Gas density is the mass of one litre of the gas you are actually breathing, at the depth you are breathing it, in grams per litre, rising in direct proportion to depth. It decides how hard a dive is to breathe, and it has nothing to do with decompression.
Denser gas flows turbulently rather than smoothly, work of breathing rises, ventilation falls behind and carbon dioxide accumulates. CO₂ retention contributes directly to oxygen toxicity and to loss of control at depth, which is why density is a planning constraint rather than a comfort issue. Helium is roughly a seventh as dense as nitrogen, so replacing nitrogen with helium is the only effective way to make a deep mix breathable. Reducing oxygen does almost nothing, because oxygen is slightly denser than nitrogen.
The two thresholds
| Density | What happens |
|---|---|
| Above 5.2 g/L | Warning, advisory only. Elevated gas density — exceeds 5.2 g/L |
| Above 6.2 g/L | Error. High gas density — exceeds 6.2 g/L |
Both thresholds are applied twice: once by the gas analyser when you inspect a mix, and once by the planner, which checks the gas you are breathing at depth and attaches a warning naming the gas and the time. The two can differ slightly on the same mix, because only the planner accounts for the water type and altitude you set.
The figures are conventional in technical diving practice, but the citations attached to them in this build are not consistent with each other, so treat their source as unconfirmed.
Air at 45 m passes the error threshold, because air is the densest thing you can put in a cylinder at that depth. On a helium mix taken deeper, density often crosses 5.2 g/L before the ppO₂ limit is reached.
Narcosis and END
Equivalent narcotic depth is the depth at which air would produce the same narcotic load as the mix you are breathing. DiveLogic works it out from the nitrogen alone: helium is treated as entirely non-narcotic, and oxygen is not counted at all. It is rounded up to the next 0.1 m, because a deeper equivalent is the conservative one. Above 30 m the analyser raises a narcosis warning.
The same number appears under two labels: EAD on the planner’s gas card, and END in the gas mix wizard and the analyser. There is a second convention in which oxygen is counted alongside nitrogen, producing a deeper figure for the same mix; DiveLogic does not use it, so an END quoted from another tool may not match this one.
Both numbers arrive on the block every gas in a plan carries. This is 21/35 judged at 45 m: Density in grams per litre and EAD in metres.
21/35
PrimaryTMX 21/35O₂21%He35%N₂44%
Open circuit
ppO₂ ≤1.40 bar
What the numbers do not tell you
Density says nothing about decompression. A dense mix and a light mix at the same depth for the same time load your tissues according to their inert fractions, not their mass.
The thresholds are not a personal limit. Work of breathing depends on the regulator, the scrubber, the exertion, the posture and the diver. 5.2 g/L is a figure below which most divers on well-maintained equipment ventilate adequately, not a line at which something changes.
END measures nothing directly either. It rests on the assumption that helium is not narcotic, and cold, workload, CO₂ and fatigue all change how narcosis presents at a given END.
Next
- Oxygen exposure is the other constraint on a mix.
- Depth limits walks through the numbers in the app.