Subject 3 articles
Scuba Breathing Gases
Scuba breathing gases are mixes of oxygen, nitrogen and sometimes helium. Each mix has a working depth range, and choosing the right one is part of planning every dive.
Articles in scuba breathing gases
- Compressed Air for Scuba Diving
What's in a scuba tank of air, how it's filled and the depth limits of air.
- Enriched Air Nitrox
More oxygen, less nitrogen: longer bottom times at moderate depth.
- Trimix and Heliox
Why deep divers add helium, how mixes are named and what it takes to use them.
Three families of gas
Almost every recreational dive is made on one of two gases. Compressed air is ordinary atmospheric air, filtered and pumped into a cylinder at high pressure. Enriched air nitrox is air with extra oxygen added, which cuts the nitrogen you absorb and lengthens your no-decompression time. Deeper technical diving adds helium: trimix and heliox reduce narcosis and make gas easier to breathe at great depth.
Why the mix matters
Each gas in a mix does something different under pressure. Nitrogen dissolves into your tissues and causes decompression sickness if it comes out too fast; at depth it also causes nitrogen narcosis. Oxygen keeps you alive, but at high pressure it becomes toxic, as explained in oxygen toxicity. Helium is not narcotic at normal diving depths, but it is expensive and carries body heat away quickly.
The key idea is partial pressure: the share of the total pressure that comes from one gas. Partial pressure equals the gas's fraction in the mix multiplied by the absolute pressure at depth. Air is 21 percent oxygen, so at 30 metres (4 bar absolute) the oxygen partial pressure is 0.21 × 4 = 0.84 bar. Divers keep oxygen partial pressure at or below 1.4 bar while working and 1.6 bar as an absolute limit, which sets a maximum operating depth for every mix. The pressure numbers come from water pressure at depth.
Comparing the common mixes
| Gas | Typical content | Main benefit | Main limit |
|---|---|---|---|
| Air | 21% oxygen, 79% nitrogen and trace gases | Cheap and available everywhere | Narcosis and short no-decompression times at depth |
| Nitrox 32 | 32% oxygen, 68% nitrogen | Longer no-decompression times at 18 to 30 m | Maximum depth about 34 m (111 ft) at 1.4 bar |
| Nitrox 36 | 36% oxygen, 64% nitrogen | Longer still in shallower water | Maximum depth about 29 m (95 ft) at 1.4 bar |
| Trimix | Oxygen, nitrogen and helium in planned ratios | Less narcosis and lighter gas at depth | Cost, cold and technical training |
Training and labels
Breathing anything other than air needs training. A nitrox course teaches you to analyse your own tank, label it with the oxygen percentage and maximum depth, and set your dive computer to match. Trimix diving is technical diving and sits several courses beyond that. Never dive a tank you haven't analysed yourself.