The 27 Ω rhodium-iron ceramic sensor

The 27 Ω rhodium-iron ceramic sensor is suitable for the measurement of temperatures from 1.2 to 300 K, and can be used up to 800 K. The sensor has been available for over 30 years, and around 12,000 units have been produced in this period. A new manufacturer of the rhodium-iron wire has resulted in different resistance-temperature behaviour with each wire batch. This has required a re-analysis of the calibration data. We discuss two types of calibration data: multi-point across the temperature range, and three-point at temperatures close to 4.2 K, 77 K, and 273 K. In both types it is shown that a reference function is of help with the data analysis. We propose a method for improving the interpolation accuracy for a three-point calibration when using a reference function.