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National Advisory Committee for Aeronautics, Technical Notes - An Investigation of Aircraft Heaters - XXXV - Thermocouple Conduction Error Observed in Measuring Surface Temperatures

When thermocouples are used to measure the temperature of surfaces
which are exposed to fluids at temperatures different from.that of the
surface, errors occur because of conduction of heat along the thermo—
couple wires. These errors have been analyzed and predicted in a
previous report; the present report presents experimental results which
verify this analysis.

The data also indicate that the electrical insulation of the wires
is not effective in reducing the errors involved when the thermocouples
are placed normal to the surface and exposed to the ambient fluids. ‘
The errors are greatly reduced, however, if the wires are maintained in
thermal contact with the plate for about h inches before allowing the
wires to pass through the fluids.

A welding device utilizing the electrical discharge from a bank of
condensers was successfully used to attach thermocouples (No. 28 to
No. #0 B. & S. gage) to the metallic surfaces.

This report concerns the experimental and analytical determination
of one of the errors involved in the measurement of the temperature of
a surface exposed to fluids existing at temperatures different from
that of the surface.

The necessity for measuring these surface temperatures arises
during thermal tests of aircraft heaters, investigation of heated
leading—edge systems for anti-icing, and many other types of thermal
analyses. (See reference 1.)

The particular error of measurement discussed herein is caused by
conduction of heat along the thermocouple wires used for the determina-
tion of the surface temperature.

Consider, for example, a plate at uniform temperature exposed to a
hot fluid on one side and a cold fluid on the other side. If thermo—
couple wires are brought through the hot fluid and attached to the
plate in order to measure the plate temperature, heat flows from the
hot fluid along the thermocouple wires and thence into the plate. This
heat flow increases the temperature of the plate at the point of attach—
ment of the thermocouple (the thermocouple Junction) so that the tem—
perature recorded by the thermocouple is too high. Conversely, if the
thermocouple is mounted on the cold—fluid side of the plate, heat is
transferred away from the point of thermocouple attachment along the
wires and then into the cold fluid so that the measured temperature is
too low.