An autoclave 1 has a water level sensor 20 in a water reservoir 5 in the pressure vessel 2. An external water tank 21 is connected to the reservoir via an electrically-operated valve 23. An electrical resistance heating element 6 in the reservoir 5 heats the sensor 20 if it indicates a full level at the start of operation, in order to dry any exposed part of the sensor and thereby prevent an erroneous output from the sensor caused by water clinging to it. This heating of the sensor can be repeated if it continues to indicate a full level. If a low level output is produced by the sensor 20, the valve 23 is opened to allow water to flow from the tank 21 to the reservoir 5. If the valve 23 remains open for longer than a predetermined time without the sensor indicating a full level, the autocalve produces a fault signal.