Certified Reference Materials (e.g. Primary Reference Gas Mixtures) produced by NPL under ISO Guide 34 will have an expiry date stated on the certificate [see ISO Guide 34:2000 para 5.9.4]. This is based on NPL's experience with the stability of the mixture concerned and follows procedures agreed with our external assessment body (UKAS).

Calibrations (or 'certifications') carried out by NPL (e.g. Calibrated Gas Mixtures) under ISO 17025 do not carry an expiry date on the certificate. This conforms to ISO 17025:2005 [para 5.10.4.4]. The principle advocated by ISO 17025 in this respect is that laboratories using such mixtures should operate a procedure to determine the on-going validity of their standards. Advice on this is included below.

Historically, NPL has issued certificates for some gas mixtures that are not covered by accreditation to either ISO Guide 34 or ISO 17025. In these cases NPL has followed one of the approaches described above.

Determination of the ongoing validity of standards

It is best practice for laboratories to operate a procedure to confirm the ongoing validity of the standards they use. For example, this requirement is defined by ISO 17025:2005:

"The laboratory shall have a programme and procedure for the calibration of its reference standards." [para 5.6.3.1]

"Checks needed to maintain confidence in the calibration status of reference, primary, transfer or working standards and reference materials shall be carried out according to defined procedures and schedules." [para 5.6.3.3]

Such a procedure may include one or more of the following:

• Maintenance of a suite of standards and routine comparisons between them with regular assessment of the results against an objective quality criterion (e.g. a Shewhart chart).
  
  
• Comparison of the standard against a standard prepared independently by a method with suitable accuracy.
  
  
• Comparison of the standard against an independent method with suitable accuracy (e.g. a dynamic method following the ISO 6145 series of standards).