Most thermopiles show a decrease in sensitivity when ambient temperature increases. By experimental study of the temperature dependence of the various factors which affect the sensitivity (radiation loss, gaseous conduction and convection loss, and losses through the solid supports) it has been determined that a major contributor to the effect at atmospheric pressure is the temperature coefficient of thermal conductivity of the thermocouple wires. Several ambient temperature independent thermopiles operating at atmospheric pressure have been made either by adjusting the size of the thermocouple wires or by addition of other solid conducting supports between the receiver and the cold junctions.
A mathematical analysis of the thermal system of a thermopile has led to a simplified formula for determining the proper materials and dimensions required to obtain ambient temperature independence. As a direct result of the analysis it can be stated that any thermopile, operated at atmospheric pressure, can be made ambient temperature independent by the proper choice of a thermal shunt between the receiver and the cold junctions. Limitations of the analysis are discussed. Experimental confirmation of the method is presented.
WILLIAM G. FASTIE, "Ambient Temperature Independent Thermopiles for Radiation Pyrometry," J. Opt. Soc. Am. 41, 823-829 (1951)