The iMERA-EUROMET joint research project for new determinations of the Boltzmann constant

The unit of temperature T, the kelvin is presently defined by the temperature of the triple point of wa-ter. Thus, the kelvin is linked to a material property. Instead, it would be advantageous to proceed in the same way as with other units: to relate the unit to a fundamental constant and fix its value. By this no temperature value and no measurement method would be favoured. For the kelvin, the correspond-ing constant is the Boltzmann constant k, because temperature always ap¬pears as thermal energy kT in fundamental laws of physics. For fixing the value, the present value of k needs to be confirmed by several independent methods. To support new determinations of the Boltzmann constant, the CCT recommended “that national labo-ratories initiate and continue experiments to determine values of thermodynamic temperature and the Boltzmann constant”, which is also asked for by the recent recommendation of the CIPM concerning preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole. As one result of a workshop held at PTB in 2005, an iMERA/EUROMET joint research project is now coordi-nating the European activities to determine the Boltzmann constant in France (LNE-INM/CNAM, Uni-versity Paris North), Italy (INRIM, Universities of Naples and Milan), United Kingdom (NPL) and Ger-many (PTB). In this major European research project nearly all partners involved use different methods, like acous-tic or dielectric constant gas thermometry, or spectroscopic and optical methods. All partners will de-termine the Boltzmann constant using their own equipment aiming at a relative standard uncertainty close to 1 ppm. These new values will be compared with the present CODATA value of k determined by acoustic gas thermometry of NIST. The aims and the progress to date of the project are reviewed in this paper.