During a GMP audit, the calibration certificate of a temperature probe is among the first metrological documents to be provided. The inspection does not merely verify that the certificate exists: it verifies that the calibration is traceable. Metrological traceability of the calibration is verified and guaranteed by using calibration laboratories accredited to ISO/IEC 17025.
An ISO/IEC 17025-accredited laboratory guarantees by definition the metrological traceability of its calibrations; however, this does not relieve the QA manager of the obligation to verify whether the certificate received is fit for purpose. Because in an audit, the question is not “is this laboratory accredited?”. The question is “can you demonstrate that this calibration certificate is adequate for your intended use?”
What metrological traceability means according to the VIM
Metrological traceability is defined by the VIM — International Vocabulary of Metrology (JCGM 200:2012) — as the property of a measurement result whereby the result can be related to a reference standard of the International System of Units (SI) through an unbroken and documented chain of calibrations, each with its own stated uncertainty. The final reference standard is a national or international primary standard; in the UK, primary standards are maintained by the National Physical Laboratory (NPL).
The chain has a precise structure: the NPL primary standard is compared with secondary-level transfer standards, which are in turn used to calibrate the reference standards of accredited calibration laboratories, against which the customer’s temperature probe is finally calibrated. Each link in this chain introduces its own uncertainty, which propagates upward through the chain.
Traceable calibration and functional verification: a substantive difference
Confusion between traceable calibration and functional verification is common and has direct consequences on the validity of metrological documentation. A functional verification — comparison of the probe against an uncalibrated internal reference thermometer, checking of the electrical signal, verification of the response at known temperatures — establishes that the instrument works, not that it measures with a documented and traceable uncertainty.
In the GMP context, EU GMP Annex 1 (2023 revision) requires that measuring instruments used for monitoring classified areas are calibrated with documented traceability. A functional check does not satisfy this requirement. The difference is verifiable in the document: a traceable calibration reports measured values with expanded uncertainty and reference to the standards used; a functional verification reports at most a conformity judgement without these elements.
The practical risk is that some maintenance service providers issue documents that resemble a calibration certificate in form, without actually meeting its substantive requirements. Checking the certificate section by section is the only means of distinguishing the two cases.
How to verify metrological traceability in the certificate: a guided reading
Laboratory identification and scope of accreditation
The first section to verify is the certificate header. It must state the name of the issuing laboratory and the accreditation number issued by an accreditation body signatory to the ILAC-MRA agreements. The accreditation number allows verification on the accreditation body’s website that the laboratory is currently accredited and that the scope of accreditation covers the calibration of temperature probes within the required measurement range.
This check is not optional.
Reference to calibration standards and their traceability
The technical section of the certificate must identify the measurement standards used for the calibration — typically platinum resistance thermometers (PRTs) — and declare their traceability to national or international standards. This declaration may appear in the reference standards section, in a footnote, or in an accompanying document referenced in the certificate.
Measured values, deviations, and expanded uncertainty
The technical body of the certificate reports the calibration results: for each calibration point, the nominal temperature value, the value measured by the probe, the error, and the expanded uncertainty U with indication of the coverage factor k and the confidence level. For temperature probes in a GMP context, the calibration points must cover the operational measurement range of the probe within the specific process.
The expanded uncertainty must be compared with the operational tolerances of the probe within the process. The criterion is the metrological coverage ratio of 4:1: as set out in the article on measurement uncertainty in the calibration certificate, the calibration uncertainty must be at most one quarter of the permissible process tolerance. For GMP monitoring of a +2/+8 °C chamber with a process tolerance of ±1.0 °C, the certificate must report U ≤ ±0.25 °C (k = 2).
Calibration conditions
The certificate must report the environmental conditions under which the calibration was performed and, where relevant, the calibration medium used (thermostat bath, furnace, dry-block comparator). These data are not ancillary: they define the conditions within which the calibration is valid and enable an assessment of compatibility with the actual operating conditions of the probe.
Implications in GMP and ISO/IEC 17025 audits
During an Annex 1 audit, the inability to demonstrate the traceability chain of a temperature probe used for monitoring classified areas is a major non-conformity. This is not a formality: without documented traceability, the monitoring data produced by that probe cannot be considered metrologically valid, with implications for process quality and, in critical contexts, for product safety.
In ISO 9001-certified systems or in ISO/IEC 17025-accredited laboratories, the absence of traceability in measuring instruments is a non-conformity against a requirement of the standard, which explicitly mandates the metrological traceability of all instruments used in the measurement processes covered by the accreditation. Non-compliance in this regard during a surveillance audit by the accreditation body may result in suspension of accreditation for the scope concerned.
Prevention is documentary: checking the certificate section by section upon receipt, before filing, is the only means that allows gaps to be identified at a stage when they can still be rectified — before the audit, not during it.
The Altiorlab team is available to provide technical consultancy for verification of the metrological traceability of temperature probe calibration certificates in use, and for documentary management in compliance with GMP and ISO/IEC 17025 requirements.
