Evaluation of the triple point temperature of sulfur hexafluoride and the associated uncertainty at NMIJ/AIST

Abstract

This study determined the triple point temperature of sulfur hexafluoride (SF6), as well as the associated uncertainty, to evaluate the potential use of this value as an alternative to the triple point of mercury as a fixed point defined in the International Temperature Scale of 1990 (ITS-90). The triple point temperature was obtained using an adiabatic calorimeter incorporating a sealed cell made from oxygen-free copper. The cell was filled with 99.9992% SF6 and included three standard platinum resistance thermometers that had been calibrated at eight fixed points defined by the ITS-90 protocol. Applying a thermal treatment to the solidified sample prior to determining the triple point temperature was found to decrease the width of the melting curve. Based on melting curves acquired following this thermal treatment, the triple point temperature of SF6 was found to be 223.556 47(53) K. The temperature scale constructed using SF6 was found to be consistent with that generated using mercury within a maximum deviation of 70 K from 13.8033 K to 273.16 K.