Metrological traceability is the property of a measurement result whereby it can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
The chain
Your instrument is calibrated against a reference standard. That standard is calibrated against a higher-order one. That one links to a national metrology institute — NIST in the United States — and through international agreements to the SI units.
Every link must be documented, and every link adds uncertainty. A chain with a missing link is not traceable, however good the equipment.
What breaks it
- A calibration certificate that does not identify the reference used
- An in-house calibration against an instrument that is not itself traceably calibrated
- A supplier’s certificate with no accreditation and no stated uncertainty
- “Calibrated at the factory” with no evidence of what against
The practical implication when buying calibration
A calibration certificate from an ISO/IEC 17025 accredited laboratory carries traceability and stated uncertainty by definition — accreditation is an assessment of technical competence to produce valid results. A certificate from an unaccredited provider might be perfectly sound, but you are taking their word for it, and your auditor may not.
This matters most in regulated and aerospace supply chains, where the customer’s own requirements frequently specify accredited calibration explicitly.
Note the terminology
Laboratories are accredited to ISO/IEC 17025, never certified. A provider describing itself as “17025 certified” is worth a second look.