Diving with mixed gases allows us to see pristine sites that are out of reach to most divers. Which gases are best, though, and what kinds of risks are associated with their uses? While technical dive courses will teach you everything you need to know about diving with mixed gases, this primer may help you decide to learn more so you can go the distance.
Air
Common “Air” used for diving is actually a blend of 21% Oxygen and 78% Nitrogen, plus 1% trace gas. While you can dive as deep as 66.2 meters on normal air, the risk for nitrogen narcosis symptoms increases greatly the moment you go beyond 30 meters. Some people are at risk for narcosis in shallower depths; breathing mixed air with lower nitrogen levels helps to increase safety by preventing the Martini Effect – that is, for every 10 meters you dive beyond 20 meters deep, you’ll feel as though you imbibed an alcoholic beverage. At 30 meters, it’s one martini. At forty, it’s two; and so on.
Nitrox
An enriched mixed gas containing more than 21% oxygen, Nitrox is often 32% or 36% pure oxygen while some nitrox mixtures contain standard air instead of pure oxygen. There are several different nitrox blends, and despite the misperception that nitrox lets you go deeper, 32% nitrox is not safe beyond about 34 meters, and 36% is designated for use at 29 meters or above. What nitrox will do for you is decrease your risk for DCS and allow you to spend less time on deco.
Trimix
As there are several favored blends for nitrox, there are also a variety of Trimix blends, each designed for use at different depths. A blend of oxygen, nitrogen, and helium, Trimix helps to reduce the risk of narcosis and it reduces the risk of oxygen toxicity. The higher the concentration of helium, the deeper you’ll be able to go, since helium reduces breathing resistance at depth. Helium also dissipates much more quickly than nitrogen does. One downside of any blend containing helium is that it conducts heat six times faster than air, making divers subject to hypothermia. Technical divers use a variety of methods to prevent themselves from becoming chilled.
Helitrox
Also known as Hyperoxic Trimix or TriOx, Helitrox is good for dives to 44 meters, but no further. A blend of 26% oxygen, 17% nitrogen, and 57% helium, this mixture decreases narcosis and allows for shorter decompression time. Some dive shops promote it, while others prefer to work with other custom blends.
Heliox and Hydreliox
Heliox is pure oxygen blended with helium; with it, you get zero risk for nitrogen narcosis, allowing you to focus on your dive. Hydreliox is Hydrogen, Oxygen, and Helium, and is used for dives beyond 130 meters. Hypothermia is a serious risk with both mixes; commercial divers often use suits heated with warm water from an umbilical to prevent it, while others inflate drysuits with separate canisters of argon, which conducts heat at a slow rate and helps prevent cold conditions from becoming problematic.
Other gases you may learn about are Hydrox, which is a special blend of hydrogen and oxygen used for deep dives beyond 500 meters, and Neox, another deep diving blend sometimes used in commercial and military applications. Special stages and rules for decompression apply to each of these gas mixes, and strategies for use include precautions with high oxygen concentrations, since flammability increases along with the percentage of oxygen. If you plan to explore deep caves or wrecks, or do any type of technical or commercial diving, then you may encounter some of these gases; recreational divers will usually stop at Trimix.