1) Leaving aside the deco issues, are there safety reasons for standardizing on EAN80 when compared to 100% O2? I understand that the standard pure O2 fill (without a booster) of an aluminum tank with a service pressure of 3000 PSI is about 2200, but then again, you need less gas volume in most profiles. Tank accidents and fires are rare, but does having 100% vs 80% make any difference?
2) Do any of you feel that there is a significant advantage of EAN80 in terms of oxygen injury (CNS clock, vascular perfusion, pulmonary injury)? Air breaks? Anything else...?
I was in the same boat when I started doing accelerated decompression. The difference between 80% and 100% is negligible, as you can verify by running through some schedules in your planning software. I do my last deco stops at 10', so I'm less than 1.6 on my ppO2 with 100%. So I don't see a safety issue here. As far as fires, I'm under the impression anything over 50% is dangerous. I also don't go to shops that don't have an O2 booster anymore. I fill my AL40 with 100% O2 and have enough for 2 dives. I don't think about it much after that. And some places want to charge you a gas blending fee if it's not something they bank or something they can just turn the valve on and ignore.
I don't see any significant advantage with using 80% over 100%. I've been over the CNS clock plenty but without issue. And my understanding is that NOAA developed it with a working diver in mind, not someone lightly finning at a deco stop.
As far as air breaks, my recollection when I looked up the studies a while back was that the testing related to pulmonary oxygen toxicity, not CNS oxtox. Pulmonary comes into play with long exposures similar to what patients experience in hyberbaric chamber treatments for ailments like diabetes. Long exposures (like over 10 hours) to high ppO2s damages your lungs and reduces your vital capacity, and so air breaks are introduced to stave off this effect. A number of these studies were done on animals breathing pure O2 for long periods (as much as 24 hours) with intervening time on air. These were also a ppO2s higher than 1.6. One of the studies look at rats and did ppO2s of 1.5, 2.0, and 4.0, for example.
There are physiologists on this board that can probably do a better job explaining it, or can correct me if I'm misremembering.
Not to put words in Steve Lewis' mouth, but here is a post on his blog about them:
https://decodoppler.wordpress.com/2...and-do-people-take-them-for-the-wrong-reason/
The GUE and UTD crowd adopted a 12 minutes on, 6 minutes off regimen, the ratio of which is related to the research I mentioned above. If you're doing multi-hour decos, it isn't going to hurt your lungs to give them a break every so often.
Edit:
I just found one of the studies from UHM 2004, Vol. 31, No. 2 - Mini-forum on oxygen toxicity and air breaks in HBO2 therapy (Hyperbaric O2). I think this is available on the rubicon project for download.
I've linked it here, so you can read it for yourself. My take is if you're doing the types of exposures you would get in a Hyperbaric Chamber, which some divers are doing, then take air breaks in a ratio less than 4:1 w/ the oxygen interval less than 60 minutes if you're at 2.0 ATA, which you shouldn't be unless you're doing in-water recompression.
Edit #2: so I just looked at some more of your posts and saw you're mainly looking to carry one deco gas
. I would just run the profile in whatever planner you have and see what gas will do the trick while only needing to carry an AL40 and give you the shortest deco time.
