Dave Sutton

Dude:

At 150 feet, 21% mix is *1.1* PP02. That's 0.1 LESS than a sort of normal 1.2 PP02 setpoint. Why in the world would you want your diluent to be LESS THAN HALF of your target setpoint? I mean, heck: Why don't you just dive *pure inert gas* as your diluent? Your 10% at 150 feet is doing you about as much good. Don't you WANT to be able to breathe that gas at the surface if you can?

Yes, a rebreather is a "gas mixing station on your back". What instructors apparently are failing to teach is that not only can you RAISE a low PP02 mix to a good mix by ADDING 02, you can also LOWER a "slightly" rich mix to a good one *in a time period that you can ignore for all practical purposes* by REMOVING 02. You RAISE PP02 with a push button or a solenoid vave. You REMOVE 02 by BREATHING for a few minutes. The issue is that the human animal wants INSTANT GRATIFICATION and is happy when he can raise the PP02 in a moment. He's unhappy when it takes a few minutes to lower it. He FEARS high PP02 *because he has not been trained that it's a soft limit* and wrongly fears it EQUALLY as much as he fears low PP02. To make it worse, he aparently fears high PP02 MORE SO than low PP02 gas because he carries 10% gas as diluent that will KILL HIM IN MOMENT near the surface to get past his fear of having a gas that "may" do something in about 4 *hours* on the bottom *maybe*. This is where he is WRONG. The only way he can meet his desire for instant gratification is to often dive a mix that will KILL HIM if breathed on it's own at the suface. But he then forgets that he can actually DIVE the same planned loop-gas by starting with a higher PP02 mix to begin with and "on a dive every now and then" end up with a mix that *for a FEW MINUTES* will be "A LITTLE HOT" in return for the safety of having good gas shallower.


Here's a practical example: If I dive 21% mix as diluent at 250 feet, I am on 1.8 *for less than a minute*. I am at 1.7 for *another minute or so*. I am at 1.6 *for another minute* (and did thousands of hours at 1.6 as a commercial diver so I am already within my "infinite time duration comfort zone"). I am on 1.5 *for another minute*. I am at 1.4 *IN ANOTHER MINUTE* and ONLY THEN does my solenoid begin to fire and maintain my setpoint PP02. If you run 1.2 it takes another TWO MINUTES at most before your loop gas is stabilized at setpoint. And this is at 250 feet! Dive shallower and the problem is of shorter duration and at about 180 feet or shallower it goes away. I know we all dream of diving the Britannic, but seriously: What fraction of your diving is done at 70 to 180 feet? My guess is the majority. For ALL of that, 21% dil is the way to go. My Doria mix? 21/50 trimix as offboard diluent, carried in a 40 cubic foot stage (plumbed to a BOV just in case too, I'm not a "pure" Alpinist, after all). 50/50 nitrox, as internal diluent (feeds my suit as it's normal job), and my 02. I have enough diluent to do a SCC bailout with EASE after loosing any one of my gas supplies (including the 02). I can "almost" do OC deco with this gas suppy if I blow off my deep stops.

NOBODY is diving *for any duration at all* at elevated PP02's using the method. It has zip to do with multi day diving or any of the other things you raise. We used to do an hour+ of *in water pure 02 decompression* EVERY DAY as commercial mixed gas divers. I just don't see the issue with a FEW MINUTES above 1.2 (or 1.4 or whatever) in return for having BETTER and SAFER gas available at all times.


Dave

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