Two of the most empowering aspects of closed-circuit rebreather diving are down to two complementary but different capabilities that amount to mini-marvels.
Whereas when we dive open circuit we are subject to a varying partial pressure of oxygen depending on the fraction present in the dive cylinder and the depth we are at - they are parameters we have virtually no control over once a dive has commenced. Any OC dive is therefore characterised by highest PO2 at maximum depth and lower PO2 during the remainder.
With a rebreather its different in two significant ways. Firstly, one can dynamically control the fraction of oxygen in the loop either automatically (via 'Setpoints') or manually by injecting additional oxygen directly into the loop. The fraction of oxygen can also be changed by diluting it with diluent gas or 'breathing it down' by closing off automated oxygen addition and waiting for the oxygen partial pressure to lower as the oxygen is metabolised. By having this control the rebreather diver is able to benefit from the effect a sustained raised Oxygen partial pressure has on decompression obligations (this in conjunction with inert gas benefits). But how can we measure oxygen partial pressure as opposed to the fraction (FO2)?
This is where the simplicity and elegance of the galvanic oxygen cell comes into play. These relatively simple electronic devices containing lead and electrolyte just happen to output an electrical potential proportional to the partial pressure of the oxygen they are in contact with. They can do so reliably for approximately a year. Hats off to the humble O2 cell which has also worked tirelessly all over the world during the COVID crisis where supplementary oxygen levels continue to be key.