NPL is leading a 21 partner EMPIR project ‘Realising the redefined kelvin’ (Real-K) which aims to take advantage of the kelvin redefinition by developing primary thermometry to the point where it can be used, in some temperature ranges, to provide traceability directly to the redefined kelvin.
Currently around the world temperature traceability is taken from one of two defined scales, the International Temperature Scale of 1990 (ITS-90) or the specialist low temperature scale the Provisional Low Temperature Scale of 2000 (PLTS-2000). The long term aim of this activity is that practical primary thermometry will increasingly be adopted, decreasing reliance on traceability to defined scales and improving the long-term reliability of measurements.
By the mid-2020s, the low temperature part of the current scales (below 25 K) could be replaced with simpler primary thermometry approaches, whilst the high temperature part of ITS-90 (above 1300 K) will be replaced by more robust indirect primary radiometry.
Specific NPL contributions to Real-K are:
- Significant input into establishing temperature traceability directly to the redefined kelvin from ~1300 K to ~3000 K through indirect primary radiometry mediated by high temperature fixed points (HTFPs).
- Research to extend the life of the ITS-90 giving users continued access to low uncertainty realisations of the scale whilst allowing time for primary thermometry methods to mature.
NPL is working with other organisations to determine experimentally low uncertainty values of the thermophysical properties of gases, specifically in NPL’s case Argon. This work will reduce the uncertainty and broaden the range of gas based primary thermometry for temperature realisation and dissemination above 25 K.