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Direct Experimental Comparison of Several Surface Temperature MeasuringDevicesAlice M. Stoll and James D. Hardy Citation: Review of Scientific Instruments 20, 678 (1949); doi: 10.1063/1.1741649 View online: http://dx.doi.org/10.1063/1.1741649 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/20/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Experimental device to measure the electrical and optical properties of radiochromic films as a function oftemperature Rev. Sci. Instrum. 80, 065103 (2009); 10.1063/1.3143565 New experimental device for infrared spectral directional emissivity measurements in a controlledenvironment Rev. Sci. Instrum. 77, 113111 (2006); 10.1063/1.2393157 A comparison of measured arrival structures with several theoretical models J. Acoust. Soc. Am. 69, S34 (1981); 10.1121/1.386283 Experimental Comparison between Several ThresholdMeasuring Procedures J. Acoust. Soc. Am. 40, 1250 (1966); 10.1121/1.1943033 A Recording Device for Surface Temperature Measurements Rev. Sci. Instrum. 23, 261 (1952); 10.1063/1.1746247
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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 20, NUMBER 9 SEPTEMBER, 1949
Direct Experimental Comparison of Several Surface Temperature Measuring Devices*
ALICE M. STOLL AND JAMES D. HARDY
From The Russell Sage Institute of Pathology in affiliation with The New York Hospital, and the Department of Physiology, Cornell University Medical College, N e"<1J York, Now Yark
(Received May 9, 1949)
A copper cylinder was constructed and to it four layers of oaktanned leather were applied. The thermal gradient through the leather was measured by means of thermocouples on the cylinder and between the layers of leather. By suitable extrapolation of this gradient the temperature of the outside leather surface was determined accurately and constituted the standard surface temperature to which the readings of the surface temperature measuring devices were compared.
Several types of surface temperature measuring devices were compared and evaluated on the basis of:
(1) accuracy as determined by agreement with the standard surface temperature,
(2) adaptability to use in a variety of experimentally produced environmental conditions similar to naturally occurring environmental conditions.
It was concluded that:
(1) The radiometric type of skin thermometer was the most dependably accurate instrument of those tested, although requiring corrections when used in "sunlight."
(2) Under normal conditions, the #4{) gauge bare wire thermocouple was of the same order of accuracy as the radiometer and was only slightly affected by environmental changes. The accuracy of the coarser bare wire thermocouple was considerably more impaired by such changes.
The thermocouple glued to the surface, or embedded in solder and taped to the surface, or mounted on copper mesh and tied to the surface was less accurate than the bare wire thermocouple. In general, increasing the effective mass of the thermocouple
INTRODUCTION
T HE importance of skin temperature in the determination of heat loss from the body has long been
recognized. An excellent discussion of the relation of skin temperature to heat loss, the factors affecting skin temperature, and the results of investigations prior to 1938 appears in Murlin's extensive review of the subject,! For any significant results in the study of heat loss the average skin temperature over the entire body surface must be known. Therefore, accuracy and rapidity of measurement are essential. Furthermore, recent advances in the evaluation of the environment with respect to its effect upon the individual have indicated the desirability of a more complete delineation of the "comfort zone." In studies of this type, skin temperature has been given weight equal to that of such fundamental physiological measurements as blood pressure, blood flow, and heart rate.2 Of these interrelated factors, it has been suggested that skin temperature may be the
* This work was supported in part by funds from the ONR of the United States Navy.
1 J. Muriin, Ergeb. d. Physiol. 42, 153 (1939). 2 Glickman, Inouye, Telser, Keeton, Hick, and Fahnestock,
"Physiological adjustments of human beings to sudden change of environment," ASHVE Journal Section, 101 (July, 1947).
decreased the accuracy of the instrument under normal conditions as well as under the changed environmental conditions.
(3) The Dermalor resistance thermometer was next in order. Errors were not greater than two percent under certain conditions but increased to four or five percent during radiation.
(4) The errors in the disk thermistor readings ranged from -1.2°C to -1.9°C under the conditions studied. The accuracy of this instrument could be increased by calibration in situ against a radiometer. However, this would not obviate the error produced by infra-red radiation, for instance. This error amounted to -o.rc.
Other types of thermistors having much smaller contact surfaces are available and it is reasonable to expect that these instruments would yield readings as accurate as those of a thermocouple. However, they too would be subject to the errors exhibited by thermocouples or any other small instruments in actual contact with the surface. The feasibility of employing a thermistor as a micro-radiometer is under consideration. It presents special advantages to use in this manner in that thermistors can be constructed with a rapid response time, and as they have a high resistance they are particularly well adapted for electronic recording.
(5) The Roll and Mold pyrometers were entirely unsuited to this measurement in the temperature range studied, showing errors of from 3°C to 7°C depending upon the experimental conditions.
(6) Mercury-in-glass thermometers were unsuited to this type of temperature measurement because of the long contact time required which affects the temperature itself, and because of the marked impairment of their accuracy by environmental changes.
best single objective index of the comfort zone.3 It is therefore of increasing importance that an accurate method of measurement be made available and that errors prevalent in commonly used methods be ascertained where possible. In this way correction factors could be found and applied appropriately so that the various results of similar studies obtained by different methods of investigation could be brought into agreement.
Many devices and methods have been developed for the measurement of skin temperature and have yielded equivocal results because of the technical difficulties involved. Foremost among these difficulties is the proper calibration of the instrument. Since the temperature to be measured is that of a boundary between two different media, the difficulty of calibration of surface temperature measuring devices other than a radiometric instrument, under actual conditions of use, is readily appreciated. That serious errors may be attributed to discrepancies in the conditions of calibration and the conditions of use has been shown.4 When contact instruments are used, the problem is further
3 "Heating, ventilating, air conditioning guide," ASHVE, New York 27, 222 (1949).
4 J. D. Hardy, J. Clin. Invest. 13b, 60S (1934).
678
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SURFACE TEMPERATURE MEASURING DEVICES 679
complicated by the effect of the applicator itself in changing the skin temperature as it measures it. A difference in the temperature of the applicator and of the skin exerts this effect. It is also known that pressure of the applicator on the skin changes the temperature. 5
This may be due to vascular changes on deformation of the skin or, possibly, local sweating. Environmental conditions also influence the surface temperature and at the same time may affect the instrument itself.
At present, because of the differences in calibration and conditions of use there is no basis for wide intercomparison of data from different laboratories. This study was undertaken to develop a method by which several types of instruments may be tested under comparable conditions and thus provide a basis of intercomparison. The same instruments were employed under a variety of experimental conditions designed to
DUCO CEMENT
-===Z7"'i~ #40 GAGE WIRE
COPPER· CONSTANTAN
;><
z
simulate actual environmental situations under which they might be used and thereby obtain an indication of the extent and direction of errors due to the influence of the environment on the instrument reading.
MATERIALS AND METHOD
Several types of instruments were selected for com-parison:
1. The Hardy Dermal Radiometer.6
2. The Dermalor-a resistance wire skin thermometer.' 3. Surface Pyrometers (Cambridge Instrument Company).
(a) The Roll Model-a flat strip thermocouple designed primarily for industrial uses.
(b) The Mold Model-a small flat bead thermocouple also designed for industrial uses.
4. Thermocouple # 5-# 40 gauge copper and constantan wire thermocouple glued to the test surface with Duco cement.
7 ?
z 7 z
#28 GAGE WIRE - COPPER'CONSTANTAN (BARE)
FLATTENED SOLDE"
7 7
J,. BEAD
-====ZZ~2 c~.~/7 #40 GAGE WIRE- COPPER'CONSTANTAN (BARE) # 28 GAGE -WIRE -
COPPER'CONSTANTAN
FIG. 1. Applicators for surface thermometers.
SURFACE PYROMETER
BIMETALLIC STRIP
THERMOCOUPLE MILLIVOLTMETER CALIBRATED IN 'F.
_40 GAGE CU-CO THERMOCOUPLE ~OLDERED TO COPPER WIRE MESH
DISC THERMISTOR
6 N. A. Remizov, Arch. Sci. BioI., St. Petersburg 38, 815 (1935). 6 J. D. Hardy, J. Clin. Invest. 13a, 593 (1934). 7 S. S. Samuels, N. Y. State J. Med. 40, 884 (1940).
(
FLATTENED BEAD jPLASTIC
~ "'""~:;.,DE==R:::M:.:A::::-LOR
RESISTANCE WIRE APPLICATOR
THERMOMETER -~
FLATTENED THERMOJUNCTION
APPLICATOR
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680 ALICE M. STOLL AND JAMES D. HARDY
5. Thermocouple #fr-#4D gauge copper and constantan wire thermocouple stretched across the surface and supported at either end with adhesive tape, leaving the thermojunction barco
6. Thermocouple # 7-# 28 gauge copper and constantan wire thermocouple imbedded in a flattened bead of solder taped to the surface with adhesive tape.
7. Thermocouple #8-#28 gauge copper and constantan wire thermocouple stretched across the surface and supported at either end with adhesive tape leaving the thermojunction bare.
8. Mercury-in-glass thermometers (a) clinical (b) general laboratory.
9. The Western Electric disk thermistor (5 mm in diameter by 1 mm thick)-applied to the surface by means of a thread drawn taut over the thermistor and fastened by adhesive tape. The leads were supported by means of a dab of wax inserted between them and the surface.
10. The Rubicon Skin Thermometer-three copper-constantan thermocouples with flattened junctions mounted in a plastic holder.
11. #4D gauge copper-constantan thermocouple-mounted on copper mesh and applied to the surface by means of ties run through the edges of the mesh and tied tightly about the cylinder.8
With the exception of the mercury thermometers and the Hardy Dermal Radiometer, these instruments are represented in Fig. 1.
In order to compare the instruments it was necessary to provide a standard surface the temperature of which could be determined accurately within ±O.05°C. This was accomplished in the following manner (Fig. 2):
THERMOS BOTTLE
GALVANOMETER
THERMOMETER
a copper cylinder, 12 inches high and 8 inches in diameter was set up and provided with a stirring device, a thermostatically controlled heater, a Bureau of Standards certified thermometer, and inlets for testing and reference thermocouples. The cylinder was filled with water and its temperature maintained constant within ±O.03°C. On this cylinder four layers of oak tanned leather, each 12 inches high, 12 inches wide and about 3 mm thick, were tightly laced. A thermocouple of # 40 gauge copper and constantan wire was inserted between the cylinder and the first layer and between each of the succeeding layers. All thermocouples used in this study were made of specially annealed Leeds and Northrup thermocouple wire. All thermojunctions were about 6 inches from the top and 6 inches from the side edges of the leather strips so that they lay in a straight line from cylinder to surface in the center of the leather covered area. The copper leads from the thermocouples were connected to a rotary switch which in turn was connected to a Type K, Leeds and Northrup potentiometer. The cold junction was inserted into the cylinder within 1 cm of the thermometer and a second thermocouple provided a check on the uniformity of the temperature within the cylinder. For convenience in recording data the thermocouples were numbered as follows:
OAK TANNED __ ....... LEATHER STRIPS
12"
III . ...-;,.·/LI--L_CORO
GALVANOMETER SCALE
LACINGS
FIG. 2. Schematic representation of apparatus for obtaining a standard surface temperature.
g E. D. Palmes and C. R. Park, Fed. Proc. 6, 175 (1947).
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SURFACE TEMPERATURE MEASURING DEVICES 681
°c CYLINDER
""'0 MM
3 9
38
FIG. 3. Method for extrapo- 37 !ation of surface temperature. Curve A (dashed line) indi-cates the temperature gradient 36 curve obtained by plotting the temperature of the surface of each leather layer against the 35 distance of that surface from the surface of the cylinder (see top of chart). Curve B (solid line), a straight line, is obtained
34
from the same data by plotting the temperature of the surface of the leather layer against the 33 logarithm of the ratio of radius I of each layer to the radius of 32 A --- TEMP. PI-OTTEa
I the cylinder (see points marked off on the abscissa). 8 -- TEMP. hOTrEO
OF RATIO r 3 I I
30
0
1/2
#O-within the cylinder at about 6 inches from the reference thermocouple,
# 1-between the cylinder and the first layer, #2-between the first and second layers, # 3-between the second and third layers, # 4-between the third and fourth layers, # 5-# 8-test thermocouples on the leather surface.
The temperature of the standard surface was then obtained by extrapolation of the thermal gradient from the cylinder to the outermost leather surface. Because of the curvature of the surfaces it was necessary to employ the equation for heat transmission through the walls of a tube in order to obtain a straight line extrapolation. This equation simplified is:9
iz- t1 =K·! loge(r2)!(rl)
where tl = temperature of cylinder, t2= temperature of leather layer, K = a constant, rl = radius of cylinder, and r2=radius of leather layer. The thickness of each layer was measured with a micrometer and the radius calculated. The temperature difference (t2- 11) of each of the first three layers was measured in microvolts, converted to degrees centigrade and plotted against the value of ! loge(rz)!(rl) for each layer, respectively. The temperature of the outside surface of the fourth layer was then found by extrapolation and was the desired standard surface temperature to which all surface thermometer readings could be referred. Figure 3 illustrates the method by which the surface temperature was determined by extrapolation of the curve obtained on plotting the observed temperatures against! log. of
• W. L. Badger, Heat Transfer and Evaporation (The Chemical Catalog Company, Inc., New York, 1926), p. 14.
I LAYER 2
rS.68 MM
I I I I I I I
AS FUNCTION OF I RAOII
AS LOGARITHMIC I FUNC'TION
!lAOll I I
2 3
LOGe R2/RI X
I LA YE R 3 If'" I' 9.66 MM
I I I I I I I I I I 1 I 1 I
4 5
10 2
6
lAYER 4
12.12 MM
the ratio of the radial distances (Curve B-solid line). Curve A (dashed line) is shown for comparison. It was obtained on plotting the same temperatures directly against the radial distances of the layers from the cylinder. It is readily seen that since Curve A is not linear, an error of as much as O.lSoC can be made on extrapolation of this line unless the mathematical equation for the curve is set up and solved. Curve B, on the other hand, is a straight line and more readily yields the correct surface temperature on simple extrapolation, therefore this method was used throughout.
In making the comparisons the desired environmental conditions were established and the apparatus was permitted to come to thermal equilibrium. The room temperature and the temperature of the cylinder as well as pertinent data such as radiation intensity or wind velocity were recorded. All thermocouples were read successively from the cylinder outward. The surface thermometers were read in an order such that the instrument having the least effect upon the surface temperature during application would be first and that having the greatest effect would be last. Thus the thermocouples affixed to the surface were read first, then the radiometer and others, with the mercury thermometers last. In the instances where the surface temperature was affected by the applicator, sufficient time for the equilibrium to be re-established was permitted to elapse before the next instrument was applied.
EXPERIMENTAL DATA
In each situation studied, preliminary trials were necessary to achieve the best technique as determined
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682 ALICE M. STOLL AND JAMES D. HARDY
by consistency and accuracy of results. The data herein presented represent readily duplicable results of the best performance of each instrument.
The following conditions were selected for test environments:
1. Normal-ordinarv !aboratorv conditions, which include natural convection and radiation -exchange between the leather surface and the walls of the room.
2. Forced convection-simulated wind, derived from an electric fan blowing air directly on the surface at the rate of four feet per second as measured by an anemometer.
TABLE L Deviation in degrees centigrade from standard surface temperature under normal conditions.
Mold Roll Thermocouple Der- pyrom- llyrom· Radiometer malor eter eter #5 #6 #7 #8
-0.22 0 +0.34 -0.14- -0.17 -0.28 -0.02 -0.22 -0.14 -0.17 -0.15 -1.00 +0.43 +0.07 +0.27 +0.66 -0.12 -0.77 -6.4- -1.4 +0.39 +0.19 +0.32 +0.84 -0.11 -0.44 -5.1 -1.1> +0.19 +0.17 +0.22 +0.32 +0.04 +0.29 +0.12 +0.12 +0.02 +0.04 +0.22 0 +0.02 -0.05 +0.12 +0.22 +0.05 +0.07 +0.07 +0.19 -\.16 -5.0 -3.9 +0.39 +0.24 +0.29 +0.24 +0.13 +0.02 -0.14- -0.10 -0.12 -0.10 -0.20 +0.22 +0.05 +0.22 +0.14 -0.11 -0.36 -5.2 -3.5 +0.39 +0.24 +0.36 +0.29 +0.03 -0.22 -5,4 -4.3 +0.24 0 +0.12 +0.02 +0.06 +0.01 -5.0 -3.3 +0.17 -0.14 +0.17 0 -0.01 -0.56 -5.6 -3,4 +0.32 -0.05 +0.41 +0.05 +0.05 +0.10 -:;.3 -3.6 +0.19 +0.02 +0.10 +0.19 -0.19 +0.16 -4.5 -3.4 +0.10 -0.07 +0.02 -0.05
Av. -0.04 -0.44 -5.3 -3.2 +0.22 +0.05 +0.14 +0.13
3. Infra-red radiation-simulating hot walls, derived from an electric hot plate directly facing the leather surface, other conditions "normal."
4. Simulated sunlight-produced by the radiation ftOm a 1500 watt lamp, other conditions "normal."
5. Simulated sunlight and forced convection-"sunlight" as above and "wind" from an electric fan blowing air on the irradiated surface at the rate of two feet per second.
The standard surface temperature under normal conditions ranged from 27°e to about 32°C. Table I presents the deviations in degrees centigrade of the surface thermometer readings from the extrapolated (standard) surface temperature obtained in a series of experiments under normal conditions.
The radiometer and thermocouple # 6 yielded the best resuits, all readings being well within the limits of experimental accuracy. Thermocouples # 7 and # 8 performed satisfactorily while # 5 (glued on surface) showed an average deviation of +O.22°C.
The copper mesh mounted thermocouple readings (not shown in table) fell about O.34°e below the extrapolated temperature.
The Dermalor, with an average deviation of -0.44°e, feU within the range of accuracy guaranteed by its manufacturers (2 percent, about O.5°e to O.6°C at the temperatures measured).
The disk thermistor was carefully calibrated in contact with the bulb of a mercury-in-glass thermometer in an air chamber rather than by immersion in a liquid in order to more closely approximate the conditions of
use. A smooth, reproducible calibration curve was obtained. In spite of this precaution, however, an average deviation of -1.21 QC (not shown in table) was observed.
The Rubicon Skin Thermometer readings (not shown in table) were in error by - 2.85°C under normal conditions. The applicator of this instrument could not be satisfactorily adapted to use on the leather surface. Because of the construction of the holder, good contact could be obtained only by pressing the applicator into the surface. This could not be done sufficiently well on the relatively unyielding leather surface. It is to be expected that on actual skin surfaces into which the applicator could be pressed, a better approximation could be attained.
Both the Roll and the Mold model pyrometers showed wide deviations from the standard which was relatively near the lower limit of their scale range (50°F to 250°F or lODe to 120+°C).
Mercury thermometer results do not appear in this table. Because of the long reading time required, about three minutes, it was deemed advisable to test them separately. Three techniques were used:
1. The thermometer was rolled on the surface. 2. Spot contact was maintained with the thermometer bulb
covered by a notched cork. 3. A combination of the above, the thermometer, under a
notched cork, was rolled on the surface.
A short series of observations was made with a clinical thermometer, a large bulb laboratory thermometer and a small bulb laboratory thermometer. It was found that under normal conditions agreement with the standard within between 0 and -O.SoC could be attained but required about three minutes contact time. Rolling under a notched cork reduced this time but introduced the possibility of errors due to friction. Forced convection or radiation falling on the surface increased the reading time by five minutes and the error by two or three degrees centigrade. Because of the long lag in response and their inadaptability to changing environmental conditions, mercury thermometers were not suited to comparison with the other instruments employed. They were therefore eliminated from the study although they could be used to advantage with a very limited set of conditions and a rigid technique developed through trial and error.
Table II presents results obtained under a variety of environmental conditions. These results are best evaluated by comparison with Table I and consideration of the factors influencing the reading of each instrument under given conditions:
1. Forced convection-To study the effects of convection currents, it was necessary to supply an air stream of constant velocity so that equilibrium was established and maintained throughout the experiment.
For accurate results with the radiometer, it was held at an angle such that it did not interrupt the stream of
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SURFACE TEMPERATURE MEASURING DEVICES 683
air, and that the air was not deflected into the reflecting cone of the radiometer. With this precaution, convection had no effect upon the accuracy of the instrument.
The Dermalor applicator must necessarily be placed upon the surface since it is a resistance coil dependent upon contact for its effect. Interruption of the air stream during application then resulted in a reading higher than the true temperature. The accuracy of this instrument remained about the same as under normal conditions although the direction of the error was reversed.
Thermocouples # 5, # 6, and # 8 and the copper mesh mounted thermocouple (not shown in table) showed an increased error of about 0.2°C, while # 7 (covered with adhesive tape) agreed more closely with the standard temperature than under normal conditions.
The Roll Pyrometer and the thermistor (not shown in table) error remained unchanged while the Mold Pyrometer showed less error than under normal conditions.
The error in the Rubicon Skin Thermometer readings (not shown in table) apparently decreased to -0.84°C during forced convection. However, this effect was only apparent since the actual temperature of the surface approached room temperature during forced convection, while it was about four degrees above room temperature under normal conditions.
2. Infra-red radiation-Accuracy within experimental limits was obtained with the radiometer when it was held in a position such that the area tested was not shaded from the radiation.
The Dermalor readings were in error by -1.75°C (average deviation) due to the shading of the test area and actual cooling of the surface by the applicator. Preheating the applicator to approximately the surface temperature (about 37°C) probably would reduce the error.
The accuracy of thermocouples # 5, # 6, and # 7 was not significantly impaired by this radiation. Thermocouple #9 (copper mesh) readings (not shown in table) were in error by - 0.4 °C while thermocouple # 8 (# 28 gauge wire) differed from the standard by as much as -OSc.
The error in the pyrometers was greatly increased. Because of the difficulty in the application of the
thermoelement of the Rubicon Skin Thermometer, these readings (not shown in table) were apparently in error by -4.93Q C. Under infra-red irradiation, however, the surface temperature was about 9°C above room temperature and the large error reflects poor contact of the thermoelement with the surface.
Since the data obtained with instruments #9, # 10, and # 11, studied after the completion of tests on the other eight, indicated that the readings were not appreciably different from those of similar instruments in the same situations, these instruments were not subjected to simulated sunshine or this radiation plus convection. It is to be expected that they would exhibit
the same type of errors as other contact instruments under the same conditions.
3. "Sunlight." In testing the performance of the instruments during
radiation of energy in the visible range, sunlight itself could not be used because of its variability. It was simulated by the diffuse radiation produced by a 1500 watt lamp about 1.5 meters from the surface, and directed toward the surface through a large lens.
Under the conditions of this experiment, reflection of the radiation into the cone of the radiometer could not be avoided without interrupting the light beam. On placing the radiometer directly on the surface to avoid reflection, this instrument with its diameter of 4 cm, shielded the surface from the radiation and thus cooling of the surface ensued during the 8 seconds required to obtain the radiometer reading. However, since cooling is more readily measured than reflection, this procedure was followed. The cooling amounted to about 0.15°C in the required reading time; the exact amount of cooling under each set of conditions was determined by experiment.
The procedure for making a surface temperature measurement with the radiometer during this irradiation was then as follows: the radiometer was placed directly over the test area and the reading noted at a definite minimal time (8 sec.). The radiometer was then removed and the radiation continued until equilibrium was reestablished. The cooling effect was then determined after the manner described by Hardy and Oppel :10 at zero time the radiometer was applied to the surface as above and at the same instant the radiation was cut off by a shutter. The radiometer was read at 8 seconds and at frequent intervals thereafter for about 2t minutes as the surface cooled. These readings were plotted against the time and the resulting cooling curve was extrapolated back to zero time. The difference between the extrapolated value at zero time and the observed temperature at 8 seconds constituted the cooling correction which was added to the reading of the radiometer obtained in making the surface temperature measurement during irradiation. Figure 4 shows two of the curves for cooling correction obtained in this manner. In order that this procedure be valid, the conditions under which the cooling correction was obtained were necessarily identical with those under which the test determinations were made.
After the addition of the cooling correction, there was no error in the radiometer measurements in "sunlight."
The Dermalor readings were in error by -1.2°C, again due to shading and the difference in temperature between the surface measured and the applicator itself.
Thermocouple # 6 (# 40 gauge wire uncovered) was not appreciably affected by this radiation, while # 5, # 7, and #8 showed between -0.4 and -0.9°C errors.
The errors in the pyrometer readings were greatly
10 J. D. Hardy and T. W. Oppel, Physics 7,466 (1936).
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684 ALICE 1\1. STOLL AND JAMES D. HARDY
36.0
u 0 35.6
I
LLI a: ::> I-<t a: 35.2 LLI Q.
::E LLI I-
LLI U 34.8 <t u. a: ::> (II
34.4
34.0L--------------------------------------------------------------------o 20 40 60 80 100 120 140 160
TIME - SEC.
FIG. 4. Cooling curves. Two curves showing the cooling of the standard surface on interruption of the radiation falling on it from the lS00-watt lamp during application of the radiometer. By extrapolation back to zero time the correction factor for cooling under these conditions is obtained.
increased in "sunlight" as they were during infra-red radiation.
4. "Sunlight" and "wind." When convection currents were superimposed on the
"sunlight" falling on the surface a new correction for cooling was determined and applied to the radiometer reading. Thereupon the radiometer readings were brought into complete agreement with the standard surface temperature.
The Dermalor appeared to be more accurate under these conditions (average deviation = -O.l°C). In this instance there was little difference between the temperature of the applicator (25° to 27°C) and the temperature of the standard surface (about 29°C), therefore, there was no appreciable cooling effect from the applicator. The error due to shading was canceled by the interruption of the air stream during measurements.
Thermocouple #6 was the most reliable, while # 5, # 7, and # 8 showed errors of from - 0.4°C to - 0.6°C.
The pyrometers under these conditions agreed more closely with the standard than under normal conditions but were still grossly in error.
A generalized summary of the foregoing data is presented in Tables III and IV.
DISCUSSION
In any study of surface temperature it is well recognized that there are factors which dictate the type of instrument most suited to the individual problem. This is particularly true when dealing with skin temperature. The measurement of surface temperature depends upon
a transfer of energy from the surface to the measuring instrument. Theoretically, surface temperature could be measured in terms of anyone of its components, i.e., radiation, conduction, convection or vapor pressure. Convection and vapor pressure measurements are technically extremely difficult to make and have not been employed for this purpose. Because of the relative ease of measurement, radiation and conduction have been
TABLE II. Deviation in degrees centigrade from standard surface temperature under conditions indicated.
Mold Roll Der- Dyrom- pyrom- Thennocou pIe
Radiometer malor etcr eter :# S #6 ,,7 #8
Air 60w-4 ft./:;cc. +0.05 +(1.30 -3.7 -2.6 -0.22 -0.17 -0.07 -0.19 +0.05 +0.60 -4.5 -3.3 -0 .. 33 -0.29 -0.14 -0.33 -0.05 +0.60 -3.0 -3.0 -0.24 -0.39 -0.02 -0.29
,\v. +0.02 +0.50 -3.7 -.3.0 -0.26 -0.28 -0.08 -0.27
J nfra-red radiation +0.12 -1.54 -6.3 -5.1 -0.07 +0.05 -O.lO -0.60 +0.02 -1.02 -8.1 -5.3 +0.02 -0.16 -0.16 -0.54 +0.02 -1.68 -4.5 -8.9 +0.02 0 -0.19 -0.34 -0.09 -2.07 -8.4 -4.5 +0.10 -0.24 -0.29 -O.Sl -0.12 -2.46 -8.8 -5.9 +0.09 -0.15 -0.17 -0.46
Av. -0.01 -1.75 -7.2 -5.9 +0.03 -O.lO -0.18 -0.4'1
1500 watt lamp radiation +0.13 -1.02 -7.7 -3.8 -0.17 +0.22 -0.73 -0.27 -0.05 -1.30 -5.9 -4.8 -0.37 0 -0.92 -0.44 -0.16 -1.59 -7.5 -7.9 -0.37 +0.07 -0.95 -0.39 -0.15 -0.37 0 -1.04 -0.56 +0.02 -1.03 -6.S -4.3 -0.50 -0.40 -0.72 -0.03 -0.98 -6.6 -3.9 -0.41 -0.10 -0.93 -0.58
Av. -0.04 -1.l8 -6.8 -SA -0.37 -0.04 -0.91 -0.49
1500 watt lamp radiation +air flow---2 ft./seC'. +0.02 -0.44 -0.27 -O.ll +0.49 -3.7 -2.6 -0.29
-0.05 -0.05 -3.5 -2.4 -0.51 -0.05 -0.68 -0.51 -0.04 +0.01 -3.5 -2.9 -0.49 -0.05 -0.58 -0.39
0 0 -3.9 -2.8 -0.41 -0.41 -0.61 -0.41 A,'. -0.05 +0.1l -3.7 -2.7 -0.42 -0.12 -0.58 -0.40
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SURFACE TEMPERATURE MEASURING DEVICES 685
used extensively. Since radiation may be measured at a distance from the surface, it would appear to be the more desirable measurement.
The formula for radiation exchange between two bodies at different temperatures may be expressed as
R=ele2S0(T14- T24),
where, in the particular instance of skin temperature, R = quantity of energy exchange, el = emissivity of a perfect black body reference block = 1, e2=far infra-red emissivity of the skin=O.989,1l So=the Stefan-Boltzmann constant, Tl = absolute temperature of the black body, and T 2 = absolute temperature of the skin. Since all the terms of the equation except Rand T2 are known, T 2 may be determined from the measured quantity R; also as R usually may be determined accurately without affecting T 2, this is theoretically a dependable measurement of skin temperature.
Conduction, on the other hand, may be expressed more simply as
C=K(Ts- T i ),
where C = quantity of energy exchange, T. = temperature of the skin, T;= temperature of the instrument, and K = a constant. However, K depends upon many factors: the contact pressure, the roughness and wetness of the skin, and the size and the conductivity of the applicator. Even when the size of the applicator is minimal and the conductivity optimal, the other factors influencing K are variable and affect the accuracy of the determination.
For practical purposes when rough, quasi-quantita-
TABLE III. Average deviation from extrapolated temperature-°c.
Experimental conditions
Infra-red 1500 Wind radiation watt
Room velocity (hot lamp Instrument (normal) 4 ft./see. stove) radiation
"Dermal" radiom-eter -0.04 +0.02 -0.01 -0.04
Thermocouple # 40 gauge wire (bare) +0.05 -0.28 -0.10 -0.04
Thermocouple # 28 gauge wire (bare) +0.13 -0.27 -0.49 -0.49
Thermocouple solder bead (adhesive tape) +0.14 -0.08 -0.18 -0.91
Thermocouple # 40 gauge wire (glued) +0.22 -0.26 +0.03 -0.37
"Dermalor' resistance thermometer -0.44 +0.50 -1.75 -1.18
"Pyrometer" strip thermocouple -3.2 -3.0 -5.9 -4.9
.. Pyrometer" solder bead -5.3 -3.7 -7.2 -6.8
Copper mesh thermocouple -0.36 -0.57 -0.42
Disk thermistor -1.21 -1.22 -1.91 Rubicon skin
thermometer -2.85 -0.84 -4.93
11 J. D. Hardy, Am. J. Physiol. 127, 3 (1939).
1500 watt lamp wind
velocity 2 fL/see.
-0.05
-0.12
-0.40
-0.58
-0.42
+0.11
-2.7
-3.7
tive measurements suffice, conduction methods are usually quite satisfactory. Because of the low cost sturdiness and ease of operation of various instrument~ based on this principle, conduction instruments have come into wide usage and consequently have been employed in studies for which they are not suited. Quantitative studies relating skin temperature to heat loss rate and to blood flow measurements, for instance, require a method of greater precision. Theoretically, the radiometric method is most suitable and this is borne out by practical experimental evidence: Of the many devices employed in measuring skin temperature, as yet no single instrument adaptable to every situation has been demonstrated. The radiometer most nearly approaches this ideal because of its accuracy and its applicability to the particular problems of this measurement. With appropriate corrections the Dermal Radiometer maintains an absolute accuracy of ±O.l°C, the limit of accuracy with which the scale of this model may be read, under all conditions of these experiments. This confirms and extends the work of a previous investigation by Hardy.4 Unlike any other instrument similarly employed, the radiometer does not come in contact with the skin; therefore changes in temperature and possible vascular changes due to pressure or contact with the skin on application of the instrument are obviated. The reading time is short, eight seconds and relatively rapid changes in skin temperature m;y be followed readily. The necessity for corrections under certain circumstances and the bulk and fragility of the instrument are decided disadvantages. Also, the radiOI~eter principle has not been adapted for measuring SkIll temperature beneath clothing or for continuous recording. However, such corrections as are necessary can be made with the radiometer itself and attempts to
TABLE IV.
Performance under experimental conditions 1500 watt
1500 lamp Wind watt wind
Room velocity Infra-red lamp velocity Instrument (normal) 4 It';sec. radiation radiation 2 [t./ser.
"Dermal" radiom- Excellent Excellent Excellent Requires Requires eter ±O.OS'C correction correction
Thermocouple Excellent Fair Good Excellent Good # 40 gauge wire (bare)
Thermocouple Good Fair Poor Poor Poor # 28 gauge wire ±0.15'C ±0.90'C (bare)
Thermocouple Good Excellent Fair Poor Fair solder bead (arlhesive tape)
Thermocouple Fair Fair Excellent Fair Fair # 40 gauge wire (v,lued)
±0.30'C
"Dermalor" Fair Fair Poor Poor Good resistance thermometer
"Pyrometer" strip Very poor thermocouple ±3.0·C
Very poor Very poor Very poor Very poor
j'Pyrometer" solder bead
Very poor Very poor Very poor Very poor Very poor
Copper mesh Fair Fair Fair thermocouple
Disk thermistor Poor Poor Poor Rubicon skin Very poor
thermometer Very poor Very poor
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686 .\LICE M. STOLL AND JAMES D. HARDY
reduce the bulk without sacrificing accuracy are now in progress in other laboratories.12
It would be of considerable practical value if these experiments could be put to use in deriving correction factors which would bring into agreement skin temperature data obtained by the various methods of measurement. The skin temperature measuring devices fall into two general classes, namely those instruments which cover several square centimeters of skin and thereby alter the normal heat flow pattern, and those which come into contact with the skin at only one point or at most in a single fine line. The bare wire thermocouples belong to this latter group, and an attempt has been made to discover whether these thermocouples are indicating subsurface temperature, surface temperature or the temperature of the air in close proximity to the surface.
Buttner13 has measured the thermal gradient through the air immediately adjacent to a wall of known temperature higher than that of the circulating air. He found a linear relationship between temperature and distance from the wall such that the temperature fell at the rate of 2°e per millimeter in still air. From this data it is possible to determine whether or not the thermocouples were reading the temperature of the air in the boundary layer. For a # 28 gauge thermocouple, 0.6 mm in diameter, one side just touching the surface, it was found experimentally that in still air this instrument was reading O.13°e higher than the actual surface temperature. It was thus evident that the thermocouple was not reading the temperature of the air in the boundary layer or the surface temperature but an equivalent subsurface temperature. It can be easily
12 M. Van Dilla and J. Bochinski, Report R252 Radiometer, A. S. F., QM Climatic Research Laboratory, Lawrence, Massachusetts (October, 1945).
13 K. Buttner, Verdunstung und Strahlung in Bioklimatologie und lllIeteorologie (Preuss. Meteorol. Institut., Berlin, 1934), Vol. X, p.5.
shown that the thermocouple was reading the temperature of the leather 0.2 mm beneath the surface, a result which is to be explained on the local banking of heat between the thermocouple and the surface. It is a fact that although thermocouples can be made with very fine wire they will indicate only the temperature of the very small junction at a spot on the skin where the wire is having the most disturbing effects. For example, the # 40 gauge thermocouple, 0.2 mm in diameter, gave a reading of the surface temperature which was o.osoe higher than the true temperature of the surface, again showing evidence of heat banking.
Buttner has also measured the "effective Grenzschicht" or thickness of the boundary layer of air at a surface upon which wind was blowing at various known velocities. Applying his data to these experiments it was found that although the thermocouples were now reading lower than the true surface temperature there was definite evidence of banking of heat between the thermocouple and the surface. From these considerations it became obvious that an exhaustive treatment of the heat flow problem would be necessary to arrive at corrections which could be applied to thermocouple measurements under various environmental conditions, to say nothing of the various methods of applying thermocouples to the skin surface. Such an approach does not appear justifiable in view of the present data. It is believed, however, that if skin thermometers were calibrated against a surface of known temperature under the various circumstances which are to be employed in the biological study, empirical corrections could be arrived at and applied so that a more accurate evaluation of skin temperature could be made. A perhaps even more accurate method would be to calibrate skin thermometers in place on the skin by comparison with radiometer measurements. Studies of this nature are now being made in this laboratory in an attempt to evaluate further the various methods now employed for measuring skin temperature.
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