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Trimix is a mix of breathable gases that is used in advanced or technical SCUBA diving. The mix consists of oxygen (up to 30%), helium (minimum 30%) and nitrogen (balance of the mix). The purpose of adding helium to the breathing mix is to reduce the percentages of nitrogen and oxygen below their ordinary atmospheric ratios. Lower nitrogen levels are required to reduce intoxication through nitrogen narcosis and other physiological effects of the gas at depth. Lower oxygen levels are even more critical as oxygen becomes toxic at depth possibly causing convulsions and death.

The exact ratio of gases in the mix is calculated to minimize narcosis and the risk of oxygen toxicity for the maximum depth of the dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum PPO2 (partial pressure of Oxygen, see Boyle's Law) of 1.0-1.2 atm and maximum EAD (Equivalent Air Depth) of 30 Meters (100 ft). For example, a mix for a 100 M (330 ft) depth might be 10% oxygen, 70% helium, 20% nitrogen.

Skills beyond those taught to basic SCUBA divers are required to safely dive trimix as the use of trimix usually assumes deeper or longer dives requiring decompression and possibly the use of several tanks on one dive containing various gas mixes to facilitate the decompression process. Training is offered by several organizations such as GUE [1] (http://www.gue.com), IANTD[2] (http://www.iantd.com) and ANDI [3] (http://www.andihq.com)).

Other versions Heliox is a blend of helium and oxygen without nitrogen. Is usually used for deeper dives (>>100 Meters) where even low levels of nitrogen can cause significant narcosis.

Normoxic trimix has an oxygen content equivalent to air, 20.9%.

See also Enriched Air Nitrox

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