The frequency with which calibrations should be carried out is an important, if sometimes difficult, question to answer. There is no fixed period for the validity of a calibration certificate - it is not like your cars MOT certificate. However, measurement results stated on certificates are usually 'on the day' values and no allowance is made for subsequent drift.
There are two main considerations when determining re-calibration intervals:
Firstly, all measuring devices - whether they are simple, 'fundamental', or sophisticated - change characteristics with time; the issue is how much do they change? New devices should be calibrated relatively often in order to establish their reproducibility - essentially their metrological stability or the change in their measuring ability between calibrations. Initial estimates of reproducibility are sometimes made using type-test data from earlier calibration results of similar instruments but the resultant uncertainty of measurement has to be cautiously higher, until real data is available.
Secondly, the required uncertainty of measurement should be assessed. If the instrument's reproducibility is shown, by successive calibrations, to be substantially better than the uncertainty required then the interval between calibrations can be extended - perhaps even up to 5 years or so, but at the other extreme - where the instrument's reproducibility approaches the uncertainty needed - the calibration intervals should be much shorter, perhaps even daily in some cases.
You should use a check box once a month and record the results and review them to see if there is a trend forming ,based on these results and how often you use the meter, you can decide how long between calibrations.
