Not to get too in depth into the tri-mix world since I'm not likely to ever need/want it, but could someone give me a high level description of the reasons behind the choices for gas? Eg "Helium reduces Ox-Tox past 150 feet, but also has some narcotic qualities that are offset by N %" or something like that...
As you indicate, oxygen toxicity is a serious danger as oxygen partial pressure increases, so the deeper you go, the less O2 you want in the mix. Nitrogen is a factor in both narcosis and in DCS, so the deeper you go, the less nitrogen you want. Helium is a good alternative to both gases for several reasons, so divers add more and more helium as they go deeper and deeper. Unfortunately, too much helium is implicated in High Pressure Nervous Syndrome (HPNS), which explains Sheck's rationale in using some nitrogen in the mix rather than going to a pure mixture of helium and oxygen.
---------- Post added March 18th, 2014 at 02:01 PM ----------
A little more information for those not familiar with trimix....
Sheck and Bowden were diving "heliair," a concept Sheck invented and which has been called "the poor man's trimix." That unfortunate nickname suggests that there is something wrong with it, but there really isn't. It is a cheaper way to make triimix than the full way of doing it, but it is an acceptable trimix blend. The thinking behind it is the same as the thinking behind the trimix blends commonly called standard gases.
To make trimix the full blown way, you first put in the amount of helium you want, and then you do a mathematical calculation to figure out how much oxygen to add. Then you top it off with air. The math involved is actually figuring out what blend of nitrox to add to the mix and then partial pressure blending that nitrox mix on top of the helium.
With heliair, you put in the amount of nitrogen you want and then top it off with air. You have essentially decided to use a nitrox percentage of 21 at all times. This saves a lot of math, and it eliminates the need to do an additional blending step requiring O2 tanks. Notice that "tanks" is plural. Because of the pressure differences in the tanks doing the filling and the tanks being filled, putting oxygen on top of helium is problematic if you are in the field and do not have an expensive oxygen booster with you. With high helium fills, you will need a whole pile of such tanks, with a whole lot of wasted oxygen.
The standard trimix blends (like 21/35) are actually done in much the same fashion. They became popular in south Florida, where lots of shops banked 32% nitrox. The standard trimix blends are helium topped off with EANx 32. Again, less math and no need for additional oxygen, as long as you have premixed nitrox 32 ready to go.