Realising the redefined kelvin

The Project

Real-K will turn the kelvin redefinition into reality


In November 2018 the General Conference of Weights and Measures (CGPM) approved a revision on to the international system of units (the SI), which is the most fundamental change of SI since its inception. The new definitions and their associated Mise en Pratiques (MePs) came into force on World Metrology Day, May 20, 2019. In the revision, all seven SI base units will be defined in terms of fixed values of seven fundamental constants of nature. For the SI quantity temperature the unit is the kelvin, which prior to the revision was defined in terms of the triple point of water with known isotopic composition. Following the redefinition, the kelvin is defined in terms of a fixed value of the Boltzmann constant (k) and is thus independent from any material properties and it does not imply any particular method or experiment for its practical realization.


Prior to the redefinition, the kelvin was realised and disseminated through two defined temperature scales; the International Temperature Scale of 1990 (ITS-90) and the Provisional Low Temperature Scale of 2000 (PLTS-2000). Following the redefinition, the realisation and dissemination of the kelvin is regulated through a broader arrangement detailed in the Mise en Pratique for the definition of the kelvin in the SI (MeP-K). The redefinition of the unit and the introduction of the Mise en Pratique opens up the possibility of primary thermometry as alternative (and even preferable means) of realising and disseminating the kelvin. This, in the longer term should lead to realisation and dissemination of the kelvin methods directly linked to the kelvin definition without recourse to the defined scales. The aim of this research is to begin to turn that important goal into a reality.


To achieve the ambitious goal of the project, the following specific objectives were set:

  1. To demonstrate and establish traceability directly to the redefined kelvin from ~1300 K to ~3000 K. Low uncertainty thermodynamic temperatures of four new HTFPs will be established. Then, through the mechanism of the MeP-K, HTFPs will be used to realise and disseminate thermodynamic temperature with uncertainties competitive with the defined scale (the ITS-90) (target U<0.05 %). [WP1]
  2. To demonstrate practical primary thermometry for realisation and dissemination of thermodynamic temperature below 25 K and so demonstrate that primary thermometry can be used, to replace the currently complex ITS-90 scale realisation arrangement below 25 K and to ensure a smooth transition to the PLTS-2000 range below 1 K (target U = 0.2 mK at 25 K and <1 % at 1 K). [WP2]
  3. To extend the life of the current defined scale (ITS-90) giving users continued access to low uncertainty realisations of the scale whilst allowing time for primary thermometry methods to mature. The troublesome issue of scale non-uniqueness will be comprehensively investigated, reducing its uncertainty by 30 %, and a suitable fixed-point replacement for the beleaguered mercury triple point identified, constructed and tested. The issue of integration of the new fixed point within the ITS-90 will be addressed. [WP3]
  4. To reduce the uncertainty in a number of different primary thermometry methods, approved for use in the MeP-K, and so begin to facilitate an extension of their applicability for temperature realisation and dissemination into the temperature region 25 K and above. This long-term objective will be facilitated through reducing the uncertainties of the calculated thermophysical properties of gases (e.g. He, Ne, Ar) used as thermometric fluids in primary thermometers. [WP4]
  5. To work closely with the CCT to issue formal recommendations and definitive guidance on the realisation and dissemination of the redefined kelvin, to ensure rapid uptake of the results of this research. Engagement with the global thermometry community by regular briefings to RMO TC-Ts, papers and conference presentations. Wide user engagement through stakeholder community and events. [WP5]