How much BCD lift do you really need?

[Don Burke]

That is surely true in a single tank straight forward recreational dive. However I would venture a guess and say that most people using a lift cap. that high are diving big doubles and carrying quite a bit of gear. Myself I find 60 pounds to be just right diving my doubles where a full set of tanks are right around 20 pounds negative before manifold, regs, ect.

The static net buoyancy of your tanks isn't an issue to be solved by a BC. Things like that are addressed as a weighting issue.

If you are dealing with twenty pounds of gas shift, that means over two hundred sixty cubic feet, which is fairly common. It is also quite a distance from sixty pounds.

Yes, I do think you are among the misguided.

 

[ScubaRon]

I agree, but it's a little misleading when BCD lift requirements are described as if wetsuit compression, tank weight, body compression, surface flotation, equipment, and bouyancy control all are needed at the same time at the same depth. For example, you need your BCD to provide postive flotation at the surface, however at that time your BCD doesn't need to provide lift for a full tank at depth as well as wetsuit compression. Conversely, at depth, your BCD doesn't need to provide surface flotation; yet people group all these factors together and lump all the lift together and come up with BCD lift requirements. Seems a little strange.

There is a lot of misguidance on this subject, which is why I posted the calculation. I see many people bypass perfectly adequate BCs and wings because they think that they need 50+ lift capacity.

The calculation only measures lift needed at depth, because that will automatically ensure enough lift capacity for surface flotation of your full tank. One consideration you should make though is where to put your weights if you have a weight integrated BC. If you're wearing 30 lbs of weights and your BC has a lift capacity of 30 lbs., don't put all your weights in the BC, because it won't be able to float your full tank if for some reason you have to take it off on the surface.

 

[DiveGolfSki]

The static net buoyancy of your tanks isn't an issue to be solved by a BC. Things like that are addressed as a weighting issue.

If you are dealing with twenty pounds of gas shift, that means over two hundred sixty cubic feet, which is fairly common. It is also quite a distance from sixty pounds.

Yes, I do think you are among the misguided.

Would you also consider this source to be misquided?

http://www.divetekadventures.com/Technical_WingSizing.htm

 

[in_cavediver]

The static net buoyancy of your tanks isn't an issue to be solved by a BC. Things like that are addressed as a weighting issue.

If you are dealing with twenty pounds of gas shift, that means over two hundred sixty cubic feet, which is fairly common. It is also quite a distance from sixty pounds.

Yes, I do think you are among the misguided.

For the record, here is my kit: Double PST LP 104's, 6lb backplate and 55lb wing. I carry 0lbs of additional weight. With just my doubles/plate/wing, it takes about 2/3-3/4 of the wing capacity to float it on the surface. With my rig and my drysuit, I can just barely carry two slung AL 80's and have positive buoyancy at the start of my dive. I really couldn't carry the second 80 unless I was planning to drop it quickly. So, net result is I need every bit of the lift I can get. Now, given that I am carrying 0lbs of droppable weight, how do I solve the weighting issues?

 

[Charlie99]

For people with integrated weight BCDs a separate calculation that should be done is how much lift you need to float your BCD + full tank + lead in pockets. That's so you can throw a BCD over the side from a small boat or a kayak and have it float.

Just as another posted has noted about the separate requirements for lift at depth and lift at the surface, this is really a third independent lift requirement. You just need to meet the highest lift of the 3 requirements, you don't add the 3 to come up with some gigantic amount of required lift.


As Don Burke said, the required lift at depth is
1. wetsuit compression +
2. weight of air in tank.

one also needs to add
3. the number of pounds that one is overweighted.
This shouldn't be more than a few pounds even when having to guess with a new configuration.