(Maybe fork this off to another thread: optimal deep bailout strategies for 100m/330ft MOD3 dives. Definitely interesting for some, but not appropriate for a "which first CCR" thread.)
Question Bailout gas configs for tech/deep chestmount rebreathers
- Thread starter DiveTucson
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mr_v
Contributor
@DiveTucson's proposition makes a lot of sense. It is a streamlined setup that yields a significant bailout volume plus an easy access to a DPV. If the overall height - chestmout CCR + doubles - are not a concern for the dive, then why not?
justinthedeeps
Contributor
A twinset of a single gas could make sense on say a Mod1 or Mod2 dive, where that one gas can basically just be your bottom gas, and it gets you easily up to that first deco gas (stated as 50%) with no real compromises on ppO2, END, or deco.
You could just do the whole dive as an OC dive if you had to, because you brought so much bottom gas. And you already are a twinset diver. But you wanted to CCR instead, because of numerous advantages, so you add on a chestmount CCR.
I'm with you!That said, you probably could have done exactly the same thing with just a single cylinder of bottom gas on your back, or the deep and deco gases sidemounted, with nothing on your back.
The twinset of bailout approach sounds pretty much the same as the GUE/DIR(?) approach of putting the bottom gas on your back as a mini twinset, so that you don't have to carry it as a stage cylinder. But this is a bottom gas.
However, once you are going to Mod3 depths (e.g. ~100 metres), there is no single 'back gas' that is going to work at the bottom, and also be ideal as a travel gas all the way up to 20 metres (the 50%, in examples given). A proposed solution earlier was to just tag on a new extra stage cylinder of deeper gas. It is easy to see how this was chosen as a simple fix for going deeper.
Unfortunately, now you are carrying a twinset of gas that used to be your Mod2 bottom gas, but is now just some kind of travel gas. And that is no longer efficient. There is no longer much justification for that extra cylinder of travel gas. You'd actually be better off with another cylinder of deeper deco gas--for example, the 32/20 that I explained in the previous example.
So we could just keep adding on cylinders, or else staging them along the way for later retrieval. I guess that is fine.
But if we are talking about what is ideal, or trying to limit the number of cylinders involved, then the twinset travel gas approach raises eyebrows from a gas planning perspective. I guess you could argue that the twinset is a mini twinset, but I am not convinced this is any better than a single full sized cylinder.
The love for manifolded twinsets is gospel in open circuit tec courses, it is a true symbol of tec. The holy 'valve drill' is like praying to the tec gods. But I've not heard of a single person ever actually needing it on a real dive. Not in recent times.
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.
I have friends that love CM+doubles for dives in the MOD2/3 range. I personally prefer diving CM with 4 side mounted cylinders (typically 2xAL80s + 2xAL40s or 4xAL80s for big dives). I choose this based on my preference to have all my tanks on the sides but as a byproduct I can better optimize the 2 deep mixes instead of having a large volume of travel mix.
Profile picture is on last deco stop of a 60m dive with 2xAL80s + 2xAL40s.
Profile picture is on last deco stop of a 60m dive with 2xAL80s + 2xAL40s.
This thread has been created from a thread diversion in a thread asking only for recommendations on a first rebreather. To allow that thread to continue on in its original vein, this spinoff thread has been created as requested.
A ScubaBoard Staff Message...
My two cents and my opinion for what it’s worth. Using the CM CCR for deep diving in open water is one of its strengths over a traditional BM unit.
For a 100M dive with an extended bottom time you could use something similar to below.
1 x AL80 Deep Bailout - Drive Gas For Unit
1 x Doubles Size Depending on Dive - Intermediate Bailou
1 x AL80 - Deco Gas 1
1 x AL80 - Deco Gas 2
With this setup and doubles on the back you can have the 2 80s left, butt clip the 3rd and still have your right side clear for DPV, Camera, etc. I’m also able to fully dress without much assistance. This same dive with a traditional BM unit is much more complicated from a bottle management standpoint.
Again just my opinion and I prefer the CM for bigger open water dives for the reason listed above.
For the record I am certified on 3 BM units, teach on the rEvo, am an IT for the CM, and also am almost teaching on the SM Liberty so a lot of experience across the different platforms and I dive and teach as my full time career.
There is not one “best” option
So your drive gas and deep bailout are on a single reg?
IMHO, on a traditional CCR, your DIL and deep bailout are on two separate first stages. This minimizes cluster****ery by providing some redundancy -- odds of losing both a DIL first stage and deep bailout first stage are minimal (and your CCR?). I guess thankfully no one has seen a first stage HP seat failure ever.
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 - backmount doubles, sidemount bottles, an H valve, whatever.
Yeah, I can see an argument that the odds of losing both that DILout first stage and the loop are minimal, but having both DIL and deep bailout on a single first stage on a dive involving deco is analogous (in my mind) to going into a cave with a single first stage, or putting deco gas in doubles and using a single stage bottle filled with trimix. I guess I'm just more conservative on this.
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 - backmount doubles, sidemount bottles, an H valve, whatever.
Yeah, I can see an argument that the odds of losing both that DILout first stage and the loop are minimal, but having both DIL and deep bailout on a single first stage on a dive involving deco is analogous (in my mind) to going into a cave with a single first stage, or putting deco gas in doubles and using a single stage bottle filled with trimix. I guess I'm just more conservative on this.
justinthedeeps
Contributor
This totally makes sense, and then in the chestmount situation, it makes me then wonder why not consider splitting up the 'back gas' into a deep cylinder and a travel cylinder, instead of a twinset of just one manifolded travel gas.
I guess the quick answer was: confusion, but I think the real answer is that we are kind of married to manifolded twinsets already because of tradition, and easy progression from traditional open circuit twinset diving.
Operationally it seems totally natural to progress divers/students from traditional OC twinsets, to adding on a chest mount CCR. Bu the key decision there is to make a manifolded twinset central to everything.
This is what I do with 4 sidemounted tanks. Losing one reg is still a problem because you lose a full AL80 of BO gas.
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