Metrological traceability under ISO/IEC 17025

Metrological traceability is the property of a measurement result whereby it can be related to a reference — normally the SI unit — through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty. The definition has three load-bearing words: documented, unbroken, and uncertainty.

What breaks the chain

• A reference whose own calibration is missing, expired beyond its documented interval, or not traceable itself.
• A comparison with no uncertainty evaluated — a 'check' rather than a calibration.
• A link that cannot be produced on request: the documentation is part of the chain.

Practical evidence

For most organisations, traceability is demonstrated by calibration certificates from laboratories accredited to ISO/IEC 17025 by a body within the ILAC mutual recognition arrangement, or from national metrology institutes. The certificate must state the uncertainty and identify the reference standards used.

The chain, link by link

Traceability is easier to check than to define, because every link has to produce the same four things. Follow it upward from the instrument in your hand and you will either reach the SI or find the break.

Each link must have a documented calibration, a stated uncertainty, a date within its interval, and coverage of the quantity and range in question. A link missing any one of those breaks the chain, and the break is not repaired by the links above it being excellent.

Traceability is per-quantity and per-range

The commonest misunderstanding is treating traceability as a property of an organisation. It is a property of a result. A laboratory with impeccable traceability for Pt100 calibration from −40 °C to 250 °C has none at all for the humidity channel on the same instrument, and none for a Pt100 used at −60 °C. Check the certificate against the actual measurement, not against the laboratory's reputation.

Checks that are not calibrations

A comparison with no uncertainty evaluated is a check, not a calibration, and a check does not create a link. This distinction catches a lot of in-house arrangements: comparing a working thermometer against a reference and recording that they agreed within 0.2 °C is a useful control, and it is not traceability unless the comparison produced a correction and an uncertainty. Interim checks are valuable precisely as checks — they detect change between calibrations — but they do not extend an interval.

Where chains break in practice

• An in-house reference used to calibrate everything else and never itself sent out. The chain stops at your own bench.
• A certificate that says traceable to national standards without naming the standards or giving an uncertainty. That sentence is not evidence of anything.
• A reference recalibrated late, leaving a period during which everything it supported has an undocumented gap.
• Adjustment at recalibration with no as-found data, which severs the link to every measurement made since the previous certificate.
• An accreditation symbol on a certificate for work outside the accredited scope — legal, common, and not what the symbol implies.
• Calibration at points that do not cover the range of use, so the result is an extrapolation the certificate does not support.