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On Mon, 4 Nov 1996, Richard Pyle wrote: > > > > 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. > So you carry another bottle of nitrox to deco with in order flush the loop. > > 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. > How does the use of the trimix instead of the heliox effect the ppN2. It seems, it isn't constant anymore. Another tangent! A while back you mentioned getting the jitters in the upper 300's and was unsure why. You said it might be some sort of proto-HPNS, but then continued to say it was kinda shallow for this. Two things: 1) Do you think the constant ppN2 may have an effect on that. If N2 helps in reducing the symptoms of HPNS, then a higher ppN2 might help, but if you lost that N2 in the loop, then symptoms might start to show. 2) I saw in the navy manual the other day, that HPNS starts to show signs of symptoms around 400' to 500'. So, considering the varibility of human physiology, why couldn't this start to show up shallower in certain individuals or conditions? How much data does the navy have documented at these depth ranges with these kinda gas mixtures? I bet they don't have much with constant ppN2, but a lot with other gas mixes. Any navy guys out there have any comments? > I'd be happy to elaborate further, but Cara just woke up from her nap, so > I gotta run... > Please elaborate as much as possible? > > Rich > Aloha Tony
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