Accuracy of Thermocouples as Surface Thermometers

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Abstract
The accuracy of thermocouples as surface thermometers when fastened to the surface by adhesive tape was assessed. The temperature of the surface as measured by a thermopile was used for the standard of comparison. Minimal error of the thermocouples was [plus or minus] 0.1[degree] C when the difference in temperature between the surface and its surroundings (the "cooling gradient") was less than 1[degree]C. As this gradient increased from 0[degree] to about 6[degree]C, the thermocouples reported temperatures progressively lower than temperatures recorded radiometrically. On a Cu surface covered by a layer of flat black paint, thermocouples covered by one layer of adhesive tape read about 0.4[degree]C less than the radio-metric temperature when the cooling gradient was 6[degree]C. This error was approximately doubled when the surface was that of a layer of rubber, 0.035 cm in thickness, backed by a Cu plate. It reached a value of about 1[degree] C when the surface was that of a piece of rubber 0.2 cm in thickness. As the cooling gradient increased beyond 8[degree]C, the error of the thermocouple tended to return toward zero. It appeared likely that thermocouples would read too high instead of too low when the cooling gradient exceeded 10[degree]C. The effect of varying the size of the wires from which the thermocouples were made is reported, as is the effect of varying the number of layers of adhesive tape by which the thermocouples are fastened to the surface. Possible explanations for these effects are presented.
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