Possible Implications of the Principle of the ‘ Mise en Pratique ’ in its Application to the Kelvin

In 2005, the CIPM, in accepting CCT Recommendation T3, made a substantial change in the definition of the quantity ‘temperature’ by transforming the written standards for temperature (the “temperature scales”) into implementations of the mise en pratique of the kelvin. Having considered “that the creation of a formal mise en pratique of the definition of the kelvin would considerably simplify and clarify statements and recommendations of the CCT concerning the realization of the definition of the kelvin and the implementation of practical temperature scales,” the CIPM decision transformed the present ITS-90 and PLTS-2000 into two ways to implement the mise en pratique, with the possibility to further define “in due course” in the mise en pratique “recommendations concerning the direct determination of thermodynamic temperature.” The paper presents the author’s views regarding possible implications for the concept of a mise en pratique when applied to the kelvin beyond the present implementation. One possibility is to promote the formal use and status of realizations of the thermodynamic scale that, in practice, today only represents the physical basis of an empirical scale like the ITS-90. Another path arises from assuming that the only technical limit placed on the addition of further methods to the mise en pratique is that they be compatible with one another, to avoid ambiguities in the definition of the temperature values, though without the constraint of having the same, or comparable, accuracy. As a consequence, other scales that are currently considered ‘approximations’ to the ITS-90, with no formal status other than possibly being ‘recommended’ by some official body, such as the CCT WG2, could be upgraded to im-