The present invention relates to a calibration method of a sensor unit connected to a reaction vessel, wherein the sensor unit comprises at least one sensor device configured to be calibrated, at least one compartment containing a calibrating agent, and a housing. The method comprising inserting a sensitive element of the at least one sensor device into a compartment of the sensor unit containing a calibrating agent, withdrawing the sensitive element from the compartment; inserting the sensitive element into the reaction vessel and locking the sensor device in position, wherein the steps of the method are irreversible.