NBSIR 79-1736

NAT L INST. OF STAND & TECH R.I.C.

A 11 10 4 5 5 1 3 7

valuation of Hand-Held

Infrared Thermometers for Wall

Thermal Resistance

Determinations

Stephen J. Treado

Douglas M. Burch

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

July 1979

o

v

%i*

£

•‘'•CAU O* ^

S. DEPARTMENT OF COMMERCE

JATIONAL BUREAU OF STANDARDS

NBSIR 79-1736

«

EVALUATION OF HAND-HELD

INFRARED THERMOMETERS FOR WALL

THERMAL RESISTANCE

DETERMINATIONS

crass® of £ bEP IT 19/

O0& ft CC - QjJ

<00x?>0

UGU

Stephen J. Treado

Douglas M. Burch

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

July 1979

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

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

Hand-held infrared (IR) non-contact surface thermometers from two manufacturers were tested to evaluate their effectiveness in measuring surface

temperatures for the determination of the thermal resistance of walls.

Two phases of the test were performed, first a laboratory test of a wood-frame wall, followed by a field test of a brick veneer wood-frame

wall. During both phases of testing, additional measurements of thermal

resistance were made for comparative purposes, using multi junction

thermopiles and heat-flow meters. An error analysis of the thermal

resistance measurement procedure utilizing IR surface thermometers was also performed.

Key words: Infrared thermometers; in situ evaluation of insulation;

R-value measurement by spot radiometer; thermal resistance

of walls.

iii
TABLE OF CONTENTS

PAGE

ABSTRACT iii

1. INTRODUCTION i

2. THERMAL RESISTANCE DETERMINATION TECHNIQUE 3

3. EXPERIMENTAL PLAN 3

4. LABORATORY TEST 5

4.1 Instrumentation and Test Set-up 5 4.2 Measurement Procedure 8

4.3 Experimental Results 9

5. FIELD TEST 11

3.1 Instrumentation and Test Set-up 11

3.2 Measurement Procedure 11

3.3 Experimental Results 13

6. ANALYSIS 14

7. SUMMARY 18

NOMENCLATURE 20

SI CONVERSION CHART 21

iv
Evaluation of Hand-held Infrared Thermometers

for Wall Thermal Resistance Determinations

by

Stephen J. Treado

and

Douglas M. Burch

1. INTRODUCTION

Recently a technique was introduced for determining the thermal resistance of walls through the use of hand-held infrared thermometers. These

devices are also known as spot radiometers or hand-held non-contact temperature indicators. A controversial issue is whether these devices can

be used effectively for this purpose. This paper attempts to evaluate

the effectiveness and accuracy of this thermal resistance determination

procedure which utilizes the infrared surface thermometers to perform

the initial measurements. This work was performed by NBS and sponsored

by DOE.

The basic function of an infrared thermometer (henceforth referred to as an IRT) is to indicate the surface temperature of objects or materials.

An IRT is a small, light, and frequently gun-shaped device (see Figure 1)

which operates by detecting infrared radiation emitted and reflected

from a surface. Performing a measurement is accomplished by pointing

the IRT at the surface of interest and depressing the on-off trigger

of the device. The infrared radiation received by the IRT is converted

into an electrical signal which is amplified by solid-state circuitry

and subsequently processed through compensation circuitry to a meter

which is calibrated to indicate surface temperature directly.

Typical specifications for IRT's include a temperature measurement

resolution within + 0.5°F, an indoor temperature range of 50°F to 100°F,

an outdoor temperature range of -20°F to 130°F, a spectral response in

the range of 10 microns, and a response time of less than 2 seconds.

Since contact with the surface to be measured is not necessary with these

devices, it is possible not only to minimize the error due to disturbing

the thermal regime of the subject surface, but also to determine the

surface temperature of otherwise inaccessible objects, such as secondstory walls. The target size increases with increasing distance from

the object to the instrument at a ratio of 1-inch diameter target per

each 15-inch operating distance. The IRT indicates an average temperature for the surface being measured over the entire field of view.

Therefore, operation close to a wall will indicate surface temperature

of a small region, while operation at a distance will indicate surface

temperature for a larger region.

1
AIMING

SIGHT

2

Figure

1.

Hand-held

Non-Contact

Infrared

Thermometer
IRT's are utilized widely for various temperature measurement applications and qualitative investigations of the thermal properties of buildings. The IRT's are also currently used to provide temperature measurements as part of a technique for determining the thermal resistances

of walls. This technique has been utilized by utility companies, insulation contractors and other individuals during energy audit studies or sales programs, and consists of the procedure described below.

Several questions exist as to the reliability of the thermal resistance

determination procedure. These include the effect of dynamic (nonsteady) conditions, temperature measurement accuracy, and uncertainty in

specifying the surface heat-transfer coefficient, the value of which is

dependent upon air flow along the surface as well as radiative heat

exchange between the surface and the surrounding surfaces that see the

subject surface.

2. THERMAL RESISTANCE DETERMINATION TECHNIQUE

In the current practice, the thermal resistance of an exterior wall is

determined by measuring the interior air temperature, the interior sur- face temperature of the wall and the exterior surface temperature of the

wall. These three measured temperatures are then substituted into an equation relating heat flow through the exterior wall to the interior

surface-to-air temperature difference and the temperature differential

between the interior-to-exterior wall surfaces. This equation is based

on a steady-state model of heat flow^ through a wall, and utilizes

assumed values for the interior surface heat-transfer coefficient and

the surface emittance of the interior side of the wall.

In actual practice, the manufacturers of the IRT devices recommend that

the interior air temperature be determined by measuring the surface temperature of a piece of furniture, followed by direct temperature measurements of the interior and exterior surfaces of the wall in question.

The attractive features of this technique are ease in operation, fast

response and portable operation in the nondestructive determination of

the thermal resistance of walls. If validated, a simple and fast thermal

resistance determination technique such as this could be of significant

value in aiding energy conservation and thermal analysis programs. The

purpose of this paper is to evaluate the accuracy and reliability of

the IRT technique in determining building envelope thermal resistance.

^ Stainton, W.D. , Heat Transfer and Building Surface Temperatures,

IRIE Conference Transaction (in preparation) 1978.

3
Consider the exterior wall schematic shown in Figure 2.

Exterior

The instantaneous heat-loss rate (q) at the inside surface is given by

q

= f

x

• (Tj - T

w

) (1)

where fj = overall heat-transfer coefficient at the inside

surface, —

h* f

t

2 * °F

Tj = interior air temperature, °F

Tw

= interior surface temperature, °F

Under steady-state conditions, the heat-loss rate through the wall may

also be determined using the relation:

q = (Tw

” T

c )/R (2)

°F*h* f t2

where R = surface-to-surface thermal resistance, —Btu

T

c = exterior surface temperature, °F.

Under steady-state conditions, we may combine eqns. (1) and (2) to give

-

< T

w

- V

£ I-'T

I - V

(3)

If this model is valid, the surface-to-surface thermal resistance of an exterior wall can be determined based on the three pertinent temperature measurements, together with an assumed value for the interior

surface heat-transfer coefficient.

4
3 . EXPERIMENTAL PLAN

The basic issue examined in this paper is the accuracy of the method

described in the previous section for determining wall thermal resistance. In addition, the temperature measurement accuracy of the IRT's

is examined, along with comparison thermal resistance determinations

obtained through the use of thermopiles and heat-flow meters.

Temperature measurements and the related thermal resistance determinations were performed in the laboratory for a wood-frame wall and in the

field for a wood-frame brick veneer wall. One half of the wall cavities

in the laboratory wall were insulated with loose-fill cellulose, while

the remaining half were uninsulated. Sets of measurements were made at

various temperature conditions under both steady-state and dynamic

conditions utilizing the IRT's and alternate technqiues. The range of

thermal resistance values determined from these measurements were then

compared to the calculated thermal resistance (based on ASHRAE Handbook

values) of the wall in question.

An error analysis was also performed, examining the parameters which

affect the accuracy of the thermal resistance determination procedure

utilizing an IRT.

4. LABORATORY TEST

4.1 INSTRUMENTATION AND TEST SET-UP

The first stage of the test involved laboratory measurements on a typically constructed wood-frame wall composed of 2x4 studs (16 in. on center), 1/2-in. gypsum board, wood-fiber sheathing and redwood siding.

One half of the wall cavities contained loose-fill cellulose insulation

while the remaining half contained no added insulation. The computed

thermal resistances for the insulated and uninsulated portions of the

wall were 13.2 and 4.2 ° F*h*f t^ respectively. (See Table 1 for computation.) Btu

The test wall was placed between two temperature controlled chambers

(see Figure 3), and subjected to various combinations of simulated indoor

and outdoor conditions. Measurements were performed under both steadystate and dynamic (non-steady) conditions. Both the insulated and the

uninsulated portions of the wall were instrumented identically. Four

heat-flow meters were connected in series and spot glued to the interior

surface at the center of each portion of the wall. Each of the heatflow meters consisted of a thin cylindrical wafer containing an imbedded

thermopile previously calibrated by the guarded-hot-plate apparatus.

The millivolt signal generated from the thermopile is proportional to

the heat flow passing through the wafer. In addition, two thermopiles,

each with three pairs of junctions, were installed on each portion of

the wall to indicate surface-to-air and surface-to-surface temperature

differences (see Figure 3). Each set of heat-flow meters was connected

5
ACCESS DOORS

TEST WALL SECTION

I

VARIABLE VOLTAGE TRANSFORMERS

INSULATED BOX

HI| 1 1 1 1 1 1 1

1

1 1 1 1 1 1 | | , 1HHHHHHI1 flHHHHHi 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 .HHHIH^HH

j

1 1 1 1 1 1 1 1 II 1 1 1 M 1

1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 i

REFRIGERATION

COIL

INTERIOR EXTERIOR INTERIOR SIDE VIEW

• HEAT FLOW METER

X SURFACE-TO-SURFACE

THERMOPILE JUNCTIONS

A SURFACE-TO-AIR

THERMOPILE JUNCTIONS

Figure 3. Schematic of Laboratory Set-up.

6
TABLE 1. THERMAL RESISTANCE VALUES

Thermal Resistance*

Material

Thickness

in.

°F*h*ft 2

Btu

Gypsum Board 0.5 0.35

Cellulose 3.5 11.99

Wood-fiber sheathing 0.75 1.95

Redwood Siding 0.75 0.89

Air Space 3.5 0.97

^insulated = 0.35 + 11.99 + 1.95 + 0.89 =

^uninsulated = 0.35 + 0.97 + 1.95 + 0.89 =

°F*h*f

t

2 15.18-^15.2 BtH

°F*h*f

t

2 4.16 4.2 Btu

Based on ASHRAE Handbook values.

to an analog integrator for the purpose of averaging the signal over

a five-minute period to minimize the error due to random surface temperature fluctuations, especially when readings are to be made at conditions

of low temperature differences. Similarly, each thermopile was connected

to its own integrator. Thermocouples were also installed to indicate

surface and air temperatures on the interior and exterior sides of

the wall. These sensors were connected to a data acquisition system

which recorded their signal levels automatically every hour and manually

before each thermal resistance determination.

To simulate winter conditions, the portion of the box surrounding the

exterior side of the wall was cooled with a specially designed refrigeration system. The main features of the system are a large evaporation

tank and a force-feed flooded coil, which was installed inside the box.

Air within the chamber was cooled by passing it over the coil with a fan. Temperature control was accomplished through the use of electric

heaters within the tank. The portion of the box surrounding the interior

side of the wall was heated with a thermostatically controlled electricresistance heater which operated in conjunction with a small fan to provide air circulation within the chamber. The voltage supplied to the

heater was controlled with a variable transformer to enable the precise

control of the cycling period needed to retain a steady-state temperature

condition regardless of the exterior condition.

For the purpose of providing dynamic test conditions, temperature fluctuations on the exterior side were accomplished through the addition of

7
electric heat on a time scale designed to simulate typical diurnal

temperature cycles. Fluctuating indoor conditions were obtained by

increasing the voltage supplied to the heater located within the interior

portion of the box, causing the air temperature to vary, simulating the

operation of a typical furnace.

4.2 MEASUREMENT PROCEDURE

The thermal resistance of the test wall was determined utilizing four

different measurement procedures. Each measurement procedure consisted

of a series of thermal resistance determinations, at various temperature

conditions, utilizing either the IRT's, thermopiles or heat-flow meters.

Initial measurements using the IRT's were performed following the manufacturers' instructions explicitly. Under this procedure, each IRT was calibrated with a factory-supplied black reference block upon which a surface thermometer indicated temperature. Calibration was accomplished

by aiming the IRT at the reference body and adjusting the IRT zero control until the temperature indicated on the IRT meter matched that of

the reference body. The distance-to-size ratio of the IRT's is 15 to 1,

providing a circular target area of 2 in. diameter at a distance of

30 in. A single calibration is said by the manufacturers to be sufficient to enable accurate temperature measurements to be made using either

the indoor or outdoor scale of the IRT. This procedure will henceforth

be referred to as the "manufacturer's calibration procedure." Following

this procedure, indoor air temperature was measured by determining the

surface temperature of a piece of foam insulation which was located

within the "indoor" side of the chamber. Wall surface temperatures were

then determined at points of the same height on both the insulated and

uninsulated portions of the wall.

The second procedure utilized was similar to the manufacturer's calibration procedure with the exception that separate calibrations were performed on the IRT's before both the interior and exterior measurements.

Copper sheets painted flat black were installed in both the interior

and exterior chamber portions, each with a thermocouple soldered to its

surface. Calibrations were performed using these sheets as references

prior to each measurement. This procedure will henceforth be referred

to as the "laboratory calibration procedure".

Additional measurements were made through the use of the two pairs of

thermopiles to indicate the temperature differences between the indoor

surface and the indoor air and between the indoor and outdoor surfaces.

This procedure was designed to demonstrate the accuracy in determining

thermal resistance which would be expected from a device which could

measure surface temperature differences with an accuracy of within + 0.3°F, approximately twice the specified temperature difference

measurement accuracy of the IRT's.

The fourth measurement procedure consisted of thermal resistance values

obtained by dividing the surface-to-surface temperature difference across

8
the wall as measured by thermopiles into the heat flow through the wall

as measured by heat-flow meters.

4.3 EXPERIMENTAL RESULTS

Table 2 gives the range of thermal resistance values determined during

the laboratory phase of the testing. The left hand column indicates the

temperature conditions under either steady-state or dynamic conditions.

Several sets of measurements were usually performed at each temperature

condition. A range of thermal resistance values is given for each of

the four different procedures. The values listed are the individual

lower and upper thermal resistance determinations; other values lie

within this range.

Considerable variation was observed between different thermal resistance

determinations using the IRT's at the same temperature conditions, as

°F * h* f t^

shown in Table 2 expressed in . Errors were larger for the

Btu

insulated portion of the wall, with thermal resistance determination

values as high as 42.5 and as low as 11.8 under steady-state conditions,

and as high as 58.7 and as low as 12.8 under dynamic conditions, compared

to the calculated value of 15.2. Errors in the thermal resistance determinations for the uninsulated portion of the wall were smaller, but

significant variation still occurred, especially under dynamic conditions

where values ranged as low as 2.2 and as high as 6.6 compared to the

calculated value of 4.2.

During the testing, a significant difference was observed between the

thermal resistances determined for the insulated and uninsulated portions

of the wall. Under the coldest conditions, some of the temperature mea- surements by the IRT's were erroneously off-scale, preventing determination of the thermal resistance under those conditions. Improved accuracy

resulted when the laboratory calibration procedure was used instead of

the manufacturers' calibration procedure.

The values obtained utilizing the thermopiles demonstrate the accuracy

expected with a more accurate temperature measurement system. If the

IRT's were as accurate as the thermopiles in determining surface temperatures, their accuracy in determining thermal resistance would be

approximately the same as that of the thermopile system. Thermal resistance determinations utilizing the thermopiles are seen to exhibit less

variation and greater accuracy than those obtained through the use of

the IRT's. The overall range of thermal resistance values is seen to

extend from 9.6 to 23.6 for the insulated portion of the wall, and

between 4.5 and 6.6 for the uninsulated portion of the wall. Since

greater accuracy is obtained through the use of the thermopile system,

inaccuracies in temperature measurements with the IRT's are partially

responsible for errors in thermal resistance determinations.

9
TABLE 2. LABORATORY THERMAL RESISTANCE DETERMINATIONS

Test Condition, °F Thermal Resistance Range1 °F*h*ft 2 Btu

IR Thermometer Thermopile Heat-Flow Meter

Thermopile

Interior Exterior Mfgrs . Calib. Lab Calib.

For Insulated Wall R=15.2

Steady-State

75 10 19.7 - 40.6* 18.2 - 25.0 15.0 14.3

75 45 16.9 - 28.3 — 15.4 - 17.5 21.5

73 33 23.1 - 42.5 26.0 - 27.6 15.1 —

78 88 11.8 - 17.9*

Dynamic

75 10-43 25.7 - 33.9 — — —

75 42-25 31.3 - 40.9 22.1 - 37.8 23.6 —

73-78 33 12.8 - 37.0 6.3 - 27.6 9.6 - 11.3 —

75 30-60 22.8 - 58.7 18.1 - 18.9 15.2 13.4

For Uninsulated Wall R=4.2

Steady-State

75 10 3.8 - 5.1* 2.5 - 3.0 4.5 4.7

75 45 2.8 - 4.2 — 5.4 - 6.0 5.1

73 33 3.0 - 4.8 2.4 - 2.6 5.0 - 5.6 —

78 88 3.2 - 4.6 — — —

Dynamic

75 10-43 3.3 - 3.8 — — —

75 42-25 4.6 - 6.6 3.1 - 4.2 5.2 - 6.6 —

73-78 33 3.0 - 3.7 2.1 - 2.8 4.5 - 4.9 —

75 30-60 2.2 - 3.5 1.4 - 1.9 4.7 4.5

^ Thermal resistance values are based on measurements and do not include air

film resistances.

Fifty percent of the temperature measurements at this condition were offscale or unmeasurable, making it impossible to determine thermal resistance.

10
The heat-flow meter thermopile combination was subjected to limited

testing, but accuracy observed was comparable to that using the thermopiles alone.

5. FIELD TEST

5.1 INSTRUMENTATION AND TEST SET-UP

The second stage of the test involved field measurements of an exterior

wall of a townhouse. The structure tested was a typically constructed

two-story unit built on a slab-on-grade (see Fig. 4). The particular

wall tested was comprised of 3/8-in. wood paneling, 2x4 wood framing, and

1/2-in. wood-fiber sheathing with a brick-veneer exterior surface. The

wall cavities contained 3-1/2-in. glass-fiber insulation. The computed

thermal resistance for the portions of the wall not containing studs is

13.2 (see Table 3 for computation).

TABLE 3. THERMAL RESISTANCE VALUES

Thermal Resistance*

Material

Thickness

in.

°F*h* ft

Btu

Wood Paneling 0.375 0.47

Glass Fiber 3.5 11.0

Wood-Fiber Sheathing 0.5 1.32

Brick 3.5 0.39

* From ASHRAE Handbook

R = total 13. 18 13.2 °F*h*ft 2 Btu

This wall was instrumented in a fashion similar to the laboratory setup,

with thermopiles to indicate temperature differentials, thermocouples to

indicate surface and air temperatures, and a heat-flow meter. Normal

thermostatic operation of the gas-fired forced-air furnace was utilized.

5.2 MEASUREMENT PROCEDURE

The measurement procedure utilized during the field portion of the test was essentially the same as used during the laboratory phase. In

addition to the manufacturers' and laboratory calibration procedures

described in the laboratory measurement procedure section, a third

11
Figure 4. Photograph of the Test House.

12
calibration procedure was used. This third procedure was called the

alternate calibration procedure and utilized a foam insulation board,

instrumented with a surface thermocouple, as a calibration standard.

Following this procedure, the insulation board was substituted for the

reference block supplied by the manufacturers and a single calibration

was performed as per the manufacturers’ calibration procedure. This procedure was used because of observed inaccuracies in the calibration

standards provided by the IRT manufacturers.

5.3 EXPERIMENTAL RESULTS

Table 4 contains a compilation of the results of the field tests of a wood-frame brick-veneer wall. The indoor air temperature listed is the

average value of the actual temperature which varied with thermostatic

operation of the furnace.

TABLE 4. FIELD THERMAL RESISTANCE DETERMINATIONS o 2 FOR WOOD-FRAME BRICK VENEER WALL R=13.2 —Btu

Test Condition, °F

Interior Exterior

Thermal Resistance RangeJ

Device A Device B

’F'h* ft"

Btu

Device C Thermopile

Mf grs.

Calib.

73.5 28.0 20.4 - 99.4 30.3 - 65.4 2.5 - 16.7

Alternate

Calib.

74.0 29.0 8.0 - 9.8 11.9 - 18.5 5.1 - 7.9

Lab.

Calib.

74.0 29.0 7.6 - 13.6 9.6 - 13.6 8.4 - 12.9

Thermal resistance values are based on measurements and do not

include air film resistances.

Devices A and B were the same IRT’s used during the laboratory phase of

the test. Device C, which was received late in the testing schedule

and subjected to a limited test regime, was a self-calibrating IRT made

13
by the same company that supplied one of the other IRT's, and was otherwise similar to the other two IRT's.

Thermal resistance determinations are listed for the three different

calibration procedures for the IRT's. Considerable variation is seen for

the values obtained using the manufacturer's calibration procedure, with

a range of thermal resistance determination values between 20.4 and

99.4, compared to a calculated value of 13.2. Utilization of the alternate or lab calibration greatly reduced the error in the thermal resistance determinations, but significant variations are still seen to occur.

6. ANALYSIS

Examination of the data reveals several major points. All of the IRT's

except the self-calibrating one experienced difficulty in accurately mea- suring low temperatures. Error increased with decreasing temperature,

as indicated in Table 2. This problem may be due to several factors.

The cold temperatures may affect the electronics of the instruments,

altering their response characteristics. The manufacturers recommend

making cold temperature measurements as quickly as possible — a technique which was utilized throughout this test. As an example of lowtemperature measurement dif ficulities , attempts to measure an exterior

wall surface at a temperature of 32°F resulted in temperature readings

ranging from 26°F to -20°F. The magnitude of the low-temperature mea- surement inaccuracy varied between different IRT's. Inaccuracies in

calibration may also strongly affect cold temperature measurements. This

possibility is supported by the significant improvement in measurement

accuracy observed during the field tests utilizing a more accurate calibration method rather than the manufacturers' calibration procedure.

Separate indoor and outdoor calibrations improved the accuracy of the

temperature measurements but had less effect on the accuracy of the

thermal resistance values. Since the thermal resistance calculation is

essentially dependent upon temperature differences, a tradeoff exists

between improving the accuracy of the individual temperature measurement on the cold side and disrupting the internal consistency of the instrument. In other words, if an IRT is reading slightly low due to inaccurate calibration, the measured temperature difference may not be as inaccurate as the temperature difference obtained after separate calibrations, because the effect of the inaccurate temperature measurements

will tend to cancel out when computing the temperature difference.

The most serious disadvantage of the measuring system concerns the transients which are apparent under almost all natural conditions. Intermittent sunlight, wind, thermostatic operation of the furnace, and outdoor temperature conditions all contribute to this problem. The thermal

resistance of a wall is not a simple function of instantaneous heat flow

and temperature difference except under steady-state conditions. An

14
example of this concept is illustrated in Figure 5. The indoor surfaceto-air temperature difference is seen to vary by almost 8 °F in response

to the furnace cycle. A smaller effect is seen in the surface-to-surface

differential measurement. The instantaneous thermal resistance computed

following the model utilized by the IRT measurement technique indicates

values ranging from 2 to 50 over a one-hour period. Since the air temperature is actually measured by determining the surface temperature

of a piece of furniture rather than the true air temperature, large

changes in air temperature over short time periods will be smoothed out

due to the thermal mass of the piece of furniture, lessening the fluctuations in the surface-to-air temperature difference.

Other factors also affect the accuracy of the measurements. Consider an insulated wall (R = 15) and an uninsulated wall (R = 5). If these walls

are exposed to a 40 °F inside-to-outside temperature difference, the

resulting heat-flow rates as given by eq.(2) are 2.7 Btu/h»ft2 for an insulated wall and 8 Btu/h»ft2 for an uninsulated wall. A representative value for the inside heat-transfer coefficient is 1.2 Btu/h»ft2 »°F

(see text below). Using eq (1), the air-to-surface temperature differ- ence (T^ - Tw

) is estimated to be 2.3°F for an insulated wall and 6.8°F

for an uninsulated wall.

The manufacturers’ literature on IRT's states that temperatures can be

measured to within +0.5 °F. Using these tolerances, the maximum

uncertainty in measuring (T^ - Tw ) and (Tw

- T

£

) is + 1 °F. Hence, the

percent error in estimating (T

± - Tw ) is 1.0/2. 3 = 43% for an insulated

wall and 1.0/6. 8 = 15% for an uninsulated wall. In the case of the

overall temperature difference across the wall (T - T ), the percent error is 1/40 = 2.5%.

The inside surface heat transfer coefficient consists of both radiative

and convective components. The radiative component (f r

) may be estimated

by the relation:

f r = 4 • a »ET

MRT

(4)

where aE

e

l e

2 TMRT

Stef an-Boltzmann constant

emittance factor, E = —, I7e

1 +l/e2 -l

interior surface emittance of wall

emittance of the surrounding surfaces

absolute mean-radiant temperature of the room.

Under typical indoor conditions, a representative value for f is

0.88 Btu/h »f t 2 »°F

.

The convective component of the overall surface heat-transfer coefficient

is a complex function of the surface-to-air temperature difference, largescale convective air movement patterns present in a room, and the height

above the floor. In the absence of large-scale convective air movement,

15
50

40

30

20

!0

0

r

34

33

32

31

30

29

28

r

r

8

7

6

5

4

3

2

I 0

A T SURFACE-TO-SURFACE

4.5 # F

H

1 H H I h

t

4 8 12 16

TIME (MIN.)

56 |60 64

HEAT

20 \ZA 28 |32

HEAT HEAT

ON OFF ON

Figure 5. Thermal Resistance Values under Dynamic Conditions.

16
a laminar f ree-convection boundary layer will exist at the top of the

wall. This boundary layer will grow in size, passing through a transition region, and then turn into a turbulent boundary layer further down

the wall. Generally, the convective component will vary between 0.2 to

0.4 Btu/h* f

t

z * °F , so that the uncertainty in specifying the convective

component of the overall surface heat-transfer coefficient is + 0.2

Btu/h*f

t

2 * °F or + 17%.

The uncertainties in specifying fj and in measuring (T^ - Tw

) and

(Tw

- T

c

) are summarized in Table 5.

TABLE 5. SUMMARY OF UNCERTAINTIES IN MEASURING THE

THERMAL RESISTANCE OF AN INSULATED WALL

Parameter Uncertainty

Normal

Range

%

f

i + 0.2 Btu/h*f

t

2 ’ °F + 17%

T

i - Tw

+ 1.0 °F + 43%

T ” T + i-1 . o

o n + 2.5% w c

2 Ku has presented propagation of error formulas for some simple func

tions. For the function form Mr . v w

by:

where x = mean value of x

y = mean value of y

2 S- = variance of x

2 S- = variance of y

M

x

*My, the variance (S^) is given

O H. H. Ku, "Statistical Concepts in Metrology", Handbook of Industrial

Metrology, American Society of Tool and Manufacturing Engineers,

pp. 20-50, Prentice-Hall, Inc., New York, 1967.

17
For the function form M

a = Mz /Mw , the variance (Sa

) is given by

where z = mean value of z

w = mean value of w

S— = variance of z

2 S- = variance of w

A formula for calculating the variance (S^) of the function form

R = z/(x-y) may be obtained by combining eqs. (5) and (6) giving the

relation:

(7)

The square root of the variance is the standard deviation. If we take

3 standard deviations to constitute the uncertainty (E^), then we have:

ER

2 + Ex

( 8 )

Substituting the uncertainty values from Table 5 into eq. (8), we find

that the uncertainty in the parameters fj, (Ty-T ), and (Tw

-Tc

) causes

an uncertainty in thermal resistance (R) of 46% for an insulated wall.

In the case of an uninsulated wall, the surface-to-air temperature difference (Ty-T ) is larger and the unceratinty in (Ty-Tw) is 13%. The

uncertainty of the other parameters remains the same. Using eq. (8), an uncertainty in R due to the uncertainties of the parameters Fy,

(T rTw ), and (Tw

~T

c

) is found to be 23%.

7 . SUMMARY

The technique utilizing hand-held infrared thermometers to determine the

thermal resistance of walls by measuring the surface temperatures and

air temperature as recommended by the manufacturers of IRT devices

18
appears to be considerably inaccurate under typical conditions for a residence. Under steady-state conditions, thermal resistance determinations utilizing the IRT's were found to have an average error of more

than 50%. Under dynamic (non-steady) conditions, this error is

approximately doubled.

Some of the error in thermal resistance determination using the IRT's is

due to inaccurate surface temperature measurement. However, even if the

IRT'S were perfect in indicating surface tempertures, substantial error

in R determination would occur due to other factors such as uncertainty

in the surface heat-transfer coefficient and emisslvity, and transients

due to cyclic operation of the furnace and outdoor conditions such as temperature and wind.

A suitable application for the hand-held infrared thermometers is as a qualitative measuring device. It should be possible using the IRT's

to determine whether or not a wall is insulated or if insulation voids

or other thermal leaks are present. Accurate calibration of the IRT

devices is essential. Separate indoor and outdoor calibrations should

result in increased accuracy. Care should be exercised to minimize

uncertainties due to transients. For greatest accuracy, measurements

should be made under conditions which produce a 20 to 30°F temperature

difference across the wall, and the cold-side surface of the wall should

be above 32°F.

19
Nomenclature

interior surface emittance

emittance of surrounding surfaces

emittance factor

overall heat-transfer coefficient

radiative heat-transfer coefficient

heat-loss rate

surface-to-surface thermal resistance

exterior surface temperature

interior air temperature

interior surface temperature

absolute mean radiant temperature

Stef an-Boltzmann constant
SI Conversion Chart

Physical Quantity

To Convert

From To Multiply by

Temperature F C T

c = (T

p-32)/l

Length in m 2. 54( 10“ 2

)

Heat Transfer Rtu/h*ft 2, F w/m2 *K 5.68

Thermal Resistance h*ft2 *F/Btu m

2 *k/w 1.76C10" 1

)

Heat-Flow Rate Btu/h*f

t

2

O w/m 3.16

21
NBS-1 14A (REV. I 1-77)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1736

2. Gov’t Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

EVALUATION OF HAND-HELD INFRARED THERMOMETERS FOR

WALL THERMAL RESISTANCE DETERMINATIONS

5. Publication Date July 1979

6. Performing Organization Code

7. AUTHOR(S)

Stephen J. Treado and Douglas M. Burch

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 742 2509

11. Contract/Grant No.

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

Department of Energy

Washington, D. C. 20585

13. Type of Report St Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

Hand-held infrared (IR) non-contact surface thermometers from two manufacturers were tested to evaluate their ef fectivensss in measuring surface

temperatures for the determination of the thermal resistance of walls. Two

phases of the test were performed, first a laboratory test of a wood-frame

wall, followed by a field test of a brick veneer wood-frame wall. During

both phases of testing , additional measurements of thermal resistance were made

for comparative purposes, using multijunction thermopiles and heat flow meters.

An error analysis of the thermal resistance measurement procedure utilizing

IR surface thermometers was also performed.

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)

Infrared thermometers; in situ evaluation of insulation; R-value measurement by

spot radiometer; thermal resistance of walls.

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