Breathing 80% O2 while waiting for buddy to finish deco

[FoxHound]

Oh goodness ... GI reruns should be banned from everywhere on ScubaBoard except the DIR forum ...

... Bob (Grateful Diver)

No kidding, I just wasted 5 min of my life reading that

 

[vjanelle]

Ah, but O2 fires can happen at 80%(or less) too, so..? What steps are you doing now to minimize the adiabatic compression?

 

[ffdiver]

The Bakers Dozen - 13 reasons why we do not use 80/20 by George Irvine

People really bought into his whole dogma thing?

 

[Adobo]

People really bought into his whole dogma thing?

Of all the content within this thread, the thing that caught your eye is George's baker's dozen which was penned probably well over a decade ago?

I see plenty of other material that quite frankly leaves me scratching my head. And this stuff was written just yesterday.

 

[Doppler]

What's slightly disturbing about the original question is that someone who is competent enough to conduct staged decompression dives on trimix to 55 metres, is asking it.

Is the use of NOAA guidelines for CNS management fallen out of favor for some reason. Do divers not understand the limits prescribed for single exposure and multiple dives during a 24-hour period.

Seriously sub-optimal dive procedures, ignorance regarding the pros and cons of breathing high PPO2, I see red flags all over this. The OP simply cannot be certified to do the dives he says he is doing.

Why are you taking air breaks while you are at 6 metres? The oxygen partial pressure at 1.6 bar ambient on EAN50 is insignificant and on EAN80 (or 80/20 heliox) is 1.3 bar for which an air break is not best practice.

 

[GrimSleeper]

Time @ 55m was about 7 to 10 mins only. Most of the dive was at 40m as my original post explains.
I dive too frequently to afford trimix every weekend, and so I must reserve helium for dives beyond 60m only : (
I'm also saving up for a CCR but it won't happen this year : (

I'll add, I'm diving warm-water (yesterday was 40C at the surface!, 24C at the bottom), with zero current and no task-loading (planned) whatsoever. Also consider that my max TTS for D1 was 16mins and for D2 13mins - this should indicate the safety built into the dive profile. Intelligently planned deep air dives with proper surface support are a good way to get the best of both worlds.

As Doppler says, so many red flags regarding procedures... Why are you all switching to 30% at 35m (1.35atm ppO2, an intelligent choice for the depth, but 50% at 21m would give you a higher ppO2 at a depth where you're off-gassing fast tissues, the inspired-dissolved inert gas pp gradient is working for you but ambient pressure is still relatively high, and it makes sense to extend the stop long enough to get a couple of good circulatory system passes, rather than switching to a lower ppO2 gas at a depth where you are probably haven't even started off-gassing, so don't want to hang around)? Why, assuming the repetitive dives are on the same wreck, which it sounds like, are you making two short bounces rather than one medium-length dive with a moderate deco obligation? Twice the number of ascents, in my opinion, offers twice as many opportunities to put yourself in the chamber.

Just to be clear, I'm not arguing about the use of air at 55m. I do it regularly, since I dive in waters that are 29C-30C all the way down and we regularly have 30m visibility, and He is near enough impossible (both logistically and financially) out here at the moment. But there's no way I'd do it as you describe. If I want to spend time at 55m, I'll do a single 25-minute bottom time, deco on 50 and O2, run-time around 73 minutes, home for lunch and done for the day. I don't use O2 because I have any DIR pretensions, but because (a) it gives me the maximum gradient between inspired and dissolved inert gas at the single depth I'll spend most time at during deco, and (b) blending 80% is a pain in the ass, whereas O2 is simply a matter of connecting a transfill whip and letting it flow. And 80% is just as likely to go 'boom' as pure O2 - potentially even more so, because you're having to blend it.

The way you describe your dives sounds like you're flying by computer rather than planning them in advance, although I could be wrong. To answer your original question, on the odd occasions when I've dived with someone who wants to deco out on back-gas, I've run the options on planning software (my preference is V-planner, but it doesn't really matter what you use): their deco requirement, my deco requirement, then their deco requirement as a series of levels using the gasses I will be using for deco. That way we can at least be running the same ascent, but I know what I'm doing to myself in terms of CNS and OTU exposure and whether I need to switch to back-gas (beyond normal ppO2 breaks) while I wait for them.

Oh, and where on Earth are you experiencing 40C surface temperatures (that's 104F for all you out there in Fahrenheit-land!) and 24C at 55m? That's an insane temperature gradient.