At the end of the nineteenth century, nearly 50 nations signed the treaty of the metre which established the international system of units or SI. Since then, national standards laboratories have worked together to develop measurement standards that are "traceable to the SI". In 1998, the Consultative Committee on Amount of Substance (CCQM) defined that traceability to the SI could only be achieved by the use of a primary method. In the case of gas standards the most relevant method is gravimetry, which when carried out to the highest metrological quality is accepted as a primary method.

Since NPL prepares its Primary Standard Gas Mixtures (PSM's) by gravimetry, they are traceable to the SI. In addition to this, they are the recognised National Standards for the UK. Any laboratory can achieve traceability to national standards through the correct use of an appropriate nationally-traceable standard from NPL.

It is necessary to note that some commercial gas companies use methodologies which do NOT give rise to nationally-traceable gas standards as they are defined internationally although they may claim traceability to national standards.

Examples of methodologies that do not provide full traceability to national standards include the use of:

• Traceable standards for mass, pressure or temperature. The use of these alone does not constitute traceability equivalent to that which would be achieved by using an independently-assessed, ISO 17025 compatible, calibration procedure which is referenced to suitable nationally-traceable gas composition standards (of the same gaseous species at similar concentrations).
• A limited number of traceable standards from one, or more, national standards institutes, irrespective of:
• Whether the gaseous species and the concentration range(s) of the nationally-traceable standards held by the commercial company encompass the range of gaseous species and concentrations that the company sells.
• Whether these nationally-traceable standards are used to certify the commercially-produced gas mixtures in a manner which would be acceptable to a recognised national-accreditation body as conforming to ISO 17025 for calibration accreditations.