Rock Bottom

The minimum gas reserve that lets two divers share gas from the deepest point of the dive and ascend to the surface with all required stops.

Rock bottom, also called minimum gas, is the gas pressure below which your cylinder no longer covers the worst plausible emergency: a buddy arriving out of gas at the deepest point of the dive and both of you ascending to the surface on your supply alone. Reach rock bottom and the dive is over, regardless of no-decompression time remaining or how good the reef looks.

The calculation is straightforward and sobering. Take an elevated consumption rate for two stressed divers, commonly double a normal SAC rate per diver, multiply by the time to solve the problem at depth, ascend at a proper rate, and complete a safety stop, all at the average ambient pressure of the ascent, then convert that volume to a pressure for your cylinder size. The result rises steeply with depth: for a typical single cylinder, rock bottom at 30 m is far more than the traditional 50 bar habit suggests.

Computing rock bottom for your usual depths and tank turns an abstract reserve rule into a personal number worth memorizing. Many divers keep a small table on a slate or in their wetnotes: rock bottom at 20, 30, and 40 m for their standard cylinder. When the plan says leave the bottom at rock bottom plus the gas needed for the swim back, the ascent never becomes a negotiation.