Realising the redefined kelvin

Real-K – Realising the redefined kelvin

On the 20th May 2019, the implementation phase of the redefinition of the international system of units (the SI) was completed and the new unit definitions came into force. For the SI quantity temperature, this means that its unit kelvin is now defined in terms of a fixed value of the Boltzmann constant (k). However, extensive research is still required in order to turn the kelvin redefinition and its associated Mise en Pratique into a reality. Currently nearly all temperature measurements performed around the world are traceable to one of the two defined scales (either the ITS-90 or the specialist low temperature scale, the PLTS-2000). However, with the impetus given by the redefinition there will be a rise in primary thermometry approaches for realising and disseminating temperature, directly linked to the redefined kelvin.

The project will test primary thermometry approaches at temperatures greater than 1300 K and less than 25 K to kick-start the process of making these methods competitive. It will also extend the life of the currently defined scale to allow time for primary methods to develop, identify a replacement for the mercury triple point and reduce the uncertainties of the calculated thermophysical properties of monatomic gases used in gas-based primary thermometry. The ultimate goal is that primary thermometry will be the basis of temperature traceability throughout the entire range. Easing the transition to primary thermometry in this way should enable in-situ traceability at lower cost, in applications such as remote monitoring in manufacturing and the nuclear power sector.

The project utilises the world leading primary thermometry skills and capabilities established through the EMRP InK1 and EMPIR InK2 projects.

For more information on kelvin, the SI unit of thermodynamic temperature, please visit