Any measurement is subject to degradation due to use, abuse, drift or aging. 

To understand this degradation, calibration at regular intervals needs to be carried out to characterize the instrument after degradation, to restore the instrument to an “as new” condition, in regards to its measurement performance and to reference the measurement to the National Standards.

ISO 9000, and many other standards, specify the maximum period between recalibration as once every two years, and more frequently if the instrument degradation is significant during that period -- typically one percent degradation.

Many users adopt annual calibration as the standard interval between calibrations. However, very few do a comparison between the current calibration and the previous calibration to ascertain the degree of degradation and determine a suitable re-calibration time period.