The triple point of water is used as the basis for the definition of the Kelvin. Different effects can influence the triple point temperature, particularly isotopic composition and impurity concentrations. For this reason, scientists want to define the Kelvin via an unchangeable fundamental constant such as the Boltzmann constant, k. Accurate determination of the Boltzmann constant is needed for this.

Christof Gaiser and co-workers, Physikalisch-Technische Bundesanstalt, Berlin, Germany, have used dielectric-constant gas thermometry (DCGT) to determine the value of k = 1.380 655 × 10⁻²³ J K⁻¹ with an uncertainty of 7.9 ppm. DCGT measures to what extent helium gas changes the capacitance of a capacitor. The main uncertainty components result from the measurement of pressure and capacitance as well as impurities contained in the gas.

The low uncertainty value achieved so far demonstrates that DCGT is suitable for determination of the Boltzmann constant at the highest level. Redefinition of the Kelvin will only be possible when the uncertainty is reduced to 2 ppm.

• Determination of the Boltzmann constant by dielectric-constant gas thermometry
  B. Fellmuth, J. Fischer, C. Gaiser, O. Jusko, T. Priruenrom, W. Sabuga, T. Zandt,
  Metrologia 2011, 48, 382–390.
  DOI: 10.1088/0026-1394/48/5/020