Voltage spectral structure of the thermocouple with temperature dependent wires

Abstract

The article presents a study of the origin of extra harmonics in the frequency spectrum of thermocouple output signal when being heated by sinusoidal waveform current. It is natural to have the first harmonic, which arises according to Ohm’s law, as a product of current and the total resistance of thermocouple and junction conductors, as well as the Peltier, Thomson effects, and the presence of the second harmonic, which is caused by a heating of junction and thermocouple conductors. The fact that third and higher harmonics arise when the resistance of conductors and junction is thermally dependent was identified. The analytical dependence of the resulting voltage across the thermocouple terminals as a function of the total resistance of conductors and junction, and the voltages due to the effects of Joule, Seebeck, Peltier and Thomson, is established. Based on the analysis of the obtained function and experimentally obtained voltage spectra across the terminal of the thermocouple, an assumption was made about the nature of the voltage spectrum that heats thermally dependent wires of the thermocouple. Recommendations are given to reduce the influence of the first, third and higher harmonics, which are uninformative, and to separate the voltage of the second informative harmonic, which is used to get frequency response of the thermocouple. Keywords: frequency response; thermocouple; EMF; spectrum; thermally dependent resistance; voltage