Stability Evaluation and Calibration of Type C Thermocouples at the Pt-C Eutectic Fixed Point

Tungsten-rhenium thermocouples (type C thermocouples) are used to measure temperatures higher than 1500 degrees C under protective, inert, or vacuum conditions in a wide range of industries, such as metallurgy, power generation, and aerospace. Generally, the measurement uncertainty of a new tungsten-rhenium thermocouple is about 1%(20 degrees C at 2000 degrees C), and a significant drift is always observed above 1200 degrees C. Recently, the National Institute of Metrology, China, has spent great efforts to calibrate tungsten-rhenium thermocouples with high-temperature fixed points of up to 2000 degrees C. In the present work, three tungsten-rhenium thermocouples made by two manufacturers were calibrated at the Pt-C eutectic fixed point (1738 degrees C) and their stability was investigated. A linear fitting and extrapolation method was developed to determine the melting and freezing temperatures of the Pt-C eutectic fixed point for avoiding the effect of thermal resistance caused by the sheath and protection tube. The results show that the repeatability of the calibration is better than 0.9 degrees C from the melting curve of the Pt-C fixed point and better than 1.2 degrees C from the freezing curve of the Pt-C fixed point, and a good agreement was obtained for the calibration with the melting and freezing temperature plateau through the linear fitting and extrapolation method. The calibration uncertainty of the thermocouples at the Pt-C eutectic fixed point was 3.1 degrees C (k = 2).