Metrologists are artists of the precise; and in the case of Boltzmann constant this precision extends up to the sixth decimal place. Determining this fundamental constant more precisely could cause a small revolution in the field of temperature measurement: The temperature unit would no longer be based on a chemico-physical material property, i.e., the triple point of water, but on an unchangeable constant.
Scientists of the Physikalisch-Technische Bundesanstalt (PTB) have now succeeded in re-determining the Boltzmann constant. Although the uncertainty of the result is not yet sufficient for a redefinition of the kelvin, it demonstrates that the procedure selected by PTB is basically applicable. The physicists expect that in the next two years they will be able to reduce the uncertainty to such an extent that a redefinition of the kelvin will be possible. These results have been recently published in the scientific journal Metrologia.
Worldwide, a number of research groups are working on the task of defining the kelvin via a fundamental constant. European research institutions are cooperating in this field in several joint EU projects. Only if several groups obtain the same result, with at least two independent methods will a "water-free" definition of the kelvin become possible. In the long term, scientists are trying to define all base units of the International System of Units (SI) with the aid of fundamental constants. In the case of the metre, this has, for example, already been achieved using the speed of light.
To determine the Boltzmann constant, many research groups are using acoustic gas thermometry, which has produced the most exact values so far. PTB has followed an alternative, completely independent path to rule out systematic error sources and, thus, to place the redefinition on a solid basis: Here, Dielectric-Constant Gas Thermometry (DCGT) is used. The method is based on the density determination of helium by means of a capacitance measurement or, in other words: The researchers measure to what extent the gas changes the capacitance of a capacitor.
The set-up now allows DCGT measurements to be carried out at the triple point of water and produces a value for k of 1.380655·10-23 J/K. With an uncertainty of 8 ppm it demonstrates that DCGT is suitable for a determination of the Boltzmann constant. However, the scientists need to achieve an uncertainty of 2 ppm. The PTB team expects that this will be possible within the next two years and that then, the way for the redefinition of the kelvin will be cleared.