I suspect you're trying to use equipment so solve a skills problem.
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)
Based on this description, I'd say that one or more of the following are happening ...
- you're not as neutrally buoyant as you think you are, and using subtle fin motion to maintain position in the water column
- you're changing depth as you re-orient yourself in the water
- you're inadvertently changing your breathing pattern (aka, "inflating your internal BCD") when inverting
Any, or any combination, of the above will change your buoyancy state under the circumstances you describe.
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.
My guess is that it's less the design of the BCD than it is the choice of this particular gear for single-tank diving. That's a HUGE air cell, and the larger the air cell the more effort you have to put into managing the bubble. This is especially true if you're carrying more weight than you actually need, as that will require a larger bubble in the cell to maintain a neutral state. The term you want to search for is "dynamic instability".
Your explanation, however, isn't accurate ... as has been pointed out several times already ... unless in the process of inverting you're also decreasing the depth, and therefore pressure, on the air in the cell. The bubble will maintain the exact same pressure as the surrounding water ... and while it will likely change shape, it will not displace any more water at a given depth regardless of the shape of the wing. Displacement is a function of pressure, and at a given depth the pressure remains the same regardless of the shape of the bubble.
Good to have a dump on the bottom...but why not design the bcd so you get the same buoyancy at all attitudes?
... all BCDs are designed to give you the same buoyancy at any attitude, given the air cell is maintained at the same depth ...
Inverting is an extreme example..even tilting a bit lets some air into the floppy part of the bladder and I find myself compensating.
That's called "taco'ing", and has to do with trapping air in parts of the cell that do not allow it to travel to the dump valve. It's not surprising you're having that problem, given the size and design of the air cell.
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....
It didn't take long for the usual "get a BP/wing" response ... and while it's a valid response (I prefer them, personally) it's not the only "best" answer. That would be "get an appropriately sized air cell for the job you're expecting it to do". For single-tank diving, 75-lbs of lift isn't just unnecessary, it makes managing the air cell far more difficult than it needs to be.
My answer would be ...
1. Decide what style of BCD best suits your diving style and preference ... back-inflate or jacket. Consider the features you want. If pockets are important to you, make sure that you can access them wearing the exposure gear you will usually be wearing (not all of them are easy to get into the pockets while wearing gloves, for example). If weight integration is important, choose a BCD that allows you the ability to use the weight you'll need for your usual diving conditions.
2. Go try on several ... think fit and function. Proper fit is, by far, the most important consideration ... it should fit snugly enough to not ride up on you when it's inflated on the surface. A BCD with a crotch strap will generally provide better stability than one without. Also, a BCD with double tank bands will generally hold your tank more snugly in place than one with a single tank band.
3. If you have access to a pool and the ability to try out one or more different models, try before you buy.
4. If it's for single-tank diving, limit your considerations to BCDs with 40 or less pounds of lift. Even 40 pounds is more than you'll need under most circumstances, unless you're diving very large cylinders and cold water exposure gear.
None of this stuff will solve the problem you describe ... only better technique will do that. But a properly sized, properly fitting, BCD will make it easier to develop that technique to a degree that the problem you've described will cease to be a problem.
... Bob (Grateful Diver)
