How much BCD lift do you really need? [Archive] - ScubaBoard - Scuba Diving Forum - Diving Social Network

View Full Version : How much BCD lift do you really need?


Sponsored Link
Fun2dive
May 11th, 2004, 05:08 PM
Hello Everyone!

I am a newly certified open-water diver who just returned from diving in ST. John. I was wondering why divers purchase BCD's that have such high lift capacities? From what I understand, the BCD's purpose is to fine tune bouyancy at depth and float you comfortably at the surface (not to say overcome the amount of weight your carrying). I guess what I'm asking is, once I'm properly weighted (regardless of exposure suit or equipment) wouldn't 5LBS. of lift make me positively bouyant?

Thanks!

ScubaRon
May 11th, 2004, 05:39 PM
I posted the weight calculation method in this thread (http://www.scubaboard.com/t39683.html) and if you read it you will see what the requirements are for.

The high lift capacities are primarily for technical diving with multiple tanks.

ScubaFriend
May 11th, 2004, 06:18 PM
Well i am very lucky i dive in warm clear seas and dont even need to wear a wetsuit at anytime of the year, i have just purchased a BCD that i am very happy with it has 18 ponds of lift....yes just 18 pounds and it is great it is enough to keep me high out of the water when i need it. Other details are i wear as i said no wet suit and 4lbs of weight ans i weigh about 200 pounds

Don Burke
May 11th, 2004, 07:01 PM
Hello Everyone!

I am a newly certified open-water diver who just returned from diving in ST. John. I was wondering why divers purchase BCD's that have such high lift capacities? From what I understand, the BCD's purpose is to fine tune bouyancy at depth and float you comfortably at the surface (not to say overcome the amount of weight your carrying). I guess what I'm asking is, once I'm properly weighted (regardless of exposure suit or equipment) wouldn't 5LBS. of lift make me positively bouyant?

Thanks!Let's look at the dive backwards.

You end up at your shallowest stop with your tank at about five hundred and your BC flat. Before the ascent to the stop, you were at the bottom with a compressed wetsuit which makes you displace less water. When you first got to the bottom, you had a nearly full tank and a compressed wetsuit.

My Bare 5/3 is about sixteen pounds positive on the surface, about twelve pounds positive at ten feet, and about four pounds positive at one hundred feet. If I'm weighted for ten feet, I'll be about eight pounds heavy at one hundred feet just due to the suit.

An AL80 is about five pounds heavier at three thousand psig than it is at five hundred psig.

That's thirteen pounds that the BC needs to carry at the beginning of the dive. Bigger tanks and thicker wetsuits will change more.

Add a little for constriction of the BC against the tank or your body. Add a little more for a little higher ride at the surface.

Generally, twenty-something pounds of lift will get the job done for me with a single tank. I happen to think those people using seventy to one hundred pounds of lift are misguided.

Fun2dive
May 11th, 2004, 09:08 PM
Thanks, for the great info. It seems then that wetsuit compression is a major factor in needing extra lift. However, this is only temporary as you return to the surface compression decreases and your bouyancy increases. In your example your Bare 5/3 loses 8 pounds between 10 and 100 ft. so it makes sense then that your BCD needs to be able to provide 8 pounds @ 100ft. But as you decrease your depth your BCD no longer has to compensate for wetsuit compression.

jbd
May 11th, 2004, 09:16 PM
are you dealing with wetsuit compression but also there is compression of your abdomen as well. Those of you who dive witha weight belt will undoubtably notice that the weight belt gets real loose the deeper you go. Much of this is due to abdominal compression.

Don Burke
May 11th, 2004, 09:22 PM
Thanks, for the great info. It seems then that wetsuit compression is a major factor in needing extra lift. However, this is only temporary as you return to the surface compression decreases and your bouyancy increases. In your example your Bare 5/3 loses 8 pounds between 10 and 100 ft. so it makes sense then that your BCD needs to be able to provide 8 pounds @ 100ft. But as you decrease your depth your BCD no longer has to compensate for wetsuit compression.True enough. That's one reason you vent air from a BC on ascent.

If you don't have enough BC to carry that eight pounds (plus whatever gas weight there is) at one hundred feet, how do you stay neutral while you're down there? The temporary problem still needs to be solved.

There's a thread with some math in it already on the board.

http://www.scubaboard.com/t39683.html

Fun2dive
May 11th, 2004, 09:46 PM
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.

Don Burke
May 11th, 2004, 09:56 PM
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.Actually, the BC does need to lift a full tank on the surface.

The displacement change due to body squeeze gets lost in the other factors.

Bouyancy control isn't an additional factor. It's what you do about all of the other factors.

I see what you're saying. It's easier than it looks.

scubacowboy
May 11th, 2004, 10:04 PM
I happen to think those people using seventy to one hundred pounds of lift are misguided.


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.

Don Burke
May 12th, 2004, 10:57 AM
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
May 13th, 2004, 06:49 PM
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
March 27th, 2006, 01:49 PM
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
March 27th, 2006, 02:03 PM
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
March 27th, 2006, 02:16 PM
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.

Sponsored Link

Search Engine Friendly URLs by vBSEO 3.6.1