Revision of the SI: The Determination of the Avogadro Constant as the Base for the Kilogram

Abstract

At least four units of the International System of Units (SI) are on the way to a new definition. Especially for the unit of mass, the kilogram, a rigorous change is considered. Instead of the current definition, a 1kg-artifact in form of a Pt-Ir-cylinder, the intended formulation relates the unit of mass to a fundamental constant. In detail this requires in a first step a measurement of the chosen fundamental constant with contemporary lowest uncertainty and best reproducibility. The constant will then be fixed to that value. As an example the metre is related to the fixed constant speed of light.For the kg there are considered two ways: one is a watt balance, which determines the mass in units of the Planck constant, h. While at present the watt balances show a heterogeneous appearance, the second class of experiment the determination of the Avogadro constant, NA, which measures the mass in terms of the number of elementary entities has reached a considerable level of uncertainty and reproducibility. The fundament of the new determination of the Avogadro constant is a highly enriched 28Si crystal. The different working groups of the Avogadro team determine molar mass and lattice parameter of the crystal, and mass and volume of two precision spheres made from different positions, but of the same crystal. All measurements are carried out for both spheres and all measurement quantities are determined at least from two independent working groups, usually of different countries.