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>>dear rich, your write-up on O2 diffusing n2 is very enlightening. i have one question though. if i chose to deco on 50% O2, we know the MOD is 22 meters with a PO2 of 1.6. as i go shallower, and the PO2 decreases, do the O2 molecules continue to "push out" the N2 molecules in the tissues? if it does or does not, up to what depth does it diffuse the N2 from the tissues? secondly, since the po2 has decreased and with 50% O2 we still have 50% N2, does that mean we still absorb N2 in our tissues?<< Hi Joel, no problem with the direct mail. I'm going to send this to the list, also, so if I'm mistaken about something, I'll be corrected (and boy, will I be corrected!!!!) >>. as i go shallower, and the PO2 decreases, do the O2 molecules continue to "push out" the N2 molecules in the tissues?< Remember, the O2 'pushing' the nitrogen out is really just a figure of speech. The nitrogen 'wants' to come out due to diffusion (and the pressure gradiant, since we are on ascent, and 'decompressing' in the pressure-sense of the word). The nice thing about 100% O2 is that we are substituting a gas that has a _minimal_ propensity to bubble, because it is metabolized normally, while we are uptaking zero percentage of N2. >>if it does or does not, up to what depth does it diffuse the N2 from the tissues?<< Remember, dive tables / computers get you out of the water with a (hopefully) _safe_ tissue gradiant of N2. That is, you _still_ have a buttload of N2 in your tissues, and are still off-gassing for hours after the dive. It's just that the _gradiant_ between the tissue N2 pressure and atmospheric gas pressure is low enough that you are not bubbling up. Two things I want to bring up right now- one, is that this thread is starting to get rather involved with decompression theory. even though I may talk a good game, I do not feel completely confidant explaining deco theory, espescially with regard to controlling tissue compartments. I have a pretty good idea, mind you, it's just that when discussions like this start getting theoretical, the fistfights over deco models begin <g>, because this is NOT an exact science. The second is that bubbles occur after most dives. There _seems_ to be a threshold at which the bubbles 'cause' DCS, but that threshold is poorly pinpointed; ie, there is a theoretical, but poor, correlation between the two. >> if i chose to deco on 50% O2, we know the MOD is 22 meters with a PO2 of 1.6.<< A pO2 of 1.6 is the recommended limit due to the risk of CNS O2 toxicity. There is a fascinating thread going on at the same time as this one, where people are yelling at each other because of a poorly written post, where it seems as if a poster is advocating keeping the pO2 at 1.6 for the entire dive. At a pO2 of 1.6, you are relatively safe from CNS O2 toxicity, but you are 'burning' up your lungs from pulmonary O2 toxicity. >>secondly, since the po2 has decreased and with 50% O2 we still have 50% N2, does that mean we still absorb N2 in our tissues?<<<< Yes, which is why using mixes other than 100% for deco is less effective. Some would say rather silly. You are simply not getting the benefit from the increased oxygen content (and therefore decreased nitrogen). While it would be better to use a 50/50 mix than air at a certain level, the added benefit from 100% O2 is worth it. So why not use both? Why not go to 50/50 at a depth of 70 fsw (1.6 ata of O2), and then switch to 100% at 20 fsw? The answer is that the concensus of smart people says that you are exposing your lungs to high pressures of O2 with less of a benefit, since you still have 50% N2 in the mix. I know, this seems rather contradictory: on one hand, I say the only thing you have to do is lower the N2 percentage to reap benefits, and then the next paragraph, I'm saying you must have pure O2! The best explanation I can offer, is that if you consider risk vs. benefit, the ratio is most favorable when using 100% O2 to deco. Again, these are layman's explanations. if any of our esteemed readership thinks I'm a stroke for my terminology, keep it to your bloody selves <g>! OTOH, if I have an error in my mental model of how this stuff works, PLEASE bring it to my attention; knowledge is power. And thanks for asking this question, Joel, you forced me to articulate and think about things I have not had to mull over in quite awhile. They say you don't really understand something unless you can teach it <g>. Hopefully my explanation above isn't too confusing or poorly written. Rich L -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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