What is a Primary Standard Gas Mixture (PSM)? (FAQ - Gas Standards)
NPL and other national standards laboratories around the world prepare primary standards of gas concentration, which they retain in-house as their ultimate reference for gas measurements. These primary standards are generally known as Primary Standard Gas Mixtures (PSMs).
These PSMs are prepared by the national laboratories using sophisticated gravimetric apparatus and rigorously quality-assured procedures. In addition, the source gases are comprehensively analysed for impurities, and the cylinder passivation used to ensure the stability of the standard is optimized individually for each species. A large number of PSMs are prepared over a long period and these are intercompared with high precision instrumentation to demonstrate their consistency before standards traceable to these are disseminated. In addition, national standards laboratories intercompare their PSMs by exchange of such standards to demonstrate their international comparability.
However, since the uncertainties estimated for the gravimetric preparation are typically an order of magnitude better than can be achieved using any analytical comparison method, they could give a misleading impression as to the true level of confidence associated with the gravimetric value. Consequently, National Metrology Institutes do not generally allow PSMs to leave the laboratory except for use in international comparisons with other key national institutes.
The above therefore summarises the rigorous procedure which is used to demonstrate the validity of PSMs at a national metrology institute. It is not unusual for such a procedure to take 3-5 years to introduce a new type or new concentration of PSM.
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