Cooperation of SMÚ in the development of the defining fixed point of SF6

Currently, the temperature scale is realized based on the globally recognized document called the International Temperature Scale of 1990 (ITS-90). One of the fundamental pillars of this realization is the use of defining fixed points (FPs), which employ highly pure substances (99.999% or higher) and their phase transitions or equilibrium states. Standard FPs include the freezing point, melting point, or triple point of a given substance, each associated with an experimentally determined temperature at a defined pressure.

One such substance is mercury (Hg), whose triple point is defined at –38.8344 °C. Following the restriction of mercury usage (EU Regulation 2017/852 and the U.S. Mercury Export Ban Act – MEBA), there has been an effort to replace this FP with an alternative that could adequately substitute the mercury point without requiring a major revision of the ITS-90.

The Slovak Institute of Metrology (SMÚ) has joined the international project InK2 and a bilateral collaboration with the French national metrology institute LNE-CNAM, contributing to the development of a FP based on sulfur hexafluoride (SF₆). Based on experimental measurements, the temperature of the SF₆ fixed point was determined to be –49.595 °C, with a measurement reproducibility of 1.0 mK.

This FP system is unique in its design, as it allows for the simultaneous calibration and measurement of long-stem and capsule-type standard platinum resistance thermometers (SPRTs).

Source Image: PAVLÁSEK, Peter, RISEGARI, Lara and SPARASCI, Fernando. Manufacturing and realization of the low volume fixed-point of sulfur hexafluoride (SF₆) [online]. 2024, no. 060003. Available at: https://www.researchgate.net/publication/385189225