At 50m the ambient pressure is 6 bar. The controller maintains the PO2 at the Setpoint of 1.3 bar - therefore, the fraction of oxygen in the loop is 1.3 ÷ 6 = approx. 0.21 or 21%. As the diver ascends the ambient pressure drops, PO2 in the loop falls and oxygen is injected by the controller until the Setpoint is regained. And so, the partial pressure of oxygen (PO2) remains the same but the percentage of oxygen in the mix rises. You can see from the table above that the diver's gas is oxygen-rich at deco depths - 80%+ rising to 100% at 3m - ideal for speedy deco.

Understanding the benefits of high FO2:

Why is oxygen-rich gas the best? Good question, and one most divers will already be familiar with:

In a nutshell… the more oxygen in the mix, the less inert gas (nitrogen, helium) for our bodies to absorb and create deco obligations. And so: Oxygen-rich = more dive-time, greater depth, less deco.

Understanding partial pressure of Oxygen:

PO2 or FO2… Where does the danger lie in terms of Oxygen toxicity? Another good one and the answer is clearly...

It is all about the PO2

After all, you can breathe 100% Oxygen perfectly safely at the surface and above 6m (as they did with early pure O2 rebreathers). Imagine for a second, you are diving with this type of historic kit...

At the surface, with an FO2 of 100% your partial pressure of O2 = 1 bar, i.e. a safe PO2.

The physiological safety band of oxygen pressure for human beings is 0.16 to 1.6 bar.

As you descend (with this fictitious pure oxygen breathing apparatus) the ambient pressure changes. At 1m for example, with this fixed 100% mix your partial pressure of Oxygen is now 1.1 bar. At 2m it increases to 1.2 bar, at 3m = 1.3 bar and so on.

So by the time you reach 6m your PO2 = 1.6. You are still breathing 100% Oxygen but now at 1.6 bar you are entering the danger zone where Oxygen becomes poisonous or Hyperoxic. Alternatively, if you were to go the other way and ascend in a hot-air balloon to over three times the height of Everest - to the point where the ambient pressure drops to 0.16 bar - your 100% Oxygen supply would now be 0.16 PO2 and would start to become Hypoxic. It's all about the PO2 .

Which is why, in CCR training we emphasise "Always know your PO2!". In contrast to open circuit scuba, where we are taught to constantly be aware of how much gas we have left, in CCR, the importance is "What gas? - what PO2 - am I breathing?".