Breathing 80% O2 while waiting for buddy to finish deco

[delta-v]

Although I refer to 80% specifically here, I've found myself asking this question with 50% too. Here are two common scenarios for me:

1. Shallow repetitive dives (30m/100ft) 4-6 dives over 2 days
2. Deep air dives (55m/180ft)

Scenario #1
Most available buddies in this scenario are single gas divers, so let's say I've a back mix of 32% plus a slung S40 with 50%.
I switch to 50% during ascent from 21m (70ft); buddy stays on 32%.
I finish deco well ahead of buddy, and am essentially killing time at the final 6m stop.

Question: Do I stay on 50% or switch to backgas?

Scenario #2
Repetitive wreck dives (minimum 2hr SI) to 55m on air and 30% for a max deco of 15-20 minutes. Most of the dive spent in the 40m range.
Everyone switches to 30% at 35m during ascent.
I also carry a slung S40 with 80%; switch at 10m.
I finish deco in roughly half the time.
I take a 1-min air break between the 30% > 80% switch, and then a 5-min air break if I'm on 80% for more than 12 minutes.

Question: Do I stay on 80% after my deco finishes, or switch to 30% while I wait for the others?

This question is prompted by the varied info I've found regarding the extended use of rich oxygen mixes. Up to the point that the richer mix is accelerating off-gassing there is no ambiguity, but once the deco is cleared, is the additional time on rich O2 a benefit (additional safety margin) or a liability (damaging effects of O2, free radicals etc.)?

Any thoughts?

 

[PfcAJ]

Why does your buddy have different gases than you?

 

[MXGratefulDiver]

Given the relatively short deco times you're talking about, I'd say it doesn't matter. Why take an air break after 12 minutes? On EAN80, I'd say anything less than 25-30 minutes isn't a concern.

I think PfcAJ's question is more relevent ... if you're carrying a deco bottle, your dive buddy should be too ... and of the same mix. Otherwise, dive planning just gets too confusing.

... Bob (Grateful Diver)

 

[MaxBottomtime]

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

 

[MXGratefulDiver]

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

... Bob (Grateful Diver)

 

[mala]

Why does your buddy have different gases than you?

good question.

2 x 55m air dives on the same day cant be too much fun either

 

[MaxBottomtime]

I should have used a smiley.

 

[delta-v]

Thanks for the quick replies.

Our core mixes and supplies are matched to the dive profile and individual/projected RMVs, with RO3rds worked in. If the dive calls for a richer deco mix, then everyone carries additional bottles. This is standard.

I carry the S40 as a personal preference. It extends my gas supply should I (or someone else need it), it allows me to off-gas quicker which means I can exit quicker if the situation demands it, it's additional O2 on board if anyone needs it on the surface.

I could leave it at home. But why do that?

For my trimix dives we match exactly with no extra add-ons. But with these quasi-recreational dives I have less choice about who I dive with, and not everyone wants to dive with multiple mixes.

But getting back to the original question, what are the specific concerns regarding free radicals etc. and the use of richer O2 mixes beyond necessary deco times?

You advise that up to 30mins @ EAN80 isn't a concern. That's great to know, but does anyone have insight into how the richer mix is affecting cellular physiology, specifically lung lining etc?

I guess the question remains: All other things being equal, is it safer to stay on the richer O2 after deco concludes, or is it better to switch to the leanest normoxic mix if you're waiting for someone else.

Things don't always go as planned. Yesterday for example, we had an OC and two CCRs as a threesome down to 85m. My OC threesome (55m) caught up with them on the deco line. One of the CCRs had an issue which required the other CCR to immediately provide assistance. I was closest to his OC buddy and he signaled me to stay with him. The CCRs left the line and went up to the boat - they had already finished their deco anyway. I had finished mine long ago since I was on EAN80, but when my buddies finished theirs I signaled them to exit while I stayed with the 85m OC (I had the most gas supply at that point). I was there for almost 20 minutes before the CCR came back and re-joined his buddy. I had switched to my 30% for the duration.

Sadly, the 85m OC had a touch of DCI (extremities only, no neuro fortunately) and did some chamber time last night. All's well now tho.

But the point is that having the S40 with EAN80 not only made me deco-free far ahead of everyone else (thus with the most options to respond to a situation), it also meant I had silly amounts of gas left over to deal with a totally unexpected eventuality.

---------- Post Merged at 06:26 PM ---------- Previous Post was at 06:14 PM ----------

good question.

2 x 55m air dives on the same day cant be too much fun either

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.

Incidentally, it was the OC trimix that got the DCI hit yesterday - not one of the 7 air divers had even a twinge : (

 

[AfterDark]

IMO as long as you're watching your total O2 exposure it's all good. I little extra O2 for a few minutes longer can't be bad.

 

[delta-v]

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

Thanks, interesting read which I admit I've not come across before.

I agree with the logic and arguments presented. However, I consider EAN80 to be less of a fire hazard than pure O2 - esp. for use in the scenarios I've outlined (i.e. where I'm using it in addition to a nominated deco plan).

Obviously, for deeper dives and longer hangs it has to be pure O2 - but even here, I've comes across some divers opposed to its use for a number of reasons (none however that the GI article refers to).

A few years ago we had a boat burn down due to a CCR fire. No casualties fortunately, but everything else apart from the boat was uninsured. Losses were in $000s - dive gear and personal possessions either incinerated or sank or both. It was a tek trip, so you can imagine the pricey equipment on board.

Fire started at the pure O2 valve on the CCR - both bottle and valve had recently been O2-serviced...

Unless absolutely necessary, I give pure O2 a wide berth. I do partial pressure blending so can't avoid it altogether, but I try to limit its use wherever possible : )