Calibration is the process of establishing the relationship between a measuring device and the units of measure. This is done by comparing a device or the output of an instrument to a standard having known measurement characteristics. For example the length of a stick can be calibrated by comparing it to a standard that has a known length. Once the relationship of the stick to the standard is known the stick is calibrated and can be used to measure the length of other things.
For many operations the quality of the calibration needs to be known and is quantified by an uncertainty estimate for the calibration. This is so important for the scientific community and manufacturing operations that it has been proposed that an evaluation of the measurement uncertainty was added as part of the calibration process.
To improve the quality of the calibration and have the results accepted by outside organizations it is desirable for the calibration and subsequent measurements to be "traceable" to the internationally defined measurement units. Establishing traceability
is accomplished by a formal comparison to a standard
which is directly or indirectly related to national standards (NIST
in the USA), international standards, or certified reference materials
Quality management systems call for an effective metrology system which includes formal, periodic, and documented calibration of all measuring instruments. ISO 9000 and ISO 17025 require effective calibration systems.
Calibration of instruments
Calibration can be called for:
- with a new instrument
- when a specified time period is elapsed
- when a specified usage (operating hours) has elapsed
- when an instrument has had a shock or vibration which potentially may have put it out of calibration
- whenever observations appear questionable
In non-specialized use, calibration is often regarded as including the process of adjusting the output or indication on a measurement instrument to agree with value of the applied standard, within a specified accuracy. For example, a thermometer could be calibrated so the error of indication or the correction is determined, and adjusted (e.g. via calibration constants) so that it shows the true temperature in Celsius at specific points on the scale.
In many countries a National Metrology Institute (NMI) will exist which will maintain primary standards of measurement (the main SI units
plus a number of derived units) which will be used to provide traceability
to customer's instruments by calibration. The NMI supports the metrological infrastructure in that country (and often others) by establishing an unbroken chain, from the top level of standards to an instrument used for measurement. Examples of National Metrology Institutes are NPL
in the UK
in the United States
and many others. Since the Mutual Recognition Agreement was signed it is now straightforward to take traceability from any participating NMI and it is no longer necessary for a company to obtain traceability for measurements from the NMI of the country in which it is situated.
See for more information on the Mutual Recognition Agreement.
See for the international metrology framework.
To communicate the quality of a calibration the calibration value is often accompanied by a traceable uncertainty statement to a stated confidence level. This is evaluated through careful uncertainty analysis.
- Pyzdek, T, "Quality Engineering Handbook", 2003, ISBN 0824746147
- Godfrey, A. B., "Juran's Quality Handbook", 1999, ISBN 007034003