Determination of the Boltzmann constant - status and prospects

The present definition of the kelvin links the unit of temperature with a material property, namely the triple point temperature of water. It would be more consistent with the current approaches to other base units of the International System of Units to fix the value of the Boltzmann constant k, instead. This would rationalize the definition and make it separate from any technique of realization. Furthermore, it is needed to improve temperature measurement, particularly at temperatures far away from the triple point of water. For this purpose, k must first be determined with appropriately small uncertainty applying different measurement methods. In this paper, the primary-thermometry methods for determining the Boltzmann constant ( acoustic gas thermometry, thermal-equation-of-state methods, radiation thermometry and methods based directly on statistics and quantization) and their state-of-the-art level of uncertainty are discussed. Special emphasis is given to the basic physics underlying these methods, the fundamental error sources and the uncertainty, which seems to be attainable on a five-year timescale in view of some new developments and the foreseeable progress. Finally, a possible simple new definition of the kelvin is proposed.