There are several factors which contribute to the discussion on re-calibration intervals for gauge blocks. We try not to give prescribed values, but to help the user come to a conclusion based on their knowledge of the gauges in question. Some general principles apply:

1. Newer gauge blocks should be calibrated more frequently than older ones, until a trend history is established. Then if the gauges appear to be stable, the period can be increased. We recommend yearly re-calibration for master gauges which are new, then if they are stable, we relax this to two years, then again to three years.
   
   
2. Gauge blocks can change size even when not in use due to changes in the material structure. This is particularly true for gauges immediately after manufacture. So even if a gauge block is not in use, it should be re-calibrated before use, if more than 3 years has elapsed since the last calibration.
   
   
3. We base the re-calibration interval on the time since previous calibration. As mentioned above, a gauge that remains unused may still drift due to material changes, so the 'clock is running' from the time it is last calibrated.
   
   
4. Gauges which are used more often, especially those used for the most critical work should be calibrated more often. If a gauge is being used for less critical work, or with a larger uncertainty allowance than its tolerance class, then the period can be extended.