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.

ITS-90 non-uniqueness and evaluation of characteristics of new 1000 Ω type platinum resistance thermometers for low-temperature measurement

“…During precise estimation of the temperature of the secondary reference points based on the ITS-90, the difference of temperature scales among CSPRTs affects the estimation values. In our earlier reports describing estimation of the triple point temperatures of CO 2 and SF 6 [21,22], we used #D, #E, #F, #G of CSPRTs and the reference CSPRT. The difference of the temperature scales among those CSPRTs at each triple point is within 0.085 mK, which is smaller than the expanded uncertainty (0.15 mK) because of the type III non-uniqueness evaluated based on the Guide to the Realization of the ITS-90 [12].…”

“…Two CSPRTs, #F and #G, were measured in different runs from that for other five CSPRTs. The CSPRTs #C, #D, #E, #F and the reference CSPRTs were used for measurement of the triple points of CO 2 and SF 6 at NMIJ/AIST [21,22]. The stability of the CSPRTs was checked by comparing resistance values at TPW before and after the comparison measurement.…”

“…In recent years, precise temperature determination of secondary reference points [16][17][18][19][20][21][22], e.g. the triple points of SF 6 , CO 2 , and Xe, based on the ITS-90 becomes important to find a candidate for alternatives to the triple point of mercury to retain low-temperature scales of the ITS-90 because the use of the triple point of mercury has been restricted due to strict regulations for mercury based on the Minamata convention on mercury [23].…”

“…the triple points of SF 6 , CO 2 , and Xe, based on the ITS-90 becomes important to find a candidate for alternatives to the triple point of mercury to retain low-temperature scales of the ITS-90 because the use of the triple point of mercury has been restricted due to strict regulations for mercury based on the Minamata convention on mercury [23]. Recently, NMIJ/AIST has specifically emphasized exploration of a better alternative candidate for the triple point of mercury to maintain a temperature scale based on the ITS-90 [21,22]. Precise determination of temperatures of secondary reference points based on the ITS-90 will require elucidation of type III non-uniqueness of the CSPRTs used for investigation.…”

“…This study compared seven CSPRTs with a reference CSPRT using the comparison system to assess type III nonuniqueness at low temperatures between the temperatures of TPeH 2 and TPW. Five of them were used to estimate temperatures of the triple points of CO 2 and SF 6 [21,22].…”