40 m for 30 min on air, no deco gas

Time to Surface plans staged decompression on back gas alone, with no deco bottle at all, if that is the dive you enter. Here is what that costs on a 40 m / 30 min air dive — and why it is not one a twin steel 12 can actually fund.

The schedule

• Descent: 4 minutes to 40 m. Time at maximum depth: 26 minutes.
• First stop: 21 m, runtime 32.1 minutes.
• Stops, all on air: 21 m × 1 min, 18 m × 1 min, 15 m × 2 min, 12 m × 3 min, 9 m × 6 min, 6 m × 34 minutes.
• Total stop time: 47 minutes. Final ascent from 6 m: 6 minutes. Total runtime: 85.1 minutes.
• CNS: 15%.

Why the twin 12 does not cover it

Minimum gas — what two divers need to leave 40 m and reach the surface with no deco bottle to switch to — comes to 310 bar on this profile, against a 232 bar rating. Add what the dive itself costs and the required fill is 520 bar. A twin steel 12 cannot hold it, and the app refuses to plan the dive on that cylinder rather than printing a number it cannot fund.

With no deco bottle, the entire ascent's minimum gas is charged at the sharing team's minimum rate all the way to the surface — including the last six metres, which the app normally slows to 1 m/min and charges accordingly. That is a much larger reserve than a dive with an EAN50 or oxygen bottle to switch to.

Inputs used

• Depth: 40 m, bottom time: 30 minutes (4 min descent + 26 min at depth)
• Bottom gas: air, no deco gas
• Back gas: twin steel 12 L (24 L water volume, 232 bar rated)
• Gradient factors: 20/85 (the app's default)
• SAC: 20 L/min per diver

Source: Time to Surface's pinned etalon suite, profile 05 ("Back-gas deco only" / "Air, back-gas deco only") in EtalonDerivedAnswers.swift, listed in docs/etalon-technical-profiles.md.

Your training, your gauge and your computer decide the dive. This is pre-dive arithmetic, not a dive computer, and it does not make any dive safe.