Question Bailout gas configs for tech/deep chestmount rebreathers

[DiveTucson]

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A ScubaBoard Staff Message...

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.

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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

 

[justinthedeeps]

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

Do you need doubles AND a deep S80? Could also carry two cylinders on the back, but operating as separate gas mixes--e.g. left deep, right intermediate?

 

[justinthedeeps]

@grantctobin I noticed your 'sadness' reaction--care to comment why one would actually need a twinset of 'intermediate mix' on their back?

I understand that people might naturally feel like it makes sense to strap a chestmount CCR on after going through twinset tec-rec, but outside of that 'progression,' I have not heard of anyone taking two separate cylinders of the same mix with them for typical 100 metre "Mod3" bounce dives.

Unless it's the 'GUE' approach, but then that's a frankentwins of deep mix, not intermediate mix.

 

[broncobowsher]

@broncobowsher I noticed your 'sadness' reaction--care to comment why one would actually need a twinset of 'intermediate mix' on their back?

I understand that people might naturally feel like it makes sense to strap a chestmount CCR on after going through twinset tec-rec, but outside of that 'progression,' I have never heard of anyone taking two separate cylinders of the same mix with them for typical 100 metre "Mod3" bounce dives.

Not sure where that came from and I have not progressed enough to be giving instructions on a 100 meter dive.

 

[justinthedeeps]

Sorry @broncobowsher looks like I misread my 'reactions.' I will re-address that so I/we can maybe learn something.

Definitely not me either! I can totally see that natural union between back mounted cylinders (twinsets by default--understandably) and chestmount CCRs though

 

[DiveTucson]

Do you need doubles AND a deep S80? Could also carry two cylinders on the back, but operating as separate gas mixes--e.g. left deep, right intermediate?

My simple answer is no. The risk of switching to the wrong gas in this situation would be too high. There are other multiple reasons as well but that’s for another day (redundancy, PO2 management of BO gas, team considerations, deco profiles, etc.)

All stuff that is discussed in the MOD3 course…

 

[Wibble]

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

However, most people doing deep MOD3 dives would not have that configuration— backmount twins??

Most would be diving with 3+ cylinders clipped to the harness and a leash, not to mention team bailout where you’d hand off cylinders to the victim.

As soon as you start diving with two cylinders on the same clip, they wrap around to the front, where the chestmount unit lives.

Short, deep dives will often be done with a scooter to maximise bottom distance. More competition for clip space and not chestmount friendly.

Chestmount CCR is compact. It doesn’t have the space for a large radial scrubber that backmount does. Deep dives means long decompression times that will exceed chestmount runtimes.

Couldn’t imagine people choosing chestmount over backmount CCR for really deep dives (specialist cave diving aside).

 

[DiveTucson]

However, most people doing deep MOD3 dives would not have that configuration— backmount twins??

Most would be diving with 3+ cylinders clipped to the harness and a leash, not to mention team bailout where you’d hand off cylinders to the victim.

As soon as you start diving with two cylinders on the same clip, they wrap around to the front, where the chestmount unit lives.

Short, deep dives will often be done with a scooter to maximise bottom distance. More competition for clip space and not chestmount friendly.

Chestmount CCR is compact. It doesn’t have the space for a large radial scrubber that backmount does. Deep dives means long decompression times that will exceed chestmount runtimes.

Couldn’t imagine people choosing chestmount over backmount CCR for really deep dives (specialist cave diving aside).

Yup I have zero experience doing 6+ hour run times on the Britannic with the exact same configuration I mentioned ;-). I also teach on the rEvo and dive at the MOD3 level with it so have some experience on that unit as well. On the last trip the other two divers AP and rEvo agreed after seeing the configuration and the ease of being more self reliant that it just works better.

I would agree that most people doing MOD3 dives don’t have that configuration but that does not means it’s not a superior way of carrying more gas for longer MOD3 dives.

If you read my post I clearly stated that it’s two bottles left and then that leaves your entire right side free for a scooter and full frame camera if needed. I’m speaking from real world experience frequently doing 100M+ dives with the configuration mentioned.

I am not saying the CM is the best first unit, the best CCR, or the best for everything. It has some limitations. I will say for carrying a ton of gas for longer and deeper MOD3 dives it makes things way simpler.

More reading here and some pictures:

Gas Planning With a Chest-Mounted Rebreather

Conducting a deep dive with a chest or sidemount rebreather using

indepthmag.com

 

[broncobowsher]

I want to get into advanced shipwreck diving, penetration diving on the wrecks in San Diego, Ca. I would love to visit the wrecks up in the great lakes (Lake Huron) at the 180-200ft (60-67m) depth. There is a lot to see here in Southern California as well. I know I wont be able to afford tec diving on OC for very long so I'm thinking I just make the transition to CC sooner than later. I can do a lot of diving on AIR DIL here locally.

Scope creep...
Back to the OP. Inquiry was for MOD2 level of rebreather diving.
Starting to get heavy into the MOD3 configurations. Pretty deep for someone looking for a first rebreather.

 

[justinthedeeps]

My simple answer is no. The risk of switching to the wrong gas in this situation would be too high. There are other multiple reasons as well but that’s for another day (redundancy, PO2 management of BO gas, team considerations, deco profiles, etc.)

All stuff that is discussed in the MOD3 course…

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 ]