Design and implementation of a dedicated calorimeter for long stem SPRTs calibrations at the argon and oxygen triple points

Abstract

Most of the thermometers used for the dissemination of the International Temperature Scale of 1990 (ITS-90) to the industry are long-stem Standard Platinum Resistance Thermometers (SPRTs). In the low temperature range, they are generally used from the triple point of water (273.16 K) down to the triple point of argon (83.805 8 K). The lack of equipment for routinely calibrating thermometers below 83 K (except comparisons at the nitrogen boiling point) limits their use at lower temperatures. The LNE-Cnam has recently developed an innovative calorimeter to improve the calibration of long-stem SPRTs at the triple point of argon and to extend the calibration range down to the oxygen triple point (54.358 4 K). Results show that at the argon triple point further investigations are needed to ascertain a reduction of the calibration uncertainty, with respect to the previous system developed by the laboratory. At the oxygen triple point, uncertainties are below the millikelvin level. With this device, the LNE-Cnam acquires the possibility to improve its calibration uncertainties for long-stem SPRTs at the argon triple point, and extends its calibration capabilities down to the triple point of oxygen.