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Determination of the Boltzmann constant k for redefining the base unit kelvin

The current definition of the kelvin depends on a property of a special substance, the triple-point temperature of water, which is presently determined to be 273.16 K by definition. Thus, the unit kelvin is linked to a material property which is rather arbitrary. 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 the value of this constant. By this, no temperature value (and no measurement method) would be favoured. For the kelvin, the corresponding constant is the Boltzmann constant k, because temperature always appears as “thermal energy” kT in fundamental laws of physics. To ensure that such a redefinition would maintain the uncertainty currently achieved for the realisation of the temperature unit, the Boltzmann constant must be known with similar accuracy. At present, however, its relative uncertainty is still about 6 x 10-7. For a redefinition, k must first be determined with appropriately small uncertainty applying different independent measurement methods. Therefore, the precise measurement of the Boltzmann constant is a challenging task of international metrology. In principle, the Boltzmann constant can be determined with any primary thermometer by measuring kT at a known temperature (ideally at the triple point of water).

The first and the second international workshop on the determination of k were held at PTB in Berlin. Representatives of worldwide leading institutes presented their talks. This allowed to discuss in detail the progress of different kinds of gas thermometry, noise thermometry and Doppler-broadening thermometry. The representative of the PTB described improvements of dielectric-constant gas thermometry leading to an essentially decreased uncertainty. Furthermore, the planned efforts of the PTB to improve the pressure measurement up to 7 MPa were outlined. It could be stated that the international cooperation has been broadened. This is very important in view of the deadline for the new definition of the base unit kelvin in 2018, for which k has to be determined with the necessary small uncertainty.

As a result of the workshops on the determination of k, in Europe an iMERA/EURAMET joint research project coordinated the activities to determine the Boltzmann constant in the European countries. For more information about this visit the homepage of the iMERA Plus Consortium "Boltzmann Constant".

After finishing the project, a task group of the Consultative Committee for Thermometry coordinates the different activities for preparing the new definition of the kelvin. An overview of the work is given in a special issue of the journal Metrologia published in 2015.