Trimix (breathing Gas) - Blending

Blending

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 mixture 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 of a sample flow and the trimix is analyzed (preferably for both 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. This is important mainly because of the high cost of helium.

Drawbacks may be that the high heat of compression of helium results in the compressor overheating (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).