Equivalent air depth, plainly

Posted 09 July 2026 · Field Note #27

Air is about 79% nitrogen. Nitrox (EAN) deliberately swaps some of that nitrogen for extra oxygen. Since it's the nitrogen that loads your tissues and drives your no-stop limits, breathing less of it means you accumulate inert gas more slowly — as if you were diving shallower than you really are.

The equivalent air depth puts a number on exactly how much shallower. EAD is the depth at which breathing plain air would give you the same nitrogen partial pressure you're actually experiencing on your nitrox mix at your real depth. Once you have it, you plan the nitrogen side of the dive — no-stop limits, decompression — as though it were an air dive to that shallower equivalent.

A worked example: 30 m on EAN32 has an EAD of about 24 m. Your nitrogen exposure matches a 24 m air dive, so you earn the longer bottom time of the shallower profile while actually being at 30 m. That extra time is the real, tangible benefit of nitrox — not more depth, more duration.

The mechanism is simply Dalton's law in action: at 30 m (4 bar) your 68% nitrogen exerts about 2.7 bar; on air, you'd have to climb to roughly 24 m to drop air's nitrogen partial pressure to that same 2.7 bar. Same nitrogen dose, shallower-feeling dive.

But here's the catch that bites careless divers: EAD only handles the nitrogen. The oxygen in your mix still acts at your real depth — and more oxygen means a shallower MOD, not a deeper one. So while your nitrogen exposure feels like 24 m, your oxygen exposure is firmly at 30 m and climbing toward its ppO₂ ceiling.

The practical takeaway: use EAD to claim your extra no-stop time, but never let it lull you on the oxygen side. Keep watching your MOD and ppO₂ at your true depth. Nitrox buys time, not depth — and only if you respect both clocks.

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⚠ Educational only — not a substitute for training or a dive computer. Verify every number independently and dive within your certification.