While there is truth in the saying that calibration is only valid at the time of implementation. In practice, the problem usually faced by equipment users and owners is how often should a re-calibration be carried out.
The equipment operator is at best position to determine calibration intervals even for organizations with quality management system that has internal specification for re-calibration, equipment ware and tear, equipment damage and change in business processes puts the operator in a best position to decide when to carry out re-calibration.
However, in setting up a time line for re-calibration, industrial best practice dictates, this is as explicitly recommended in the ISO 10012, that it is more sensible to allow a greater number of measurements or specific time intervals between two calibrations. If the deviations observed during this intervals lies within the metrological requirement then the measurement result obtained are justifiable. If however the deviations are greater, than the question is whether measurements are only meaningful to the extent limited by time, quantity and frequency of usage and calibration be repeated. However, the decision must take into account the cost of re-calibration, time , cost during calibration and the possibility for possible worthless measurement results, re-measurements, sanction, recalls and the likes.
For equipment where premium are placed on precisely known measuring properties calibration must be carried out before and after every important measurement.
An important aspect here also is the chances for change in the measuring properties that may result in significant deviations in calibration results between calibration intervals , conditions like high operating hour ( shift operations) extreme temperature , dirt and moisture . Here the operator has to make conscious effort in tracking the calibration results , observing the long term behavior of the equipment under the operating conditions valid for the application.
The significance of the operating conditions naturally means that a re-calibration should be implemented in all cases where a measuring equipment is subjected to stresses that lie outside its intended use, this range from overload, excessive impact extreme temperature condition and major repairs.
