How does this work? continued...

This can be better understood if you imagine a diver ascending from 50m...

At 50m the ambient pressure is 6 bar. The Inspiration controller maintains the PO2 at the chosen default setpoint of 1.3 bar* - so, the fraction of oxygen in the loop is 1.3 ÷ 6 = approx. 0.21 or 21%. As the diver ascends the ambient pressure drops so oxygen is injected by the controller in order to maintain the PO2 at 1.3 bar. So, as the diver ascends the percentage of oxygen in the mix rises. For example: at 30m (4 bar) you would breathe 1.3 ÷ 4 = 32%, at 16m : 50%, at 6m : 81% and at 4m : 93% O2.

*APD CCRs have 2 default setpoints: 0.7 bar used at the surface and the descent and 1.3 bar for the dive. Both setpoints are user adjustable (underwater if necessary): the lower from 0.5 to 0.9 bar and the higher from 0.9 to 1.5 bar.