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> > Now let's take the reverse. Suppose a diver starts at 20 feet with a > > certain PCO2 in the loop, and then descends to 200 feet. The volume of the > > loop is held constant by the addition of diluent and O2. Unless there is > > an appreciable amount of CO2 in the diluent or O2 supply cylinders, then > > the PCO2 will NOT increase on the descent, because the "n" will not > > increase (where would the addition of CO2 molecules come from?). Ya with me? > > > Rich, > > Correct me if I'm wrong, but this is the same principle that > allows you to dive with a constant ppN2, right? Yes. > My question is a tangent, but once you ascend you lose most of > those N2 molecles,right. So is your decompression mostly heliox and how > does that affect your deco profile, if at all? Alright, for the benefit of anyone else following this, let me explain the constant PN2 stuff. I start my descent with a setpoint of about 1.4 and air as diluent. When I get to about 100 feet or so, I switch the diluent feed to heliox and keep going. This "locks in" a certain number of N2 molecules in the loop, which means the PN2 stays relatively constant regardless of how deep I go (yes, yes, I know some of the N2 is lost by absorption into my blood, and some more is lost everytime I clear my mask, but this is pretty-much trivial from a decomprssion standpoint, and when it's not trivial - I handle it in a different way - see below). When I ascend, both N2 and He are vented from the loop, while the solenoid does what it can to keep the PO2 up. When I get to about 130 feet of so, I flush the loop with air, so the helium is all gone, then finish the deco on nitrox/oxygen (yes I know that some helium is trickling out of my body into the loop on decompression, but given the extremely low solubility of helium in blood, this is a very small volume). The computer, of course, is keeping trak of all this. > Also, do you lose most of the N2 on dives where you have to go up > and down(just 20" or 30') or do you avoid that kind of profile? Excellent question. That's where the problem comes in - sawtooth profiles. If I have a fixed PN2 in the loop at 300 feet, then ascend to 200 feet, a good portion of the N2 is vented. If I then return to 300 feet, only heliox is added, so the PN2 goes down. I used to compensate for this by manually adding N2 to the loop and gauging the PN2 by the level of narcosis. But I decided this wasn't scientific enough, so i did some calculations, and I now use a trimix with about 5-10% N2 instead of heliox. I'd be happy to elaborate further, but Cara just woke up from her nap, so I gotta run... Rich
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