AlllOD TTSSSfl

Radiation Errors in Air Ducts

Under Nonisotliermal Conditions j Using Thermocouples, Thermistors,

And a Resistance Thermometer \

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ICT 0 6 1978 UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary

kTIOMAL BURWAij

tat BTANDARD8 NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Radiation Errors in Air Ducts Under Nonisothermal

Conditions Using Thermocouples, Thermistors, and

A Resistance Thermometer

Joseph C. Davis

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Building Science Series 26

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 26, 14 pages (November 1969) CODEN: BSSNB

Issued November 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 (Order by SD Catalog No. C 13.29/2:26), Price 25 cents
Contents

Page

1. Introduction 1 2. Description of apparatus 2

3. Calibration 5

4. Method of testing 8

5. Results 9

6. Errors in measurement 11

7. Summary and discussion 11

8. References 12
Radiation Errors in Air Ducts Under Nonisothermal Conditions

Using Thermocouples, Thermistors, and a Resistance Thermometer

Joseph G. Davis*

studies were made to determine the radiation error in temperature measurements made with thermocouples, thermistors, and a i-esistance thermometer in moving air at

velocities ranging from 300 to 1300 fpm when the temperature of the duct wall surrounding

the air stream was from 0 to 50 deg F higher than that of the air in the center of the duct. To eliminate all but the variable under study, conduction errors were minimized to a point where they were almost nonexistent by using Chromel P-constantan thermocouple

wire and by employing other techniques. Radiation effects were studied when the probe

housing the three types of temperature sensors was unshielded and again when it was

shielded. The studies showed that when the sensors were unshielded and the temperature

difference between the duct wall and the air was 50 deg F (28 K, approximately), the

error in the sensors was about 3.8 deg F (2.1 K) for an air velocity of 300 fpm (1.5 m/s)

and 1.0 deg F (0.6 K) for an air Telocity of 1300 fpm (6.6 m/s). When the sensors were

shielded, the error was about 0.2 deg F (0.1 K) for 300 and 500 fpm velocities and the same

duct wall air-temperature difference. Tests were not performed at 1300 fpm with the sensors shielded because theory indicated that radiation error would be negligible at this

velocity. Under the test conditions that prevail in the testing of air conditioners and heat pumps in laboratories, it should be possible to reduce the error in temperature measurement

of the moving air to about 0.2 deg F (0.1 K) by a suitable combination of air mixers, duct

insulation, radiation shields, and calibration techniques.

Key words : Conduction error ; radiation error ; resistance thermometer ; temperature measurement ; thermistor ; thermocouple.

Present address: 4534—47th St., N.W., Washington, D.C. 20016.

1. Introduction

Accuracy of measurement of the temperature

of moving air depends, among other things^ on the

effectiveness of precautions taken to minimize con- duction and radiation errors. It is known that in

the determination of the thermodynamic properties of moving air, these errors can be significant

at air velocities below 1000 fpm and when the

temperature of the surroimdings, such as a duct

wall bordering a stream of moving air, is 20 deg

F different from that of the air at the position of

the sensor. However, the literature does not show

much information on the magnitude of error at these velocities and temperature differences.

The error in determining the capacity of an air

conditioner or a heat pump in a laboratory can be

as high as 5 percent if the error in the temperature

measurement from these sources is neglected, even though the temperature difference between the air

and the duct wall may not exceed 6 deg F. Similarly, the measurement of the moisture generation

capacity of a humidifier can be as much as 10

percent in error if corrections are not made to the

observed temperature.

In a previous paper by Davis, Faison, and

Achenbach [1]^ showing the results of the study

of the errors of thermocouples, thennistors, and

mercury-in-glass thennometers used in moving

air, and where the temperature of the duct wall was essentially the same as that of the sensors, it was shown that the principal errors under those

conditions were due to change in performance of

the sensors between calibrations, and to false readings due to theiTnal lag of the sensors. Other

smaller sources of error found in the study were

self-heating of the thermistors, parallax difficulties in reading the mercury-in-glass tliermometers,

orientation of the thennometers, and impact error due to the energy of motion of the air stream. Shielded and unshielded temperature sensors are widely recommended in various standard test pro1 Figures in brackets Indicate the literature references on page 12.

1
cedures for air conditioning, heating, and refrigerating equipment for indicating the temperature of

moving air in ducts under conditions where the

temperature of the duct wall may be significantly

different from the air temperature in the duct.

This study was designed to evaluate the magnitudes of the radiation errors for three common

types of sensors, used with or without shielding,

for a range of air velocities and a range of temperature differences between the duct wall and the

moving air, and to indicate application techniques

that would minimize conduction errors. In the

previous studies, thermocouples, thei-mistors, and

mercury-in-glass thermometers were used. In the

study now being reported,^ thermocouples and

thermistors were used, but because of placement

problems, and because a better calibration refer- ence was needed, a platinum-resistance thermometer was used instead of a mercury-in-glass

thermometer.

- Throughout this paper, the more substantive results are given not only in the British units now customary in this country, but also in the International System of Units (ab- breviated as SI). This is done in recognition of the position of the USA as a signatory to the General Conference on Weights and Measures, which gave official status to the SI system of units in 1960. To assist readers interested in malilng use of the coherent system of SI units, the exact conversion factors used

in this paper are : Length 1 inch = 0.0254 meter. Velocity tOO ft/min = 9.0508 meter/second Temperature difference 1 deg F = 5/9 deg C = 5/9 K (kelvin)

2. Description of Apparatus

The tests employed a well-insulated, round

metal duct with an air-mixing device near the inlet. Air was drawn through the duct at velocities

sufficiently high to produce turbulent flow. All

temperature measurements were performed at a

station seven duct diameters downstream of the

mixing device. The duct walls were heated for a distance of 3.7 duct diameters upstream and 3.7 downstream of this station. A temperature boundary layer along the duct wall started at the leading edge of the heated section, and grew in

thickness in the flow direction. At the test station,

this boundary layer was still quite thin. The re- mainder of the volume of air was essentially

isothermal so that the conduction of heat through

the air to the temperature sensors was not signifi- cant. All temperature measurements at this station were made near the axis at the center of the

isothermal volume of fluid. The magnitude of the

radiation error was directly assessed by obtaining

measurements with and without radiation

shields and comparing them with measurements

made with specially constructed reference sensors. Two nearly identical thermocouples, two nearly

identical thermistors, and the resistance thermometer comprised a probe which was part of a

rigid assembly designed to prevent flexure of the

leads and to minimize conduction losses. The probe was placed longitudinally in the center of the

duct and parallel to the direction of the flow of

moving air. Figure 1 shows the assembly consisting of the probe ( A) ; a cage-like structure housing

bare lead wires for minimizing conduction errors (B) ; a hollow, phenolic cylindrical structure (C),

which served to thermally insulate the plastic

tube carrying the leads through the duct wall and

which was useful in determining if there were any conduction errors involved during testing

(experiments performed to make this determination are described later) ; and a wooden cabinet

(D), which housed an ice bath and a junction

box. The junction box, which had gold-plated

connectors to minimize corrosion, was used to connect the thermistor leads to the thermistor bridge

Figure 1. T/ie probe housing two thermocouples, two

thermistors, and a resistance thermometer.

The Ice-bath for the thermocouples is housed in the cabinet in the lower portion of the figure. Conduction errors were mini- mized with the use of the cage-lilte structure housing the bare

lead wires.

in the instrument room which was remotely lo- cated. Wire leads to the measuring equipment in

the instrument room for the thermocouples and the

resistance thermometers were permanently con- nected to the assembly.

Details of the probe are shown in figure 2. Thei

two thermocouples, the two thermistors, and the

resistance thermometer were fixed in a rigid man2
DUCT \ / WALL

5"

2 THERMISTORS

RESISTANCE THERMOMETER THERMOCOUPLES

Figure 2. Diagram of the prohe-end showing the two

thermocouples, one of the two thermistors, and the

capsule-type platinum resistance thermometer.

ner at the end of the probe, and the leads, insulated

in vinyl plastic sheathing, were fixed rigidly

through the center of thin-walled methyl methacrylate tubing bent at a right angle (fig. 1) . Holes

about 0.1 in diameter were placed in the tubing

to allow the flow of air across the insulated lead

wires thereby further reducing conduction and

errors due to reradiation of heat from the plastic

tubing to the wires (figs. 1 and 2)

The leads running from the probe to the ice-bath

and the junction box in the assembly were rigidly

fixed into the U-shaped methyl methacrylate tubing between (C) and (D) by filling the tubing

completely with epoxy resin. This rigidization re- duced cold-working of the wires which could

cause a change in results from calibration to cali- bration. The tubing was insulated with spongerubber and with a reflective covering.

The thermocouples in the probe were of No.

30 B and S gage (0.010 in) Chromel P and con- stantan wires. Each junction was about % in long.

All-copper switches were used in the electrical cir- cuit of the thermocouple system. The ice bath

consisted of two wells filled with oil inserted in an

insulated Dewar flask containing slushy ice. The

wells were immersed to a depth of more than 6

in in the ice, and the junctions were placed near

the bottoms of the wells. This procedure was

recommended by Caldwell [2] . The floating ice in

the bath was maintained at a sufficient depth to extend below the bottoms of the wells at all times.

The ice bath was stirred about every 3 hr. The use

of Chromel P-constantan thermocouple wires further helped minimize conduction error because of

their low thermal conductivity, which was about

6 Btu hr ft deg F-^ for chromel and about 13

Btu hr ft deg F'^ for constantan as compared

copper which is about 200 Btu hr ft deg F-\

The wires connecting the assembly to the switch

and to the potentiometer in the instrument room

were copper. A precision-type laboratory jDotentiometer was used for measurment of voltage. It was

capable of direct reading to within I/jlV and of

interpolation to within 0.1/aV, corresponding to

0.03 deg F and 0.003 deg F, respectively, when

Chromel P-constantan theraiocouples are used.

Each of the two bead-type thermistors had a nominal resistance of 2000 at 77 °F. Dimensional details of one of the thermistors are show n

in figure 3. The two lead wires within the glass

stem were made of untinned wire having a low

thermal conductivity, and having a coefficient of expansion approximately equal to that for glass.

According to the manufacturer, the "dissipation

constant" in still air is 25 sec; beta ^ (a material

constant) at 25 °C (77 °F) is 3465 ±175 K; and

.1, • , Ro (0 °C) Ho (32 °F) . the resistance ratio, j^^j^^ or ^^^^^22^ is

7.1. The parameter Ro is the resistance of the

thermistor measured when a low level of electric power, small enough so as not to heat it appreciably, is applied. The manufacturer advised that

the thermistors supplied had been aged at ele- vated temperatures, a precaution which is re- garded as necessary to impart stability. [3] A

modified ratio bridge was used with the thermistors. The bridge was constructed so that the

dial reading was nearly linearly related to the

temperature of the air. [4] Shielded microphone

cables were used to connect the wires from the

junction box in the probe assembly to the thermistor switch, in the instrument room. Goldplated connectors were used at the thermocouple

selector switch.

-2" APPROX.- -2 APPROX.-

-O.IOO" MAX.

0.07" DIA. GLASS STEM T

0 012 TINNED LEADS

Figure 3. Physical dimensions of each of the thermistors. A capsule-type platinum resistance thermometer

1.63 in long with a diameter of 0.25 in was used.

The capsule was made of j^olished platinum. Four

lead wires from the capsule were connected to long copper wires leading to the IMueller bridge

in the instrument room. These four lead wires are needed in Mueller bridge measurements to compensate for effects in lead wire resistances. The

bridge was capable of reading to the nearest 0.0001

O, corresponding to a temperature resolution of

0.002 deg F. A schematic drawing of the apparatus is shown

in figure 4. The test duct, 12 ft long, was made of

brass, had a wall of i/g in thickness, and had a diameter of 10 in O.D. It was heated around the

measuring section with electrical heating cables

for a distance of 3 ft upstream and 3 ft downstream from the probe. The duct was insulated

along its entire length. There was an access door

at the top of the duct with an access mechanism

for placing the probe in the duct. This door had

3 Beta is approximately constant. It appears in the equation i?=i?oe where T Is any Kelvin temperature, and To is the temperature at which i?o was determined.

J
HEATED ACCESS DOOR WITH COPPER- CONSTANTAN THERMOCOUPLES

TO BLOWER

AND EXHAUST

FOR MEASURING TEMPERATURE

OF AIR AT INLET

THE VERTICAL DIMENSCN IS EXAGGERATED FOR THE PURPOSE

OF CLARITY. DETAILS OF THE PROBE ARE NOT SHOWN.

O RESISTANCE HEATER CABLES • COPPER-CONSTANTAN THERMOCOUPLES FOR MONITORING TEMPERATURE ON INSIDE OF DUCT WALL OR INSIDE OF ACCESS DOOR

+ CHROMEL P-CONSTANTAN THERMOCOUPLES

FOR MONITORING AIR MIXING (17 THERMOCOUPLES IN EACH ARRAY).

FiGXJBE 4. Schematic diagram of duct and associated air-mixing, heating, and

temperature-sensing equipment.

a heating cable of its own. Copper-constantan thermocouples were placed in two rings around the in- side surface of the duct, one upstream and one downstream from the probe, as well as on the

inner surface of the duct along the bottom of the

entire length of its measuring section, and on the

bottom of the access door. These thermocouples were used to monitor the temperature of the duct

wall.

The temperature of the air in the room and sur- rounding the duct was within 2 deg F of the temperature of the air at the intake to the duct. The

air intake was from a large reservoir of air that

had been held constant to within 0.15 °F for a period of at least 1 hr before each test. The air was

exhausted from the duct to a remote part of the

large air reservoir. The instrument room, remote

from the room housing the duct, was maintained

at about 75 °F throughout each test. A concentric-louvered, metallic air-mixing device developed at the National Bureau of Standards [5] was used to insure thorough mixing of

the inlet air. This resulted in a uniformity of air

temperature at the entrance to the heated section.

To observe the degree of temperature uniformity

throughout a test, constant monitoring of air mixing took j)lace using three arrays of 17 buttwelded Chromel P-constantan thermocouples in cross sectional areas in the duct. The butt-welded

thermocouples were approximately 2 in from each

other, and each one on the outer periphery of the

array was approximately 1 in from the inside sur- face of the duct. One thermocouple was in the

center. One array was upstream in the section of

the duct that was unheated, another immediately

upstream from the sensors in the heated section,

and a third downstream, also in the heated section.

No. 36 B and S gage wire (0.005 in diam) was used

for the thermocouples. The use of the fine-gage,

butt-welded thermocouple wires reduced conduction and radiation error. The flow was turbulent

in the region close to the duct wall even at 300 fpm

and the low time constant of the thermocouples

caused difficulty in making the thermocouple readings in this region. This difficulty was overcome by

the use of an integrating type digital voltmeter by

which an accurate average microvolt value for

each thermocouple in this region was obtained.

Due to the multiplicity of thermocouples, a zone box was used along with a single, common refer- ence junction in an ice bath. The circuitry for

using a zone box wih Chromel P-constantan

thermocouple is described by Roeser [6].

There were radiation and conduction errors within each array, but these were sufficiently small

so that the arrays served as satisfactory indicators

of air mixing and radial temperature gradients.

Throughout the testing program, when there was

a measurable difference in temperature between

the moving air and the duct wall, there were low

radial temperature gradients and almost no fluc- tuation in temperature in the region immediately

surrounding the probe. Throughout the rest of the

volume of the stream of moving air, the air exhibited temperature fluctuations in increasing magnitude and frequency in locations near the duct wall.

Reference sensors were used as a means for obtaining temperature values almost completely unaffected by radiation and conduction and for making comparisons Avith the values obtained with the

probe. These consisted of three thermocouple junctions fabricated from No. 36 Chromel P-constantan wire supported by a plastic cage-like structure. The six lead wires were bare and were strung

back and forth in the cage so that about 5 in of

the wire was exposed to the moving air. This technique, together with use of fine wires, minimized

conduction error. The three junctions were butt

welded to minimize radiation error. The average

of the three thermocouple readings was used for

the reference value.

Details of the experimental program required

that two different types of reference sensors be

used. The first, used for the tests where the probes

4
were shielded, fitted snugly into the inner of two

10-in-long concentric shields just upstream from

the probe. This reference sensor and the concentric shields are shown in figure 5. The two shields were made of polished aluminum. The diameter

of the imier shield was 2 in and that of the outer

shield was 3 in. The second reference sensor, used

for the tests where the probe was not shielded,

fitted snugly into the inner of two 3-in-long concentric shields fabricated especially for the refer- ence sensor. The sensor and its shields were placed

immediately upstream from the probe. These are

shown in figure G. The diiiirieter oi' the inner sliield was 1 in and of the outer shield was 2 in. These

small diameters were used to rediKMi radiation end

effects. Both of the concentric shields were made

of polished aluminum.

The effectiveness of the radiation shields for the

reference sensors was determined by special tests and the errors were found to be very small but

measurable. Details on how measurements and cor- rections for the small errors were made are given

later.

Figure 5. Large shield and the reference sensor used

in the tests where the test proie was shielded.

Figure 6. Small shield and the reference sensor used in

tests rvhere the test prohe loas unshielded. Removable legs for the shield are shown In lower left.

3. Calibration

The errors in uncalibrated temperature-sensing

devices are usually determined by comparison of

their indications with those of a secondary or primary standard whose errors have previously been

analyzed. These comparisons are typically made

in an apparatus in which there is minimum opportunity for heat transfer to or from the devices

by radiation, conduction, or convection and in

which the change of temperature with time is also minimized. An example is calibration of thermistors in a water bath whose temperature is close to the temperature of the room. In the present study

the errors of several types of sensors were to be

evaluated under conditions in which there was opportunity for heat transfer to or from the devices

by radiation, convection, and conduction. In such

a situation, a secondary or primary standard would

also have unknown errors, so a simple comparison was no longer adequate.

Therefore, the following plan of attack was

followed

:

a. The platinum resistance themiometer used in

the test probe was calibrated by the NBS

Thermometry Laboratory, using the normal

procedures described by Riddle [7]

.

b. Because of the well known stability of platinum resistance thermometers, the platinum

resistance thermometer used in the test probe was used to calibrate the thermocouples and

thermistors and also the two tln-ee- junction

reference sensoi'S in still air under isothermal

conditions.

c. The magnitudes of small errors of the two

three-junction reference sensors under nonisothermal conditions in the test duct were

determined by comparing temperature indications when the junctions were aspirated at a velocity of 1400 fpm and when they were not

aspirated.

d. The magnitudes of the lai'ger errors of the

thermocouples, thennistors, and the resistance

thermometer in the probe wei'e determined

under nonisothermal conditions in the test duct for a range of air velocities by comparison with the calibrated reference sensors.

e. At the close of the program, the observed

readings of the resistance thermometer in the

probe were compared in still air with observed readings of another resistance ther- mometer calibrated by the NBS Thermometry

5
laboratory. At the four temperatures within

the range of temperatures of 75 to 80 °F at which the comparison between the two re- sistance thermometers was made, there was

less than 0.02 deg F (0.01 K) difference

between the observed readings of the two

thermometers.

The thermocouples, thermistors, and the large

reference sensor were calibrated four times within

a year, and the small reference sensor twice, one

year apart. These sensors were calibrated in air,

shielded from ambient temperatures by a large in- sulated Dewar flask. This flask is shown in figure 7. An insulated cover was over the flask during

calibration.

Figure 7. Calibration chamler for calibrating the thern>

istors, thermocouples, and the ttvo reference sensors in

still air.

The changes in performance for the thermocouples, thermistors and the reference sensors be- tween calibrations are shown in table 1. The

changes listed are consistent with those shown in

isothermal tests reported by Davis, Faison, and

Achenbach [1].

Before each calibration in still air, the Dewar

flask housing a probe or the reference sensor re- mained for at least 24 hr at an ambient temperature constant within ±0.06 deg F and usually

within ± 0.04 deg F. After the calibration started

there was usually a small rise in temperature in

the room due to the heat from the lights and from

the observers. Full consideration was therefore

given to errors which might have been caused

by the order in which the thermistors or the thermocouples were read. Analysis showed that the

contribution of self-heating of the resistance

thermometer or the thermistors due to current flow was negligible.

When a reference sensor was calibrated, the

cage-like structure was maintained at a center

position in the flask so that the three No. 36 buttwelded Chromel-constantan thermocouple junctions were about % in from the end of the

resistance thermometer. Figure 7 shows the ref- erence sensor for the 10-in set of shields as it was

placed inside the Dewar flask ready for insertion

of the probe which housed the resistance

thermometer.

The test probe with its three types of sensors, and the two reference sensors, were each calibrated

as an assembly to avoid the problems of disassembly. These assemblies were calibrated in still air

rather than water to avoid the need for insulating

the sensors electrically.

When determining the magnitudes of the small

errors in each reference sensor by the aspiration

technique, the velocity of the air moving across the reference junctions within the inner shield was increased to 1400 fpm using a vacuum pump.

Special adaptors were used for connecting the

air-line from the inner shield to the vacuum pump.

A comparison was made between aspirated and

non-aspirated values at selected levels of temperature difference (A^) between the duct wall and the

moving air at the probe as measured by the re- sistance thermometer. Based on well-established

theory [8, 9], calculations showed there was no

appreciable error in the reference sensor due to radiation or conduction at a velocity of 1400 fpm,

even for a of 50 deg F. This comparison showed that the error of the

large reference sensor in the long 10-in set of

shields reached a value of 0.08 °F when the differ- ence in temperature between the duct wall and

the moving air was 50 deg F and the air velocity was either 300 or 500 fpm. The errors of the small

reference sensor in the small set of shields were

considerably greater than for the large reference sensor under comparable conditions. Curves showing the errors determined in this manner, and used

for corrections for both reference sensors, are shown in figure 8. A small uncertainty is indicated

at the bottom of the curves for each reference sensor by a horizontal line across the figures at a distance about 0.04 deg F above the zero error

line at the 0 °F point. For both reference sensors, the indicated temperatures when aspirated were

0.04 deg F lower than when unaspirated with no

temperature difference between the duct wall and

the moving air. This disparity is the reverse of

what would be expected as a result of impact effects. While the cause is not known, it is probable

that there was a change in the air mixing pattern

within the shield when the air velocity was

changed. Since the error involved was not due to radiation it was not considered in the corrections.

6
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ol \ I I \ I I I I 0 10 20 30 40 50 60 70 DIFFERENCE IN TEMPERATURE BETWEEN THE MOVING

AIR AND THE DUCT WALL (AT), deg F

FiGTJKE 8. Errors in the two reference sensors as a function of the difference in temperature between the duct wall and the air at the reference

sensors. These errors were used for making corrections in calculations.

4. Method of Testing

Tests were performed at 300 and 500 fpm with

and without shields around the probe. Tests at 1300 fpm were performed only without shields,

because the theory and results from the tests at 300 and 500 fpm showed that the radiation error

at 1300 fpm with shields would be almost nonexistent even for a At of 50 deg F, Tests for each

condition were performed with none of the sen- sors aspirated, and during a continuous 12-hr

period to reduce the effect of differences in ambient

conditions, thereby facilitating better comparisons.

Before the design of the probe was fixed, calculations had been made to determine if conduction

of heat from the duct wall along the wires was

significant [9,10]. Before the tests, experimental

verification was made. Hot metal, about 50 deg F

hotter than the temperature of the duct wall, was

applied suddenly m the hollow phenolic tube

(designated as C on figure 1) to the plastic tube

housing the lead wires. There Was no indication

of temperature change on any of the indicating

instruments for the thermocouples, the thermistors, or the resistance thermometer. The same technique was used for the reference sensors. In each

case there was no indication of temperature

change.

All air velocity measurements were made with a

pitot tube and a self-calibrating manometer which

could be read to the nearest 0.01 in water gage.

Tests with the long metal shields around the

probe were performed first. The sensors of the

probe were not aspirated. The reference sensor as- sembly was placed in the end of the inner shield.

Each test was performed at a selected level of

velocity of 300 or 500 fpm. For these tests the

difference in temperature (A^) of the moving air

and the duct wall was held at 0, 10, 20, 30, 40, or 50 deg F for a period long enough to assure

steady-state conditions. The value of M was con- sidered to be the difference between the reading of

the resistance thermometer and the average of the

readings of all the thermocouples in the rings

around the duct, at the bottom, and on the access door. Steady-state conditions were obtained for

each test, but no attempt was rnade to obtain

values of any closer than within 1 deg F of the

preselected values. At each condition of velocity

and temperature difference, 15 readings were

taken for the thermocouples, the thermistors, and

the resistance thermometers. The readings were 2

min apart.

Before taking the readings, readings of the

thermocouples of the three arrays along the duct were taken to determine if the air mixing pattern was satisfactory. A satisfactory and randomly

chosen cross-sectional temperature pattern from a

test is shown in figure 9. This pattern was obtained

during a test with Ai=40 deg F and with an air

8
NUMBERS. FiGUKE 9. Map of air-tempcratures in Fahrenheit degrees

of a cross-sectional area during a typical test when A^ =

40 deg F. Data obtained from an array of butt-welded chromel constantan thermocouples fabricated from No. 36 wire (AWG). Array was

immediately upstream from the probe.

velocity of 500 fpm. It was for the array of thermocouples immediately upstream from the probe.

The cross-sectional temperatures, it will be noted, were essentially uniform in the central area for the seven thermocouples nearest to the center of the

duct. Analyses of comparable data for the other

two arrays along the length of the duct during the same test showed a similar and satisfactory pattern. This is illustrated in table 2, showing temperatures for selected corresponding positions in

the arrays. The mixing was satisfactory for all tests. Comparison made of the pattern m figure 9 with two

other determinations, made at the same station at 40 deg F ^.t and at 500 fpm, showed that in each

case the maximum difference between any of the

seven center readings was less than 0.30 deg F.

The difference between the average of these 7 readings and the center reading was ahvays within

0.10 deg F. For the tests at 50 deg F M at the same

velocity, the maximum difference between any of

the seven readings was about 1.0 deg F. The difference between the average and the center was

0.7 deg F. For the tests j^erformed at smaller

values of the differences were considerably

smaller.

The temperature of the inlet air coming into the

duct was monitored using a copper-constantan

Table 2. Air temperature for corresponding positions in the three arrays along the length of the duct, 500 fpm

A<=40 deg F, Duct wall temperature= 119 °F, Inlet Air Temperature=78.3 °r

Position No. in

ii/acu Array

First Array

(Upstream

from heated

section of duct)

Second Array

(In heated

section of duct,

upstream

from probe)

ThirH Arrav (Downstream

from probe

in heated

section of duct)

1 78. 34 80. 59 83. 97

2 78. 46 78. 64 79. 14

3 78. 37 81. 36 83. 99

4 78. 34 78. 58 78. 76

5 78. 43 80. 77 85. 30

6 78. 43 78. 67 79. 53

7 78. 46 79. 88 82. 31

thermocouple in conjunction with the digital volt- meter. There were unavoidable small cycles in the

temperature of this inlet air, and to minimize any

effect of these cycles, determination of the air pat- tern was made only at instances in time when that

temperature was constant within 0.1 deg F. The

mass in the metallic air mixers tended to dampen

the cycles and help even more. The readings for

the thermocouples in the arrays were taken in

rapid succession.

After the tests with the 10-in shields were per- formed, tests without shielding around the probe

but with small shields around the reference sensor were made at 300, 500 and 1300 fpm. The small

reference sensor, in its special set of sliields (fig.

6), was placed immediately upstream from the

probe. The same test conditions as used for the

tests with the 10-in shields were used. For the test at 1300 fpm it was not possible to obtain readings

for At=^50 deg F because of shortage of electrical power in the test facility.

The temperature of the moving air was steady near the axis of the duct; therefore it was possible

to determine the readings of the fine-wire, buttwelded, thermocouples in the reference sensors with the manually operated potentiometer.

5. Results

The results of the studies are shown in figures

10 and 11. Figure 10 includes curves showing the

effect of radiation when the three sensors were

measuring the temperature of the moving air at 500 and 1300 fpm, and figure 11, curves showing

the effect when the sensors were measuring this

temperature at 300 fpm. For the unshielded sensors under the conditions of 1300 fpm (6.6 m/s)

air velocity and a of 50 deg F (28 K), the

observed error was about 1 deg F (0.6 K). For

500 fpm (2.5 m/s) and 300 fpm (1.5 m/s), under

the same test conditions, the error was about 3.0 deg F (1.65 K) and 3.8 deg F (2.1 K), respectively. When the sensoi'S were shielded, at both 300

and 500 fpm and under the same test conditions,

the error was less than 0.2 deg F (0.1 K)

9
DIFFERENCE IN TEMPERATURE (K)

5 10 15

10 15 20 25 30 35 40

DIFFERENCE IN TEMPERATURE OF DUCT WALL AND MOVING AIR AT THE CENTER OF THE DUCT ( deg F

)

Figure 10. Observed radiation error for the three types of sensors in the test proVe, when

used in air having velocities of 500 and 1300 fpm.

DIFFERENCE IN TEMPERATURE IK) 5 10 15

10 15 20 25 30 35 40

DIFFERENCE IN TEMPERATURE OF DUCT WALL AND MOVING AIR AT THE CENTER OF THE DUCT ( deg F

)

> 10

Figure 11. Observed radiation error for the three types of sensors in the test probe, when

used in air having a velocity of 300 fpm.
The indicated radiation error was greater for

the thermistors than for the other two types of

sensors for all but one test condition. However, the

indicated radiation errors for the thermocouples

and the resistance thermometer change in magnitude relative to each other from one year to the

next. The cause of the change is nbt understood,

but it is probable that from the spacing on figures

10 and 11 between the thennistor curves and the

resistance thermometer curves, and from the con- stancy of their relative positions, that the thermocouples had changed, not the thermistors or the

resistance thermometer.

6. Errors in Measurement

Uncertainties occurring in the experimental

work resulted in small errors in the observed results. For the tests where the ])robe was shielded,

the possible error in experimental results was rela- tively large when compared to the radiation error

reported. This uncertainty was principally due to the drift in observed values between calibrations

for the thermocouple wire in the reference probes.

Table 1, giving the change in performance of the

probe sensors and the reference sensors, shows that

the performance of the reference thermocouples

sometimes changed 0,06 deg F between calibrations,

and for two measurement temperatures changed — 0.10 and —0.12 deg F, respectively. Another uncertainty was involved in using the aspiration

technique on the reference sensors where the

measured radiation error was 0.04 deg F at A;^=0.

It is possible that some of the experimental

errors for the shielded probe could have been cumulative and could have totaled as much as 50 percent

of the reported radiation error of 0.2 deg F at M

= 50 deg F. However, in view of the several

possible sources of error it is more probable that

there were compensating effects and that they

totaled something less than 50 percent. Even so, a reported radiation error of 0.2 deg F at A^=50

deg F with an experimental uncertainty of 50

percent is not serious for most laboratory

measurements.

For the tests where the probe was not shielded,

the possible error in experimental results was rela- tively small when compared to the radiation error

reported. The same causes for experimental error occurred for these studies as for the studies when

the probe was shielded, but the effect of cycling of

the temperature of the air stream was more evi- dent. Another contributing factor seemed to be the

scatter of readings in the thermocouples on the

duct which made it difficult to know the difference

(A^) between air temperature and duct surface

temperature for a given test with an accuracy of

better than 1 deg F. Because of the difficulty, a

choice was made after testing to plot the curves

of figures 10 and 11 using the preselected nominal

values of A^. These factors contributed to the dis- persion of the plotted error values in figures 10

and 11.

7. Summary and Discussion

Results showed that the three types of sensors in

the probe were materially affected by radiation.

At 50 deg F (28 K) difference in temperature

between the duct wall and the air at the center of

the duct, the error for all three unshielded sensors due to radiation was about 3.8 deg F (2.1 K) at 800 fpm (1.5 m/s) 3.0 deg F (1.7 K) at 500 fpm

(2.5 m/s). At 1300 fpm (6.6 m/s) the expected

error with the sensors unshielded would be about 1 deg F (0.6 K). Radiation error for all three

shielded sensors was less than 0.2 deg F (0.1 K)

even at as low a velocity as 300 fpm (1.5 m/s) and

for a Ai5 of 50 deg F (28 K)

There appeared to be little change in calibration

of the thermistors during the period of two years

in which the studies were made. The change in the

thermocouples, including the reference thermocouple ranged from 0.0 to about 0.10 deg F (0.06

K). The change in calibration of the resistance

thermometer during the period was not significant.

The use of Chromel P-constantan thermocouples

dE

has the advantage of a high -r^ and of low

thermal conductivity. They were also found to be

easy to use during fabrication. They were rugged

despite their small diameter. The electrical re- sistance of Chromel P-constantan wires is high,

however, and when used with a digital voltmeter,

without a preamplifier of high input impedance,

such resistance may cause significant error. The

Chromel P and the constantan wires should be

tested for inhomogeneities [11]. It is recommended that a shielded probe as de- scribed be used for measuring temperature of

moving air with either thermocouples, thermistors, or a resistance thermometer when the temperature of the duct is 5 deg F (2.8 K) or more above

or below the temperature of the moving air in the

center of the duct. Any of the three sensors could

be used with greater accuracy, at low air velocities

when aspirated in a shield. The investigator can

determine the approximate degree of error in- volved from the use of the curves of figures 10 and

11. It is immaterial whether the duct wall is colder

or hotter than the moving air. If colder, the same

11
magnitude of error determined from the curves

will apply but will have a negative sign.

Under conditions often occurring in a psychrometric calorimeter for measuring the capacity of

air-conditioning equipment when the air velocity is 1300 fpm and A#=5 deg F, the expected radiation

error for unshielded sensors would be about 0.1 deg F. For 500 and 300 fpm, the error would be

0.2 deg F and 0.25 deg F, respectively. For all of

these conditions the expected radiation error for

shielded sensors is less than 0.1 deg F (0.06 K).

Under the test conditions that prevail in the testing

of air-conditioners and heat pumps in laboratories,

it should be possible to reduce the total error in

temperature measurement of the moving air to about 0.2 deg F (0.1 K) by a combination of suit- able air mixers, duct insulation, radiation shields

and calibration techniques. This value is in agreement with the value recommended by Davis,

Faison, and Achenbach [1] for Temperature

Measuring Standards.

If all other conditions remained the same, in- creasing the diameter of the test duct beyond the

10 in size used for this study would decrease the

magnitude of the radiation errors of a shielded

probe somewhat because of the smaller radiation

effects through the open ends of the shields, but

would have very little effect on the errors of an

unshielded probe. The curves for the shielded condition are applicable only if shields of the same

material and low temperature-emissivity characteristics are used. A probe fabricated with material

having a lower emissivity than methyl methacrylate, which has an emissivity * of about 0.7 to 0.9, or one which is wrapped with a covering of reflective

foil, would have a smaller radiation error when

used as an unshielded probe. In an actual application, the temperature sensor would probably consist of a single element of considerably smaller

diameter than the entire probe used for the study.

Such an element would probably have a somewhat

more favorable relationship between radiation

heat gain and convection heat loss than the test probe. Presumably individual sensors could be

designed which have no more error than the refer- ence sensors. It is important that the design of a temperature-sensing probe incorporate prmciples

for minimizing conduction errors similar to those

used in this investigation.

Appreciation is expressed to Messrs. Thomas K.

Faison, Walter M. Ellis, and Jesse Dungan of the

NBS staff for their assistance in performing the

tests and in difficult fabrication of some of the

pieces of equipment.

* As determined from materials such as paints -which have

similar radiation characteristics at the temperatures under

consideration.

8. References

[1] Davis, J. C, Faison, T. K., and Achenbach, P. R., Errors in temperature measurement of moving air under isothermal conditions using thermocouples,

thermistors, and thermometers. ASHRAE Trans.,

vol. 73, Part I pp. VII 1.1-1.10 (1967).

[2] Caldwell, Frank R., Temperatures of thermocouple

reference junctions in an ice bath, J. Research

NBS c. Eng. and Instr. 69C, No. 2 (April-June

1965).

[3] Friedberg, S. A., Semiconductors as thermometers.

Temperature, Its Measurement and Control in Science and Industry, 2, Reinhold Publ. Corp.

(New York, N.Y., 1955).

[4] Drums, C. R., Thermistors for temperature measurements. Temperature, Its Measurement and Control in Science and Industry, 3, Pt. 2, Reinhold

Publ. Corp. (New York, N.Y., 1962).

[5] Faison, T. K., Davis, J. C, and Achenbach, P. R., Performance of louvered devices as air mixers. NBS Building Science Series Report (in preparation).

[6] Roeser, W. F., Thermoelectric thermometry. Temperature Its Measurement and Control in Science and Industry, 1, Reinhold Publ. Corp. (New

York, N.Y., 1939).

[7] Riddle, John L., Notes to supplement resistance thermometer reports. Obtainable from Temperature

Physics Section, Heat Division, Institute of Basic

Standards, National Bureau of Standards, Washington, D.C. 20234.

[8] Eckert and Drake, Mass and Heat Transfer, pp. 426-428, McGraw Hill Book Co., Inc. (1959).

[9] Fiock, E. F., Olsen, Lief O., and Freeze, Paul D., The

use of thermocouples in streaming exhaust gas, Volume of the Third Symposium on Combustion,

Flame and Explosion Phenomena at University

of Wisconsin, pp. 655-658 (1948)

.

[10] Kreith, Frank, Principles of Heat Transfer, pp. 376 and 377, International Book Co. (1958).

[11] Powell, Robert, Caywood, Lindsay P., Jr., and

Bunch, M. D., Low-temperature thermocouples.

Temperature and Its Measurement and Control

in Science and Industry 3, Pt 2, Reinhold Publ. Corp. (New York, N. Y. 1962).

12
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