Gas density¶

Gas density at depth is one of the limiting factors of deep tech diving — and one of the easiest to get wrong if you don't actively check it.

Why it matters¶

As depth increases, the absolute pressure rises (≈ 1 ata per 10 m), and the gas you breathe gets denser proportionally. Denser gas means:

Higher work of breathing — your respiratory muscles do more work per breath
CO₂ retention risk — harder to ventilate adequately, especially under exertion
Faster onset of exhaustion — silent and hard to detect underwater
Greater susceptibility to narcosis effects — though density and narcosis are partly independent

Above a certain density, the risk profile changes from "uncomfortable" to "dangerous." Two thresholds are widely cited:

Density	Status	Reference
≤ 5.2 g/ℓ	Recommended ceiling	Mitchell & Anthony, 2018
6.2 g/ℓ	Hard upper limit	Same source — beyond this, performance degrades sharply

How AeroPlus Deco reports it¶

The Gas plan card → Limits table lists the gas density at the deepest planned depth for each gas:

Depth (m)	Mix	Density (g/ℓ)	PPO₂
75	16/58	5.14	1.34

Color coding:

Green / black — within the 5.2 guideline
Orange — exceeds 5.2 (warning)
Red — exceeds 6.2 (limit)

The same density value also appears as a warning in the Critical warnings banner at the top of the plan when it exceeds either threshold.

Reference temperature

Densities in the app are computed at 15 °C (a typical cold-water reference). Warm-water densities are slightly lower — the values reported are conservative.

Managing density with helium¶

Helium is much less dense than nitrogen, so swapping nitrogen for helium reduces gas density at depth. This is why trimix exists — narcosis management is one reason, but density management is the other half.

Rule of thumb¶

For every 10 m of additional depth on a nitrox-heavy trimix, you typically need ~5–10 % more helium to keep density in check. Use the Best mix calculator with Limit gas density toggled on to compute the right percentages automatically.

The Best mix density-aware option¶

When Limit gas density is enabled in the Best mix calculator:

The optimal mix targets both your narcotic depth (EAD) and a density ≤ 5.2 g/ℓ
Whichever requirement needs more helium wins
The result may have a shallower EAD than your target, but the density is safe

Same logic applies for Best diluent in CCR mode — the loop density is computed at the bottom setpoint and used as one of the constraints.

Density vs MOD¶

A common confusion: a gas can have an acceptable PPO₂ at depth (so its MOD is fine) and still be too dense. PPO₂ and density are separate constraints. Check both.

For example: EAN32 at 40 m has PPO₂ = 1.34 (fine) but density ~5.0 g/ℓ (also fine, just). At 45 m, density rises to 5.5 g/ℓ — already over guideline — even though the PPO₂ is still under 1.6. The MOD doesn't catch that.

Practical implications¶

Use trimix earlier than narcosis alone would dictate. By 40 m on air you're at 5+ g/ℓ density even though the EAD is shallow.
Density-check every bottom mix before going deeper than planned.
CCR loop density is calculated for the loop gas composition at depth, not raw diluent. The loop tends to have less helium than the diluent at high setpoint (because the setpoint is more O₂), so loop density can be higher than diluent density.