Trimix and Heliox

What the mixes are

Trimix is a blend of three gases: oxygen, helium and nitrogen. Heliox is a blend of two: oxygen and helium, with no nitrogen at all. Both are used for diving deeper than air or nitrox allow.

Trimix is named by its oxygen and helium percentages, with nitrogen making up the rest. "Trimix 21/35" contains 21 percent oxygen, 35 percent helium and 44 percent nitrogen. "18/45" contains 18 percent oxygen, 45 percent helium and 37 percent nitrogen.

Why add helium

On deep dives, two gases cause trouble. Nitrogen becomes strongly narcotic, as described in nitrogen narcosis, and oxygen becomes toxic at high partial pressure, as explained in oxygen toxicity. Helium solves both problems: it isn't narcotic at the depths where narcosis becomes a problem, and replacing some of the oxygen and nitrogen with it keeps both within safe limits.

Helium is also a very light gas. Gas gets denser as you go deeper, which makes it harder to breathe and can let carbon dioxide build up. A helium mix stays easier to breathe at depth, which reduces the work of breathing and the risk of carbon dioxide build-up.

Hypoxic mixes

To keep oxygen partial pressure safe at great depth, deep mixes often carry less than the 21 percent oxygen in air. A mix with too little oxygen to support life at the surface is called hypoxic. Divers using hypoxic mixes must switch to a richer travel or decompression gas in shallow water, because breathing a hypoxic bottom mix near the surface can cause unconsciousness without warning.

Drawbacks

• Cost. Helium is expensive and not available everywhere.
• Cold. Helium conducts heat far better than air. Divers in drysuits often inflate them with argon rather than their breathing gas to stay warm.
• Decompression. Helium enters and leaves the body faster than nitrogen, which changes decompression planning. Deep trimix dives usually need long decompression stops on several gases.
• Complexity. More gases mean more cylinders, more switches and more ways to make a mistake.

At extreme depths, usually well beyond 100 m and mostly in commercial or research diving, rapid compression on helium mixes can cause high-pressure nervous syndrome (HPNS), with tremors and nausea.

Who uses them

Technical divers use trimix to explore deep wrecks and caves, such as long cave dives and deep wreck dives. Commercial and military divers use heliox, often supplied from the surface or in saturation diving, where divers live under pressure for days or weeks.

Training

Trimix diving is technical diving. It is taught as a series of courses after advanced nitrox and decompression procedures, usually with a sequence from normoxic trimix (enough oxygen to breathe at the surface) to full hypoxic trimix. Nobody should breathe a helium mix without that training.

Frequently asked questions