Minimum gas from 40 m

40 m is close to the deep end of what a recreational team plans. The reserve two divers need to reach the surface from there is bigger than the 30 m figure, and it still depends on the cylinder and on how hard the team breathes.

The figures

• SCR 15 L/min (the app's Light tier) — 46 bar from a 15-litre steel, 62 bar from an aluminium 80.
• SCR 20 L/min (Typical) — 61 bar from a 15-litre steel, 83 bar from an aluminium 80.
• SCR 27 L/min (Heavy) — 83 bar from a 15-litre steel, 112 bar from an aluminium 80.

Against the 50 bar habit most divers were taught, only the lightest breather on the 15-litre steel comes in under it, at 46 bar. Every other combination here asks for more — up to 112 bar, more than double the habit, for a heavy breather on the aluminium 80.

Inputs used

• Depth: 40 m
• Ascent style: NDL with safety stop (9 m/min to 5 m, 3 minutes at 5 m, then to the surface)
• Cylinders: single 15-litre steel (232 bar) and single aluminium 80 / 11.1-litre (207 bar)
• SCR: 15, 20 and 27 L/min — the app's Light, Typical and Heavy tiers
• Two divers sharing one supply, at the reserve's own ascent rate

A 15-litre steel at 232 bar and an aluminium 80 at 207 bar both hold enough for this reserve at every tier above — the question the app answers is how much of the fill it takes, which is why the cylinder you dive and how hard you breathe both change the number your dive plan is built around.

Source: the app's own pinned test suite for the figures it is sold on (PublishedReserveFiguresTests.fortyMetres).

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.