Deco Gas

An oxygen-rich mix, commonly EAN50 or pure oxygen, breathed during decompression stops to speed inert gas elimination and shorten deco time.

Deco gas is an oxygen rich mix carried specifically for the decompression phase of a technical dive. By switching from the bottom mix to a gas with far less inert gas, the diver steepens the pressure gradient that drives nitrogen and helium out of the tissues, which shortens the required stops considerably. The classic choices are EAN50, first usable at 21 m at a PO2 of 1.6, and pure oxygen at the 6 m stop.

Deco gases are carried in clearly marked stage bottles, labeled with the analyzed mix and its maximum operating depth, and rigged so the diver can reach and identify them without ambiguity. Gas switches follow a strict protocol, typically confirmed by a teammate, because breathing a rich deco gas below its MOD is one of the most dangerous mistakes in technical diving: a PO2 spike can trigger a convulsive oxygen toxicity hit with little warning.

Dive computers and planning software account for each programmed switch, so the run time you follow assumes the deco gas actually gets breathed on schedule. Losing a deco bottle therefore has real consequences: contingency plans include longer stops on the remaining gases, and teams carry enough combined deco gas that any single lost cylinder still leaves a workable, if slower, way to the surface.