National Physical Laboratory

Classes of NPL gas standards

NPL gas cylinders


NPL provides the following classes of gas standards:


NPL Primary Reference Materials (NPL PRMs)

NPL PRMs (previously PRGMs) are prepared gravimetrically in accordance with ISO 6142 and are validated against NPL in-house PRMs. The certificate provided with a NPL PRM reports the gravimetric value with an uncertainty that represents the best achievable analytical uncertainty. It also contains an expiry date for the mixture, which is assigned from data obtained from stability studies of similar mixtures.

These mixtures are the top of the traceability chain with the smallest uncertainties. It is only by comparison of these primary realisations of the mole with other NMIs and DIs (Designated Institutes) around the world, using the measurement redundancy that this provides, that the full uncertainty of the realisation can be achieved. This takes place as part of Key Comparisons performed under the CIPM Mutual Recognition Arrangement (MRA) and results in Calibration and Measurement Capabilities (CMCs) claimed by the NMIs/DIs which are peer-reviewed and, when agreed, stored on a database as Appendix C to the CIPM MRA. This process provides the uncertainty with which these standards can be realised and disseminated.

NPL PRMs provide the primary realisation of the mole in gas mixtures at a national level which is only available from NMIs/DIs.


NPL Calibrated Gas Mixtures (NPL CGMs)

NPL CGMs are mixtures that have been supplied by commercial organisations and individually certified against NPL in-house PRMs. The certificate provided with a NPL CGM reports the certified value determined by the analytical method used, and an analytical uncertainty (which is typically larger than that provided for a NPL PRM). A stability guarantee is not normally given for a NPL CGM due to a lack of knowledge of the history of the cylinder and the gas mixture.


NPL Certified Reference Materials (NPL CRMs)

NPL CRMs (previously SGSs) contain reactive and unstable components. These mixtures are prepared in the same way as NPL PRMs but due to initial changes in amount-of-substance fraction after filling, they are provided with a certificate that reports the value obtained from the analytical certification process by comparison to NPL in-house PRMs. The certificate may also contain an expiry date for the mixture, which is assigned from data obtained from stability studies of similar mixtures.


NPL's Gas Standards are supplied in 10-litre cylinders, filled in excess of 100 bar*. The cylinders are sold, eliminating cylinder rental costs. This proves financially beneficial when customers require gas standards over significant periods of time.

* A minority of NPL gas standards are filled to less than 100 bar due to technical reasons.


Contact

Please email gases@npl.co.uk or telephone Customer Services on +44 20 8943 6151.

Last Updated: 23 Jan 2017
Created: 17 Sep 2013

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