A works calibration documents the current status of a test object at the time of calibration by recording the pressure readings of the test object compared to the real pressure reading of a reference gauge. So any calibration is only kind of a snap shot.
Dependent on the application the calibrated instrument can remain within the specified variation for a long time. However, in specific applications (e. g. with a high amount of oil vapors) a very fast contamination is possible which results in a changed characteristics of the gauge. Hence a period of validity cannot be specified for any calibration. Based on our long-term practical experience we recommend an annual calibration.