Posted by Kendall Raine on March 23, 2001 at 09:08:18:
In Reply to: trimix or nitrox? posted by mdfixitman on March 23, 2001 at 07:31:40:
There are operational, cost and physiological aspects to consider.
Operationally, adding helium complicates the filling and gas management picture. With Nitrox, you can use a membrane to dial in the mix every time, or partial pressure fill where you only worry about how much O2 to add. 1-FO2=FN2. Since trimix blending is more complex than nitrox blending, the level of skill of the blender needs to be higher. This means fewer shops will invest in the capability to do it right. It's not particularly difficult per se, but most dive shops are pretty low tech.
Cost wise, helium is more expensive than oxygen.
Physiologically, helium is believed to have certain benefits over nitrogen at elevated partial pressure. It is non-narcotic at any depth. It's solubility in lipid tissue is much lower. It does not seem to create the same RBC rigidity problems at depth that N2 causes. Being much lighter than N2, He is easier to breathe and reduces the degree of CO2 buildup from work of breathing. There may be more. The degree to which these benefits really change the diving experience at depth less than, say, 100 fsw, is open to question. Is narcosis present in 60 fsw? Of course. Are most divers endangered by that narcosis at 60 fsw? I guess that depends on the diver and the conditions.
Are there applications for both? Yes. Is trimix better for all applications? That depends. Does trimix have drawbacks? Yes. Practically speaking, is trimix used on dives of 100 fsw or less? In the past, almost not at all, but this is changing as the myths are exploded and people become more comfortable with the physiological issues of helium. Will either nitrox or helium replace air for no D diving? They'll make headway, but probably not totally replace air.