Characteristics of Rhodium-Iron Resistance Thermometers and Interpolation Properties from 0.65 K To 24.5561 K

Resistance thermometers using wires of rhodium with 0.5 mol % of iron (Rh-Fe thermometers or RIRTs) were first made by H Tinsley & Co in 1969 for applications at low temperatures, below the range where standard platinum resistance thermometers can be used, and down to 0.5 K or lower. They were investigated at NPL and found to have good sensitivity and excellent stability. Subsequently RIRTs have been used as the standard thermometers which record and compare the results of experiments in thermometry particularly below 24.5561 K, the triple-point of neon. From 1969 until the early 2000s, when Tinsley ceased to manufacture them, several hundred RIRTs were made and many were calibrated at NPL, NIST and elsewhere. In order to document the resistance-temperature characteristics of the production, and indicate the variability from batch to batch, the present paper analyses representative data for the resistance at the triple-point of water and the low-temperature calibrations of a number of thermometers produced at various times. The opportunity has been taken to include data for three RIRTs which were made independently, two in Russia and one in China.