What is calibration?

Calibration is the comparison of a measured value with the correct value under specified conditions, documenting the deviation, calculating the measurement uncertainty, and issuing the certificate. 

Increasing quality demands and strict regulations lead to more and more requirements in production and manufacturing. This means that quality relevant measuring tasks must be monitored, documented and carried out with the correct test equipment. This requires not only many widespread standards such as the ISO 9000 series, but also industry specific standards such as IATF 16949 or the EU GMP guidelines. For this reason, all measuring instruments used for quality relevant tasks must be calibrated. This is imperative, because even the smallest measurement errors can have drastic effects on the safety of production processes or on the quality of products.

Calibration may be carried out by an external laboratory (e.g. calibration service provider) or by an internal body within the company. 

Calibration

Adjustment is setting to the smallest possible deviation from the correct value. When adjusting, it is necessary to intervene on the measuring device. In contrast to this, nothing is changed on the measuring instrument itself during calibration. Both processes are used to detect and document deviations. If a deviation from the permissible tolerance value is determined during calibration, the measuring instrument is adjusted so that the values are within the permissible tolerances. 

The distinction between a calibration and a verification is more a regulatory and organisational distinction than a technical one. Verification is the "official certification or official testing of measuring instruments to ensure measurement reliability carried out by verification authorities". Calibration, on the other hand, can be performed on any measuring instrument.

In a traceable calibration, the calibration item can trace back to a national or international standard over an uninterrupted series of calibrations. Each of these calibrations must have the property of being traceable. ISO calibration certificates are proof by accreditation that this calibration is traceable.

Accreditation is the formal recognition of the competence of a calibration laboratory to perform certain calibrations according to specified standards (UKAS guidelines). This formal determination is made by a body authorised for this purpose. In the United Kingdom this is the British Accreditation Body in the UK

A frequently chosen calibration interval is 12 months. In fact, the calibration interval should be defined individually for each measuring device on a risk-based and application specific basis. Further information on this topic can be found in our technical article.

Those measuring instruments which are used to carry out a test for compliance with a quality requirement after the measuring process. The linking of measuring with testing for compliance with a quality requirement turns a measuring device into a test device. 

A calibration should always cover the entire operating range of a sensor. Accordingly, two calibration points are usually already set - the lowest operating point of the sensor and the highest. Since it cannot always be assumed that the measuring line of the sensor is linear within these two points, at least a third calibration point should be added. This is ideally located in the middle of the work area.