected. The rate of heat release for each line-source heating

element is denoted by 2m.

A-l
LINE SOURCE

HEATING

ELEMENTS

- , dt, d ,, dt. , k, di

,L -dr (k"dr ,

- dx,L

AIR

I Theater j

INSULATING

BOARD

LINE SOURCE

HEATING ELEMENTS

Figure A-l. Simplified mathematical model for thin-foil heater.

When the back-side losses are small, a heat balance on an elemental

slice of the fin may be approximated with the second-order differential

equation:

hf .(t-t

a

) = k-L.^J (A-2)

The coefficient of heat transfer (hf) is an overall heat-transfer coefficient which includes both the effects of radiation and convection. The

foregoing differential equation must satisfy the boundary conditions:

L*k"-r- = m at x = dx

dt

dx

= at x = d/2

(A-3)

(A-4)

Introducing the following dimensionless parameters

n = x/d, o = (t - t )/(m-d/L«k), and H = h «d /k«L)

3. L.

A-2
we obtain the dimensionless equations

u d

2 u H * u = —

T

(A-5)

dn 2

|^=0 at n = 1/2 (A-6)

du^ = -1 at n - (A-7)

The general solution to (A-5) is

u = A-cosh(/H*n) + B-sinhCv^-n)

Application of (A-6) and (A-7) gives

u - l. cosh[/H-(l-2n)/21

u - — Q ^ (A-8)

/H sinh[/H/2]

1 ,1 - ! cosh (v^l/2)-lr

, 9

x

|n = jn = % /H sinh (/H/2)

As the parameter (h f -d2 /k-L) becomes small, the temperature difference

along the surface becomes very small. This may be accomplished by

decreasing the spacing (d) between the separate line-source heating

elements and increasing the thermal conductivity of the heater. The

thin-foil heater used for the present study has line-source heating

elements spaced 0.147 inch on center.

A sample calculation was performed for the thin-foil heater. Taking

the overall heat-transfer coefficient to be h

f = 1.46 Btu/h*ft 2, F, the

thermal conductivity of the heater to be k = 6.233 Btu/h»ffF, and the

thickness of the heater to be L = 0.01225 ft, the maximum temperature

variation is found to be At = 0.44°F at a front-side heat-flux rate of

3.415 Btu/h*ft2 .

The temperature-profile adapter was used to measure the temperature

profile across the thin-foil heater of the HFRP. As seen in figure A-2,

the temperature is uniform across most of the heater, with the exception

A3
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-P

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-P

ft

CMI <U

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A- 4
of a narrow region at the edges. The temperature variation within

the center portion of the profile is seen to be within a 0.36°F

temperature band. It should be pointed out that the resolution of

the camera is no better than 0.36°F. Since the heat-loss rate at a surface is proportional to the surface-to-air temperature difference,

a uniform surface temperature implies a uniform heat-loss rate. Thus,

these measurements suggest that a very uniform front-side heat flux is

being produced by the HFRP.

4. Response Time

Another important feature for a HFRP is that it have a fast response

time; that is, very little time be required for the front-side heat flux

to reach equilibrium after changing the input power from one level to

another. A fast response is desirable so that the successive adjustments required for an IR heat-loss measurement will not be time consuming. For a thin-foil heater mounted on a perfect insulating board,

the electric power released by the thin-foil heater must be either lost

by convection to the ambient air or stored within the heater. An energy

balance of the system results in the following expression:

p.A-L - A.hf .(t - t ) = A-L-p-CD -~ (A-10)

where p = heat generation rate per unit volume

A = surface area of the heater

L = thickness of the heater

t = temperature of the heater

t a

= temperature of the ambient air

hf= overall heat-transfer coefficient

p = density of the heater

C = specific heat of the heater

P

T = time

Here hf is the overall heat-transfer coefficient which includes both

radiation and convective heat transfer rates. The heater must satisfy

the initial condition:

t = t i for t <_ o

Introducing the following dimensionless parameters :

9 = (t - t )/(p-L/h), Fo = T/(p-L-C )/h ), and V = (t . - t )/(p-L/h,)

A-5
we obtain the dimensionless relations

d0

dFo

+9=1 (A-12)

G = V for Fo < (A-13)

which has the solution:

0=1+ (V-l)-e-Fo (A-14)

The response of the pad is seen to be a first-order exponential function

with a thermal time constant (p-L-Cp/hf). The response time can be reduced

by decreasing the time constant. This may be accomplished by making the

heat capacity per unit area (p-L'Cp) small. In actual practice, when a thin heater is attached to an insulating board, back-side losses and

thermal storage within the insulating board have a significant effect on

the response of the HFRP.

The actual response of the experimental-prototype HFRP to a step

increase in the rate of electrical-power input is shown in figure A-3

.

The normalized front-side heat-flux rate (q/qf) is plotted as a function

of time in minutes, where qf is the final steady-state front-side heatflux rate. The actual response measured with the IR television camera is

in good agreement with the theoretical response predicted by equation (A-14)

during the first portion of the transient adjustment. After approximately

one minute the actual measured response begins to lag behind the theoretical response, due to the back-side losses of the thin-foil heater.

The theoretical response as predicted by equation (A-14) treats the insulating board as a perfect insulator; that is, heat transfer into the insulating board is neglected. The sluggish response of the actual HFRP, due to

the rather large time required for the back-side losses to reach a steadystate condition, poses a serious problem in determining the front-side

heat-flux rate from the rate of electrical-power input to the HFRP.

The measured response determined by the heat-flow meters is also very

slow, owing to the rather large time constant of the heat-flow meters.

The results of figure A-3 indicate that it is necessary to wait 40 minutes

before a steady-state condition is reached. The response may be substantially reduced by using a thin heat-flow meter having a short time

constant.

5. HFRP Size Requirements

It is also important that the size of a HFRP be large enough to

permit its temperature to be resolved by the IR camera. When attempting

to view very small objects with an infrared television system, the level

of the video signal obtained for the thermal image of such objects may

introduce temperature measurement errors. These errors become significant

only when the angle subtended by the object approaches the order of

magnitude of the IR camera's instantaneous f ield-of-view. What happens

A-6
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is, the object image size in this case becomes smaller than the IR

detector with the result that the detector no longer "sees" only radiation

emitted by the object of interest, but also radiation coming from sources supposedly eclipsed bjr it during the instant of scanning.

Since the detector of the IR camera is square, the instantaneous

scanning spot viewed by the detector is also square. Based on information*

supplied by the manufacture of the IR camera, the surface temperature of

a square-shaped object can be resolved when an edge of the square subtends

an angle more than 4.5 x lO- -^ radians for the 10° field of view lens and

more than 11.0 x 10~3 radians for the 25° field of view lens. The edge

length (s) of the smallest square for which the temperature can be

resolved is related to the object distance (r) by the relation:

s = x r (A-15)

where x is the angle subtended by an edge of a square scanning spot. A

plot of the minimum size square (expressed in edge length) for which the

surface temperature can be resolved as a function of object distance is

given in figure A-4 . Note that the surface temperature of a 1-ft. square

surface can be resolved at 91 feet with the 25° field of view lens.

The above analysis is based on the assumption that a perfect lens and

scatter-free scanning system is used. No real optical system is perfect.

There is always some blurring of the image due to diffraction. To this

must be added the blurring due to aberrations in the lens and scattering

at each reflecting or refracting optical surface. Also, since a dynamic

scanning system is involved, the time constant of the electronics (detector,

amplifier and recorder or display) will make a contribution to the recorded

or displayed image.

Assume that a vertical bar of uniform radiance, higher than the radiance

of the background by AL, is scanned by a noise-free scanner with perfect

optics and electronics with a zero time constant. If the band is of a width such that its image exactly fills the detector, the signal produced

as its image is scanned across the detector will be two sides of an isosceles triangle, of width equal to twice the width of the detector

and the image of the bar, and of height AL. With a real system, the

image of the bar will be enlarged, because of the blurring at its edges,

and the signal produced by a noise-free system of zero time constant will

be distorted. The corners of the triangle will be rounded, so that its

base is longer, and its peak lower than those of the perfect system.

However, the area under the two curves will be the same. When noise is

added to the signal, and particularly when the signal-to-noise ratio is

low, as it is with the infrared scanner, it becomes very difficult to

evaluate the height of the peak. In essence the uncertainty is equal to

approximately half the width of the noise band.

AGA Operation Manual for Thermovision System 680 , AGA Corporation,

550 County Ave., Secaucus, NJ O709I+.

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When an area of uniform radiance of width such that its image is twice

the width of the detector is scanned, its ideal signal will be in the form

of a truncated triangle with a base equal to three times the width of

the detector. With a real noise-free system, the signal still has a flat top at a height equivalent to AL, of width equal to slightly less

than that of the detector. When noise is added to the signal, it can be

averaged over the flat peak, and the uncertainty is reduced, by an amount roughly proportioned to the square root of the length of the

flat top.

6. Effect of Surface Emissivity and the Rate of Convective Heat

Transfer on IR Heat-Loss Measurement

This section presents a theoretical analysis for predicting the error for an IR technique for measuring heat-loss when the emissivity of

the surface is different from that of the reference pad, and when the

surface and the reference pad are exposed to different rates of convective heat transfer. Several sample problems are then worked out.

The thermal energy (E )* radiated from a surface is equal to the

g self-emitted radiation due to the temperature of the surface and reflected

thermal radiation from surrounding surfaces, or

Es = e s -a-T s 4 + (1 - e s

) •

e

a -a-Ta

4 (A-16)

where E = thermal flux radiated from a surface

s

£ = emissivity of surface

s

All surfaces are treated as gray bodies which are opaque to

infrared radiation.

A- 10
T = absolute temperature of surface

e a

= emissivity of surrounding surfaces

a = Stefan-Boltzmann constant

T = absolute weighted-average temperature of the surroundings,

3. Radiation in the visible spectrum has very little energy in the thermal

range and will therefore not contribute to the thermal radiation exchange

process given by equation (A-16) . Similarly, the thermal flux (E ) radiated from a HFRP is given by:

ER

= e R

-a-TR

4 + (1 - e R)-e a

-a-T

a

4 (A-17)

In the IR technique for measuring heat loss, both of these surfaces are included in the thermal picture. The difference between the level of

video signals for the surface and the HFRP (AIRC.) is approximately equal

to:

AI

RS

- Z.(ER

- Eg) (A-18)

where Z is the transfer function for the IR television camera.

Substituting equations (A-16) and (A-17) into (A-18) gives

AIRS = Z.[e R-a-TR

4 + (1 - e R

) •

e

a .a-Ta

4

4. a

- e s 'o--V " (1 " eJ-£a -a-T„ ] (A-19)

Approximating the emissivity of the background to be unity, e = 1.0,

and defining f (T) = Z-a-T 4

, we have that

AIRS - e R[f (TR

) - f (T a )] - e s

[f'(T s

) - f (T a

) ] (A-20)

The function f(T) can be determined experimentally and is shown for the

IR television camera used for the present study in figure A-5 which can be represented by the relation f (T) - -86.3 + 0.2-T over the temperature

range (470 < T < 495°R or 10 < t < 35°F). In the IR technique for measuring

heat loss, the reference pad is adjusted so that it has the same radiance

temperature as the surface (Al = 0). Setting Al = 0, we obtain:

Ko Kb

f (TR

) = ^-f (T ) + (1 - -%'f (T a ) (A-21) k e R

s e R

Substituting the linear relation f(T) - -86.3 + 0.2-T into equation (A-21),

the following relation is obtained :

e R'<TR " V = £ s'(Ts " T a) ( A" 22 )

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The heat-loss rates from the HFRP and the surface are given by:

q s = e s -a-(Ts

4 - T a

4

) + hs -(T s - Ta ) (A-23)

1r = e R' a '(TR

4 " T a

4

) + hR

'(TR

- T

a

) (A-24)

where hs and hR denote the convective heat-transfer coefficient of the

surface and the reference pad, respectively. The radiation terms in

the foregoing equations may be linearized by noting that

e s .o--(T s 4 - T a

4

) - e s -F-(T s - Ta ) for T

g

- T

a

= T

(A-25)

e s -o-(TR

4 - Ta

4

) - £

S 'F-(TS - T a ) for TR

- T

a

= T

(A-26)

3 where F = 4«a»T . Over the temperature range (470 _< T < 495 °R or

10 < t < 35°F), F is a weak function of T and may be treated as a constant

for the purposes of deriving a simple error expression. Substituting

the foregoing linearized radiation terms into equations (A-23) and (A-24) gives

q s = (e s .F + hs ).(T s - Ta ) (A-27)

qR

= (e R

-F + hR)-(TR

- T

a

) (A-28)

The error is defined by the relation:

qs : qR

Error = (A-29)

q s Substituting (A-27) and (A-28) into the foregoing equation yields

But,

whence

,

(e

s -F + hs ) - (e R

-F + hR) Error = X^L- —^ (A-30)

e s .F + hs

^R ^a = fs ( e q. A-22)

T

S " T

a

£R

V£s " hr /e* A(h/e)/(h /e ) Error = _^_§ r_R =

8 s (A_ 31)

h

s /e

s +F 1+F-e/h.

A- 13
Thus, we see that the error in the infrared technique for measuring

heat loss is the percent deviation of the factor (h/e) for the pad

from the corresponding factor (h/e) for the building surface, divided

by the factor (1 + F-e c /h ). & s The foregoing error expression is plotted in figure (A-6) . This

figure shows that the effect of the factor (h^/e ) for the HFRP departing

from the factor (hs /£ ) for the building surface decreases as the

outdoor convective conditions approach a still-air condition. For

instance, at approximately a still-air condition (hs /e s = 0.5), when

the factor (h^/eft) for the reference pad deviates 10 percent from the

value for the building surface, the error is only 3 percent. However,

at approximately a 15- mph wind condition the same deviation produces

an error of 8.7 percent. This analysis clearly indicates that the

optimum outdoor condition for which the most accurate heat-loss measurements can be performed are still-air conditions.

Perhaps the most crucial issue that needs to be addressed is the

typical accuracy that one might expect to achieve in performing a field measurement. To assist in several sample calculations, infrared

emissivities for various building surfaces and paints are given in

tables A-l and A-2 , respectively.* Several sample calculations are given in the following example;

Consider an exterior wood-frame wall shown in figure A-7 . Take

the outdoor temperature to be 10° F. The percent error is calculated

below for still air and 15'mph conditions for cases in which the

emissivities of the building surface are 0.90 and 0.85. The emissivity

at the surface of the HFRP is taken to be 0.96.

Still-air conditions

For still- air conditions, it is very likely that the heattransfer coefficients at the building surface and the HFRP would be

very nearly the same. Hence, h^ = h = 0.5.

1. For the case e s = 0.90

hc /e = 0.5/0.90 = .556

s s

hR/eR

= 0.5/0.96 = .521

A(h/e)/(hs /e

s

) = .063

Error =2.5% (figure A-g

)

See pages A-22 , A-2 3, A-2U.

A-14
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2. For the case e s = 0.85

hs /e s = 0.5/0.85 = .588

hR

/eR

= 0.5/0.96 = .521

A(h/e)/(hs /e s ) = .114

Error =4.7% (figure A-6)

15-mph condition

For this sample calculation, it shall be assumed that the HFRP

is in a more exposed condition. The heat-transfer coefficient at

the surface of the HFRP will be taken to be 30 percent higher than

the value at the surface of the building. Therefore, hs = 1.3(5) = 6.5 Btu/h-ft 2 -F.

1. For the case e s = 0.9

hs /e

s = 5.0/0.9 = 5.56

hR

/£R

= 6.5/ - 96 = 6.77

A(h/£)/(hs /£ s

) = -.218

Error = -19.1% (figure A-6)

2. For the case e s = 0.85

hs /£ s = 5.0/0.85 = 5.88

hR /eR

= 6.5/0.96 = 6 . 77

A(h/e)/(hs /e

s) = _.i5i

Error = -13.3% (figure A-6)

The foregoing sample calculations suggest that, when the infrared

technique is used to perform heat-loss measurements under still- air

conditions, the error will be quite small and will probably be less than

5 percent. On the other hand, when measurements are performed under

high-wind conditions, precautions need to be taken to insure that

both the building surface and the HFRP are exposed to the same con- vective conditions, in order to obtain accurate measurements. It was

shown that exposure of the HFRP to 30 percent greater convective

heat-transfer conditions could result in a measurement error of

19 percent.

A-17
7. Errors in the IR Technique for Heat-Loss Measurement Due to the

Minimum Detectable Temperature Difference of the IR Camera .

As the surface temperature of an object is reduced, the amount of

radiated energy is reduced. The minimum temperature difference which

can be detected by an IR television camera depends on the ratio AE/AT,

where AE is the change in radiated energy and AT is the change in object

temperature. Over the spectral range for the IR camera, 2 to 5.6 um,

the ratio (AE/AT) decreases as the object temperature decreases. Therefore, the minimum temperature difference which can be detected by an

IR camera increases as the object temperature decreases. A plot of the

minimum detectable temperature difference for the IR camera used for the

present study as a function of object temperature is given in figure A-8.

The accuracy of the IR technique for heat-loss measurement strongly

depends on the precision to which the radiance temperature level of the

HFRP can be adjusted to the level for a building surface. The uncertainty

in performing this adjustment will in turn depend on the minimum detectable

temperature difference.

When the emittance and rate of convective heat transfer for the HFRP

and the surface of a building are identical (h = h and e = e ), the

expression for error given by eq. A-30 reduces to:

T - T

Error = -= — = AT /(T - T ) (A-32)

T - T m s a

where AT is the minimum detectable temperature difference for the IR camera and (T - T ) is the temperature difference between the building

surface and the ambient outdoor air.

Under steady-state conditions, the heat-flux rate (q/A) through a wall is governed by the relation:

q/A = U • (T. - T ) = h-(T - T ) (A-33) n l a s a

where U = thermal transmittance of the building component

h = overall heat-transfer coefficient

T. - T = air-to-air temperature difference across the building

component

T - T surface-to-air temperature difference.

S 3.

A-18
i

o

roOCVJ

o 0} o oJO

o S

0> 0)

<u o .1-1 00

La. U

o o

lu eratu ture. o •=> a

g

CO £ C a) cc

»le t

O tem LU

Q_2

LU +i u o H «- 1- 0! O

o

OLU

CD

mum

d

tion

of

i

o

o

CM

I^H <+-4 o • 00

1 <

o

oCVJ

I

<0 •o *: to cm do^dlQ dl/\G± 3naVlD313Q wwiiniin

A-19
Introducing equation (A-33) into (A-32) gives

Error = AT -h/U- (T . - T ) m 1 a (A-34)

The crucial issue to be examined is the amount of measurement error

due to minimum detectable temperature difference that occurs in typical

applications. Measurement error due to minimum detectable temperature

difference is investigated for an uninsulated wood-frame wall (U = 0.2

Btu/h-ft 2 -F) and an insulated wood-frame wall (U = 0.065 Btu/h-ft2

-F)

.

Measurement errors were calculated for these two walls using equation A-34

for still-air conditions (h = 1.46 Btu/h-f t *F) and 15-mph wind conditions

(h = 6.0 Btu/h'ft^'F) . For these calculations, the temperature of the

indoor air was taken to be 75°F. Calculations were performed for outdoor

air temperatures of 32°F and 0°F. The results of these calculations are given in table A-3.

Table A-3.

Measurement Errors Due to Miniumum

Detectable Temperature Difference

Still-Air Condition

(h = 1.46 Btu/h-ft 2 -F)

15-mph Condition

h = 6.0 Btu/h-ft-F

Description of Wall Temperature, °F Temperature, °F

32 32

Insulated wall _ (U = 0.065 Btu/h-ft -F) 11.7 11.4 48.3 46.7

Non- insulated wall

(U = 0.2 Btu/h-ft -F) 3.8 3.7 15.7 15.2

The results dramatically illustrate that measurement error due to the

minimum detectable temperature difference of the IR camera increases substantially as the rate of convective heat transfer at the surfaces of the

HFRP and the building increase. This is due to the fact that the surfaceto-air temperature difference decreases as the rate of convective heat

transfer increases. Also, the IR technique can more accurately measure

heat-loss rate for non-insulated walls than for insulated walls. Again,

this finding is due to the fact that the surface-to-air temperature

difference decreases as the heat-loss rate decreases.

It was pointed out during the review of this paper that the infrared

television system used for the study could have been modified to permit

distinction of much smaller temperature differences. One way of accomplishing

A-20
this would have been to specially process the video signal of the single

trace line displayed on the cathode-ray tube of the temperature profile

adaptor. By averaging the signal from various portions of the trace

line, the signal-to-noise ratio could have been enhanced, permitting

smaller difference in signal level to be detected. However, such modifications were beyond the scope of the present study.

The error analysis presented in the last two sections showed that

much more accurate measurements could be attained under still-air conditions, it was shown that an upper limit on the error due to emissivity

was 5 percent and the error due to minimum detectable temperature difference was 12 percent. The net error due to the combination of these sources

is given by the square root of the sum of the squares of the separate

error sources. Thus, the net error due to these sources under stillair conditions is 13 percent.

A-21
TABLE A-l

Infrared Emissivities of Common Building Surfaces

Emissivity

Total Normal Hemispherical

Building Surface E (°F) Ref. eA°F)

n h

Ref.

BRICK

Brick wall

red rough

type not specified

red

94 (68)

93 (70) 0.93-0.95(32-392) 5 0.93(70) 6

0.96(68) 3

0.93(32-199) 4

WOOD

type not specified

sanded spruce

oak, planed

.80-0.9(68)

0.91(100)

0.82(199)

.895(32-199)

.89(32-392)

.895(70)

CONCRETE

rough 0.94(32-199)

PLASTERING MATERIALS

Gypsum

Plaster, rough lime

Plaster

Plaster, rough

0.9(100)

0.8-0.9(68)

0.91(100)

0.93(100)

0.91(50-190)

0.91(32-392)

0.91(32-199)

GLASS

type not specified

polished

0.9(100) 10

0.94(68) 1

MISCELLANEOUS

roofing paper 0.91(68) 0.91(70)

A-22
TABLE A-2

Infrared Emissivities of Common Paints

Type Paint

Emissivity

Total Normal Hemispherical

e n

(°F) Ref. e h

(°F) Ref

.

OILS

type not specified 89-. 97(68) 1

16 different colors

all colors

.92-. 96 (212)

.92-. 96 (32-199)

10

8

ENAMELS

white

camouflage green

green

.92(125)

.85(125)

.82(125)

2 11

11

LACQUER

flat black

black

white

matt black

.96(125)

.925(212)

.97(176)

12

13

13

.96(199)

.95(68)

7 14

ALUMINUM

all types .27-0.60 15 .4-. 7(0-100) 17

3-. 5(68) 16

A-2 3
References for Emissivity Tables

1. D'Ans, J. and Ellen Lax, "Taschenbuch fur Chemiker and Physiker,"

Springer - Verlag, Berlin (1943).

2. Schmidt, E. , Gesundheite - Ingenier, 20, (1), 1-23 (1927).

3. Randerson, H. Y. , "Australian Sanitary Engineering Practice,"

3-d Ed. Angus and Robertson, Sydney, (1949).

4. Forsythe, W. E. and Worthing, A. G. , "Properties of Tungsten -Wand Characteristics of Tungsten Lamps" Astrophysical Journal,

61:146-185 (1925).

5. Cammerer, J. S. "The Thermal Properties of Wood and Materials

Containing Wood, "Holz als rohund Wenkstoff, 1(6): 206-213 (1938)

6. Grant, G. R. "Design of Simple Shelter from the Sun. The Role of

Emissivity and Graphs to Facilitate the Use of Available Data,"

University of Queensland Interim Report No. 1, Project 103, (1949)

(Mimeo)

.

7. McDermott, P. F. , "An Apparatus for Measurement of the Total

Emissivity of Sheet-Materials in the Temperature-Range 140-500F."

Review of Scientific Instruments, 8, (6), 185-192 (1937).

8. Anon. "Heat Transmission and Radiation," Heating and Ventilating,

43, 72a-72b, (1946).

9. Wamsler, F. , Feitschr, Ver. Deutsch. Ing. 55, 599 (1911).

10. McAdams, W. H. , "Heat Trans-transmission" quoted from Standard

Oil Development Company (Personal Communication to H. C. Hottel,

1928) 4-d Ed., p 393-396, McGraw Hill, New York and London (1942).

11. Boelter, Bromberg, and Gier. (N.A.C.A. Wartime Report, 19. An

Investigation of Aircraft Heaters. Part XV. The Emissivity of

Several Materials. University of California (1944).

12. Hluika, F. , Annal .d. Physik, 96, 230 (1933).

13. Eckert, E. , "Introduction to Transfer of Heat and Mass," McGraw

Hill Book Co., First Edition (1950).

14. Littleton, J. T. Jr., Phys. Rev., 33, 453 (1911).

15. Standard Oil Development Company, Personal Communication (1928).

16. Allcut, E. A. "The Use of Heat Insulation in Building - Design and

Instruction," Royal Architectural Institute of Canada, Journal,

24 90-100, (1947).

17. Fishenden, M. and Saunders, 0. A., Introduction to Heat Transfer,

Clarendon Press, London (1950).

A-2U
NBS-1 i 4A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

I. PUBLICATION OR REPORT NO. NBS TN-933

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

AN INFRARED TECHNIQUE FOR HEAT-LOSS

MEASUREMENT

7. AUTHOR(S)

D.M. Burch, T. Kusuda, andD.G. Blum

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

12. Sponsoring Organization Name and Complete Address ("Street, City, State, ZIP)

Federal Energy Administration

Washington, D.C. 20461

3. Recipient's Accession No.

5. Publication Date April 1977

6. Performing Organization Code

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 462 6417

11. Contract/Grant No.

13. Type of Report & Period Covered Final

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 paper describes a newly developed technique for measuring heatloss rate utilizing an infrared television system. A device called a heatflow reference pad was developed that makes it possible to measure

quantitatively the heat-loss rate through the surface of a building without

the need for a conventional heat-flow meter to be mounted on the surface.

Technical considerations for the design of a heat-flow reference pad are

presented. The infrared measurement technique predicted heat-loss rates

in the laboratory and field within approximately 12%.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Heat-flow measurements; infrared heat-loss measurement technique;

measurement technology; thermography.

J. AVAILABILITY [j£ Unlimited

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P>? Order From Sup. of Doc, U.S. Government Printing Office - Washington, D.C. 20402, SD_Cat. No. en .46:933

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