According to the Article 14 of the Basic Law of National Standard, "To improve the traceability between measurement instruments used in all fields of national measurement standards and national society, measurement instruments owned or used should periodically be calibrated to maintain traceability among measurement instrumentsby applying and operating its own policy for calibration subjects and application scopes according to Article 40 of the "Guidelines for the Operation of the National Calibration Institution Designation System".
Measurement instruments that can maintain acceptable accuracy must be used for testing and measuring. The accuracy can be validated based on the allowable rate of tolerance required by the specification. This accuracy measurement is achieved by periodic comparison and calibration with higher standard instrument. Therefore, calibration subject should be selected based on the credibility that the performance can be maintained within the allowable rate of tolerance required by the individual measurement instruments.
However, there may be cases where calibration is not required due to performance or structure of the measurement instruments. Furthermore, absence of national measurement standard or appropriate calibration procedures could also result in inability to calibrate. (i.e. surface characteristics, non-destructive test areas, etc.)
To set an appropriate calibration frequency, it is desirable to set the frequency based on the survey data of usage conditions and measurement data of each department for a certain period of time. Therefore, the basic matters to be considered are the acquisition of past accumulated measurement data, which is the basis for frequency adjustments. Therefore, it is essential that the optimum calibration frequency is set considering user required uncertainties, frequency, usage method, and stability of measurement instruments. One of the most important decisions in calibration is when and how often it should be performed.
Article 40, Paragraph 2 of the Operating Points of the National Calibration Agency Designation System states that "When a person who owns or uses measurement instrument intends to establish a calibration frequency, he or she shall take into account accuracy, stability, purpose of use, environment, and usage frequency of measurement instrument to set a standard that is scientific and reasonable. However, scientific and reasonable calibration frequency cannot be determined, then calibration frequency stated in the National Technology Standard shall be applied."
The calibration cost is generally not negligible in setting the calibration frequency, but if the calibration frequency is longer than the recommended; then it may seemingly alleviate the high cost at the time. However, the risk of quality service increases as the uncertainty of measurement also increases.