Gas density explained: why deep gas is hard to breathe
Gas gets heavier the deeper you go, and heavy gas is hard work to breathe. The figure to watch is grams per litre, and the line most divers draw is 5.2.
What gas density is
Every gas has a weight per litre, and the pressure of the water multiplies it. Air at the surface is 1.2 g/l. At 30 m, with four times the pressure, it is about 5.2 g/l. Breathing dense gas needs more effort, and carbon dioxide can build up. Past about 5.2 g/l the effort rises steeply; 6.2 g/l is the hard limit in the work Loggerfish follows (Anthony and Mitchell, 2016).
Air, nitrox and trimix
Density follows the depth and the gas. Helium is far lighter than nitrogen, so it is the way to bring a deep mix down.
| Gas | Depth | Density |
|---|---|---|
| Air | 18 m | 3.6 g/l |
| Air | 30 m | 5.2 g/l |
| Air | 40 m | 6.4 g/l |
| 21/35 trimix | 50 m | 5.5 g/l |
| 18/45 trimix | 60 m | 5.6 g/l |
Air reaches the advised limit at 30 m and the hard limit before 40 m. That is the second reason, after narcosis, why deep dives move to trimix.
Try it with your numbers
The calculator finds the best mix for a depth: the most oxygen and the least helium that respect your ppO₂ and your narcotic limit. At 60 m with a narcotic limit of 30 m, it gives 20/43, with a density of 5.8 g/l.
- Best mix
- Trimix 20/43
- Oxygen
- 20%
- Helium
- 43%
- Equivalent narcotic depth (END)
- 29.9 m
- MOD of this mix at ppO₂ 1.4
- 60.0 m
- Gas density at depth
- 5.8 g/l
Dense gas: 5.8 g/l at 60.0 m, over the advised 5.2 g/l. Add helium or plan shallower.
What the number does not tell you
- It is a calculation at one depth. Density rises and falls through the dive.
- It does not tell you whether you are trained for the mix. Trimix needs a course.
- Analyse every mix before you breathe it.
These calculations are planning aids, not a dive computer. Your training and your computer decide the dive.
Check your mix and its density in Loggerfish
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