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Our normal breathing gas, air, conists of oxygen 21% and nitrogen 79%. The primary purpose of adding helium to the breathing mix is to reduce the percentage of nitrogen and oxygen to below that of air to allow the mix to breathed safely at greater depth. Lower nitrogen level is required to reduce intoxication, referred to as nitrogen narcosis and other physiological effects of the gas at depth. Lowering oxygen content is necessary as oxygen becomes toxic as pressures from depth increase, 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], IANTD[1], TDI/SDI [1] and ANDI [1]).