Calibration and traceability explained
Calibration helps you understand what an instrument's reading means. It establishes a relationship between the instrument's indications and reference values, with their uncertainties, under stated conditions. That relationship can then be used to obtain measurement results from the instrument. See the JCGM definition of calibration.
Calibration, adjustment and verification
These activities answer different questions:
| Activity | Question it answers | Thermometer example |
|---|---|---|
| Calibration | How do the readings relate to reference values? | Compare the thermometer with a reference at selected temperatures and report the differences and uncertainty. |
| Adjustment | What change to the instrument will alter its readings? | Change its offset setting. |
| Verification | Does it meet the stated requirement? | Check whether the result meets the permitted error using the agreed decision rule. |
A calibration does not necessarily include adjustment or a pass/fail decision. If an instrument is adjusted, keep the evidence of its condition before and after the change. The JCGM calibration notes explain why calibration should be distinguished from adjustment and verification.
Error and correction: a small example
Imagine a reference temperature of 100.00 °C and a thermometer indication of 100.30 °C. Using indication minus reference, the indicated error is +0.30 °C. An additive correction at that point would be -0.30 °C. The reference and comparison still have uncertainty; this is not proof of an exact true temperature.
Measurement error is a difference relative to a reference. A correction compensates for an estimated systematic effect. Always check the certificate's sign convention and the conditions under which a correction applies. See measurement error and correction in the JCGM vocabulary.
What makes a result traceable?
Metrological traceability connects a measurement result to an identified reference through a documented chain of calibrations. Every calibration in that chain contributes uncertainty. A certificate number or asset history alone does not establish this connection. See the JCGM definition of metrological traceability.
For example, a working thermometer may be compared with a laboratory reference thermometer, which has its own calibration against a higher-level reference. The supporting records need to explain those relationships and their uncertainties.
How these terms fit into Metra
| Metra record | What it represents |
|---|---|
| Equipment | The individual instrument being managed. |
| Calibration requirement | The recurring work that must be done, including its schedule. |
| Calibration event | One occasion on which the work was performed. |
| Procedure | The controlled instructions for performing the work. |
| Reference standard | A reference used to support the measurement, with its own capability and evidence. |
| Certificate | Retained documentary evidence of the work. |
One thermometer can have an annual external calibration requirement and a separate intermediate verification requirement. Their events and evidence remain distinct.
Metra's traceability view helps review the saved relationships. A complete structural chain does not itself establish accreditation, fitness for use or a conformity decision.