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A Note on Adjustment and Calibration

Adjustment ≠ Calibration

The terms ‘adjustment’ and ‘calibration’ are often used interchangeably in common parlance. However in scientific terms this is incorrect. ‘Adjustment’ and ‘calibration’ are distinct and defined procedures.

Adjustment

Bringing a measurement instrument into a state of performance suitable for use.

Calibration

Establishing, under specific conditions, the relationship between the values of quantities indicated by a measurement instrument and the corresponding certified values of standards.

It is often the case that both procedures are required, adjustment before calibration.

The importance of calibration records

Both adjustment and calibration procedures must always be documented. Calibration records are necessary to establish the stability and performance of a measurement instrument, and may also help to determine whether an instrument has been damaged or ill-treated. In extreme circumstances, the calibration history of a measurement instrument may even become a vital piece of evidence in a dispute or court case.

Documenting the calibration is as important as the calibration itself.

Frequency of calibration

There is no hard-and-fast rule which determines the required frequency of calibration for any given measurement instrument. There are two major factors which must be considered:

1. Drift

ALL instruments drift with time. New instruments should be calibrated more frequently to determine their drift characteristics. The environment in which the instrument is used also contributes to drift. Instruments which are used in harsh or potentially damaging environments will require more frequent calibration.

2. Required measurement uncertainty

If a low measurement uncertainty or high confidence level is required, the instrument should be calibrated frequently. A longer recalibration interval is sufficient in cases where a larger measurement uncertainty is acceptable.