Deco gas switch depths: EAN50 at ~21 m, O₂ at ~6 m

Posted 10 July 2026 · Field Note #28

EAN50 at ~21 m, O₂ at ~6 m

A decompression gas is a separate cylinder of oxygen-rich mix you switch to during your ascent to off-gas faster. Each one can only be breathed down to a certain depth, because its oxygen content sets a maximum operating depth — go deeper and the ppO₂ climbs into toxic territory. The clever part of standard deco planning is choosing mixes whose MODs land on tidy, memorable stop depths.

Why a depth limit at all? Oxygen at partial pressure is a toxin. Most agencies cap deco ppO₂ at 1.6 ata. Run the numbers and EAN50 reaches that 1.6 ceiling at about 22 m, so the conventional switch depth is 21 m — the deepest you breathe it. Pure O₂ hits 1.6 at about 6 m, which is why it lives on the shallow stops. These aren't arbitrary traditions; they fall straight out of the ppO₂ maths.

The reason you switch the moment each mix becomes legal is off-gassing speed. A richer mix means less inert gas going in, a wider pressure gradient out of your tissues, and faster decompression. Waiting longer than you have to just wastes deco efficiency — and gas.

But gas switches are also one of the classic places divers die. Breathe EAN50 or pure O₂ too deep — grab the wrong bottle, misread a label — and the elevated ppO₂ can trigger a CNS oxygen-toxicity convulsion underwater, which usually means drowning. The defence is procedure, not memory. Most guidance has you physically confirm the cylinder: read the MOD label, check the analysed mix, and verbally confirm "switching to fifty at twenty-one" with a teammate before the regulator goes in your mouth.

The practical takeaway: standardise your switch depths, label every bottle with its MOD in big numerals, and treat each switch as a deliberate, verified, two-person checkpoint. Know the depth, name the gas, confirm the bottle, then breathe. The maths gives you the depths; discipline keeps you from dying on them.

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