Why do a Calibration?
Apart from the fact that all manufacturers dictate that this is to take place at least 6 monthly, there are very good reasons for doing it.
All sensors have an amount of drift over time. Calibration verifies that the instrument is reading as accurately as possible.
Situations like over-exposure and the introduction of poisons can have heavy impacts, along with extreme environmental changes, all can cause sensors to be less accurate.
Calibration will correct for these potential uncontrollable effects.