highly reliable components, but adding to the task loading of the diver. A hazard specific to closed circuit rebreathers is failure of the oxygen partial pressure control system. The breathing gas mixture in a diving rebreather loop is usually measured using electro-galvanic oxygen sensors, and the output of the cells is used by either the diver or an electronic control system to control addition of oxygen to increase partial pressure when it is below the chosen lower set-point, or to flush with diluent gas when it is above the upper set-point. When the partial pressure is between the upper and lower set-points, it is suitable for breathing at that depth and is left until it changes as a result of consumption by the diver, or a change in ambient pressure as a result of a depth change. Accuracy and reliability of measurement is important in this application for two basic reasons. Firstly, if the oxygen content is too low, the diver will lose consciousness due to hypoxia and probably die, or if the oxygen content is too high, the risk of central nervous system oxygen toxicity causing convulsions and loss of consciousness, with a high risk of drowning becomes unacceptable. Secondly, decompression obligations cannot be accurately or reliably calculated if the breathing gas composition is not known. Pre-dive calibration of the cells can only check response to partial pressures up to 100% at atmospheric pressure, or 1 bar. As the set points are commonly in the range of 1.2 to 1.6 bar, special hyperbaric calibration equipment would be required to reliably test the response at the set-points. This equipment is available, but is expensive and not in common use, and requires the cells to be removed from the rebreather and installed in the test unit. To compensate for the possibility of a cell failure during a dive, three cells are generally fitted, on the principle that failure of one cell at a time is most likely, and that if two cells indicate the same PO2, they are more likely to be correct than the single cell with a different reading. Voting logic allows the control system to control the circuit for the rest of the dive according to the two cells assumed to be correct. This is not entirely reliable, as it is possible for two cells to fail on the same dive. Surface oriented surface supplied diving Surface-supplied diving