A certificate without a stated uncertainty and without a traceability chain is paper, not evidence.
A quality audit does not ask whether the instrument was calibrated. It asks whether you can prove it.
Traceability is a chain
Traceability is the unbroken chain of comparisons linking your measurement to the national standard. Every link carries an uncertainty, and those uncertainties accumulate.
In practice: the plant transmitter is calibrated against a working standard; the working standard is calibrated at an accredited laboratory; the laboratory keeps standards calibrated against the national metrology institute. If any link is lost, the chain breaks.
What to look for on the certificate
- Identification of the standard used and its own certificate number. Without this there is no chain.
- Stated measurement uncertainty, with the coverage factor (usually k=2, ~95%).
- As-found and as-left values. The as-found reading is what tells you whether the instrument was conforming during the period it was in service.
- Environmental conditions of temperature and humidity during calibration.
- Calibration date — and note that the certificate does not set an expiry. The interval is set by the user, based on criticality and drift history.
Uncertainty ratio
The rule of thumb in industrial metrology is that the standard should be three to four times more accurate than the instrument being calibrated. Calibrating a 0.5% gauge with a 0.5% standard proves nothing: the standard's uncertainty consumes the entire tolerance.
That is why the working standard costs more than the process instrument — and why it needs periodic calibration too.
