An Introduction Trimix

Where air contains primarily oxygen and nitrogen Trimix is a mixed gas containing oxygen, helium and nitrogen. Helium an odorless, tasteless gas, has little narcotic effect when breathed at elevated pressures. As such, by replacing a percentage of the nitrogen with helium in the diver's breathing gas narcosis is greatly reduced on deep dives. 

Of course narcosis isn't the only concern for deep divers. As the partial pressure of inspired oxygen increases with depth the diver is faced with the risk of Central Nervous System (CNS) oxygen toxicity. Because of this factor Trimix typically contains a percentage of oxygen less than found in atmospheric air.
For most technical sport diving applications, Trimix will contain 16 percent oxygen or higher. Applying a 1.4 atm maximum partial pressure exposure limit to such a mix the maximum operating depth for a trimix containing 16% oxygen would be around 255 fsw.

On some occasions sport divers involved in scientific research or exploration may use mixed gases containing less than 16% oxygen. When breathing mixes with less than 16% oxygen divers can suffer the affects of anoxia while swimming at the surface and at shallower depths. To avoid these problems a small tank of "travel gas" containing a breathing gas with a higher oxygen content is carried.
Helium is a very small molecule in comparison to oxygen or nitrogen molecules. Because of this it moves in and out of tissue very quickly when used as a diving gas. This factor, combined with concerns about oxygen toxicity and the other risks associated with deep diving help to emphasize the need for specialize training before attempting to execute deep dives.
Specialized training for Trimix diving is offered by a variety of training agencies including NAUI and IANTD. Prerequisites for these courses usually include some substantial diving experience, 200 to 250 logged dives, and previous training at the technical diving level. You can learn more about Trimix in this site's "Training" section, by checking for articles in our "Library" section and by visiting some of the links provided to other sites.