Recommendation 2 of the 25th CCT (2010)

Considerations for a new definition of the kelvin

The Consultative Committee for Thermometry (CCT),

recalling its previous Report to the CIPM in 2007, entitled “Report to the CIPM on the implications of changing the definition of the base unit kelvin”, TG-SI/docs05, and

considering

• further discussion at its 24th and 25th meetings held in 2008 and 2010,
• recent progress in experimental determinations of the Boltzmann constant, k, as reported at the 3rd and 4th International Workshops on Progress in Determining the Boltzmann Constant held in 2008 and 2009 and
• other experimental progress allowing a mise en pratique for the new definition of the kelvin already established and presently extended to cover direct measurement of thermodynamic temperature,

noting

• that various experiments, such as acoustic gas thermometry, dielectric constant gas thermometry, Johnson noise thermometry, total radiation thermometry and Doppler broadening thermometry represent distinct routes to determining the Boltzmann constant,
• that the experiments currently underway to measure k need another two years before CODATA can recommend a robust value for k with a relative standard uncertainty about a factor of two smaller than the current u_r of approximately 2 × 10⁻⁶.
• that a relative standard uncertainty of 1 × 10⁻⁶ in k corresponds to a standard uncertainty of about 0.25 mK in the temperature of the triple point of water after the redefinition,

appreciating the considerable progress of ongoing experiments to determine the Boltzmann constant in order to improve confidence in the present value,

recommends

• that before proceeding with the redefinition of the kelvin a relative standard uncertainty of the value of k of order one part in 10⁶ be obtained, based on measurements applying different methods of primary thermometry,
• that these measurements ideally include at least two fundamentally different methods such as acoustic gas thermometry and dielectric constant gas thermometry and be corroborated by other measurements such as Johnson noise thermometry, total radiation thermometry or Doppler broadening thermometry,
• that the CODATA recommended value be adopted for the Boltzmann constant.

DOI : 10.59161/CCT2010REC2E