The term ‘traceable’ has a very specific definition. It means that there is an auditable chain of direct measurements linking the value of the calibration material (in this case the concentration of the calibration gas) back to primary standard gas mixtures and ultimately to the SI units.
Primary Standard Gas Mixtures (PSMs):
National Measurement Institutes (NMI) worldwide, for example, NPL in the UK, NIST in USA and VSL in the Netherlands, prepare gravimetric primary standards of gas concentration, which they retain in-house as their ultimate references for gas measurements. These standards are known as Primary Standard Gas Mixtures and sit at the highest echelon of gas standards.
Primary Reference Gas Mixtures (PRMs):
PRMs are secondary gas standards that are certified by a National Measurement Institute (NMI) directly against a selection of PSMs using in-house validated analytical methodology, which has demonstrated repeatability and linearity.
Certified Reference Mixtures (CRM):
CRMs may be prepared and supplied by commercial gas companies that have an ISO17025 laboratory with a calibration accreditation. The CRMs are prepared using audited methods to compare the gas concentration to Metrology PRMs.
CRMs have a valid traceability in terms of the ISO17025 and MCERTS Accreditation and the analytical result is stated on the Calibration Certificate with a calculated uncertainty of the measurement. This uncertainty measurement is very important with regards to adhering to the standard reference methods (SRMs) whilst conducting QAL2 and AST procedures.
Air Liquide offers a wide range of CRGMs that are ISO17025 accredited.
Traceable Gas Mixtures:
Air Liquide also offers calibration mixtures that are analysed against benchmark standards such as Standard Reference Materials (PRMs & CRMs) obtained from the U.S. National Institute of Standards and Technology (NIST), or other recognized metrology institutes such as NPL (National Physical Laboratory) in the UK and VSL (formerly NMi) in the Netherlands.
These standards are analysed and certified in accordance with ISO6143. Alternatively, calibration gases that contain stable, inert components can be manufactured using high-precision weight (gravimetric) techniques and these are traceable to primary mass reference standards from NPL. These calibration gas mixtures are manufactured and certified in accordance with ISO6142. Traceable gas mixtures are supplied with a certificate of composition that conforms to the ISO6141 standard.