Gas mixtures with the analytical method A-CRM

Analytically calibrated metrologically traceable gas mixtures are defined A-CRM: the certified concentration and uncertainty are determined by comparison with the Centre's traceable samples (G-CRM or CRM) or with standards certified/produced by a National or Primary Metrology Institute (VSL, NPL).

The metrologically traceability derives from the mixtures used in the calibration phase.

The international standard used for analytical calibration is ISO 6143. The instrument calibration is performed for each mixture using the method of generalized least squares (GLS): usually at least 3 reference gas mixtures are used (G-CRM, CRM or VSL and NPL mixtures).

The sample to calibrate may be supplied by the customer or produced by SIAD: the Centre provides only the analytical calibration for the component required by the customer. If the mixture is produced by SIAD, the stability guarantees of Specialty Gas mixtures can be apply. In the hierarchy of the traceability of the Centre's products, A-CRM mixtures are metrologically inferior to the gravimetrically calibrated G-CRM mixtures: the calibration is carried out for comparison and not with a primary method; therefore also the minimum certified uncertainty is greater than for the latter.

The main recommended applications are:

• verification of the calibration status of the analytical measurement equipment;
• calibration of equipment for which the performance isn’t so strictly binding.

Gravimetric calibration and analytical