A protocol for accurately calibrating thermocouples at cryogenic temperatures

Thermocouples are affordable temperature sensors with low self-heat capacity and a quick response time, making them well-suited for investigating the cooling behavior of materials in cryogenic liquids and performing fast and precise transient measurements. To ensure precise temperature measurements, it is essential to calibrate newly acquired thermocouples or recalibrate existing ones. For this purpose, a common practice is the use of fixed-point temperatures such as the ice-water slush (0 °C), the normal sublimation temperature of dry ice (-78.5 °C), and the normal boiling temperature of liquid nitrogen (-196 °C). However, uncertainties are associated with the fixed-point temperatures of dry ice sublimation and liquid nitrogen boiling due to the inherent heat and mass transport phenomena occurring at the phase-changing interface and within the bulk medium. Therefore, it is important to address these uncertainties and define a protocol for calibrating thermocouples using these fixed-point temperatures.