Stage Bottle

An additional cylinder clipped to a diver's side that carries extra bottom gas or a dedicated decompression mix on technical dives.

A stage bottle is an extra cylinder a technical diver clips to the left side of the harness to carry gas beyond what the back-mounted tanks hold. Depending on the plan it may extend bottom time with more back gas, carry a travel gas for the descent, or hold a rich decompression mix such as EAN50 or pure oxygen that is breathed only during the staged stops on ascent.

Rigging is standardized so any teammate can read your setup: an aluminum cylinder, typically 5.7 or 11 liters (40 or 80 cubic feet), with a regulator whose hose is bungeed along the tank, clipped to chest and hip D-rings. Every bottle is analyzed after filling and marked with its contents and the maximum operating depth of the mix, because breathing a rich deco gas deep is an oxygen toxicity emergency.

Gas switch procedure is a core technical skill: confirm depth against the MOD label, have a teammate verify the bottle, then switch. In cave diving a stage may also be dropped along the line and picked up on exit.