NATIONAL BUREAU OF STANDARDS REPORT

3761

DESIGN AND PERFORMANCE OF REMOTE THERMOSHIELD,

NAX-2-51

Richard N. Jansson

Oliver N. McDorman

Henry E. Robinson

Report to

Bureau of Aeronautics

Department of the Navy

Washington 25, D, C.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

by
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1003-20-4^32 October 21, 1954

NBS REPORT

3761

Design and Performance of Remote Thermoshield MAX-2-51 MODELS 001 and 002

by

Richard N. Jansson

Oliver N. McDorman

Henry E. Robinson

Heating and Air Conditioning Section Building Technology Division

to

Bureau of Aeronautics

Department of the Navy

Washington 25, D.C.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, re

unless permission 1

25, D, C. Such pe

cal ly prepared if I

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

r In part, is prohibited

itandards, Washington

eport has been specifireport for its own use,

DESIGN AND PERFORMANCE OF REMOTE THERMOSHIELD,

NAX-2-51 MODEL 001 AND MODEL 002

by

Richard N, Jansson, Oliver H. McDorman, Henry E. Robinson

Abstract

A temperature reference unit, called a thermoshield was developed which is believed to indicate the true dry bulb temperature of atmospheric air within ± 0.1 deg. F, even when the

instrument and its proximity are in strong sunlight. The instrument is suitable as a standard

reference unit for calibrating other weather instrument shelters. Another model has been constructed for remote installation aboard ship to

be used in conjunction with automatic indicating

and recording equipment located at a central monitoring station. Both models employ blowers to

circulate ambient air over their sensing elements

which are shielded from radiation by l/2-liter

Dewar flasks.

1. INTRODUCTION

The development of a remotely-located miniature thermoshield with electrical recording devices is the result of a

number of parallel events of the past several years.

In 1949 the Aerology Section of the Bureau of Aeronautics, while collaborating with the National Bureau of Standards on the thermoscreen project, suggested the replacing

of the thermoscreen, where the added expense was justified,

with an overall system consisting of a remotely located

thermoshield and a recording system placed conveniently for

observation. At about the same time the Air Force asked the

Signal Corps to produce a small remote temperature measuring

system for airborne use. The results of this development are

still pending.

In 1950 the Aerological Section of the Naval Research Laboratory started the development of a remote system for recording

weather data for aircraft using a vortex cooling principle to

compensate for frictional skin heating on aircraft* This unit,

near completion, is called the Vortex Thermometer. A groundbased model of this recording system, which is called the Aerological Recorder, is also nearing completion.

During the summer of 1952, as a part of the continued thermoscreen program carried on at the National Bureau of Standards, the

further development of ventilated ambient temperature measuring

units was found to be desirable. Throughout the tests of a prototype thermoscreen under the auspices of the Bureau of Aeronautics,

the thermocouples inside the thermoscreen read consistently about

l/2 deg. F lower than the air temperature reference that had been

used up to that time. This led to the development of the ambient

air temperature sensing device described here.

The primary objective of the design was an instrument capable

of indicating the temperature of atmospheric air without errors

or effects due to solar or other radiation. The design was intended

to provide not only practically complete shielding of the temperature

sensing elements from outside radiation, but also prevention of heat

transfer. to or from the air stream between the inlet of the unit

and the temperature sensing elements.

2. DESCRIPTION OF INSTRUMENTS

Model 001 Unit

Figure 1 shows a vertical cross-section of the model 001 type

unit. It consists of a l/2-liter Dewar flask inverted over a 2-inch

0D thin-wall bakelite tube, a motor driven blower for air circulation

and an outside enclosure. The tube supports the flask with a plastic

hoop at the top and has small pieces of cork to keep the flask concentric. The tube is secured to a black bakelite disc at the bottom

with glue, and a 4-1/2 inch 0D brass tube surrounds the flask unit.

The motor and blower are mounted at the top of the exterior tube.

The whole unit is suspended from a hook at the top of the motor, with

the axis of the unit vertical. The air flow through the unit was

approximately 90 cfm corresponding to a velocity of abuut 4200 fpm

in the intake tube.
'

.
- 3 -

For the tests at the National Bureau of Standards the

001 unit was instrumented with a copper-constantan thermocouple that was "loaded" with a 3/3-inch brass ball. The

thermocouple was located at the upper end of the internal

tube so it was exposed to direct radiation from below only

within the small solid angle formed by the thermocouple and

the bottom of the tube.

Model 002 Unit

Recently a model 002 unit was built for the Naval Research Laboratory. The design of this unit was similar to

that of model 001 except that certain features were modified

to make the model 002 unit suitable for shipborad use. It

was built with the understanding that the Naval Research

Laboratory would develop the instrumentation for indicating

and recording the temperatures and the National Bureau of

Standards would develop the thermal aspects of the enclosures

for the sensing elements.

The model 002 unit is ventilated by a self-contained

motor-driven blower housed within a 4f=In. square aluminum tube

12-inches long (Figures 2 and 3 )» Also mounted within this

tube is a spring-mounted Dewar flask which serves as an effective thermal shield for the temperature-sensitive element.

The motor is bolted directly to the aluminum top cover to permit it to dissipate some of its heat to the outer walls and

still be sheltered from weathering effects. The blower exhausts through the supporting strut a 2-inch diameter tube

14-inches long connected to a 4-inch transition piece (Figures 2 and 3) so that recirculation is prevented between the

exhaust and the intake. The Naval Research Laboratory is

proposing to use this exhaust air for humidity measurements.

* The electrical connections are made with standard AN

type Cannon connectors that are housed in the support strut

flange. All connectors are mounted from the inside of the

thermo shield housing so that any component ( motor , thermometer, or defroster) may be removed without detaching any wires.

Figure 4 is a view of the model 002 thermoshield disassembled to show the principal components. The air enters
.

'
= 4

-

the bottom horizontal surface of the unit through an opening in the white nylon bottom plate. The air is given a

circular motion as it flows upward through the inner aluminum tube by a plastic vane element from a Vortex Thermometer.

This circular motion of the incoming air forces any entrained

water particles away from the sensing element and toward the

outer wall. The vane element supports and positions the temperature sensing element, which for test purposes consisted

of a thermocouple attached to a brass slug of a thermal mass

equivalent to that of the resistance thermometer element to

be inserted by Naval Research Laboratory. This assembly,

consisting of tube, vane, and thermometer, is surrounded by

the inverted Dewar flask. After passing the temperature

sensitive element the air passes downward between the tube

and flask and upward around the outside of the flask to the

blower.

This model was developed for mounting in remote locations

on a ship with the recording systems located conveniently for

the observers. The unit should be cleaned and oiled at intervals of 4 to 6 months. It may be removed from its support tube

by releasing the latch, removing the electrical connectors,

and removing the hinge pin (Figure 3).

Air flow measurements on the unit showed that the flow

was about $ cfm with a velocity at the thermometer of about

1200 ft/min.

Reference^) found in the literature suggested the use

of a Dewar flask as a thermocouple radiation shield 0

3. TEST PROCEDURE AND RESULTS

Air Recirculation

Smoke flow tests and comparison of their temperature

indications were made on the model 001 and 002 units. The

smoke flow tests were used to determine whether recirculation

occurred between outlet and intake of the units and to study

the amount of air sampling. One of the original premises in

the measurement of outdoor air temperature was LhaL air masses

move in small eddies or thermal streams and that a temperature-

sensing unit would have to move relatively large amounts

of air to obtain a fair sampling of the air and an average

of the air temperature in the vicinity of the device,, A

hand-type smoke generator was used to indicate air motion

in the vicinity of the thermoshields.

It was observed that the model 001 unit sampled and

mixed large amounts of air drawn from quite a distance

from its inlet and exhausted the air horizontally at a high

velocity above the intake level so that the exhaust air was

thoroughly mixed with the ambient air before it had a chance?

to reenter the inlet.

The model 002 unit, on the other hand, circulated much

less air and the tests indicated that only the air quite

close to the intake opening was sampled. The air from this

unit was exhausted at a much lower velocity and in a downward direction from the end of the support tube. The air

was forced down to the level of the intake and recirculation

could and probably did occur occasionally when the intake was

down wind from the exhaust.

Temperature Tests

The temperature tests were conducted on a comparative

basis between the two models because no absolute standard of

greater precision was available for calibration purposes.

For these tests the two units were mounted side by side on a

counterweighted boom arm so as to sample the same air stream.

This arm was rotatable so the units could be oriented in a

desired direction with respect to the sun. The thermocouples

in the two units were connected as a differential thermocouple

with the leads connected to a portable potentiometer with a

sensitive external galvanometer. The temperature differences

were read as deflections on the galvanometer scale which had

previously been calibrated in deg, F per millimeter scale

division.

Preliminary tests were conducted in the very early morning using heat lamps as artificial radiation sources to selectively produce the solar heating effect upon the thermoshields. This method of test was discontinued because it was

recognized that the heat lamps did not produce the same

radiation effects on the thermoshields as daytime solar

radiation, particularly in regard to radiation from the

ground. Also, the radiation intensity was unknown and

could not be easily determined. Other tests were conducted in sunlight by alternately shading the units with large

cardboard sheets. This method was unsatisfactory because

only the units were shaded and because the shields were

warmed above air temperature and induced stray thermal currents that at times affected the readings of the units under

test

.

Since the 001 unit circulated a greater quantity of air

and produced greater air movement near the unit it is probably better from a heat transfer standpoint and thus more

accurate than the 002 unit which was modified for shipboard

use. The 001 model should probably be used where greater

accuracy is desired and when serviceability is not a determining factor. Because of the appreciable difference in air

circulation comparative tests were conducted on the two units

using the 001 unit as a reference.

Table 1 is a summary of the test conditions under which

the two thermoshields were compared and the differences in temperatures indicated by the two units.

Data taken under test condition A were obtained in the

morning about 9«00 AM EST on clear sunny days, but the units

were in such a location with respect to local shrubbery that

adequate and well-ventilated shade was provided for both units

and their environment. The units were exposed to very little

radiation under this test condition and the difference in indicated temperature was much less than 0.1 deg. F.

The data recorded on sunny afternoons with the grass sward

under the units in strong sunlight, summarized under Condition B

in Table 1, show that radiation increased the difference in the

indicated temperature of the two units quite markedly. The

temperature difference ranged from 0.10 to 0.24 deg. F for this

group of tests with model 002 indicating the higher temperature.

Data summarized under condition C in the afternoon show that an

overcast sky decreased the temperature difference between the

- 7 -

two units. These results indicate that model 002 was adversely affected by solar radiation to a greater degree

than model 001 o

Special tests under condition D were made to discover

the cause of the temperature rise in model 002 when strongly

irradiated. By removing the vortex vane from the intake tube

of model 002 the air flow was approximately doubled with a

consequent decrease in the temperature indicated by this model.

Under these conditions model 002 indicated a temperature 0,03

deg. F lower than model 001 on the average when both were exposed to strong afternoon sunshine.

To further show the effect of the air flow rate on the

accuracy of the thermoshields 9 the air flow rate in model 001

was drastically reduced and a comparison made with model 002

without the vane in the latter unit. Under these conditions

the average difference in temperature of the two units was

0.22 deg. F with the 002 unit reading the lower value.

These results indicate the desirability of a high air flow

rate through the thermoshield and show that the vortex vane

had an adverse effect on the accuracy of the model 002 unit in

bright sunshine. The vortex vane is considered desirable 9 however ? in wet weather to deflect entrained water droplets

from the temperature sensitive element.

The rate of response of the temperature sensitive element

was much higher in the model 001 unit than in the other unit.

This was attributable to the higher air flow rate and to the

lesser thermal mass of the sensitive element in the model 001

unit. The thermal mass of the vortex vane in the 002 unit would

also make the sensing element in that unit less responsive to

sudden temperature changes. The test in which the air flow in

the 001 model was reduced further illustrated the importance

of air flow rate on the rate of response because no sudden

variations in temperature were indicated during this test.
' ’ • . .
- s -

TABLE 1

COMPARISON OF TWO THERMOSHIELDS

MODELS 001 , 002

Difference

Time Wind* in Indicated

Date EST Knots Sky Tempb deg, F

Condition A? Units Shaded

9-23-53 9° 25 2-3 Clear <<0„la

9=29=53 9: 05 calm Clear << 0„X

9-30-53 9:05 2-3 Clear <<0„1

Condition B: Units in Bright

Sunshine

9-20-53 13=40 2-3 Clear 0 o l6

9=29=53 13:15 slight Clear 0.18

9-30-53 13:00 2-3 Clear 0 o 20

2-5=54 13:15 5-15 Clear QolQ

2-7=54 14:22 gusts Clear 0 o 24

Condition C: Sky Overcast

2-8-54 15:45 0-5 Very Hazy 0,07

Condition D: Special Tests

Model 002 with vane removed:

2-7=54 15 “ 00 0-5 Clear -Q 0 Q$

Model 002 with vane removed; Model 001 with restricted air flow

2-7-54 14:45 0-5 Clear -0 o 22

a. The symbol <2<C 0 o l means n mueh less than tf one tenth degree

Fahrenheit „

b A positive difference means model 001 indicated a lower

temperature

„

- 9

-

4* DISCUSSION

There are four principal ways in which heat may enter

thermoshield NAX-2-51 to cause an error in the indicated temperature by raising the temperature of the air flowing past

the temperature-sensing element*

1) By transfer from the exterior enclosure

„

2) By radiation reflected or emitted from the ground

and absorbed on the under surfaces of the thermoshield housing or entering the air inlet*

3) By conduction of heat along the bottom surface of

the thermoshield from the outer casing toward the

inner tube*

4) By circulation of exhaust air that has been warmed

by motor heat or radiant energy absorbed by the

thermoshield

„

The following equation can be used to express the relation

between the heat transferred to the entering air and the temperature rise of the air:

q = mc£±t

where q

= heat absorbed, Btu/hr

m = mass flow of air, Ib/hr

c = specific heat of air under test conditions = 0,24

A t - difference between indicated temperature and true

air temperature, deg, F,

If it were assumed that the same amount of heat was transferred to the air entering each of the two models of the thermoshield, the following relation would exist:

m^cAt^ - n^eAtg

or

ra 2

At2

Xti

n

v2

where v^ and V2 ” volumes of air circulated in models

001 and 002, respectively, efm.

10 -

Under the conditions that produced the greatest temperature

difference shown in Table 1 9 A to = + 0 o 24 9 V]_ = 90 efm s and V2 “ $ cfm n

Thus Ah. + 0 o 24

llo3

- 0 o 023 dego F

At2 " 0 o 26 dego F

q - 2.2 Btu/hr

Based on these assumptions the error in the model 001

thermoshield for the worst conditions reported in Table 1

was about 0 o 02 deg 0 F and the amount of heat transferred to

the entering air was 2 0 2 Btu/hr , whereas the error in the

model 002 thermoshield was about 0„26 deg, F„

Calculations 9 with certain assumptions „ were made to

estimate the approximate magnitudes of the first three ways

of heat transfer to the inlet air listed above „ Heat transfer

from the housing through the Dewar flask to the inner tube was

estimated to be very small s both because the Dewar flask has a

very low heat transmission coefficient , and because there were

four surfaces between the outer surface of the unit and the

inner surface of the inlet tube from which heat was removed by

the high velocity air stream after it had passed the temperature

sensing element,, The third item listed,, the conduction of heat

along the bottom surface of the thermoshield from the outer easing toward the inner fcub@„ also was found by computation to be

very small

„

Radiation absorbed by the under surface of the thermoshield

can be separated into two kinds (1) radiation emitted by the

ground or grass surface and (2) solar and sky radiation reflected

from the ground or grass surface 0 Assuming the grass surface

to have been about 10 deg, F warmer than the air „ radiation

emitted from it and absorbed by the under surface of the model 001

thermoshield would have been about 0„9 Btu/hr. Assuming that at

the time of the test under consideration the total radiation incident on a horizontal surface was about 145 Btu/hr per square

foot„ and that from 5 to 10 percent of this was reflected upward ? the absorption of such radiation by the undersurface of the Model 001

thermoshield would have been from 0„7 to 1.4 Btu/hr „ The total

11 -

heat transferred by absorbed radiation is estimated therefore to have been from 1,6 to 2,3 Btu/hr, This is in reasonable agreement with the air heat gain of 2 C 2 Btu/hr calculated previously from the difference in the temperature indications of the two units.

These figures provide reason to believe that the error

of this type of thermoshield is due chiefly to heat gain by

absorption of radiation incident from below ? assuming a constant rate of air flow through the unito If the thermoshield

were mounted over a ship ? s deck, or over bare or paved ground,

the temperature difference between the surface and the thermoshield could be much greater than 10 deg, F and the heat transfer by directly-emitted radiation would be correspondingly

greater. The absorption of solar radiation reflected from the

surface could also be markedly different , depending on the solar

reflectivity of the surface. Absorption of either kind of radiation could be minimized by making the under surface of the

thermoshield as reflective as possible to both solar and low

temperature radiation c Elevating the thermoshield to a greater

height above the surface 9 if the latter were of large extent s would not affect the gain of heat by radiation from the surface

,

but might reduce errors , not considered in this report , due to

sampling of air heated by proximity to warmer surfaces when the

objective is to ascertain the temperature of the atmosphere not

affected by local conditions,, Further research might reveal

that a radiation shield could be designed that would appreciably

reduce the incident radiant energy from bel©w s but the problems

in such a design appear to be very difficult

.

The amount of recirculation of exhaust air was variable

depending on the velocity and direction of the wind. The smoke

tests indicated that the model 002 thermoshield was somewhat

more susceptible to recirculation than model 001, However,

this factor was probably of little significance, because the

difference in indicated temperature for the two units was consistently less than 0,1 deg F for the tests made in the shade

or with cloudy skies.

Not all of the heat entering the thermoshield by the first

three ways described above would necessarily effect a rise in

indicated temperature unless the air stream was well-mixed as

it passed through the inlet tube. Model 002 equipped with the

vortex vane probably mixed the air in the inlet tubes very thoroughly.

- 12 -

As pointed out in Test Procedure, no attempt was made

to provide an absolute calibration of either model of the

thermoshield. It is believed, however, that an artificial

environment could be created in a specially-designed enclosure that would provide accurately-controlled and steady

air temperatures and in which known values of radiation could

be imposed on the devices under test to simulate the effects

of solar radiation without destroying the constancy of the

air temperature. In such an environment, which would probably

be quite costly, a laboratory model of the thermoshield with

better radiation shielding than either of those described in

this report could be used as a reference unit for calibration

of the models designed for shipboard or shore use.

The results showed that these devices conformed reasonably

well to their design specifications. However, some modifications of the model 002 unit would probably increase its

accuracy. These modifications include increasing the air flow

by means of a larger blower or by reducing the resistance to

air flow of the vortex vane, relocating the exhaust outlet

to reduce the likelihood of recirculation, and devising a

satisfactory shield, if possible, to reduce radiation from

below.

( 1 ) Huebscher, R. G. and Parmelee, G. V.

’’Shielding of Thermocouples from the Effects

of Radiation.” Transactions, A.S.H.V.E., 52

183-190, 1946
\ . •' ’ ..
FIG. I
I
FIG* 2

FIG, 3

htj

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

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