Please use following aspects to determine re-calibration intervals for each measuring device and instrument separately. Please do not consider uniform frequency of re-calibration of all instruments.
Similarly window period should be based on practical aspects of conducting re-calibrations especially, availability of service engineer / calibration technician and other logistics aspects. Generally, for monthly or quarterly calibration intervals the window period should be lesser than yearly or 6-monthly calibration intervals.
Measurement processes are dynamic systems and often deteriorate with time or use. The design of a calibration program is incomplete without some established means of determining how often to calibrate instruments and standards. A calibration performed only once establishes a one-time reference of uncertainty. Periodic recalibration detects uncertainty growth, serves to reset values while keeping a bound on the limits of errors and minimizes the risk of producing poor measurement
results. A properly selected interval assures that an item will be recalibrated at the proper time. Proper calibration intervals allow specified confidence intervals to be selected and they support evidence of metrological traceability.
Calibration intervals may be determined (either lengthened or shortened) by performing a
technical and statistical analysis, taking the following factors into consideration as appropriate, and fully documenting the data and assessment:
• Calibration history of that instrument;
• Measurement assurance data;
• Interlaboratory comparisons;
• Data for the population of similar standards, equipment or technologies;
• NIST / Regulatory recommendations;
• Statistical analysis methods; and
• Manufacturer′s recommendations.
Also consider following factors,
What type of instrument is it?
What is the extent and severity of use and what are the environmental conditions?
Does it have a tendency to wear of drift?
What is the measurement uncertainty required?
What is the risk of an instrument exceeding the maximum permissible error?
What is the cost of correction and recall measures when the instrument is found to be
“out of calibration?” (Criticality)
What does previous calibration data tell you?
Are intermediate checks / verification performed?
Have the personnel been trained appropriately?
Guidelines for the determination of calibration intervals of measuring instruments ILAC-G24, Edition 2007 (E) -INTERNATIONAL LABORATORY ACCREDITATION COOPERATION
Good Measurement Practice for Assignment and Adjustment of Calibration Intervals for Laboratory Standards- GMP11, R-6969-2019