Gas density: the limit you don't plan for
Most divers plan for oxygen and narcosis. Fewer plan for how thick their gas is at depth. Research published in 2016 suggests they should, especially on rebreathers.
Read this first Information, not instruction. Nothing here replaces training with a qualified instructor.
Gas gets denser with depth. At 30 m, air is four times as dense as at the surface. Dense gas is harder to move in and out of the lungs, and the harder breathing gets, the harder it is to clear carbon dioxide.
That matters because carbon dioxide build-up, called hypercapnia, makes almost everything else worse. It deepens narcosis, raises the risk of oxygen toxicity, and can lead to a frightening loop in which a diver breathes harder, clears less CO₂, and feels an overwhelming need to breathe.
The numbers
In 2016, Gavin Anthony and Simon Mitchell published a review of respiratory physiology in rebreather diving. Based on the available data, they suggested keeping gas density at or below 5.2 grams per litre, and not going above 6.2 g/L.
Those figures are surprising when you first calculate them for common mixes.
| Gas | Depth | Density |
|---|---|---|
| Air | 30 m | 5.2 g/L |
| Air | 40 m | 6.4 g/L |
| EAN32 | 33 m | 5.6 g/L |
| TX 21/35 | 45 m | 5.0 g/L |
| TX 18/45 | 60 m | 5.6 g/L |
| TX 15/55 | 75 m | 5.8 g/L |
| TX 10/70 | 100 m | 5.7 g/L |
Air reaches the suggested limit at 30 m and passes the hard limit before 40 m. A trimix chosen only for narcosis, such as 18/45 at 60 m, already sits in the caution band. Oxygen is denser than nitrogen, so nitrox is slightly denser than air at the same depth.
Try it yourself
The calculator below shows density alongside ppO₂ and narcotic depth. Try air at 40 m, then 18/45 at 60 m, then add helium until density drops under 5.2.
Why rebreathers are more exposed
On open circuit, the regulator does much of the work. On a rebreather, the diver moves gas through hoses, a scrubber and counterlungs. Resistance adds up. Rebreather divers also tend to do longer, deeper dives, and a scrubber that's nearing the end of its life lets CO₂ through. Dense gas adds to all of this.
Gas density was one of the factors examined in the analysis of Dave Shaw's death at Boesmansgat in 2005, where carbon dioxide retention at depth is thought to have played a central role.
Planning
Should density limit your plan?
The research is newer than most dive planning rules, and divers treat it differently.
Plan to density
Choose helium content so the bottom mix stays at or below 5.2 g/L, and treat 6.2 g/L as a hard limit.
Why divers choose itAddresses CO₂ retention directly, which ties together narcosis, oxygen toxicity and breathing control.
Watch out forNeeds more helium, which costs more and makes some dives colder. The evidence base is thinner than for oxygen limits.
Plan to END, check density
Keep planning around narcosis and oxygen, and use density as a warning rather than a rule.
Why divers choose itMatches most training. Accepts that many successful dives have exceeded 5.2 g/L.
Watch out forYou may be carrying a risk you can't feel until it's too late.
Practical steps
- Add density to your dive planning sheet next to ppO₂ and END.
- Treat heavy exertion at depth, such as swimming against current or towing a diver, as a density problem as well as a gas-consumption one.
- On rebreathers, respect scrubber duration and check your loop's work of breathing.
- If you feel short of breath at depth, stop, rest and slow your breathing before continuing. Don't push through it.