Bottom Gas vs Deco Gas: Why Two Different Mixes?

The principle: one gas can't do both jobs

On any dive past recreational no-decompression limits, the optimal gas at depth and the optimal gas during the ascent are different gases. The bottom gas must keep ppO₂ safe at maximum depth — pushing the O₂ fraction down — while the deco gas must accelerate inert-gas off-gassing during the ascent — pushing the O₂ fraction up. Trying to do both with a single mix means compromising on both ends: the bottom phase becomes deeper than safe, or the ascent becomes longer than necessary. Doing the dive with two (or three) gases is the standard technical solution, and the partial-pressure trade-offs explain why.

Bottom gas: keep ppO₂ ≤ 1.4 under load

The bottom gas is breathed while you're working — finning into current, carrying camera or scooter, managing buoyancy at depth, navigating. Working ppO₂ tolerance is conservatively 1.4 ata because CNS-tox risk rises sharply at higher fractions when CO₂ production is elevated. For a 50 m wreck dive this means O₂ fraction ≤ 28 % (EAN28 or trimix 21/35 if helium replaces some N₂ for density and narcosis). The bottom gas also bears the burden of breathing density at depth: target ≤ 5.2 g/L (Anthony 2018, DAN). Above 6.2 g/L work of breathing dominates and CO₂ retention becomes likely — that's why air bottom gas past 40 m is increasingly regarded as substandard practice.

Deco gas: maximise off-gassing at rest

The deco gas is breathed at rest, hanging on a line at a stop. Resting ppO₂ tolerance is 1.6 ata, so the O₂ fraction can be much higher. The faster the inert-gas partial-pressure gradient between tissues and breathing gas, the faster off-gassing happens — and oxygen replaces inert gas at 1:1. EAN50 at 21 m gives ppO₂ 1.55 ata (right under the 1.6 limit) and roughly halves shallow-stop time compared to continuing on the bottom gas. Pure O₂ at 6 m gives ppO₂ 1.6 ata and is the most efficient final-stop deco gas. Some agencies allow up to 1.6 at 21 m, others cap at 1.45 — follow your training agency's standard.

Choosing the right deco gas (EAN50 vs O₂, ICD)

The two most common deco gases are EAN50 (switched at 21 m) and pure O₂ (switched at 6 m). Carrying both shaves the most time off shallow stops, but doubles cylinder count. Many tech dives use EAN50 only and accept ~5 minutes longer at 6 m. ICD (Isobaric Counter Diffusion) becomes a concern when switching from a high-helium bottom gas to a low-helium deco gas: Doolette's 1:5 rule says the change in inert-gas fractions ratio must stay below 5:1. A switch from trimix 18/45 to EAN50 at 21 m typically fails this rule and risks vestibular DCS. Use the icd-calculator to validate each switch before the dive, and consider helium-rich intermediate deco gases on heavy trimix dives.

Practical recommendations

For 30-40 m no-stop dives: a single EAN32-36 covers bottom and ascent without a switch — no deco gas needed. For 40-50 m with light deco: bottom EAN28 + EAN50 at 21 m is a common setup. For 50-70 m trimix: bottom 21/35 + EAN50 at 21 m + O₂ at 6 m. For 70 m+: add a travel/intermediate deco gas to bridge the ICD gap. Never breathe a deco gas at a depth past its MOD — labelling the cylinder neck with maximum operating depth (e.g., "21 m" for EAN50) is the standard safeguard. Plan every switch in the dive-planner with explicit switch depths and verify ICD in the icd-calculator.