GlossaryKind of term: Manufacturing termCalibration
What is calibration?
Calibration compares a measuring instrument, under stated conditions, with a traceable reference and records the deviation with its measurement uncertainty.
What is compared is the instrument's indication against the known value of the reference; traceable means that value can be related to a national or international standard through an unbroken chain of calibrations. The result is usually stated in a calibration certificate, giving the deviation or correction and the uncertainty at each measuring point. Calibration on its own changes nothing about the instrument: setting it right is adjustment, and whether it then meets its limits is a separate judgement.
What does calibration depend on?
A calibration is only useful when it is traceable: a documented, unbroken chain of calibrations to a national or international measurement standard, each with a known uncertainty. The calibration uncertainty feeds into the uncertainty of every measurement you then make with the instrument. For dimensional measurement the reference temperature is 20 °C; when instrument, workpiece or reference differ from it, temperature and the different expansion coefficients come into play. On a coordinate measuring machine, verifying the machine is something other than qualifying the probe before a measurement, although shop floors often call both calibration.
What does the standard say about calibration?
The terms are defined in the International Vocabulary of Metrology (VIM, JCGM 200:2012), which separates calibration from adjustment and from verification. ISO/IEC 17025 sets requirements for laboratories that calibrate; with an accredited calibration, an accreditation body has assessed the laboratory's competence for that measurement, including traceability and uncertainty. Where measurement traceability is a requirement, or the organisation considers it essential, ISO 9001 requires measuring equipment to be calibrated or verified at specified intervals, or before use, against traceable measurement standards. None of these standards prescribes a fixed interval.
How does calibration work in practice?
Measuring equipment carries a calibration status and a due date; an instrument past its date has no place in final inspection. A laboratory can calibrate and adjust in one job; the certificate then gives the values before and after adjustment, as found and as left. If the as-found values were out of limits, you assess what has been accepted with the instrument since the previous calibration. Customers sometimes ask, in an inspection report or first article inspection, which instruments were used and when they were calibrated. The cost of calibration, subcontracted or in-house, usually sits in overheads rather than in the estimate of one part. A gauge needed for one part only, such as a special thread gauge, can go into the estimate or be charged as a one-off cost.
What is calibration often confused with?
Calibration is not adjustment: calibration establishes the deviation, adjustment corrects the instrument. Nor is it legal verification, the statutory check of instruments used in trade, such as scales. And in estimating the word is sometimes used for tuning standard times to post-calculation results; that is an estimating convention, not a metrological calibration.
Frequently asked questions
How often should an instrument be calibrated?
No standard prescribes one fixed interval. You set it from use, wear, stability shown by earlier calibrations, customer requirements and the risk of a wrong measurement, and adjust it when the results give reason to.
Is a calibrated instrument accurate enough?
Not automatically. A calibration states the deviation and the uncertainty. Whether that is enough depends on the tolerance you measure: the measurement uncertainty must be small compared with that tolerance.
What does calibration mean in estimating?
There it means tuning standard times to post-calculation results, an estimating convention. It is usually done per operation, and only when a deviation repeats across several orders; one outlier, such as a broken cutter, is no reason.