The unit of mass, the kilogram, is the mass of the international prototype of the kilogram kept in air under three bell jars at the BIPM. It is a cylinder made of an alloy for which the mass fraction of platinum is 90 % and the mass fraction of iridium is 10 %.
Due to the inevitable accumulation of contaminants on surfaces, the international prototype is subject to reversible surface contamination approaching 1 µg per year in mass. For this reason, the CIPM declared that, pending further research, the reference mass of the international prototype is that immediately after cleaning and washing by a specified method (PV, 1989, 57, 104-105 and PV, 1990, 58, 95-97). The reference mass thus defined is used to calibrate national standards of platinum-iridium alloy (Metrologia, 1994, 31, 317-336).
The masses of 1 kg secondary standards of the same alloy as the international prototype are compared in air with the mass of the international prototype by means of balances with a relative uncertainty approaching 1 part in 109. In the case of stainless-steel 1 kg artefacts, the relative uncertainty of comparisons in air with respect to secondary standards made of platinum-iridium alloy is limited to about 1 part in 108 by uncertainty in the correction for air buoyancy. The results of such comparisons made in vacuum, though unaffected by air buoyancy, are subject to additional corrections to account for changes in mass of the standards when cycled between vacuum and ambient air.
Mass standards representing multiples and submultiples of the kilogram can be calibrated by a conceptually simple procedure.
[last updated: 26 September 2005]