Constant Partial Pressure of Oxygen (PO2)

The APD controller maintains a constant PO2 at the chosen setpoint throughout the dive. This gives the advantages of oxygen-rich diving at every stage of the dive down to 50m.

How does this work?

On open circuit scuba the gas mix is constant (approx. 21% O2 in air) at all depths while the PO2 is variable – i.e. 0.21 bar at the surface, 0.42 at 10m, 0.63 at 20m and so on.

With APD CCRs it is the opposite case: the PO2 is constant and the mix or fraction (FO2) is variable. The PO2 is held at the chosen setpoint by the controller injecting oxygen into the breathing loop. The result is that the FO2 varies with depth.