Why 1.4 and 1.6
Posted 10 June 2026 · Field Note #3
ppO₂: the 1.4 / 1.6 rule
Oxygen keeps you alive, but at pressure it behaves like a drug, and the dose isn't the percentage in your tank, it's the partial pressure you're actually breathing. Partial pressure (ppO₂) is just the fraction of oxygen in your mix multiplied by the ambient pressure. The deeper you go, the higher the ambient pressure, so the same gas delivers a bigger oxygen dose the deeper you take it.
Why care? Because too much oxygen, sustained, can trigger central nervous system oxygen toxicity. The frightening form is a sudden convulsion, with little or no warning, and a convulsion underwater usually means a drowning. This isn't about how much oxygen is good for you on the surface; it's about a pressure dose your body wasn't built to tolerate.
So divers and agencies cap the dose. Most agencies set the working ppO₂ limit at 1.4 bar for the active, task-loaded part of a dive, and allow a higher contingency or decompression ceiling of 1.6 bar for the calmer, shallow phase. Above 1.6, risk climbs steeply, and seizure risk also rises with exertion, carbon dioxide retention, and time, which is why the working number is more conservative than the deco one.
That cap is exactly what sets your maximum operating depth. Run the numbers and the rule becomes intuitive. EAN32 reaches a ppO₂ of 1.4 at roughly 33 m, so that's its sensible working depth. Pure oxygen, by contrast, hits 1.6 at just about 6 m. That single fact explains a cornerstone of technical diving: we breathe pure O₂ on the shallow decompression stop, never at depth, because anywhere deeper it's already over the line.
Practical takeaway: before any nitrox or mixed-gas dive, calculate your ppO₂ at your planned maximum depth, confirm it sits at or under 1.4 for the working portion, and label the cylinder's MOD. Let the gas dictate the depth, not the other way around.
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⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.