Logo PTB

From defining the units to their realization: the new “mises en pratique”


In addition to the revision of the International System of Units (SI), the supporting documents (mises en pratique) for the practical realization of the SI units were also adapted to the revision of the SI and were published on 20 May 2019 as well.

From 1889 to 2019 the international prototype of the kilogram (“Le grand k”) was the measure of all masses, but since May 2020 the definition of the kilogram has been based on the Planck constant h. In the new mise en pratique for the kilogram, two methods are described which are capable of realizing this unit with uncertainties within a few parts in 108. The first of these methods uses high-accuracy electromechanical balances (Kibble balances or watt balances) and the second one is based on experiments which focus on determining the number of atoms in single-crystals.

This revision was governed by the principle that “realizing an SI unit” means that the measurement is to be understood as defined by means of “primary methods”, and that influence parameters on the respective measurement uncertainty as well as possible corrections must be specified.

In the new, fundamentally revised mise en pratique for the SI unit of length, the meter, the reference to the primary measurands was, for example, clearly developed for the first time and the achievable measurement uncertainty as well as the influence parameters and corrections to be considered were named. Furthermore, methods for the secondary realization of the meter in the field of nanometrology were included in three guidelines of the Consultative Committee for Length. These methods were based on the work carried out in the scope of the International Avogadro Cooperation Project, among other things.