Trimix Diving. Trimix is a breathing gas, consisting of oxygen, helium and nitrogen, and is often used during the deep phase of dives carried out using Technical diving techniques. Our normal breathing gas, air, consists of about 21% oxygen, 78% nitrogen, and 1% of other gases. 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 gas mix to be breathed safely at greater depth. A lower proportion of nitrogen is required to reduce the intoxication that is referred to as nitrogen narcosis and other physiological effects of the gas at depth. Lowering the oxygen content is necessary because oxygen has a maximum operating depth before it becomes toxic as pressure, due to depth, increases. Oxygen toxicity can cause convulsions, which if they occur underwater can lead to drowning. When breathing helium gas mixtures at depths below 130 meters / 429 feet, High Pressure Nervous Syndrome is a danger. 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 partial pressure of oxygen (PPO2Boyle's Law) of 1.0-1.2 bar and maximum equivalent air depth of 30 meters (100 ft). For example, a mix suitable for a 100 meters (330 ft) dive might be 10% oxygen, 70% helium, 20% nitrogen - this mix is named "trimix 10/70". At 100 meters / 330 feet, this has a PPO2 of 1.1 bar and an equivalent air depth of 18 meters /60 feet. In open-circuit scuba, two types of trimix are commonly used: "normoxic" trimix is used in the 30 meters / 100 feet to 60 meters / 200 feet depth range and "hypoxic" trimix is used for deeper diving, as a "bottom" gas only. Also, in rebreathers that use trimix diluents, the mix will be "hyperoxic" in shallow water. See breathing gas for more information on the composition and design of gas blends. PADI 5 Star National Geographic Instructor Development Center. 49 Thaweewong Road, Patong Beach, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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