Recommendation 1 of the 29th CCT (2021)

Requirement for new determinations of thermodynamic temperature above 400 K

The Consultative Committee for Thermometry (CCT), at its 29th meeting in 2020/2021,


  • the CCT Declaration on the 27th meeting in 2014 “Requirement for new determinations of thermodynamic temperature”,
  • the CCT Recommendation to the CIPM in 2017 “For a new definition of the kelvin in 2018”, CCT T 2 (2017);


  • the Resolution 1 of the 26th meeting of the CGPM (2018) “On the revision of the International System of Units (SI)”, which now links the unit of temperature to the Boltzmann constant;


  • the discussions held at the 27th, and 28th meetings of the CCT in 2014 and 2017,
  • that experimental progress has allowed the development of a Mise en Pratique for the realization of the kelvin (MeP-K), which has been extended to cover direct measurement of thermodynamic temperature, T, after the redefinition of the kelvin;

noting that

  • Resolution 1 of the 26th meeting of the CGPM (2018) stated “that Member State NMIs take full advantage of the opportunities for the realization and dissemination of thermodynamic temperature afforded by the kelvin redefinition and the MeP-K”,
  • in the last years, primary thermometry by acoustic gas thermometry, dielectric-constant gas thermometry, and refractive-index gas thermometry have yielded low uncertainty thermodynamic-temperature data and so improved considerably the knowledge on T – T90 below 400 K (T90 is the temperature according to the International Temperature Scale of 1990, ITS-90),
  • above 400 K, there is a dearth of new accurate measurements of thermodynamic temperature, which are urgently required,
  • primary thermometry approaches to facilitate implementation of the MeP-K by thermodynamic thermometry and to evaluate T – T90 in this important temperature region need significant further development;


  • that Member State NMIs improve their capabilities in primary thermometry, by various means, above 400 K to improve determination of T – T90, accompanied by appropriate research to ensure that International Temperature Scale realization remains fit for purpose, allowing access to lower uncertainty thermodynamic temperature values over a wide range for a broader community.

reference The reader should note that the official version of this Resolution is the French text

CCT logo