Trimix is a mixture of compressed air, helium, and oxygen. It is used by divers, usually technical divers, to reduce the amount of nitrogen absorbed into the bloodstream while underwater. It eliminates the danger of nitrogen narcosis during deep dives. It is also easier to decompress from than standard air, since helium is slower to absorb and faster to off-gas from body tissues. It is mixed by adding a predetermined amount of helium and oxygen to a SCUBA tank and topping it off with air. The exact amounts of helium and oxygen added to the mix are determined by the maximum depth the diver wishes to operate at. The maximum depth at which a mix can be used is determined by the amount of oxygen in the mixture; oxygen becomes toxic at partial pressures of 1.6 ATM and higher, so mixes with less oxygen can be used at greater depths. However, caution must be observed when using low-oxygen mixtures at the surface, since mixes of 16% O2 and less can cause hypoxia at sea level.

Trimix should not be confused with its two close relatives, heliox and heliair. The former is made by mixing helium with oxygen, and thus contains no nitrogen, while the latter is made by mixing helium with air. Because helium conducts heat five times faster than air, heliox can aggravate hypothermia underwater. It is also quite expensive to mix due to its high helium content. Heliair is not often used due to its lower (typically 10%-12%) O2 content, which makes it hypoxic at the surface.

Trimix is relatively easy to mix at home; all that is needed is a helium tank, an oxygen tank, an oxygen analyzer (can be had for \$100), and a filling whip. Basic math skills and knowledge of Boyle's Law will suffice to calculate the proper amount of helium and oxygen to add; the oxygen analyzer is for verification, since it's a great comfort to know exactly what you're breathing when you're 230 feet underwater. An ideal mix would give you a partial pressure of around 1.2-1.4 ATM of oxygen at your maximum planned operating depth and a nitrogen equivalent depth of 100 feet or less, corresponding to about 3.2 ATM ppN2. Too much helium is expensive and requires more oxygen added, and too little helium may not eliminate enough nitrogen to alleviate narcosis. Add the helium first; it gives you more control over the final mixture, and it allows you to add the helium when the tank being filled is at its lowest pressure, thus allowing you to get the most out of your helium bottle. The oxygen is mixed in next, and the bottle should be topped off with air. Allow the tank to cool, and then take a measurement of the O2 content. Minor corrections can then be made by bleeding the tank and adding oxygen or helium.