Manufacturing and realization of the low volume fixed-point of sulfur hexafluoride (SF6).

In the recent years a significant effort was invested in the development and characterization of a novel low temperature fixed-point cell that would replace the currently used ITS-90 [1] defining fixed-point cell of mercury (Hg). These attempts were dominantly motivated by the restriction of heavy metals use in multiple countries. The European Union mercury regulation 2017/852 [2] can be used as an example of these restrictions. In order to have a minimal effect on the current International Temperature Scale of 1990 several materials with similar triple point temperatures were considered. These were CO2 (216.59 K) [3, 4] and SF6 (223.555 K) [5, 6, 7, 11]. In this contribution we will be focusing on the later of the two and on the performance of a low volume SF6 cell with a total volume close to 0.05 l. This cell has been specifically designed to enable simultaneous calibration of capsule and long stem type of SPRTs (Standard Platinum Resistance Thermometers) in different cooling apparatuses (cryostats, baths etc.). The conducted measurements that are going to be presented and discussed were realized in adiabatic conditions and in quasi-adiabatic conditions. Simultaneous multiple realizations with capsule type SPRTs and long stem SPRTs have been done with subsequent evaluation of suitability of this type of fixed-point cell for routine calibrations. The usage of capsule type SPRTs in a quasi-adiabatic system has shown a repeatability which was within 1 mK. The average triple point value measured was in this case 223.55523 K. In the case of using a long stem SPRTs in the quasi-adiabatic system a higher spread of the realizations was observed with a level of repeatability of 1.5 mK. The average triple point temperature of 223.55595 K was measured which was on average 0.72 mK higher indicated temperature when compared to the measurement with capsule type SPRTs. This shift in temperature indication and a lower observed repeatability can be explained by a more significant effect of hydrostatic head and the heat load from the SPRT stem. It should be noted that the latter is considered the more dominant effect. [ABSTRACT FROM AUTHOR]