Rock bottom gas

Posted 18 June 2026 · Field Note #11

Rock bottom: the gas you never plan to spend

Start with the plain idea. "Rock bottom" (also called minimum gas) is the amount of breathing gas you set aside so that, if everything goes wrong at the worst moment, two divers can still reach the surface together. It is not your reserve for comfort. It is the gas you treat as if it isn't yours to touch.

Why two divers? Because the scenario you plan for is an out-of-gas teammate at the deepest point of the dive. From there you both ascend while sharing a single gas supply, you stop for any safety or decompression obligations, and you do all of it breathing harder than normal because you are stressed. Real numbers, not optimistic ones.

Here is the mechanism that trips people up. The gas a stressed diver moves at depth is large, because every breath is compressed by the surrounding pressure. At 40 m the water presses at about 5 bar, so each lungful contains roughly five times the surface gas of the same breath at the top. Double the divers, raise the breathing rate for stress, and add the time it takes to ascend slowly through stops, and the required reserve climbs steeply with depth. This is precisely why a habitual "I turn at 50 bar" fails: 50 bar may be generous at 18 m and dangerously thin at 40 m.

The practical takeaway: calculate rock bottom for the actual depth and team you are diving, not from habit. Most technical training has you compute it from a stressed breathing rate, your real ascent profile, and your stops, then set your turn pressure comfortably above that figure so you begin heading up with margin in hand. When your pressure gauge reaches rock bottom, the dive is over. No negotiation, no "one more minute."

Gas planning is the cheapest insurance in diving. Do the arithmetic on the surface so you never have to do it underwater.

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