In brief
Calibration is determining and documenting the deviation of a measuring instrument from a traceable reference standard under specified conditions.
Contents
What is Calibration?
During calibration, a measuring instrument is compared with a more accurate reference instrument whose values are traced back to national or international standards through an unbroken chain. The result is the deviation found, together with the measurement uncertainty. An important distinction: calibration does not alter the instrument. The subsequent setting to reduce the deviation is called adjustment; legal metrological verification is a legally regulated inspection for certain measuring instruments, for example those used in trade.
Typical items calibrated include pressure gauges, pressure transmitters, temperature sensors, flow meters, torque wrenches and quality assurance test equipment. The calibration interval is set by the operator, depending on manufacturer information, stress, criticality and previous stability. The calibration certificate documents measured values, deviations, measurement uncertainty and the reference standards used. Accredited laboratories work in accordance with ISO/IEC 17025 (DIN EN ISO/IEC 17025).
In maintenance, calibration is part of test equipment management. Sensors that read incorrectly lead to control errors, quality deviations or unnecessary part replacement. When replacing a measuring instrument, it should be clarified whether it is supplied calibrated and whether a calibration certificate is required for in-house test equipment management.
In practice: maintenance and procurement
When ordering measuring and test equipment, state explicitly whether a factory calibration certificate or a certificate from an accredited laboratory is required, and for which measuring range. Calibration status and the next due date belong in the test equipment management system.
Synonyms and abbreviations: instrument calibration · factory calibration · accredited calibration · UKAS calibration · test equipment calibration
Frequently asked questions
What is the difference between calibration, adjustment and legal verification?
Calibration determines and documents the deviation of a measuring instrument without altering it. Adjustment intervenes and sets the instrument so that the deviation is as small as possible. Legal verification is a mandatory inspection by weights and measures authorities or officially recognised bodies for measuring instruments whose use is governed by legal metrology law.
How often does a measuring instrument need to be calibrated?
There is no universally valid interval. The operator sets it, taking into account manufacturer recommendations, operating conditions, required accuracy, criticality of the measurement and the results of previous calibrations. If an instrument shows stable values over several calibrations, the interval can be extended with justification; if deviations occur, it is shortened.
What is an accredited calibration certificate?
It is issued by a calibration laboratory whose competence has been confirmed to ISO/IEC 17025 by an accreditation body, in Germany by DAkkS and in the UK by UKAS. The certificate states the metrological traceability and the measurement uncertainty. A manufacturer’s factory calibration certificate, by contrast, is not accredited but is sufficient for many applications.