PfcAJ
Contributor
Care to enlighten us all with your omniscient understanding of the relationship between inspired PO2 and time of exposure? Because until you do, I’m a bit confused as to your insistence that an arbitrary PO2 limit is really important for avoiding a CNS hit, but that length of exposure to PO2(s) is irrelevant to avoiding the same thing.
I’m perfectly happy to answer for you: you don’t understand how it works, but so far ignoring the clock has worked for the size of the dives you’re doing, so it’s not a hard and fast rule. No disagreement from me, I’ve done dives where my CNS ended in the 400% range. You won’t catch me doing that kind of stuff back to back to back, though, and I make no suggestion that just because I’ve gotten away with it, the whole CNS clock concept is “bunk.”
PO2 matters, but it's obviously not everything - or I'd be dead from a lot of air dives in the 240' range. Length of exposure matters, but it's obviously not everything, or a whole bunch of us would be dead from just doing our deco. But the idea that CNS is somehow completely useless is horse.
I'll indulge you for one reply more, since I should have maybe used smaller words in my earlier post. You submit that if 80% were "better" than 100% it'd be used in chambers, and then seem to think you've spiked the football by pointing out that OMG it isn't! However, you're missing (for your sake, I hope deliberately) the fact that in water deco and a chamber are two very different environments.
In the chamber, you’re likely warmer, certainly not submerged, and a whole lot less likely to have elevated CO2 levels due to breathing through a regulator – three things that, shockingly, have all been shown to increase human tolerance to elevated PO2s. And of course, you won’t
Now, pay attention, because this next bit involves you…where it really comes in handy is when you want to use a big word like “better” without sounding like you may have, on occasion, lost points otherwise awarded for not drooling on the paper. This is because determining whether one option (say, 80% O2) is better than another option (say, 100% O2) really requires that one look at all the possible risks and benefits involved in a given context. So, while the marginal deco efficiency of 100% O2 might make it better than 80% in a chamber, where CNS hits are much less likely and much less of a problem, the same marginal deco efficiency difference might be quite irrelevant in the water, where CNS hits are significantly more likely and generally result in death, and you’re not trying to treat existing bubbles to boot.
Did I clear that up for you, or would you like to try being glib once more?
I think I figured it out. We're arguing 2 different things. I will agree that multiple days of high ppo2 diving is bad. I think we agree on that. But I suggest that the "CNS Clock" is not a valid way of tracking this.
You state that you've done a dive with a CNS of "400%". That's 4 time the limit. And yet, here you are. What's the deal? I too have done a few dives with high CNS %'s. In fact, my last dive's CNS exposure was over 5,000% according to deco planner. That's not a typo. Five THOUSAND percent. That's 50 times the 'limit'. So once again, wtf is up with these limits if they don't mean anything?
The little CNS % thing is worthless. Its invalid on the low end (you can tox without it exceeding 100%) and invalid on the high end (obviously because we've both went way over the 100%). The idea of a CNS% "half time" is arbitrary with no data to support it, and a 24hr wash out is equally arbitrary. I CHALLENGE you to find some data to support these ideas. We know that o2 can get you. NOAA needed some sort of a limit to cya, so they came up with this clock thing. That doesn't make it real.
The only proven way to deal with o2 is to limit the po2 and do gas breaks. Ppo2s of 1.0-1.3 on the bottom are pretty proven, as is limiting deco ppo2 to 1.6 when resting, and doing gas breaks to the lowest ppo2 gas you have available at intervals (we can debate the intervals somewhere else) during the 1.6 stops and before switching to a high ppo2 gas (at 30ft before the o2 switch, for instance), and keep your ppo2s even lower for multi day diving. That's it. You CAN'T track o2 exposure in a meaningful way currently. It sucks, but its where we are in diving knowledge. You can choose to follow the CNS% thing, and its pretty safe generally. But its far from some sort of a gold standard.
"People don't think the universe be like it is, but it do"
