Sidemount rebreathers and hydrostatic lung loading

[crazyduck]

There was a comment made on the NACD cave meeting that some people felt that sidemount rebreathers (we are talking round all inclusive units) with the counterlung in the bottom of the case causes a problem with the pressure of breathing depending on the position of the rebreather. Instead they wanted to maintain the counterlungs on the body and have the canister mounted up front or make the canister moveable.
I found this interesting and was curious about people thoughts.

Thanks, Andrew
Have a great Thanksgiving weekend.

[FW]

My first sidemount rebreather had-over the-shoulder counterlungs. I guess that is OK, if you never do any actual sidemount diving. I discovered very quickly, that if you get into a tight passage, you collapse the counter lung, and can't breathe. All my sidemount rigs since then have had the counterlungs inside a tube.

Yes it is true that the position of the rebreather is critical, but that is actually good, since you can adjust the Work Of Breathing by positioning the rebreather. When I have been on a really long dive, I can inhale and exhale with negative WOB, by pushing the canister up and down with my elbow. I would like to see any over-the-shoulder counterling do that :-D

[farmerted]

The work of breathing in a sm rebreather with the lung(s) inside will breathe really well whilst in trim. When you break trim is when problems of static lung loading will arise, just as it will if that same unit was on your back. I have been diving my sm rig with top of shoulder lungs for over 3 years now and can wiggle into just about anywhere I can on oc sidemount.

[tflaris]

Brett Hemphill told me the same thing. Works great while in trim otherwise poor static lung loading. He felt they were mission specific.


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[apitkin]

They are very much mission-specific tools. Putting the lungs in the case by your side means that if you twist sideways to go through a vertical fissure the lungs will either collapse and add gas via the ADV (or you'll have to add diluent if you have a manual add system), or the counter lungs will try to blow the air out through your mouth. Neither is very comfortable at all. If you're used to using well-placed counterlungs (e.g. over-the shoulder or back mounted) it's a revelation how variable the breathing performance can be. Descent and ascent can also be challenging, depending on the design.

For "general purpose" side mount use, the Bill Gambrill approach of keeping the counterlungs on the chest and sidemounting the canister is pretty streamlined and is probably the best overall compromise between performance and streamlining. Only the really hardcore explorer will need something else. The main problem is that if the cave is really small you still have to get your bailout in with you, so it's often better just to use open circuit anyway.

That's why I think Brett is right. If you need a totally self-contained side mount rebreather, you probably have a specific task in mind.

Andy

[johnny boy]

When i was watching a recent video of the SF2 in action on Vimeo, i thought the same as Brett. Here's the link for everyone... http://vimeo.com/groups/hdtdv/videos/29626117

[Slüdge]

I can inhale and exhale with negative WOB, by pushing the canister up and down with my elbow.

Wouldn't it take a whole lot less effort to let your lungs do that for you? :rollguy

[FW]

Wouldn't it take a whole lot less effort to let your lungs do that for you? :rollguy
Try using a rebreather with a high W.O.B. then ask me that :-D