Calibration and Standards

Calibration and traceability

Calibration establishes the relationship between reference quantity values and instrument indications, including their uncertainties. It is distinct from adjustment, which changes the instrument, and verification, which checks specified requirements. Calibration does not mean the result has zero error. Metrological traceability is a property of a measurement result: a documented unbroken chain of calibrations relates it to a reference, with each link contributing uncertainty. The reference is often an SI unit realization. A label stating “calibrated” alone does not describe the full measurement uncertainty.

Standards

A primary measurement standard is established using a primary reference procedure or by convention. A secondary standard is calibrated with respect to a primary standard for a quantity of the same kind. A working standard is routinely used to calibrate or verify instruments. These terms describe roles and relationships, not a guarantee that every standard is held by a national laboratory.

Error and correction

Define indication error e = M − S, where M is the indicated value and S the reference value. The additive correction is c = S − M = −e. Relative error is e/S when S is nonzero; multiplying by 100 gives percent. State the sign convention explicitly. Error is not synonymous with uncertainty.

Worked example

A voltage reference indicates 120 V while the instrument reads 118 V. Indication error = 118 − 120 = −2 V. Correction at this calibration point = +2 V. Relative error = −2/120 = −0.01667 = −1.667%. This single point does not establish a constant +2 V correction across the whole range. A calibration result also needs uncertainty and applicable conditions; none can be calculated from these two values alone.

Good practice

Record the range, method, reference, environmental conditions and uncertainty. Check calibration validity against use, drift and required tolerance. Include loading and operating conditions when applying a correction to an actual measurement.

Quick check

1. Does calibration necessarily adjust the instrument? No.
2. What is the correction for an indication 0.3 V high? −0.3 V at that point.
3. Does traceability eliminate uncertainty? No; uncertainty accompanies the chain.