CL position, back or over-the-shoulder?

[O2BBubbleFree]

A couple of years ago I read an article arguing that over-the-shoulder counterlungs (CLs) were preferred, because in the most common diving attitude (face-down) the CLs are in the same plane as your lungs, thus reducing the work of breathing (WOB). Makes sense to me; if your CLs are either a few inches deeper or a few inches shallower than your lungs, WOB will be increased.

However, most modern RBs seem to have gone with rear-mounted CLs. In that case your lungs and the CLs are in the same plane when horizontal and facing to the side, or when vertical. These two attitudes would be in the minority, especially using horizontal ascents and descents.

As far as I can determine, there are two advantages to rear-mounting:

1. They are enclosed, so better protected. Wouldn’t want one to get punctured, would we?

2. It also seems to me that rear-mounting would get the CLs out of the divers way, but I’m not sure that over-the-shoulder CLs would be all that inconvenient.

So, the question as I see it is:

Do the advantages of rear-mounting outweigh the increased WOB?

BTW, I also remember reading that a certain WWII-era military O2 RB (sorry, I don’t remember the model) had some sort of mechanical device to compensate for the pressure difference between the lungs and the CL. Do modern RBs with rear-mounted CLs have some sort of device that compensates for the increased WOB? I haven’t put much thought into it, but it seems to me any simple device that could reduce WOB on the inhale side would increase it on the exhale side, and vice versa.

Seems to me that the optimum layout would have lungs, CLs, and diluent regulator (if used) all in the same horizontal plane during the majority of diving.

Thanks in advance for all the input. I’m not yet a RB diver, just trying to learn all I can while scavenging pennies.

 

[caveseeker7]

Where the counterlungs should be is above all a personal prefernce I believe. You'll find divers preferring either position, to the point of it being a major purchase criteria. And, I asume, some who couldn't care less.

However, most modern RBs seem to have gone with rear-mounted CLs.

Cis-Linar, UTx, Inspiration, Prism, Megalodon, Evolution, Shadow Pack, DiveRite all have or had, or will have o-t-s bags.
MKx, CCR2000, KISSes and Ouroboros, Shadow Pack and Nemesis have or had, or will have backmounted lungs ... seems like a fairly even split.

For some odd reason on SCRs there only o-t-s models that come to mind are the Ray and Voyager.

1. They are enclosed, so better protected. Wouldn’t want one to get punctured, would we?

Correct, but unless you're squeezing through extremly tight spaces and/or dive 'hostile' environments that level of protection might not be necessary. O-t-s bags are usually made from pretty sturdy materials. Breathing hoses (as well as gauges/displays) are still in harm's way, as is your wing, unless you want to return to the 70s BCDs.
Again, the wants and needs of the diver are the most important thing.

2. It also seems to me that rear-mounting would get the CLs out of the divers way, but I’m not sure that over-the-shoulder CLs would be all that inconvenient.

Some dislike the clutter that comes with o-t-s bags so much it's a deal breaker. There was a guy here locally who dove a Meg recreationally, including caves, and CCR2000s professionally, when filming UW. He used large camera housings and o-t-s didn't work for him.

Having had a Prism and MK16 in the pool side by side, there is quite a diffence between the two. I breath more comfortable on the Prism. But the MK16 certainly breathed well enough (especially in it's sweet spot) that I would dive one without worrying about it.

The uncluttered chest area is great, but the Prism's bags didn't bother me enough to decide on a rig with backmounted lungs.

Aside from the Prism, a 15.5 would suit me fine, and the Ouroboros looks mighty good, too.

Best advice: Try before you buy !
Not just because of WOB, hydrostatic differences, but also size, weight, balance, pre- and post-dive maintainance and general ease of use.

 

[O2BBubbleFree]

Thanks, caveseeker7. I'm starting to look to you as the best source of info on all things rebreather!

 

[caveseeker7]

Thanks, caveseeker7. I'm starting to look to you as the best source of info on all things rebreather!

Thanks, I appreciate that.
But I'm not, there are plenty of people here with a whole lot more experience and/or knowledge than I have.
But when I can be of help I try.

The subject of rebreathers isn't an easy one because there are so many things to consider ... one diver's MK-5p is another diver's SportKiss.

The best thing you can do is research the subject and units, look at your dives and the ones you're planning to do, and research, including intros, some more to make sure you decide on what you want and need.

Some units are pretty extreme designs, on either end of the scale, while others make great allround rigs that will come up somewhat short when pushed to either end.

Luckily, these days the choice is there, and with every new model hitting the market your chance to find the right one for yourself increases.

If you get the chance, do intros. They're fun and you learn a lot.

 

[O2BBubbleFree]

The best thing you can do is research the subject and units, look at your dives and the ones you're planning to do, and research, including intros, some more to make sure you decide on what you want and need.

//snip//

If you get the chance, do intros. They're fun and you learn a lot.

Thanks, again, for the good advice.

I've been studying rb design/philosophy for the last couple of years, and will probably have to 'study' for a while longer, since I have a new house with a new baby in it, and limited income!

So I guess I'm a 'cyber RB diver' for the time being, stuck blowing bubbles, but a guy can dream and plan...

I hadn't thought of the intro idea, though. Maybe that can help fill the gap. I've only seen one RB in a LDS, and it was a unit that the LDS bought so that they could conduct training. They were selling it due to lack of interest in said training. I'll have to check on the availability of intros in the Austin area.

 

[caveseeker7]

IANTD lists 4 RB instructors in TX, two each for Dolphin and Inspiration, one each in Houston. On the map that doesn't look all that far away, but I hear it's a large state.

BigJet here on the board is the Inspiration instructor and might have suggestions, too. One of those people I mentioned earlier vastly ahead of me.

 

[BigJetDriver]

BigJet here on the board is the Inspiration instructor and might have suggestions, too. One of those people I mentioned earlier vastly ahead of me.

Caveseeker,

I don't know about the "vastly" part, but I do appreciate the kind words!!

3Dent,

My units of choice are the Inspiration, and the Shadow Pack III.If I can help with info or suggestions, please feel free to call me on my toll-free voice pager at 888-535-5890 and leave a voice message along with your number.

Caveseeker is a good source of knowledge about the Prism, and has close ties with its creator, Peter Readey. He has also spent much time researching a lot of the other units that are (or were) available.



Cheers!

Rob
IANTD I.T.
Inspiration Instructor
Shadow Pack III I.T.

 

[O2BBubbleFree]

My units of choice are the Inspiration, and the Shadow Pack III.If I can help with info or suggestions, please feel free to call ...

Hi BigJetDriver,

Thanks for the info, and the offer. I hadn't really investigated the Shadow Pac. I don't have any questions right now, but if you don't mind I'll keep your number for future use.

Thanks, guys!

 

[jagfish]

Gosh
This thread raises a great question that I am beating myself up over at the moment as well....