ISO/IEC 17025 calibration explained

ISO/IEC 17025 is the international standard for the competence of testing and calibration laboratories. An accredited calibration is one performed within a laboratory's accredited scope, under a management system assessed by a national accreditation body (for example UKAS in the United Kingdom).

What the standard asks for

• Impartiality and confidentiality.
• Competent personnel with defined responsibilities.
• Suitable facilities and environmental conditions.
• Equipment that is calibrated, maintained and controlled, with records.
• Metrological traceability of results to the SI or to agreed references.
• Validated methods, and evaluation of measurement uncertainty.
• Technical records, reporting requirements for certificates, and control of nonconforming work.
• A management system with internal audits and corrective actions.

What an accredited certificate gives you

Traceable results with a stated uncertainty, produced under an assessed system. Where the certificate includes a statement of conformity, the decision rule applied must be stated. The accreditation symbol on the certificate is what distinguishes an accredited calibration from a non-accredited one performed by the same laboratory.

What it does not give you

Accreditation does not mean the instrument is fit for your purpose, that the calibration interval is right, or that the reported uncertainty is small enough for your application. Those are the customer's decisions. ValiTrac is designed around these workflow elements — traceability, uncertainty, technical review, records — but is not itself an accredited laboratory and makes no compliance claim on your behalf.

Accredited and non-accredited, from the same laboratory

A laboratory can issue both, often on the same day and on the same bench. The distinction is the accreditation symbol and the scope it refers to, not the quality of the work or the letterhead. A calibration falls outside the accredited scope whenever the quantity, the range, the point or the method is not listed in it — and laboratories routinely take that work, because customers ask for it.

So the check is specific: find the scope on the accreditation body's website, and confirm that the quantity, the range and the points on your certificate appear in it. A laboratory accredited for Pt100 from −40 °C to 250 °C is not accredited for the humidity channel on the same instrument, and the certificate covering both will say so if you read it carefully.

What accreditation does not decide for you

Three judgements stay with the customer no matter how good the certificate is. Whether the instrument is fit for your purpose: that depends on your tolerance and the reported uncertainty, which is the test uncertainty ratio, not something the laboratory can know. Whether the calibration interval is right: that depends on how the instrument is used and how it has drifted historically, which only your records show. And whether the uncertainty is small enough: a perfectly valid ± 0.5 °C certificate is useless for policing a ± 0.3 °C limit.

This is why an as-found result matters so much. It is the only evidence about whether the instrument was fit during the interval that just ended, and the only input that can set the next interval on something better than habit.

On-site calibration

• On-site work can be accredited, and many scopes cover it explicitly — check whether yours does, because a laboratory accredited in its own premises is not automatically accredited in yours.
• Environmental conditions on site are worse and more variable, so the achievable uncertainty is larger. A site certificate quoting the same uncertainty as the laboratory one deserves a question.
• The advantage is real, though: an instrument calibrated where it works, at the immersion and in the installation it actually has, describes the measurement you actually make.