Boltzmann constant determined
All obstacles to the redefinition of the unit of temperature, the kelvin, have been removed
For a unit to be based on a fundamental constant, the latter should, as a matter of principle, be measured by means of two methods which are independent of each other and have a comparable uncertainty. As early as 30 years ago, the Boltzmann constant k had already been determined with a relative uncertainty of 1.8 ppm by means of the acoustic gas thermometer. Over the past decade, this method was further refined by various metrology institutes, with the most Three times a year, PTB News provides topical information from the varied spectrum of activities of the Physikalisch- Technische Bundesanstalt (PTB) consisting of fundamental research, legal metrology and PTB's various activities in the service of the economy. accurate result exhibiting an uncertainty reduced by a factor of 2. These results allow the first condition of the CCT for the new definition to be met, namely obtaining an averaged value for the Boltzmann constant with an uncertainty of less than 1 ppm.
An independent method of doing this is dielectric-constant gas thermometry, which PTB has been using for many years. This method consists in determining the pressure of the measuring gas, helium, in a gas-filled capacitor. This approach is based on the fact that helium, as a dielectric, changes the capacitance of the capacitor. At pressures up to 7 MPa, the uncertainty of the pressure measurement had to be reduced by a factor of 4. This has now been achieved with a worldwide unequalled relative uncertainty of 1 ppm. In order to measure the capacity changes, relative uncertainties of a few parts per billion may not be exceeded. Moreover, the material parameters for the capacitors – at these high pressures – had to be determined at the metrological limit, and a gas purity of better than 99.99999 % had to be ensured. This could only be achieved thanks to various cooperation projects within PTB (with the two working groups “Pressure” and “Geometrical Standards”) and thanks to large-scale international cooperation.
Now that the Boltzmann constant has been determined with sufficient precision by means of both methods, CODATA will compute the final value of k in September 2017. This will pave the way for the redefinition of the kelvin based on a fundamental constant. Presumably in the fall of 2018, the whole International System of Units (SI) will rest upon a new basis.
Contact
Christof Gaiser
Department 7.4
Temperature
Phone: +49 (0)30 3481-7349
christof.gaiser(at)ptb.de
Scientific publication
C. Gaiser, B. Fellmuth, N. Haft, A. Kuhn, B. Thiele-Krivoi, T. Zandt, J. Fischer, O. Jusko, W. Sabuga: Final determination of the Boltzmann constant by dielectricconstant gas thermometry. Metrologia 54, 280−289 (2017)