Question Bailout gas configs for tech/deep chestmount rebreathers

[kensuf]

Yes. If rebreather still works, I just plug my next gas. If it fails during a BO, then it's a bigger problem.

So let's say you've got 21/35 and you're doing a 150' helitrox dive. If you lose that bottle of 21/35 you'd plug in a 50% bottle?

 

[Wibble]

Am curious about acceptable CNS and POT (pulmonary oxygen toxicity) limits for MOD3 dives.

What limits would you use for planning 100m/330ft, 125m/410ft and 150m/500ft dives?

 

[Whitrzac]

Am curious about acceptable CNS and POT (pulmonary oxygen toxicity) limits for MOD3 dives.

What limits would you use for planning 100m/330ft, 125m/410ft and 150m/500ft dives?

E: None of the above.
Its a known risk factor at that point.


Every 300ft+ dive I've done would run over the CNS limit if a bailout occurred. Prioritizing getting out of the water as fast as possible over lung stress.

 

[Whitrzac]

These threads are always fun considering one of the most well known instructors out there teaches mod3 as a 2 al80 bailout. Dives are all planned within the limits of BO limitations, but still odd.

 

[grantctobin]

These threads are always fun considering one of the most well known instructors out there teaches mod3 as a 2 al80 bailout. Dives are all planned within the limits of BO limitations, but still odd.

He’s also has a history of bringing open water divers into the Nest during those courses, but it is neither here nor there.

 

[inquis]

On DILout configs, where DIL and bailout are the same gas source, I think a better option would be to put the deep bailout on the redundant setup

That certainly solves the simultaneous loss of loop & DIL/BO reg problem, but it seems like there could be a volume issue if breathing rate is elevated after "just" a loop failure. There's obviously lots of DIL/initial BO, but that is quite inefficient for off-gassing, which probably increases later stages of deco beyond a couple of 80s. (At least it does for the 20 minute 330 ft example we have going.) Of course, this should be apparent during the dive planning.

@justinthedeeps solves this with team BO, but that is mandatory with his gases. (His Diver1 needs a SAC of 0.4 to do it with what they were carrying.) A team strategy where everyone could make it individually but as a team could be out even sooner seems like a better approach (if possible). (In fairness, Justin didn't specify the bottom time for his plan, but I'm assuming relevance to the example.)

It could also be solved with 18/45 in doubles. Getting onto this mix a few minutes after the initial BO takes the pressure off the later deco gas. You're still rather deep and breathing hard, so doubles gives plenty of gas. 80s of 50% and O2 get you home. (It's been more than 30 mins, so I'm assuming breathing is below 0.7 cfm/20 lpm by then.) This choice avoids having to get gas from anyone.

The claim was made early on (it may be stuck in the parent thread) that CMs aren't really suitable for deep, which was disputed. This is an example where it's not only suitable, it seems better in the more likely (single-failure) scenario. Yes, a teammate would be needed for a simultaneous double failure.

I don't wish to appear like I know the definitive answer here. I don't know what @DiveTucson puts in his doubles, and I don't even dive a CM CCR. The above is just my take on something feasible within the constraints of what he outlined and observations about the alternatives being discussed. Like most things, I think your own priorities will play a role in what is best for you.

 

[inquis]

Am curious about acceptable CNS and POT (pulmonary oxygen toxicity) limits for MOD3 dives.

You realize we just derailed that other thread, right?

 

[mr_v]

Granted, there could be other reasons why you want all that double share of extra mid-depth gas, like if you think it's possible that you could get delayed from leaving the top of a wreck, or your cave exit is at a certain depth, etc.

From my perspective, that is it. What are the odds of bailing out on a deep dive and then saying "oh, it is a bummer that I had all that extra gas?" However, I am not sure if I'd bring this setup for cave diving - the doubles and the CM unit add to the profile height.

 

[kensuf]

You realize we just derailed that other thread, right?

Thread derailed? It's the scubaboard way...

So my thoughts on all of this stuff that's cropped up.

In the ocean, I don't get carried away with deco. 3.5 hours is the max runtime I'm willing to do and that's gotta be a special dive. You can do a lot of deco with 3x AL80s.

In a cave you can drop lots of deco bottles (and heat and food). I'm happy to do ~6+ hour runs in a cave, my longest was only 8.5 hours, but it was enough for me.

In terms of team bailout, I won't do team bailout in SE Florida because I consider team separation in the gulf stream to be a realistic possibility, even with very cohesive teams. I've been working on MOD1 with a guy that spends his summers as a dive guide in the WPB/Pompano area and we discussed team bailout strategies and I asked him "how often do you see teams get separated down there" and his answer was "the question should be when don't they get separated!" - it's just a fact of life down there, so I wouldn't want my deco gas on a guy that may or may not be with me.

I will do team bailout for deco gases in a place that I am confident we will not get split up.

Oxygen exposure: In terms of CNS load, even with bailing out to OC on a 3.5 hour run you shouldn't be exceeding 100%. If you are, gas breaks on OC help for managing it. In terms of pulmonary, I honestly don't pay attention to it unless I'm on a multi-day diving binge. Single day exposure is 850 OTU's - that's a big dive, if you hit that number you're not diving the next day (and if you are, HOLY CRAP MAN!). Multi-day excursions you eventually get limited to 300 OTU's, that's still around 4 hours underwater continuously at 1.3PO2. If you're doing 300 OTUs a day for 8-9 days straight, take a day off in the middle.

 

[LFMarm]

So let's say you've got 21/35 and you're doing a 150' helitrox dive. If you lose that bottle of 21/35 you'd plug in a 50% bottle?

Was referring to deeper dives where I have 10/70 and 21/35.

If I am doing a 50m dive on 21/35 and 50% and I lose 21/35, 2 options (a) if rebreather is still working, I go up and start the deco, (b) if I am on BO, I swim up to 21m and go on 50% OC (first stop would be a 18m). Definitely not ideal but also unlikely to have 2 major failures concurrenlty.