The New SI System of Units—The SI of 2018

Abstract

This chapter is devoted to the new, revised SI system, eventually accepted in 2018, and to the phases of its conception and creation. In this context we present the need for improvement of the system of units, and the international efforts to create a new system. There was unanimous consent in the metrological community that a new system of measurement should be defined on the basis of fundamental physical and atomic constants, as it has been put forward for 180 years, since the first proposals by K. Gauss and J. Clerk Maxwell. We discuss the proposed redefinition of four SI units: kilogram, ampere, kelvin and mole. We present the particular importance of the new definition of the kilogram and the difficulties encountered in the development of a quantum standard of the unit of mass. The practical realization (mise en pratique) of any unit generally means establishment the value and associated uncertainty of a quantity of the unit of the same kind. Mise en pratique for realization of the units of the SI were prepared by the Consultive Committees of the CIPM. They are presented in Sects. 3.5–3.8.