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Verifications

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.



Summary
The BIPM watt balance
BIPM ensemble of mass standards
International Avogadro Project
Calibration services TEMPORARILY SUSPENDED
International prototype of the kilogram
Verifications
BIPM calibrations and services in mass and related quantities
Density determinations of prototypes and mass standards
Frequently asked questions about the international prototype of the kilogram
Recent publications
Staff of the Department
On the possible future revision of the SI
Director's Report 2013
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)
Cleaning of standards
The BIPM watt balance
International Avogadro Coordination (IAC) project