Primary Reference Gas Mixtures from NPL are prepared, in a similar manner to the PSMs. However, a much greater number of analytical comparisons are made than for NPL Secondary Standards, involving a wider range of PSMs and more purity checks, to demonstrate that the gravimetric composition value of this prepared standard is valid. The NPL Certificate then reports the gravimetric value with an uncertainty that represents the best achievable analytical uncertainty after a substantial number of repeated measurements. This is smaller than that associated with a comparable SGS, but not as small as the estimated gravimetric uncertainty.

NPL supplies 'Primary Reference Gas Mixtures' to customers requiring the lowest uncertainties.

See also:

• What is the NPL Gas Standards Hierarchy? (FAQ - Gas Standards)
• What is a Primary Standard Gas Mixture (PSM)? (FAQ - Gas Standards)
• What is a Secondary Gas Standard (SGS)? (FAQ - Gas Standards)
• What is a Certified Reference Gas Mixture (CRGM)? (FAQ - Gas Standards)