A miniature high-temperature fixed point for self-validation of type C thermocouples

Reliable high-temperature (>1500 °C) measurement is crucial for a wide range of industrial processes as well as specialized applications, e.g. aerospace. The most common type of sensor used for high-temperature measurement is the thermocouple. At and above 1500 °C, tungsten–rhenium (W–Re) thermocouples are the most commonly used temperature sensors due to their utility up to 2300 °C. However, the achievable accuracy of W–Re thermocouples is seriously limited by the effects of their inhomogeneity, drift and hysteresis. Furthermore, due to their embrittlement at high temperature, the removal of these thermocouples from environments such as nuclear power plants or materials processing furnaces for recalibration is generally not possible. Even if removal for recalibration were possible, this would be of, at best, very limited use due to large inhomogeneity effects. Ideally, these thermocouples require some mechanism to monitor their drift in situ. In this study, we describe a miniature Co–C eutectic fixed-point cell to evaluate the stability of type C (W5%Re/W26%Re) thermocouples by means of in situ calibration