Static characteristics calibration of Pt/Au thin-film thermocouple by means of laser heating and temperature extrapolation method

Abstract

Thin-film thermocouples (TFTCs), owing to their fast response, show promise for use in temperature measurements with high spatial and temporal resolution. Because of the huge difference in the temperature profiles in the calibration and application, in the strictest sense, the standardised thermocouple calibration method is no longer suitable for TFTCs. However, this issue is always ignored in some TFTC research literature. To realise accurate calibration of a TFTC, a novel method is proposed in this article. The highlight of this method is the use of a laser to heat the hot junction of the TFTC. Then, according to the surrounding calibrated platinum resistance thermometers and the temperature distribution model, the temperature of the hot junction is calculated with an extrapolation method. Finally, the static characteristics of the TFTC are calibrated through establishing the function between the calculated temperatures and the electromotive forces (EMFs). In order to verify the applicability of this method, a Pt/Au TFTC sensor is designed on a thin cylindrical substrate. Through numerical simulation, a logarithmic model is obtained to describe the temperature distribution regularity of the sensor under laser heating. With screen-printed sensor fabrication and experimental setup realisation, the EMFs and resistances of the platinum resistors are recorded simultaneously. The results leads to the following three conclusions. First, this method can calibrate the Seebeck coefficient of the legs of the TFTC, where the temperature gradient is concentrated in applications. Second, the method shows good repeatability and stability in three repeated experiments. Third, the value of the calibrated temperature and the EMF curve of the TFTC are similar to those of bulk Pt/Au thermocouple. This signifies that the proposed calibration method is reliable and effective.