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Additional recommended knowledge
The main reason for adding helium to the breathing mix is to reduce the proportions of nitrogen and oxygen, below those of air, to allow the gas mix to be breathed safely on deep dives. A lower proportion of nitrogen is required to reduce nitrogen narcosis and other physiological effects of the gas at depth. Lowering the oxygen content increases the depth and duration of the dive before which oxygen toxicity becomes a limiting factor. Also, the nitrogen in trimix can prevent High Pressure Nervous Syndrome, a problem when breathing heliox at depths below 130 meters (429 feet).
Conventionally, the mix is named by its oxygen percentage, helium percentage and optionally the balance percentage, nitrogen. For example, a mix named "trimix 10/70" consisting of 10% oxygen, 70% helium, 20% nitrogen is suitable for a 100 meters (330 feet) dive.
The ratio of gases in a particular mix is chosen to give a safe maximum operating depth and comfortable equivalent air depth for the planned dive. Safe limits for mix of gases in trimix are generally accepted to be a maximum partial pressure of oxygen (ppO2 - see Dalton's law) of 1.0-1.6 bar and maximum equivalent air depth of 30 to 45 meters (100 to 150 feet). At 100 meters (330 feet), "12/52" has a PPO2 of 1.3 bar and an equivalent air depth of 40 meters (132 feet).
In open-circuit scuba, two classes of trimix are commonly used: "normoxic" trimix - with a minimum PO2 at the surface of 0.18. A Normoxic mix, such as "19/30", is used in the 30 meters (100 feet) to 60 meters (200 feet) depth range and: "hypoxic" trimix - with a PO2 less than 0.18 at the surface. A hypoxic mix, such as "10/50", is used for deeper diving, as a "bottom" gas only and cannot safely be breathed at shallow depths where the ppO2 is less than 0.18 bar. In rebreathers that use trimix diluents, the mix will be "hyperoxic" in shallow water because the rebreather automatically adds oxygen to maintain a specific ppO2. See breathing gas for more information on the composition and choice of gas blends.
Gas blending of trimix involves decanting oxygen and helium into the diving cylinder and then topping up the mix with air from a diving air compressor. To ensure an accurate mix, after each helium and oxygen transfer, the mix is allowed to cool, its pressure is measured and further gas is decanted until the correct pressure is achieved. This process often takes hours and is sometimes spread over days at busy blending stations. A second method called ' continuous blending' is now gaining favor. Oxygen, helium and air are blended on the intake side of a compressor. The oxygen and helium are fed into the air stream using flow meters, so as to achieve the rough mix. The low pressure air is analyzed for oxygen content and the oxygen (and helium) flows adjusted accordingly. On the high pressure side of the compressor a regulator is used to reduce pressure and the trimix is metered through an analyzer (preferably helium and oxygen) so that the fine adjustment to the intake gas flows can be made. The benefit of such a system is that the helium delivery tank pressure need not be as high as that used in the partial pressure method of blending and residual gas can be 'topped up' to best mix after the dive. Drawbacks may be that the increased compressibility of helium results in the compressor over-heating (especially in tropical climates)and that the hot trimix entering the analyzer on the high pressure side can affect the reliability of the analysis. DIY versions of the continuous blend units can be made for as little as $200 (excluding analyzers).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Trimix". A list of authors is available in Wikipedia.|