What the EAN50 deco bottle needs

The deco bottle's fill is walked level by level from the schedule that was actually solved, not estimated from a flat guess — and it carries a margin on top of what the dive is expected to cost, in case the ascent runs long.

Worked for the reference dive

45 m for 25 minutes on 21/35, switching to EAN50 at 21 m, in an aluminium 80 deco bottle (11.1 L, 207 bar rated):

• Minutes breathed on EAN50: 23 minutes — from the 21 m switch to the surface, ramp included.
• Litres consumed: 920 L.
• Reserve on top (×1.5): 1,380 L.
• Required fill: 130 bar of the AL80's 207 bar rating.

The bottle does not need a full fill for this dive — 130 bar leaves headroom inside a 207 bar cylinder. A longer or deeper EAN50 obligation asks for more; the app checks the fill you actually plan to use against what the dive needs, not the other way round.

Inputs used

• Depth: 45 m, bottom time: 25 minutes
• Bottom gas: 21/35, deco gas: EAN50 switching at 21 m
• Deco bottle: aluminium 80 (11.1 L, 207 bar)
• SAC: 20 L/min
• Gradient factors: 20/85 (the app's default)

The deco bottle picker offers three aluminium sizes — AL80, AL50 (7 L) and AL40 (5.5 L) — because steel is unsuitable for a bottle slung at a stop rather than worn on the back. Which one covers a given EAN50 or oxygen obligation depends on the deco gas and the dive, not on the depth alone.

Source: Time to Surface's pinned etalon suite, profile 07 ("The baseline") in EtalonDerivedAnswers.swift.

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.