text: Breathing gas composition monitoring The partial pressure of oxygen in anaesthetic gases is monitored by siting the cell in the gas flow, which is at local atmospheric pressure, and can be calibrated to directly indicate the fraction of oxygen in the mix.
  The partial pressure of oxygen in diving chambers and surface supplied breathing gas mixtures can also be monitored using these cells. This can either be done by placing the cell directly in the hyperbaric environment, wired through the hull to the monitor, or indirectly, by bleeding off gas from the hyperbaric environment or diver gas supply and analysing at atmospheric pressure, then calculating the partial pressure in the hyperbaric environment. This is frequently required in saturation diving and surface oriented surface supplied mixed gas commercial diving.
  Diving rebreather control systems
  The breathing gas mixture in a diving rebreather loop is usually measured using oxygen cells, 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.