Gas Density at Depth

Thresholds (Anthony & Mitchell, 2016): Keep gas density ≤ 5.2 g/L at the deepest planned depth, with 6.2 g/L as a hard ceiling. Above 6.2 g/L, work of breathing rises sharply and CO₂ retention becomes likely. The model uses 0°C reference densities (O₂ 1.429, N₂ 1.251, He 0.179 g/L); cold water is slightly denser, warm water slightly less.