WP3: Extending the life of the International Temperature Scale of 1990

The aim of this work package is to extend the life of the current defined scale (ITS-90) in the Standard Platinum Resistance Thermometer (SPRT) range –248.6 °C to 962 °C by reducing its non-uniqueness uncertainty by a target of 30 % and provide the experimental and mathematical foundation for a suitable replacement to the mercury triple point.

Reducing the ITS-90 non-uniqueness uncertainty

The aim is to establish upper limits (reduced by at least 30 % with respect to literature values) for the Type 3 non-uniqueness from a set of high-quality comparison measurements performed in four different laboratories. Determine Type 1 non-uniqueness for a large sample of high-quality SPRT European data. Type 1 non-uniqueness uncertainty estimates will be determined for the relevant overlapping ITS-90 sub-ranges. Establish a new approach to reducing non-uniqueness uncertainty based on redundant fixed points and least squares fitting.

Replacing the mercury fixed point with alternative fixed points

The aim is to provide the experimental and mathematical foundation to the replacement of the mercury fixed point as defining fixed point of the ITS-90 with alternative fixed points. Two alternative high performance fixed points, the SF6 and the CO2 triple points, suitable for the calibration of long stem SPRTs will be constructed and metrologically characterised. A recommendation report to CCT on the replacement of the mercury as defining fixed point of the ITS-90 will be prepared.