Counter-lung question...

[Sounder]

Just curious...

Do your counter lungs have enough lift to float you at the surface? Is there an additional bladder for buoyancy besides the drysuit? How does the lift work when you've got multiple bail-out bottles and need more lift than, I'm assuming, the counter lungs provide.

Thanks!

[Dmitchell]

You still have your wing and your drysuit so your positive buoyancy ability is the same.

The counterlung does have a huge effect on Buoyancy. The idea is to keep your loop volume at the minimum you need to breathe comfortably. Think breathing into a paper bag and when you empty it you hit a "wall" you want just a hair more than that to breathe comfortably. When you ascend or descend that volume will change due to pressure change so you have to add or subtract gas to correct the volume and maintain buoyancy. The loop has a OPV on it to hopefully keep you from hurting yourself if you forget to vent on the way up.

The awesome thing is that when you get perfect buoyancy in you suit/wing and loop you truly hover motionless. No up and down as you breathe.

Clear as mud?

Dave

[Joshua Smith]

Even with 2 al 80 stage bottles, along with all my other gear, I still need weights to be able to sink on my Meg- the counterlungs are buoyant, when full, but then there's the rest of the loop as well- including the can with the scrubber, that's full of gas, not water. In and of themselves, CCRs are fairly buoyant.

[Dmitchell]

I wear 2- SS backplates and about 8 lbs of lead and I think I'm a few pounds over-weighted.

DM

[camerone]

When you ascend or descend that volume will change due to pressure change so you have to add or subtract gas to correct the volume and maintain buoyancy. The loop has a OPV on it to hopefully keep you from hurting yourself if you forget to vent on the way up.

...and that makes bailout scenarios of breathing open circuit, with the loop closed off, even more fun, as you're now trying to manage the lung volume, your BC, and your drysuit, all expanding at once as you ascend.

Counterlung capacity is around 5-7 litres for each of the two counterlungs, which translates into roughly a 25-30lb buoyancy swing if you're not on the loop and you expand all the way from nil, as would be the case with completely flattened lungs expanding from gas in the non-compressible portions of the loop. That's like a full wing, so you need to manage it, unless you like doing dolphin-at-SeaWorld imitations when you ascend.

Although minimum loop volume is the "goal" on a CCR (not so semi-closed), there are situations where it is advantageous to have slightly more gas in the loop, and the lungs can be used, ever so slightly, to fine-tune buoyancy on occasion.

FWIW, in heavy undies, normal "tech gear," and my CLX450, it takes about 20 lbs + my DSS steel backplate to sink my Evolution, or 5-7 lbs less if I ditch the yellow box and use the travel frame (but it's heavy!) Some of that needs to be up by the shoulders, as most over-the-shoulder counterlung rebreathers, while being easier to breathe, definitely want to turn you a bit head-up/butt-down.

[airsix]

The awesome thing is that when you get perfect buoyancy in you suit/wing and loop you truly hover motionless. No up and down as you breathe.

I was wondering about that the other day. I was thinking how hard it would be to get perfectly neutral without having your lung volume to use as trim. So I took my rig off in the pool and tried to get it neutral. It was weighted about 2 pounds negative with the combo of wing/harness/plate/nearly-empty AL80. I mention that so you know there a little air in the wing to swing the volume as depth fluctuated a foot or 2 as I was working on this. Anyway, for the life of me I could not get it neutral no matter how small the adjustments and no matter how careful I was. There's a lot of pressure change in a foot or two when your at the 10ft depth I was doing this, but it still left me thinking I'd be frustrated trying to get neutral in a CCR.

...On the other hand I came away from reading that CO2-loading article in DiveTraining magazine the other day thinking that buoyancy-trim breathing we do on OC is probably a significant setback to offgassing of CO2 and that CCR divers are likely getting a better CO2 flush (ignoring work of breathing as a factor since I don't know how well CCR's are in that regard)

-Ben

[camerone]

There's a lot of pressure change in a foot or two when your at the 10ft depth I was doing this, but it still left me thinking I'd be frustrated trying to get neutral in a CCR.

You'll find that there's one other complication here. The volume is approximately constant, but not exactly. Your loop volume actually diminishes over time as you metabolize oxygen, and then will "spike" a little bit as the solenoid replenishes the O2 with a little bit of overshoot to try to average out the PO2 to whatever your setpoint is.

With a manual unit, there's a constant trickle of O2, but you hand-fly the addition of O2 to play "human solenoid", if you will. It helps that the trickle is there, if the unit is properly adjusted, to reduce those spikes to a minimum, but they're not non-existent -- and the shallows are the trickiest part of it all, from a buoyancy and PO2 maintenance point of view.

As you ascend, even in a normal situation, you are forced to add (or the computer adds) quite a bit of O2 to make up for the fact that as the absolute pressure on you, the diver, decreases, so does the PO2, which, on a CCR, is designed to be held constant. In doing so, the counterlungs will also be expanding, so you end up adding O2, moving things around the loop by continuing to breathe, and exhaling through your nose, or, alternatively, by opening the BC-style dump valve on the lungs, if you have one.

The reality is that this is all second nature after you've been diving them for a while.

[Tangfish]

I switched from neoprene counterlungs to ballistic nylon ones.

[Joshua Smith]

I switched from neoprene counterlungs to ballistic nylon ones.

Let's go ride bikes!

[Burntchef]

I switched from neoprene counterlungs to ballistic nylon ones.

Let's go ride bikes!

im a big boy!!!

[Sounder]

Thanks! That totally answers my question.

The buoyancy without fine-tuning through breathing volume sounds like quite the learning curve.

[Tangfish]

I switched from neoprene counterlungs to ballistic nylon ones.

Let's go ride bikes!

im a big boy!!!

I like tacos!

[Dmitchell]

Thanks! That totally answers my question.

The buoyancy without fine-tuning through breathing volume sounds like quite the learning curve.

It's very different and it does take a while to get used too, it but like anything it comes with practice.

Calvin - I thought the Neoprene CL were in high demand? Why would you want the Nylon ones? they seem way too bulky. The OTS CL were one of the reasons I didn't get the Meg.


DM

[Joshua Smith]

The neo cls for the Meg are kind of weird- they don't really attach to your harness, and kinda flap around in the breeze, sorta. I think that's why Calvin switched.I've become sort of used to the ballistic ones- I try to work with them by using them to attach gear, like my Z knife. And I really do like having my ADV, 02 add, and OPRV right there on my chest. And An extra port for plumbing in offboard gas is a nice bonus, too!

[Tangfish]

I think I opted for the neo CLs in the beginning because having a loop and CLs just seemed like there was so much stuff around your head. After diving the CCR for awhile, I realized that I didn't notice much difference in terms of being less burdened by the different types of CLs. The neo ones don't have all the ways to attach stuff, like Nailer said, and didn't strap onto the shoulder straps, so they were kind of loose. Plus, and I could have imagined this, it felt like they didn't have the right amount of volume for my lung capacity. Still, some people swear by em so I guess to each his own.

[Gill Envy]

Thanks! That totally answers my question.

The buoyancy without fine-tuning through breathing volume sounds like quite the learning curve.

after a couple of years thinking it was just a matter of time before I got back to the comfort level with Buoyancy I had on OC i've concluded it will never be the same, it's just an area i'll always have to be more vigilant about. having three bladders of air: CL, suit and wing definitely requires more juggling.

the upside is that while at a constant depth the feeling of bliss is hard to beat, being truly neutral, just hanging freely in the water, not having to focus so much on breathing and moving slowly (or getting so cold) with so much worry about gas consumption, not the headaches, the dry mouth, the nitrogen fatigue, the muscle aches... oh, right, this was about counter lungs.
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