The basic units of the SI system are defined with the fundamental physical constants with suitable small measurement uncertainties. Only the kilogram, the present SI base unit for the mass, is still defined by an artefact (alloy of 90% Pt and 10% Ir) which is kept at BIPM (Bureau International des Poids et mesures) in Sevres in France. The uncertainty of the mass of the artefact is zero by the definition but due to the atmosphere changes, use of this artefact for periodical verifications of national standards and cleaning itself, its instability in the mass increased to about 5.10-8kg.Thus the uncertainty of the mass is not anymore zero and definition of the kilogram was decided to be changed. The definitions of some SI units (e.g. ampere, Kelvin, mole and kilogram) are likely to be revised in 2011 and as regards the kilogram, the experiments aiming the linking the mass of the prototype of the kilogram to the fundamental physical constants thus have to be completed by then.

INKRET Monika; 

2009-01-22

University of Ljubljana

JRC49970

https://publications.jrc.ec.europa.eu/repository/handle/JRC49970,