Question Bailout gas configs for tech/deep chestmount rebreathers

[DiveTucson]

Great to hear ways that chestmount tech CCR is developing.

So the doubles comes into it mostly because most of us got put through diving courses with manifolded doubles. It's "tech." I see that. Doubles are everywhere! It's an obvious pairing for anyone coming thru from OC tech.

Has anyone here ever actually needed that manifold? It is vestigial for 99% of divers. You will only ever touch it during a 'valve drill.' Having more gas might be good, but what gas?

For your config as above, what do you put in those doubles? Presumably deep mix breatheable at the bottom of your dive? As the GUE/DIR CCRs do?

Or else, when would you actually need doubles of a travel gas? Maybe if you are in a specific kind of cave with a transit at that specific MOD?

Re: gas switching, PO2s, team, profiles etc, this is protocol. You decide on a plan and protocol and you practice it...

I bring up these questions for thought, since the approach for gases on chestmount CCRs seems like relatively fresh ground.

[Bring on the 'sadness' reactions, fellas ]

All stuff that is covered in courses and way to much detail and time for me to address.

Maybe someone else will jump in.

 

[justinthedeeps]

All stuff that is covered in courses and way to much detail and time for me to address.

Maybe someone else will jump in.

Fair enough....

I think as @broncobowsher points out, your earlier post here begs a "bailout gas configs for tech/deep chestmount rebreathers" subthread--could be a hot topic this year and beyond!

 

[inquis]

For your config as above, what do you put in those doubles?

It's an intermediate mix, getting off the diluent gas with the extremely inefficient OC PO2 to shorten the deco. There are also redundancy and elevated breathing rate considerations.

It's a really long way from 330 ft to 70 ft.

FWIW, I don't see the use of doubles as a holdover from traditional tech. It's simply an efficient use of limited real estate.

 

[flymolo]

Fair enough....

I think as @broncobowsher points out, your earlier post here begs a "bailout gas configs for tech/deep chestmount rebreathers" subthread--could be a hot topic this year and beyond!

i would imagine it's a problem that's been solved quite a few times already

 

[justinthedeeps]

It's an intermediate mix, getting off the diluent gas with the extremely inefficient OC PO2 to shorten the deco. There are also redundancy and elevated breathing rate considerations.

It's a really long way from 330 ft to 70 ft.

FWIW, I don't see the use of doubles as a holdover from traditional tech. It's simply an efficient use of limited real estate.

Thanks for jumping in to discuss

I am aware of the intermediate gas, I have switched to it myself on ascent from 100 metres during bailout practice, as soon as its ppO2/MOD was suitable.

Never in our calculations did we need a doubles of this gas--not even at 40 litres per minute SAC/SCR--but it was a simple open water vertical ascent.

Optimal utilization of that now available space on the back seems precisely the matter when considering chestmount deep diving.

Why are we assuming that only a twinset of travel gas is the right way to do that? That seems to be the assumption so far in this thread.

You can't use that gas during your bottom phase because of ppO2. But you will still keep it lean, so that you can switch to it sooner. So it's a poor gas for deco. How long were you planning to breathe that travel gas?

It is some kind of worst-case planning, like you lost your deco gas(es) and need to surface on your travel gas alone?

 

[justinthedeeps]

Mods(?) maybe please move us to a separate "deep bailouts" thread ?

On a recent ~100m dive, we had the following bailout gases:
[diver '1' carried:] Al80 8/68, Al80 32/30, Al80 79/00
[+ team bailouts available:] Al80 14/55, 50/00

These being standalone cylinders, rearrangement would be easy, for example if I preferred to carry the 14/55 on my person, or if we need to donate.

This was with standard backmounted CCRs.

Presuming that is an acceptable selection of gases, how would we have distributed these gases if we had been diving chestmount CCRs?

With our backs now free, but our chest D rings cluttered, what goes on our backs?

The deep bailout (8/68) sounds like an obvious candidate. I am not handing that away to anyone. Worst case, I can breathe that all the way up to a 33 or 20 metre deco stop. Now I have one cylinder on my back. The rest could probably get variously staged clipped. -OR- I could consider getting another one of those cylinders onto my back. The simplest candidate would be the travel gas.

So I gather that the gas plan looks different under the chestmount + twinset config. What mixes are they?

 

[inquis]

Never in our calculations did we need a doubles of this gas--not even at 40 litres per minute SAC/SCR--but it was a simple open water vertical ascent.

The scenario given was one that required 80s of 50% & 100%. If you have a bottom time requiring that (e.g. 20 min @ 330 ft), I think you'll see why doubles of intermediate mix are attractive.

 

[justinthedeeps]

The scenario given was one that required 80s of 50% & 100%. If you have a bottom time requiring that (e.g. 20 min @ 330 ft), I think you'll see why doubles of intermediate mix are attractive.

I am still not sure I follow... What is the ascent rate on that intermediate gas, what is the mix (O2%/He%), and from what starting to ending depths? Why is the first deco gas at 20 metres (50/00), when the first deco stops are much deeper than that?

Quick run through the Subsurface planner here. Leaving the bottom at ~20 mins on bailout, the single S80 of intermediate bailout is enough to reach the first deco gas (here 32/20) at ~40 metres.

Not sure how visible that is, but it's at the 40 litres / minute 'panic sac'

I guess the key difference here then is the presence of another actual deco gas that can be used as deep as 40 metres, rather than spending that whole time breathing down a twinset of a deeper mix.

It's probably not optimal to use a single 'intermediate deep' mix all the way up to 20 metres/70ft, if there are significant stops below that. A good 'second deepest' gas isn't going to have an optimal ppO2 for those deeper deco stops, and if the END was appropriate for a deep switch, it will also have too much helium in it, slowing down decompression.

 

[broncobowsher]

m

I’ll disagree on this. We should all start with the end in mind. Some divers never get to the MOD3 level.

My two current frequent dive buddies got there in about 18 months. In that time they managed about 450 hours on their “first” unit in extremely diverse conditions all over the world.

There is nothing wrong with looking forward to see what future limitations or opportunities may arise. The best answer to this thread is there is no best rebreather.

The answer is get the one that makes the most sense for you and then dive the **** out of it.

I never ruled out MOD3 level of diving. But the discussion of what first rebreather to get has sidelined into a massive 100m bailout thread. For the most part every rebreather in this discussion can go to 100m in a safe fashion. There is a good configuration for doing that with all of these rebreathers.

I just find it odd that for a first rebreather the decision criteria has turned into the ideal bailout from 100m and then find a rebreather that matches that bailout configuration. Really, that is what this post has turned into. I (that is myself, not everyone) feel that is not the way to pick a rebreather. Make a plan of where you want to go (The OP did, 200' wrecks off California and great lakes) and pick gear that can do that with enough margin to go further. There will be a lot of good times at the range of the goals. And in the future there is a desire to go further, it's possible. By that time there may be a new flavor of Kool-aid that will be the latest and greatest. If you ever get to that point.

 

[justinthedeeps]

Sorry guys, I have gone too deep into the 'twinset is an ideal bailout' claim.
I understand that the first mention of it wasn't intended that way!
Maybe it was that 'sadness' reaction to my initial question.