While this post is most certainly trolling, I do think there are some aspects of CM units that often get overlooked in the zeal of hopping on the bandwagon (seems like every day I’m seeing a new CM unit hit the market). And no, I’m not talking about the most common pros/cons thrown around here on SB. I find it interesting that people don’t discuss soft palatal air leaks which can be caused by the positive pressure of the CM counterlungs. From my experience and talking to people who dive CM units, and from a rare thread here or there on SB, it seems like CM units cause a higher number of these incidents over more traditional BM or OTS counterlungs. I owned a CM unit (my first unit, I.e., getting caught up in the hype) and experienced a number of these incidents over the course of owning it, with little to no common denominator on the type of diving to cause it specifically. It would just happen regardless of how easy or strenuous the dive was. It almost ruined rebreather diving for me since it became a nagging thought on every dive due to how uncomfortable it felt. I sold the CM unit and bought a BM unit and I absolutely love it. WOB is great, and with no more CM counterlungs, I have yet to experience the same palatal leaks, even on dives significantly longer and fatiguing.
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Gone for diving
Contributor
Not sure, but it will be roughly when back mounted doubles are fazed out...
justinthedeeps
Contributor
Is this when unconscious voluntary closing of the nasal passage is overwhelmed or fatigued, resulting in uncontrollable loop volume loss through the nose/mask?
Have experienced that on backmount CCRs most notably during rescue drills while surfacing inverted as the 'unconscious victim' with an overfilled loop.
Does anyone know what kinds of pressure values/limits are involved? Probably simple to estimate positive (or negative) pressure of a counterlung as a function of its difference in depth from the lungs.
Presumably this is covered in the courses with regard to positioning the counterlungs?
First thought is what a load of bollocks. Shirley sidemount rebreathers are the only true way to dive.
The context of that instructor’s comment would be important. Chestmount has some benefits, mainly for lightweight travel at moderate depths. Outside that chestmount isn’t the best by a long way.
DiveLikeAMuppet
Contributor
Anyone wants to trade a mildly used useless JJ for DiveTalk Go? 
That's right - when I first encountered the issue and began to research it, there seemed to be little discussion around the topic. I posted a thread here on SB over a year ago about the issue and got a lot of input and perspectives from other rebreather divers. While it does seem to happen on both CM and BM units, people generally agreed it is more prone to happen on CM units due to the positive pressure created by the counterlung positioning (which makes sense if you've dived a CM unit). This is also touted as a benefit to WOB since it requires less effort to inhale (but more effort to exhale). It might be a non-issue for most people, but I found it very uncomfortable and would usually terminate the dive it if did not stop. I've found BM counterlungs to not have the same effect on me, but I'm sure in certain circumstances (like you said above) it can still happen.
As previously mentioned, the WOB is always effected by positioning of the counter lung(s) it’s very basic physics,, everything about them is to do with their proximity to the divers lungs and if they are under slightly more or less pressure of water it’s an inescapable fact that you trade one aspect for another be it easy breathing in and having to push a little to exhale because the lung(s) are chest or vice versa for backmount..
you pick what works best for yourself in your chosen environment and run with it,, (but I am sick to death of hearing W is better than Y,,, they all have flaws and the better ones all have positives too,)
you pick what works best for yourself in your chosen environment and run with it,, (but I am sick to death of hearing W is better than Y,,, they all have flaws and the better ones all have positives too,)
justinthedeeps
Contributor
How's €1.000Anyone wants to trade a mildly used useless JJ for DiveTalk Go?
The Euro's dropped; needs to be greenbacks
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