Trimix

A breathing gas of oxygen, helium, and nitrogen used in technical diving to reduce narcosis and gas density on deep dives.

Trimix is a three gas blend of oxygen, helium, and nitrogen used for deep technical diving. Helium replaces part of the nitrogen to reduce nitrogen narcosis and part of the oxygen to keep oxygen partial pressure safe at depth. A mix is named by its oxygen and helium percentages: trimix 18/45 contains 18 percent oxygen, 45 percent helium, and the balance nitrogen.

Divers distinguish normoxic trimix, with enough oxygen to breathe safely at the surface, from hypoxic trimix such as 10/70, which does not contain enough oxygen for shallow depths and requires a travel gas for the descent. Beyond narcosis, helium lowers gas density, which reduces breathing effort and CO2 buildup at depth; a common guideline keeps density ideally below 5.2 g/L. The cost is real: helium is expensive, and it speeds up some tissue loading, so trimix dives are decompression dives planned with dedicated software and deco gases.

Trimix use requires technical training and, in most curricula, comes after a decompression procedures course. Entry level normoxic trimix ratings typically cover dives to around 60 m, while full trimix certification extends to hypoxic mixes and greater depths. Because helium costs keep climbing, many divers reserve trimix for dives where an air or nitrox bottom mix would be genuinely impaired, roughly beyond 40 m for most people.