What Calibration Is

Definitions worth getting right

• Calibration — comparing an instrument's indication with a reference standard of known accuracy, and recording the difference. Strictly, calibration is comparison and recording; adjustment is a separate step that may or may not follow.
• Adjustment — changing the instrument so it reads correctly.
• Verification — confirming the instrument is within its stated tolerance, without necessarily adjusting.
• Rerangeing — changing the range of a smart instrument by configuration. This is not calibration and proves nothing about accuracy.
• Zeroing — setting the output to zero at a known zero input. A useful routine check, but only one point of the range.
• Bump test — applying a stimulus to confirm the instrument responds. A functional check, not a calibration.

Conflating these produces records that claim more than was actually done, which is worse than no record at all.

Traceability

A calibration means nothing unless the reference is itself traceable. Traceability is an unbroken chain of comparisons, each with stated uncertainty, back to a national or international standard. The working standard used in the field is calibrated against a laboratory standard, which is calibrated against a national standard.

The practical consequences:

• Every reference instrument carries a calibration certificate with a date, an expiry, the uncertainty, and the standard it was compared against.
• An out-of-calibration reference makes every measurement it produced questionable, which is why finding a reference out of tolerance triggers a review of all work done with it since its last good calibration.
• Certificates are kept, not filed and forgotten. In a dispute, the certificate is the evidence.

Uncertainty and the accuracy ratio

Every measurement has uncertainty, and the reference must be substantially better than the instrument it is checking. The conventional guidance is a test accuracy ratio of at least four to one — the reference should be at least four times more accurate than the tolerance being verified.

Using a reference of similar accuracy to the instrument under test means a result within tolerance may actually be outside it, and a result outside may actually be within. The measurement produces a number and no confidence, which is the worst of both worlds.

Tolerance

Before calibrating, know the tolerance for that instrument — the maximum acceptable error for its duty. It comes from the process requirement, not from the manufacturer's specification: a fiscal meter and a tank level indication have entirely different requirements, and applying a single site-wide tolerance to both is either wasteful or unsafe.