Want BCD that gives me same buoyancy at any pitch angle

[Diver8492]

Ok, so I have good neutral buoyancy and excellent trim just "hanging in the water" with fins down. When I invert myself, I get extreme positive buoyancy. (of course its worse in shallower water)

My guess is that it is the design of the Genesis Recon BCD. When I hang, the air is at the top the bladder. The top of the bladder is wound tightly with several shock cords, so the bladder can only displace a certain amount of water.

When I invert, the air goes to the bottom of the bladder. There are no shock cords there. The air can expand much more at the same depth because there are no shock cords compressing the air. So now more water is displaced, and I get (huge) positive buoyancy.

Good to have a dump on the bottom...but why not design the bcd so you get the same buoyancy at all attitudes? Inverting is an extreme example..even tilting a bit lets some air into the floppy part of the bladder and I find myself compensating.

Is there a BCD Make/Model that gives you the same buoyancy at all attitudes? Now that the Recon is 11 years old, I am looking around....

 

[Jim Lapenta]

Back plate and wing.

 

[herman]

I have to disagree with Jim, the answer is none and all of them. You buoyancy is not going to change due to position more than slightly, what will change is you. You don't realize it but you are changing some thing in the way you breath. If you are neutral in one position you will be neutral in another if you don't change anything like your breathing. You trim will shift around if you allow an air bubble to move around in your BC but again that has nothing to do with the type of BC, although how you manage it will vary some between BCs and bladder types for that matter. Take a close look at what you are doing, you are changing something and no BC will fix that.

 

[doctormike]

Another issue is that any shift in the position in the gas in the wing (with respect to your center of mass) is going to be more pronounced the bigger that bubble is. The only reason to have any gas in your wing at all during your dive is to offset the two things that change during the dive - exposure suit compression at depth and the weight of the gas that you consume.

This bubble should be kept to an absolute minimum. Many divers are overweighted, and compensate by adding gas to the wing, which makes this problem worse.

Also, backplate and wing. Not so much because of this issue, but because [insert links to last 8,000 threads on this topic].

 

[RJP]

The shock cords will not compress the gas in your wing... unless the wing is completely full. If there's room in the wing for the gas to move, it will simply move rather than being compressed.

The likelihood is that you're buoyancy control isn't as good as you think. Your trim certainly isn't if you're "just hanging in the water, fins down" as you say. From a physics standpoint there's no explanation for "extreme positive buoyancy" to magically appear out of nowhere simply from rotating your body from head-up to head-down orientation.

 

[Kharon]

Ok, so I have good neutral buoyancy and excellent trim just "hanging in the water" with fins down. ...

I would beg to differ. If you have excellent trim you can hover in any position. If you can only hover with your feet down you need to move weight around until your center of gravity is in your center. Trim is distinctly different from buoyancy.

 

[divad]

BP/Wing is the closest thing to the Magic Carpet you're looking for............will someone please put the lamp away

 

[FinnMom]

Seriously, try a wing and you will be amazed at how small the air bubble seems to be and at how nicely it forms a horizontal plate above your back.

When I first tried one (after maybe 60 dives and 12 mo.s of diving), I felt like I'd been played for a fool not having this equipment and being criticised by people wearing BPW setups. Might as well have competed for speed with everyone but me wearing fins. Later I learned to control a BCD equally as easily, but esp. as a new diver I found it vastly easier to attain and maintain a horizontal position using a BP/W. Staying horizontal is one one of the key points in achieving bouyancy control.

 

[Eric Sedletzky]

First of all, the size of the bubble changes with depth not by squeezing in on it with bungees. If you squeeze a bubble with bungees it will just find somewhere else to go, so by inverting yourself (if you haven't changed depths) the bubble will be exactly the same size. You also didn't mention what type of suit you have. Are you using a drysuit? and what depths are you doing this. 30 or less is where most of the pressure changes take place so if you're in a drysuit and get inverted maybe you have air going up into your legs which will be higher in the water column and at a shallow depth that could be enough to make things change.
Going to a BP/W isn't going to solve anything until you figure out your buoyancy issues. Many of us who have been diving for a long time have figured out to weight ourselves as light as we can get away with because any added balast means it has to be made up with air in the BC (wing) which means the more air in the BC the more yo yo-ing effect at shallower depths.
As a rule of thumb, you should get your weighting dialed in to where you can hold a stop at 15 feet with no air in your BC at the end of a dive with a near empty tank. And if you have a drysuit it should be the same except you should have just enough air in your suit to prevent shrink wrap and be comfortable. If you can do this then you know your weights are down as light as they can be. With this in mind there will be no unneeded air in a bladder that can expand and contract at that critical 15 ft mark (the most critical - that's why it's a stop).
Also, many of us who have gone to a BP/W do so because of the many options available in wings and wing design. I personally like to use the smallest wing (BC part) I can get away with because it means the air that I do put in will have a smaller area to spread itself out so that means the wing will be more evenly inflated instead of using a huge air cell and having the bubble all collected in one area. That's part of the problem we've found with those huge BC's like the Recon (and similar) is they have too much lift that never gets used. Many people who dive doubles don't even use that kind of lift.
Let me guess, that thing has about 70# lift, am I right?

 

[Bob DBF]

I'd start with doctormike's advice and insure you are weighted properly for the first step.

Come back from a dive and at your 15' safety stop, with 500# in your tank you should be able to hover with an empty BC breathing normally. At this point you will probably be handing your buddy your excess weights to make it happen. I usually wear 2 or 3 pounds over this ideal weighting, but no more. The amount of weight determines how big a bubble you need to be neutrally buoyant, the bigger the bubble the harder it is to control, especially when shallow.

Be aware of where the bubble is in relation to you. say you are horizontal and come to a vertical position, I would bet your head and torso would be higher than your horizontal plane and then the BC bubble would move up to a position that is higher and would expand. If you rotated up so the bubble stayed on the same plane it would not expand, however your head position would be deeper than the previous example to achieve that.

Keep your eyes open for someone that looks good in the water and start asking questions. If you start out with "you look great in the water. How do you do it?" I'm sure it will go well from there. The other answer is to find a PPB class or workshop in your area, the folks on ScubaBoard could point you in the right direction.

As for any particular BC solving your problem, I believe that an experienced diver can wear any pos BC and make it look good.


Good Luck

Bob
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