Minimum gas: C × A × T

Minimum gas is the volume two divers on one back-gas supply need to leave the deepest point and reach the first decompression gas, at the minimum ascent rate rather than the planned one — a sharing team does not move at planned speed. The formula is the workbook's own: C × A × T.

Worked for the reference dive

45 m for 25 minutes on 21/35, EAN50 deco gas, twin steel 12s:

• C — team consumption: 40 L/min (two divers sharing one supply, 20 L/min each).
• A — mean absolute pressure over the ascent to the gas switch: 4.3 bar.
• T — minutes from 45 m to the switch at 21 m, stops included: 10 minutes.
• C × A × T = 40 × 4.3 × 10 = 1,720 litres.
• In the twin steel 12s, that is 80 bar.

Add what the dive itself costs to breathe (118 bar on this profile) and the required fill for the back gas comes to 200 bar, inside the twinset's 232 bar rating.

Inputs used

• Depth: 45 m, bottom time: 25 minutes
• Bottom gas: 21/35, deco gas: EAN50 switching at 21 m
• Back gas: twin steel 12 L (24 L water volume, 232 bar)
• SAC: 20 L/min per diver
• Gradient factors: 20/85 (the app's default)

The endpoint is the gas switch, not the surface — this dive carries a deco bottle to change onto. A dive with no deco gas at all charges the reserve all the way to the surface instead, at a much larger cost.

Source: Time to Surface's pinned etalon suite, profile 07 ("The baseline") in EtalonDerivedAnswers.swift, the workbook's MG = C × A × T rule reproduced against its own printed table before any etalon value was written.

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.