Many divers simply call the gas in their tank “oxygen” or “air” without really knowing what it’s made of. What’s the difference between scuba diving gas mixes?

Many new divers incorrectly call their diving cylinder an oxygen tank. For some, it’s just a turn of phrase; they know full well that the standard diving gas is good, old-fashioned air. But divers can breathe other gases, including the commonly used nitrox, as well as technical mixtures. What’s the difference between all these scuba diving gas mixes anyway?

Air (79 percent nitrogen, 21 percent oxygen)

It almost seems silly to mention air, but some readers may want to start diving and haven’t yet signed up for an open-water class. Air is the gas of choice for every dive resort in the world. Why? It contains enough oxygen to sustain us underwater, and since it’s all around us, it’s cheap. The downside is that during our descent and bottom time, our tissues are absorbing nitrogen. On ascent, tissues release this extra nitrogen in a process called off-gassing. The nitrogen load limits recreational divers’ bottom times and how fast they can ascend. There are depth limitations too, as nitrogen becomes narcotic the deeper you go. This becomes increasingly debilitating, so the maximum depth for recreational diving is 130 feet (40 m). Air itself becomes toxic as we go beyond 184 feet (56 m).

Availability: Hopefully everywhere
Best used for: All recreational diving down to 130 feet (40 m).
Cost: Always included in the price of a course or fun dive at resorts. Dive shops will charge a small fee to fill a cylinder.

Nitrox (up to 40% oxygen)

Nitrox contains from 22 to 40 percent oxygen. The most common mixes are 32 percent and 36 percent oxygen, also known as EAN32 and EAN 36. Many people think that nitrox allows you to dive deeper and stay down longer, but this is simply not true. Nitrox is a shallow-diving gas. While it can extend no-decompression limits (NDLs) compared with air, you still breathe at the same rate, so you’ll still breathe the tank down at the same rate.

Because nitrox contains more oxygen, it contains less nitrogen. This means that your body absorbs less nitrogen at a given depth compared with diving on air, which explains why you can extend your NDL. But staying down for longer with less nitrogen in the tank will still produce the same decompression risk as using air for a shorter amount of time at a given depth. Because oxygen becomes toxic with increasing depth, every nitrox mixture has a maximum depth for a particular dive, with shallower maximums than air.

Availability: Of the scuba diving gas mixes, this is one is most widely available at busy resorts. More remote diving locations may not offer nitrox. Check ahead if you’d like to use it. Remember to show proof of certification to dive this gas.
Best used for: Repetitive and multi-day diving, such as on a liveaboard. The nitrox specialty course covers everything you need to know to use nitrox safely.
Cost: The cost will be included in your nitrox course and you’ll often pay a small fee per dive at a resort. Dive shops will charge per cylinder.

Trimix

Now we’ve firmly entered technical-diving territory. Trimix is a mixture of oxygen, nitrogen and helium. Why add helium? Well, we know that nitrogen becomes narcotic as we go deeper. Helium is an inert gas, meaning it does not react with our bodies at conventional scuba depths. So if we replace some of the nitrogen with helium, we can go deeper and still have a clear head. So-called normoxic trimix does allows divers to do just that. A common mixture is 21/35, which has 21 percent oxygen, 35 percent helium and 44 percent nitrogen. Another common mixture is 18/45, with 18 percent oxygen and 45 percent helium. These mixtures allow technical divers to hang around at up to 197 feet (60 m) — and actually remember their dive.

Dives any deeper than 197 feet require hypoxic trimix. In this mixture, helium replaces some of the nitrogen and oxygen content. Doing so reduces the risk of oxygen toxicity at depths of around 328 feet (100 m). A typical hypoxic mixture at this depth would be 10/70 (10 percent oxygen and 70 percent helium).

Availability: Widely available since the mid-2000s, but only for technical divers at resorts that offer technical diving courses as well as fun diving. Divers need to complete numerous tec courses before doing a trimix course. Remember to show proof of certification to dive this gas.

Best used for: Dives beyond 130 feet (40 m), or if you have a high susceptibility to narcosis.

Cost: Helium is quite expensive, and the cost of filling tanks will vary greatly depending on the depth and equipment configuration. People who do a lot of deep diving use rebreathers to keep gas costs down.

Decompression gases: Nitrox 40 to 99 percent and 100 percent oxygen

If you’re decompression diving, using air for your deco stops is a bad idea for many reasons. Mainly it’s terribly inefficient — your deco stops will be quite long. For reasons beyond the scope of this article, all the nitrogen your body accumulates during a deco dive can be teased out a lot faster if you breathe high percentages of oxygen on your shallow deco stops. Common mixtures are 50 percent or 80 percent nitrox and 100 percent oxygen. However, you must be a trained technical diver to understand the risks and benefits associated with breathing higher percentages of oxygen, and obtain the skills necessary to dive safely.

Availability: Only available to qualified technical divers at resorts that offer technical diving courses. You’ll need to be qualified as an advanced nitrox/Tec 40, or decompression procedures/Tec45 diver to rent tanks. Remember to show proof of certification to dive this gas.

Best used for: Decompression diving

Cost: Slightly more expensive than nitrox

Commercial and exotic gases

The commercial diving industry commonly uses heliox in place of trimix. This mixture of helium and oxygen can be used at depths of up to 984 feet (300 m). Any deeper than that requires divers to replace helium with hydrogen. Helium becomes narcotic at these depths, and the body becomes susceptible to High Pressure Nervous Syndrome (HPNS). Hydrox is a mixture of hydrogen and oxygen. Experimental commercial divers, laying oil and gas pipeline at a depth of 1,752 feet (534 m), used this gas. But that’s nothing if your name is Theo Mavrostomos. He achieved a record simulated dive of 2,300 feet (701 m) in an onshore hyperbaric chamber.

Availability: Ask Doc Brown

Best used for: Fun diving the Titanic in the 22nd century.

Cost: Make sure your great-grandchildren are sitting down

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