Metrological traceability in ISO/IEC 17025 (clause 6.5)

Clause 6.5 requires the laboratory to establish and maintain metrological traceability of its measurement results by means of a documented unbroken chain of calibrations, each contributing to the measurement uncertainty, linking them to an appropriate reference.

Accepted routes to the SI

• Calibration provided by a competent laboratory — in practice, one accredited under the ILAC arrangement or an NMI.
• Certified values of certified reference materials from a competent producer.
• Direct realisation of SI units ensured by comparison with national or international standards.

ILAC-P10 sets the accreditation-body policy that gives these routes teeth: a certificate from a non-accredited laboratory generally cannot be used to demonstrate traceability without the laboratory itself demonstrating that provider's competence.

Reading a certificate for traceability

Traceability is a property of a chain, and the chain is only as good as its weakest documented link. Four things have to be present on a certificate before it can support a traceability claim, and a certificate missing any of them is a gap you will have to close yourself.

The scope is narrower than the laboratory

A laboratory is not accredited; a scope is. The same laboratory may hold accreditation for Pt100 calibration from −40 °C to 250 °C and none at all for humidity, and it will happily perform the humidity work on a plain certificate. Check the point, the range and the quantity against the published scope rather than against the letterhead. This is also where an expired accreditation hides: the symbol is printed on the template long after the body has suspended it.

When you use a non-accredited supplier

ILAC-P10 is the policy that gives clause 6.5 teeth, and it does not absolutely forbid a non-accredited provider — it moves the burden onto you. If a laboratory uses a calibration from an unaccredited source to support traceability, it must itself demonstrate that provider's competence, traceability and uncertainty. In practice that means an assessment of the supplier, a review of their method and budget, and evidence of their own traceable references. That is a serious piece of work, and it is almost always cheaper to use an accredited supplier.

Where the chain usually breaks

• An in-house reference used to calibrate working instruments, but never itself sent out. The chain stops at your own bench.
• Certificates filed but never read, so nobody noticed that the calibration covered 0–100 °C and the instrument is used at −30 °C.
• A statement of traceability with no uncertainty, which cannot contribute to a budget and therefore cannot form a link.
• Reference standards recalibrated late, leaving a period during which every result they supported has an undocumented gap.
• Adjustment at recalibration without as-found data, which severs the link to everything measured since the last certificate.