With a mixture of three gases it is possible to create mixes suitable for different depths or purposes by adjusting the proportions of each gas.
The lower density of helium reduces breathing resistance at depth.
Helium off-gasses rapidly and it does not enter slow tissues as readily as nitrogen.
Helium conducts heat 5 times faster than air; often helium breathing divers carry separate gas supplies to inflate drysuits.
Helium on-gasses rapidly and usually requires deeper decompression stops than a similar decompression dive using air.
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 43 meters (140 feet).
In open-circuit scuba, two classes of trimix are commonly used: "normoxic" trimix - with a minimum PO2 at the surface of 0.18 and "hypoxic" trimix - with a PO2 less than 0.18 at the surface. A Normoxic mix, such as "19/30", is used in the 30 meters (100 feet) to 60 meters (200 feet) depth range and 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 fully closed circuit rebreathers that use trimix diluents, the mix can 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.
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).
Source: Trimix and heliox diving. Retrieved on 2008-10-07..
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