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> I do not switch to air and then >nitrox to increase the inert gas gdiffeential, but as a delivery >vehilce fot he higer oxygen, the most improtant factor. It is >nice not to be accumulating more of the same inert gas, so >switching between helium and nitrogen is helpoful there. I think I begin to understand. Please check my reasoning and correct my misconceptions. The basic concept is to maximize the ppO2 at each deco stop, subject to the 1.4 ATA ppO2 hard limit, and keeping in mind that narcosis is not the greatest idea in the world. This requires "something" at 240', which would be a hypoxic mix at a shallower depth if it were 1.4 ATA ppO2 at 240', and bottom trimix is conveniently available. (You've been at 300' for a while, so you are still going to outgas some at 240'.) Air at 190' is 1.4 ppO2, close enough, and meanwhile the high ppN2/zero ppHe means that you WILL be outgassing He, at the possible price of high narcosis. As you go to shallower stops, you shift to progressively richer nitrox mixes, still observing the 1.4 ATA ppO2 constraint. You are still going to get longer and longer stops as you get shallower and shallower, just because the basic outgassing process is exponential in the ratio of partial pressures in and out of solution, and you necessarily have less differential pressure to play with at the shallower depths. Is this what is going on?
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