Logo PTB

Temperature metrologists prepare for the redefinition of the Kelvin


Figure 1: Differences between thermodynamic temperatures T and temperatures T90 determined in accordance with the International Temperature Scale of 1990, ITS-90. The bars show the corresponding uncertainties.

The national metrology institutes prepare intensively for the redefinition of the base units which is scheduled - from 2015 on – to be traced back to fundamental constants. For that purpose, recommendations for implementation - so called Mises en Pratiques - are elaborated and published. According to the planned redefinition of the Kelvin - the base unit for the temperature - temperature measurements will, in some temperature ranges, be carried out increasingly with so-called primary thermometers which directly determine thermodynamic temperatures.

Primary thermometers do not require any calibration. However, for a wide temperature range, the current International Temperature Scale of 1990 (ITS-90) will still be valid. Therefore, to guarantee the uniformity of metrology, the differences between the temperatures which have been determined thermodynamically and those which have been determined as usual in accordance with the ITS-90 must be clearly defined. For that purpose, a working group of the International Committee for Thermometry of the Metre Convention (CCT WG4) has - under the direction of PTB - critically evaluated all the data available of the past 30 years and published these differences with well-founded uncertainty statements. The most important results can now be retrieved online in the up-dated Mise en Pratique for the Definition of the Kelvin as an official recommendation of the CCT. This allows any user to freely select the temperature measuring procedure most convenient for him.


J. Fischer, Department 7.4, E-Mail: Joachim.Fischer(at)ptb.de