Preparative Steps Towards the New Definition of the Kelvin in Terms of the Boltzmann Constant

The International Committee for Weights and Measures (CIPM) approved, in its Recommendation 1 of 2005, preparative steps towards new definitions of the kilogram, the ampere, the kelvin, and the mole in terms of fundamental constants. Within the Consultative Committee for Thermometry (CCT), a task group (TG-SI) has been formed to consider the implications of changing the definitions of the above-mentioned base units of the SI, with particular emphasis on the kelvin and the impact of the changes on metrology in thermometry. The TG-SI has presented the results of its deliberations to the CCT and to the Consultative Committee for Units, CCU, and worked with them to prepare a report to the CIPM. This contribution, authored by the members of TG-SI, solicits input from the wider scientific and technical community on this important matter at the TEMPMEKO 2007 conference. For this purpose, the main details of the report to the CIPM are presented. The unit of temperature T, the kelvin, can be defined in terms of the SI unit of energy, the joule, by fixing the value of the Boltzmann constant k, which is simply the proportionality constant between temperature and thermal energy kT. Currently, several experiments are underway to determine k. The TG-SI is monitoring closely the results of all experiments relevant to the possible new definition of the kelvin, and has identified conditions to be met before proceeding with the proposed redefinition. The TG-SI considers that these conditions will be fulfilled before the 24th General Conference on Weights and Measures in October 2011. Therefore, the TG-SI is recommending a redefinition of the kelvin by fixing the value of the Boltzmann constant. A new definition of the kelvin in terms of the Boltzmann constant does not require the replacement of ITS-90 with an improved temperature scale nor does it prevent such a replacement.