Most calibration certificates get filed without being read. They arrive, somebody checks there is a tick in the pass box, and the folder goes back on the shelf until the auditor asks for it. That is a shame, because the certificate is the only evidence you have that your measurements mean anything — and a surprising number of them do not say what people assume they say.
The four things worth checking
- As-found and as-left values. Both, as numbers, not as a verdict. If only as-left appears, you cannot tell whether the instrument was drifting.
- Measurement uncertainty. A stated figure, per point. "Within manufacturer's specification" is not an uncertainty.
- Traceability. Which reference standard was used, and its own certificate number. The chain has to be unbroken all the way to a national standard.
- Scope. What was actually calibrated. A multimeter certificate covering only DC volts tells you nothing about the current ranges you have been using.
Omissions that should worry you
A missing uncertainty figure is the most common gap and the most consequential. Without it you cannot tell whether a reading near a limit is inside or outside — and "near a limit" is exactly where decisions get made. If a certificate gives you a tolerance and a result but no uncertainty, it has quietly left the hardest judgement to you.
The second is a pass verdict with no as-found data. It usually means the instrument was adjusted first and measured afterwards. That certificate proves the instrument is accurate today. It proves nothing whatsoever about last Tuesday.
Accreditation is not a single yes or no
A laboratory's accreditation covers a defined scope: specific quantities, specific ranges, specific uncertainties. A laboratory can be genuinely accredited and still perform work outside that scope — legitimately, but the resulting certificate is not accredited work. Look for the scope, not just the logo, and check the discipline and range you care about actually appears on it.
If you are unsure what you are looking at, send us the certificate. We will tell you what it covers and what it does not, whoever issued it.
