A controller (not shown) for controlling a breathing apparatus 1 (e.g. a rebreather), and method of controlling a breathing apparatus, comprises the provision of a plurality of oxygen sensors 19a, 19b, 19c, each for sensing an oxygen content of a gas in the breathing apparatus, the controller being configured to control the operation of the breathing apparatus (for example the oxygen content of a gas in the apparatus) based on the output of (i) one or more of the said oxygen sensors and (ii) additional information relating to the operation of the breathing apparatus. The additional information may relate to an installation/manufacturing date of a sensor, the operational lifetime of a sensor, time limits for duration of operation of a sensor or the initial maximum input of a sensor. This information may be stored in advance of operation in a memory of the apparatus or derived/obtained during operation of the apparatus.
