There is disclosed a method and system for determining the partial pressure of at least one gas in a mixture of gasses contained in a pressure vessel, the mixture being pressurised to a level which is above local atmospheric pressure. The method comprises the steps of positioning a gas analysis sensor (14) within a pressure vessel (10)； exposing the sensor to the mixture of gasses at the pressure level found in the pressure vessel； operating the sensor to measure the actual partial pressure of the at least one gas in the mixture contained in the vessel； and periodically calibrating the sensor by directing a calibrating gas mixture (20, 22) to the sensor in the chamber, the calibrating gas mixture being breathable by a human being.
