Calculating BCD Lift for Future Purchase

hijax · Jun 17, 2012

Hey all,

I went through the SB archives looking for info on how to determine BCD lift capacity and came out a little confused so I was hoping I could provide details of my setup and someone could give me an idea of how much lift I would need:

Here are the details:

Dive in tropical waters specifically Ft. Lauderdale, FL area
I'm 6'4", 240lbs. Weight is mainly due to fat, not muscle
I use 16lbs of weight
3mm full wetsuit (size XXL) (Don't know weight when compressed)
Single tank aluminum 80s filled to between 2500-3000psi (Nitrox)
Regulator set weighs 6lbs
Attached to the BC: compass, surface sausage (w/ SS attachment hook) & spare mask (~3-4lbs)
Depths range from 30-120'
I DO NOT do any tech diving, just recreational.

If there's anything that I'm missing, please let me know. I'm considering getting my own BC and would like to make the right decisions.

Thanks.

Rey

mala · Jun 17, 2012

you have to allow for the loss of lift of the exposure suit when under pressure
add the total weight of the gas
add a bit to make you positive at the surface.

with your kit i would start around the 35lbs mark.

less is more ... at the end of the day you will have to push it through the water

decompression · Jun 17, 2012

I think 35lbs might be a bit much. To give some perspective I dive double steel 120's sidemount in a drysuit, I use a 6lbs backplate and a 37lb lift wing. Obvious the smaller (less lift) results in less drag and less instability of the air moving around. Your weight/body composition has little to do with weight unless it's an extreme case, as your body is mostly water any way. The exposure suit is the main factor, a 3mil suit will not change as much as thicker suits with depth so a very easy test to see how much weight your suit require is to place weights on it enough to sink it in the water you'll be diving in. Do the same with your empty of air BCD. That will give you an idea of lead which combined with the positive vs negative ( not just surface weight) of the rest of your gear for what size bladder you'll need.

Comparison;
I'm 6'3" 260lbs, with a 3mm suit in salt water I use 12lbs with AL80, bladder size 20lbs (actually I have a wing that is adjustable from 20-37lbs, very handy).

chrpai · Jun 17, 2012

My concern about this thread is that your focusing on trivial details and looking for a magic number when it's really more important to understand the "why".

For example. Your reg weights 6lbs and you wear 16lbs of weight. These are unimportant details because if your reg weighed 5lbs you'd need 17lbs of weight. And that's assuming that 16lbs of weight is the correct amount of weight for you anyways.

You're 6'4". Doesn't really matter. You weigh 260lbs.. again, doesn't matter.

What does matter?

a) make sure you are weighted correctly. If you are overweighted that's more you'll need in your BCD.
b) the buoyancy shift of your wetsuit. As this compresses you put air into your BCD.
c) the buoyancy shift in your AL80 tank. It starts off heavier ( you start with air in the bcd ) and it swings positive ( you no longer need that air in your bcd )

Based on all of this, and again assuming you aren't overweighted to begin with, the smallest lift sizes should be more then enough for you.

decompression · Jun 17, 2012

You're 6'4". Doesn't really matter. You weigh 260lbs.. again, doesn't matter.

What does matter?

a) make sure you are weighted correctly. If you are overweighted that's more you'll need in your BCD.
b) the buoyancy shift of your wetsuit. As this compresses you put air into your BCD.
c) the buoyancy shift in your AL80 tank. It starts off heavier ( you start with air in the bcd ) and it swings positive ( you no longer need that air in your bcd )

Based on all of this, and again assuming you aren't overweighted to begin with, the smallest lift sizes should be more then enough for you.[/QUOTE]

hijax · Jun 17, 2012

chrpai:
My concern about this thread is that your focusing on trivial details and looking for a magic number when it's really more important to understand the "why".

I appreciate the feedback but I do have to reply to this. I'm not focusing on trivial details or looking for a magic number. I'm providing details of data that have been mentioned in other threads which seemed relevant to determining what the best baseline amount of lift should be. Perhaps some of it may not be as important but I don't believe that it's trivial.

My main focus is to try to determine where I should be at as a minimum so I can make a smart decision from there. The set of conditions you mentioned later were appreciated.

Thanks.

Rey..

---------- Post added ----------

@decompression: In your last message, it looks like you were trying to reply to a post and something went wrong.

GrumpyOldGuy · Jun 17, 2012

If you are correctly weighted, the only thing your BCD really needs to compensate for on a normal dive is the amount of buoyancy lost as your wet suite compresses at depth plus the weight of the gas you use. This is pretty much what others have said. Its not very much in your case, unless you decide to dive a 5 or 7mm in cold water someday.

The other things not mentioned is it needs to float you comfortably in the water high enough and also float your rig should you need to take it off on the surface. Again, not very much unless you use integrated weights, then you need to make sure it can lift you weights plus maybe another 5-6 lbs.

Most divers tend to get too big of a BCD the first time around thinking more is better. I have a couple of seldom used BCD's with 55lbs of lift - been there, done that.

I use a 30lb wing for single tank diving with thick 8mm wet suit, (220lb male, 30lbs lead, etc...)

If I was dedicated to diving warm water in 3mm I would go for 15-17lbs of lift and have a smaller profile.

chrpai · Jun 17, 2012

My point is if you have a math formula that says X+Y=Z and you are trying to solve for Z it really does no good at all to mention the data A B and C as that is trivial, superfluous information.

decompression · Jun 17, 2012

Ya, quick fingers. So, simply ensure that your positive capability (lift of wing) is more than your negative. Keeping in mind, the positives and negatives of your gear at the beginning and end of your dive. Your positive (lift) must be able to keep you neutral until the end of your dive(out of the water). The benefit of a balanced rig is that as you have no buoyancy redundancy (drysuit) it will enable you to remain neutral and avoid a bouyant ascent. If you'd like a little presentation I do on a balanced rig let me know. By the details you've given, a BCD with aprx 20lbs should be fine.

boat sju · Jun 18, 2012

hijax,

How much of your weight is integrated and how much is on your belt? GrumpyOldGuy's point of floating your rig with a full tank will probably be the ruling factor. Plus a few extra pounds of buoyancy as a factor of safety is a good idea too. Integrated weight plus negative buoyancy of a full tank (6#) plus 6# for your reg/light/line cutter etc. = ??????

boat

Calculating BCD Lift for Future Purchase

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hijax

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mala

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decompression

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chrpai

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decompression

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hijax

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decompression

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