Want BCD that gives me same buoyancy at any pitch angle

[Diver8492]

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.

Lots of good advice here, but I have to disagree with this instructor.
If you take a gallon sized steel cylinder and fill it with air, its going to produce 8 lbs of buoyancy at any reasonable depth.

But if you have a cylinder that that can expand depending on whether its up or down, like a cylinder with a flexible bladder in its bottom, the buoyancy will change as the gas pushes up on the bladder. Thats because more water will be displaced.

No doubt bouyancy control is a skill to continually improve, but if you tried this for yourself with the Recon, you would exhaling everything and then pulling on the rear dump when you put your fins up.

I could try to lose 3 lbs of lead...right now my hood fits poorly, catches the exhaust and gas gets trapped in it. Im no expert, but Im sensitive to stuff like that.

 

[Eric Sedletzky]

Lots of good advice here, but I have to disagree with this instructor.
If you take a gallon sized steel cylinder and fill it with air, its going to produce 8 lbs of buoyancy at any reasonable depth.

But if you have a cylinder that that can expand depending on whether its up or down, like a cylinder with a flexible bladder in its bottom, the buoyancy will change as the gas pushes up on the bladder. Thats because more water will be displaced.

No doubt bouyancy control is a skill to continually improve, but if you tried this for yourself with the Recon, you would exhaling everything and then pulling on the rear dump when you put your fins up.

I could try to lose 3 lbs of lead...right now my hood fits poorly, catches the exhaust and gas gets trapped in it. Im no expert, but Im sensitive to stuff like that.

Just a wild guess, but I'm going to go out on a limb and say that I think RJP maybe has a tad more experience and firm grip on physics a little better than you do.
You can't change the laws of physics, even with a good attorney.
Your wing isn't a steel cylinder, it's a soft bag, and just because one end has bungees making it smaller just means that the air will find somewhere else to go. Just like a balloon, fill it up half way and squeeze in the middle, what do the ends do?
This is junior high stuff, and it also doesn't belong in advanced scuba discussions.

 

[DaleC]

Yes, the bungees will not compress the air in the BCD. The air will just move to an area with less restriction. You also cannot compress the air because almost all BCD's have over pressure relief valves (OPV'S), that vent when the pressure gets beyond a few lb's, so your BCD won't pop.

Another effect on buoyancy and orientation is where that bubble is in the water column. If you are horizontal the bubble will be roughly at the same depth as your body (and the depth gauge you are looking at). If you go heads down, and keep the same depth, the bubble is actually centered a foot or two higher than before, because that depth gauge is down low and the bubble is riding high. It may seem you are at the same depth but just your gauge is, you have moved the bubble up and depending on how much air it contains that may create a positive lift effect. If you typically do the "superman" pose, your depth gauge could be 3' lower than the bubble.

I experienced this when trying to learn how to back kick. I would involuntarily go a little heads down, the bubble would ride up the wing and I would kick upwards instead of backwards. To correct it I had to lose the size of bubble (become more neutrally buoyant through proper weighting) and stop going heads down (extend/arch the neck/back a bit).

I disagree with the notion that a wing would be any better than a jacket in this regard. A bubble is a bubble, no matter the packaging. A bigger difference would be the comparison between horseshoe and donut designs, which do have some effect on bubble travel/shape.

 

[diversteve]

Let me guess, that thing has about 70# lift, am I right?

Close - see the 75 printed on the top of the jacket...according to LeisurePro -

Genesis Recon BCD with CQR Weight Pockets (75 lb. Lift)
This back mounted compensator with weighted integration system is a must for any technical diver The recon BCD consists of a back inflation air cell with single bladder technology and buoyancy of 75 lbs

That's bigger than my doubles wing. I think you might've hit on part of the problem...

 

[Jim Lapenta]

I had to say it as soon as possible.

But if that doesn't work, and it won't for some, sidemount.

 

[The Chairman]

The problem is not in the BC, but in the regulator. If the second stage regulator is shallower than your lungs, the tendency is to overfill the lungs. If it is deeper, the tendency is to inadequately fill them. Unless the difference is quite dramatic, you might not even notice it much... but it will affect buoyancy. Change your breathing to compensate.

 

[Rooster59]

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.

Another characteristic of split fins.

 

[Diver8492]

Just a wild guess, but I'm going to go out on a limb and say that I think RJP maybe has a tad more experience and firm grip on physics a little better than you do.
You can't change the laws of physics, even with a good attorney.
Your wing isn't a steel cylinder, it's a soft bag, and just because one end has bungees making it smaller just means that the air will find somewhere else to go. Just like a balloon, fill it up half way and squeeze in the middle, what do the ends do?
This is junior high stuff, and it also doesn't belong in advanced scuba discussions.

Yes that was a wild guess.

Lets take your own balloon as an example. Wrap it with rubber bands on one end. Put a little air in it.

Take it under water. Hold it rubber band side up. The bubble will compress and the loose end of the balloon will collapse. Now turn the balloon upside down. If you cant understand that the loose end expands and displaces more water because there is no force holding it back like the other side, then you should get out your junior high school books.

Or admit that you were schooled here in Advanced Scuba Discussion.

 

[RJP]

Yes that was a wild guess.

Lets take your own balloon as an example. Wrap it with rubber bands on one end. Put a little air in it.

Take it under water. Hold it rubber band side up. The bubble will compress and the loose end of the balloon will collapse. Now turn the balloon upside down. If you cant understand that the loose end expands and displaces more water because there is no force holding it back like the other side, then you should get out your junior high school books.

Or admit that you were schooled here in Advanced Scuba Discussion.

I think you need to try an actual execution of your hypothetical experiment. The reality of physics doesn't match up with the fantasy of your example.

Forget what happens to the balloon between the surface and any given depth; that's immaterial. The orientation of the balloon at any given depth will have no impact on the amount of water the balloon displaces, because the volume of air in the balloon will not change. As one side expands, the other side contracts. Net zero change. Accordingly, the orientation of the balloon will have no impact on its buoyancy.

 

[Diver8492]

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Just a wild guess, but I'm going to go out on a limb and say that I think RJP maybe has a tad more experience and firm grip on physics a little better than you do.
You can't change the laws of physics, even with a good attorney.
Your wing isn't a steel cylinder, it's a soft bag, and just because one end has bungees making it smaller just means that the air will find somewhere else to go. Just like a balloon, fill it up half way and squeeze in the middle, what do the ends do?
This is junior high stuff, and it also doesn't belong in advanced scuba discussions.

Yes that was a wild guess.

Lets take your own balloon as an example. Wrap it with rubber bands on one end. Put a little air in it.

Take it under water. Hold it rubber band side up. The bubble will compress and the loose end of the balloon will collapse. Now turn the balloon upside down. If you cant understand that the loose end expands and displaces more water because there is no force holding it back like the other side, then you should get out your junior high school books.

Or admit that you were schooled here in Advanced Scuba Discussion.

If you want to use a plastic bag instead of a balloon, the idea still works.

Anyhow it sounds like a wing is something to try. Thanks to all the helpful people.