National Physical Laboratory

International Comparisons - worldwide metrology

International comparisons play a vital role in maintaining the international metrology system; providing confidence in the metrology undertaken by the NMIs involved, and are also a necessary activity for the mutual recognition of measurement standards across international borders.

International comparisons are organised between NMIs for a range physical and chemical quantities. The NMIs involved all measure the same artefact or sample (or a small number of artefacts or samples) under strictly controlled conditions, quoting a result and the associated uncertainty.

The NMI leading the comparison is termed the 'pilot' laboratory and is responsible for organising and scheduling the activities, drawing up the technical protocols of the comparison, and reporting the results.

Comparison of measurement standards may be organised in the form of a round robin, star, petal or bilateral depending on the number of laboratories involved, the stability of the transfer standards or samples and the time taken to make all the measurements. The majority comparisons take place with a strict schedule. Usually the Pilot will be the first and last to make measurements, with the standard being sent around all participants in turn.

Comparisons tend to fall into two main categories:

  • Regional (organised within a particular RMO)
  • International (involving NMIs across more than one RMO)

In some cases a regional comparison may include an NMI for outside that region in order to form a link between two different regional comparisons.

Participation in comparisons represents a significant undertaking by the NMIs and in particular to pilot and comparison requires a great deal of resource. Participation in international comparisons is an obligation on all signatories and participants in the CIPM MRA. NPL takes part in and pilots many International comparisons

Visit the BIPM website to see more specific Guidelines for CIPM key comparisons

Last Updated: 17 Apr 2012
Created: 4 Oct 2007


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