– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
Search facility:

| Site map | News | Contact us

On three occasions, roughly 40 years apart, the mass of the official copies, the national prototypes and the working standards of the BIPM have been compared with the mass of the international prototype. On the last of these occasions (1988-1992), 34 standards from national laboratories were cleaned with solvent and then steam-cleaned with double-distilled water before the definitive mass comparisons.

The results of comparisons between the official copies and the international prototype show some divergence with time. The graph below shows changes of about 5 x 10–8 in the mass of the standards since their first calibration. All measurements are with respect to the international prototype. For this reason, the mass of the international prototype defines the y-axis of this graph.

There is no reason to suspect that the mass of the international prototype of the kilogram is more stable than its official copies. The question of stability can only be answered definitively by comparison with a fundamental constant of nature (such as the Planck constant or the mass of an atom of 28Si). Such experiments are being carried out in several national laboratories.

The BIPM watt balance
BIPM ensemble of mass standards
International Avogadro Project
Calibration services
BIPM calibrations and services in mass and related quantities
Recent publications
International prototype of the kilogram
Frequently asked questions about the international prototype of the kilogram
Density determinations of prototypes and mass standards
Staff of the Department
On the possible future revision of the SI
Director's Report 2014
Related articles
The third periodic verification of national prototypes of the kilogram (1988-1992) (Metrologia, 1994, 31, 317-336)
The washing and cleaning of kilogram prototypes at the BIPM (Monographie 1990/1)