Final Report, 21 March 2005
The temperature of the triple point of water is presently determined to be
To ensure that such a redefinition would maintain the relative uncertainty currently achieved for the realisation of the temperature unit, the Boltzmann constant must be known with similar uncertainty. At present its uncertainty is still about
PTB will contribute to the reduction of uncertainty with another variant of the gas thermometer, the dielectric constant gas thermometer (DCGT) with helium – in particular, because the polarisability of the helium atom can now be calculated very precisely applying quantum mechanics by scientists of University of Delaware. With this method the temperature- and pressure-dependent dielectric constant of helium is determined from the small change of capacitance when evacuating a capacitor filled with helium gas. For many years PTB has successfully applied the DCGT in the low-temperature range and is aiming at to further improve this technique. Spectral radiation measurements based on the Planck law will support the project.
NPL uses the measurement of the total radiation without spectral selection based on the Stefan-Boltzmann law and a consortium of BNM-INM and University Paris North determines the spectral Doppler broadening of a laser absorption line in a gas cell.
It was concluded that the project to redefine the kelvin is extremely ambitious and will take a couple of years for finalisation. The final aim can be achieved only by tight world-wide cooperation which has been started by PTB with this workshop and will continue. The program of the workshop and all presentations can be found at http://www.berlin.ptb.de/en/org/7/74/k_Workshop_2005.html. A scientific summary prepared by the organisers of the workshop is submitted to the 23rd meeting of the CCT and will be available as CCT document.
The temperature of the triple point of water presently defines the unit of temperature, the Kelvin. 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 corresponding constant is the Boltzmann constant k. To ensure that such a redefinition would maintain the uncertainty currently achieved for the realisation of the temperature unit, k must be known with similar uncertainty. At present its uncertainty is still about one order of magnitude higher. Therefore, k has to be determined with distinctly lower uncertainty than presently possible.The workshop aims at to exploit the uncertainty limitations of the methods capable to contribute to an improved value of the Boltzmann constant.
The project is expected to further extend to participants outside the EU.