Realization and evaluation of the triple point of sulfur hexafluoride

Abstract

Abstract In this study, we realized the triple point (TP) temperature of sulfur hexafluoride (SF6) at the National Institute of Metrology (NIM), China. To this end, we designed an iso-thermal and immersion type apparatus with an open cell with a height of 18 cm, diameter of 5 cm, and a capacity of approximately 275 ml. The cell was designed with four thermometer wells for the simultaneous temperature measurements with four standard platinum resistance thermometers (SPRTs). The cell is connected with two buffer tanks that allow a sufficient amount of SF6 to surround the thermometer wells during the solidification and melting processes to ensure stable temperature plateaus with long durations. An SF6 sample with a purity of 99.9997%, which is the highest purity currently available, was supplied and analyzed from the Korea Research Institute of Standards and Science. The SF6 TP temperature was determined using four long-stem SPRTs. The melting plateaus obtained had a duration longer than 15 h, and the TP temperature of SF6 was determined to be 223.556 03(54) K (k = 1). These results are consistent with those of previous studies and further indicate that the SF6 TP is a good candidate to replace mercury TP, which is one of the fixed points defined in the International Temperature Scale of 1990, for the calibration of SPRTs.