Calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, a sound tone, or a physical artefact, such as a metre ruler.calibration means just the act of comparison, and does not include any subsequent adjustment.

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 (such as NIST in the USA), international standards, or certified reference materials. This may be done by national standards laboratories operated by the government or by private firms offering metrology services.

Calibration may be required for the following reasons:

• * A new instrument
• * after an instrument has been repaired or modified
• * when a specified time period has elapsed
• * when a specified usage (operating hours) has elapsed
• * before and/or after a critical measurement
• * after an event, for example
• * after an instrument has been exposed to a shock, vibration, or physical damage, which might potentially have compromised the integrity of its calibration
• * sudden changes in weather
• * whenever observations appear questionable or instrument indications do not match the output of surrogate instruments
• * as specified by a requirement, e.g., customer specification, instrument manufacturer recommendation.