At what interval should I calibrate my instruments? It's a question that's always repeated, to be on the right track in terms of quality and even quantity. Please read Part 1 before starting this section.
Another important aspect here is the probability of changes in measurement properties that can give rise to major discrepancies in calibration results between one calibration and another that follows it.
Qualitatively, it is easy to verify that certain conditions may require more frequent calibrations.
For example, high operating hours, extreme temperature conditions, long-term alternate loading operations with dirt and moisture. However, to produce quantitative statements about the measuring device used with the help of manufacturer's data, comprehensive statistical data would be required for each type of sensor or electronic measurement, which is not normally available. The operator can then get a very good idea, by continuously monitoring the calibration results, of the long-term behavior of the equipment he is using under the operating conditions valid for the application.
In other words: If a thermometer is used in a site where operating conditions are difficult and costs high, and the measurement results are presented at a later stage to be trustworthy, it may make sense to implement recalibration after 6 or even 3 months.
If, however, it is clear from the first or second calibration that the measurement properties remain stable, then it is likely that the thermometer will also remain stable, and it can then be decided to extend the calibration interval. Such a procedure for determining the conditions under which the calibration interval can be extended should be an integral part of the QM system. And it should at least also manage the shortening of calibration intervals, for example due to wear and tear or behavioral drift.
Comparative measurements of several calibrated test instruments against each other are another decision aid for adapting calibration intervals. For example, if a test laboratory uses several hygrometers and has the necessary equipment for comparison measurements on hand. These comparisons can show whether a calibration interval which, initially, may have been rather generously calculated should be shortened in a particular case.
The importance of operating conditions naturally means that recalibration must be implemented in all cases where a measuring tool is subjected to stresses that lie outside its intended use. These range from higher overloads, falling, extreme temperature conditions to interventions in the equipment for repair purposes.
In general, no specific calibration intervals are recommended for measuring instruments, devices or standards. Specific recalibration intervals depend on a number of factors, including
In the absence of other external requirements for specific intervals, leading metrology institutes recommend that laboratories adopt in-house measurement assurance programs that include cross-comparisons of primary and secondary standards. Data recording and analysis results in control charts that can be used to characterize the short- and long-term behavior of specific devices.
This time-dependent behavior can be compared with the accuracy requirements of the particular application to determine an initial recalibration interval. Subsequently, recalibration reports should include "as submitted" and "post calibration" accuracy and precision data to validate and/or refine the interval.
