Ten VIM terms every calibration user should know

• Measurand: the quantity intended to be measured — e.g. the air temperature at a defined location, not 'the thermometer reading'.
• Indication: the value provided by the instrument.
• Measurement error: measured value minus a reference value; only estimable.
• Correction: the compensation for an estimated systematic error, applied to an indication.
• Accuracy: closeness of agreement between a measured value and the true value — a qualitative concept.
• Trueness: closeness of the average of many measurements to the true value; related to systematic error.
• Precision: closeness of agreement between repeated measurements; related to random error.
• Resolution: the smallest change in the quantity that causes a perceptible change in the indication.
• Instrumental drift: continuous change in indication over time due to changes in metrological properties.
• Metrological traceability: relation to a reference through a documented unbroken chain of calibrations, each contributing uncertainty.

Using these words as the VIM defines them removes most arguments about what a certificate or specification actually claims.

The distinctions that settle arguments

Most disputes about what a certificate claims come down to three pairs of words being used interchangeably when the VIM separates them.

The sign of a correction

This is worth spelling out because it is the error that survives review most often. If a thermometer indicates 20.5 °C when the reference says 20.3 °C, the error is +0.2 °C and the correction is −0.2 °C. A certificate may report either, and it must say which. Applying a stated error as if it were a correction moves the result the wrong way by twice the error — a 0.4 °C mistake from a 0.2 °C finding, and one that will never be caught by the arithmetic because the arithmetic is correct.

Measurand, and why it is the first line of any budget

The measurand is the quantity intended to be measured, and it is not the thermometer reading. The air temperature at the geometric centre of the cold room at product height is a measurand; the indication of logger S07 is an indication. Naming the measurand precisely decides what belongs in the budget: if the measurand is air temperature at a position, spatial representativeness is a component; if it is the correction to the logger's indication, it is not.

Terms people use loosely

• Calibration establishes the relation between indication and value with uncertainty. It does not include adjusting the instrument — that is adjustment, and a certificate reporting only post-adjustment values has withheld the as-found information.
• Verification confirms that specified requirements are met. Calibration produces a number; verification produces a verdict against a requirement, and needs a decision rule to do it.
• Trueness is closeness of the average of many results to a reference value. It is the word missing from ordinary speech, which is why accuracy gets asked to mean two things at once.
• Resolution is the smallest change that causes a perceptible change in indication — a property of the instrument, not of the measurement, and a contributor to uncertainty rather than a statement of it.
• Metrological traceability is a property of a result, not of a laboratory, and it is specific to a quantity and a range.