Buoyancy with a 'breather...

mort

I have been doing a lot of reading online about rebreathers and one question persists in my mind:
How do rebreather pilots control their buoyancy?
Do you do it with loop volume, or do you use a wing bcd that I can't seem to see in pictures, or do you use your dry suit?


(Pardon the questions if this has already been asked.)

rigdiver

Good question. The first few Rebreather dives are trying as far as buoyancy goes. The more OC experience you have the worse it is. Forget the inhale=rise/exhale=sink. When you are neutral, you are neutral. Period. One should try to dive minimum loop volume at all times. This helps with buoyancy control. Use the BCD just like in OC. Using the counter lungs for buoyancy creates a lot of problems. Too much gas in CL=hard to exhale. Using CL for buoyancy control should be a last resort. As far as a dry suit, you could use it, but then you are dealing with gas in 3 different places. If diving wet and the BCD is not functioning, then you have no choice. Try not to make too many depth changes. You have at least 2 different gas spaces/volumes to deal with. Depth change causes one to at least deal with loop volume and gas in the BCD.
The first 3-5 dives on a Rebreather will really make you unhappy with yourself as far as buoyancy control. After about the 8th dive it gets much better.
Rebreather's have many benefits. Getcha one.
Bill

ROB DAVIE

Mort,

Actually, the question is not often asked. As an instructor, I have found that students come to the rebreather class with the expectation that bouyancy is a "given", and the rest of the information is what they need to learn.

As it turns out, the first few dives on a rebreather are very humbling, with lots of fumbling about and crashing into things, like the bottom!

This brings about sort of a sudden depression on the part of the student. They begin to question themselves, and the choice they made to come to the "Dark Side" and to take up diving with one of these "damned infernal machines"!

Interestingly, the better the student is, as a diver, the worse the depression gets. I taught a student recently who was, and is, one of the best divers I have ever had the pleasure of diving with. He is a diver with great experience, and is a Course Director for a major training agency (not PADI).

He was thoroughly and utterly disgusted with himself at one point, but we worked our way through it. I encouraged him, and ran another dive where we worked on the bouyancy problem. It was really fun to see the light come on, and to be able to say: "By George, I think he's got it!!! "

As Rigdiver says, the essence is to keep the gas in the various spaces down to a minimum, to minimize your management problems. Work to get your trim EXACT. And be of good cheer! Keep working at it. The light WILL come on!

Rob

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[SIZE=2]"CC Rebreathers will become a viable part of tech diving [U]WHEN PIGS FLY[/U]!!"--GI3[/SIZE]

edster

As a UK diver, theres no question that you have to be diving in a dry suit, so as mentioned above, you dont want to have gas in 3 places - personally I have the wing totally empty, just there for emergencies, and dive my suit. But it takes a while to get your wieghting right, so its a tough process to start with - you might feel like things are happening all a bit too suddenly

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Odin

I always use my drysuit for buoyancy control.
Less task loading with only the suit + counterlungs to think about.
and as you should be aware of how much your counterlungs are affecting your dive, set the dump on the suit to dump at the rate you feel in control.

EBT

People use what works for them;

Some use CL volume (bit daft, since you lose all the benefits of MLV)
Some use suit, keeps you warmer, but makes for harder to manage gas bubbles.
Some use the wing, puts it in a good place, but then some folks dont like to manage suit and wing.

Personally I use the wing, its put the bubble in the right place, is fast and easy to dump and I cave, so I find that easier.

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MLV= Minimum loop volume

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mort

If you use your drysuit for buoyancy, are you able to carry stage bottles/pony bottles without looking like the Michelin Man? What do you do to keep your neck seal from "burping"? And, do you then have enough buoyancy to haul stuff you find (your buddy's boat anchor that broke loose, the fabled golden treasure from a pirate ship), or do you have to use a lift bag for that sort of stuff?
More importantly do you have enough drysuit buoyancy to lift a rebreather with a flooded loop?

Genesis

One thing that's obviously different is that on OC you have to have your wing in active use to compensate for "mass of gas consumed" as the dive progresses.

The "bigger" the dive, the more of an issue this is. On a dive with "full" 104s you might have up to 20lbs of buoyancy shift from the backgas tanks alone (you MUST be able to hold stops with dry tanks - so its not safe to weight for your "reserve" still being in the can) - and this ignores the shift from stage/deco tanks you're carrying. I often go in on the start of a dive OC with a 50lb wing half-inflated, and come out of the water with a LOT less in there. No way around it.

On a Rebreather there's very little "mass of gas" involved. Two 19s full is only about a three pound shift (!) in buoyancy from full to empty. You still must account for bailout gas buoyancy shift, but your actual required buoyancy compensation is MUCH lower.

I can see where you'd get right frustrated though with buoyancy control as a "new" Rebreather diver. I remember how badly I sucked on OC when I started, and I see how badly others usually suck too. It takes a while before you have decent control, and quite a while before you have EXCELLENT (e.g. glide a few inches off a silty bottom without kicking it up) control.

I suspect its radically different getting used to it on a CCR, as the unconscious use of your breathing to make fine adjustments doesn't do anything.....

On the other hand if you're weighted properly neutral is neutral and doesn't change appreciably from start to end of dive. This means that provided the only expandable airspace is in the counterlungs and a very small amount of air in your wing there's not much to get out of control as there is on OC - that should balance it out.

ROB DAVIE

The one thing that people who are starting out on rebreathers forget is that changes in "altitude" cannot be made with a simple slight breath, either in or out.

The more experience that the diver has, the more that this is done on a sub-conscious level. Thus, it is a harder job to retrain one's self.

And, of course, it is fun to watch....!

Rob

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[SIZE=2]"CC Rebreathers will become a viable part of tech diving [U]WHEN PIGS FLY[/U]!!"--GI3[/SIZE]