Realizations of the triple points of carbon dioxide and sulfur hexafluoride-ITS-90 temperature determinations and interpolations.

The triple points of carbon dioxide (TP CO2) and sulfur hexafluoride (TP SF6) have been studied over the last decade as alternative fixed points to the triple point of mercury (TP Hg, 234.3156 K) on the International Temperature Scale of 1990 (ITS-90). NIST has been developing TP CO2 and TP SF6 cells to be employed as drop-in replacements for TP Hg cells for calibrations of standard platinum resistance thermometers (SPRTs). This work presents the results of the assessment of new immersion-type CO2 and SF6 triple point cells built at NIST and the results of their use in ITS-90 calibrations of long-stem SPRTs. It was demonstrated that a very well-controlled realization technique can result in measurements of the triple points of CO2 and SF6 within 25 μK and 75 μK of reproducibility, respectively, with estimated temperatures of 216.591 21(15) K and 223.556 22(33) K for F= 1, respectively. The large volume expansion of CO2 and SF6 upon melting in large immersion cells is responsible for a progressive head correction that increases the realization slopes of the triple points of CO2 and SF6 by 0.1 mK and 0.46 mK, respectively. Calibrations of two SPRTs on different ITS-90 subranges using either TP Hg, TP CO2 or TP SF6 cells showed that the interpolated temperatures using either triple point agreed within their realization uncertainties. These results demonstrate that both fixed points can be used as alternatives for the Hg TP in a future revision of the ITS-90.