The measuring quantity temperature in the revised International System of Units

Since 20 May 2019, the temperature unit kelvin is no longer defined by fixing the value of the temperature of the triple point of water, but by a fixed value of the Boltzmann constant (k = 1.380649 x 10(-23) J/K). Through this, mechanical and thermal energy are directly connected, which is physically a principal progress. However, the new definition is very abstract because there is no reference to rules for the realization of the unit. Practical recommendations ("Mise en Pratique") have been, therefore, prepared by international committees. Primary thermometry, which is directly based on the laws of thermodynamics, is of special importance for this. But the new definition does not change the role of the International Temperature Scales. Thus, the efficient system of traceable temperature measurements is maintained.