Workshop on methods for new determinations of the Boltzmann constant

Final Report, 21 March 2005

The temperature of the triple point of water is presently determined to be 273.16 K by definition. Thus, the 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. A possible new approach would be to define the kelvin as the change of temperature leading to a change of thermal energy kT of 1.38065·10-23 J.

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 2·10-6. Therefore, k must first of all be determined with lower uncertainty. For this reason experts of the most important national metrology institutes met on 20 and 21 January 2005 at the Institute Berlin of PTB. In principle, the Boltzmann constant can be determined with any primary thermometer by measuring the product kT at a known temperature – the triple point of water – and calculating k from it. The present value of the Boltzmann constant is based on the acoustic gas thermometry at NIST, a method measuring the speed of sound in a gas which is also pursued at IMGC/IEN.

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.

Proposal

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.

Further Information

The project is expected to further extend to participants outside the EU.

Subjects
Thermometry (T)
Coordinator
Joachim Fischer, PTB (Germany)
Coordinating Institute
PTB (Germany)