How much BC lift do I require?

[ScubaRon]

Since you're getting the answer piecemeal, here's the complete calculation for recreational diving:

Following is the formula to calculate the lift capacity of your BC for recreational diving:

weight of the air in the largest tank you'll dive with at max. pressure minus 500 psi [A]
+ buoyancy of the thickest wetsuit you'll dive with x 0.7
+ buoyancy of any articles you may be carrying or picking up UW [C]
+ safety margin [D] )
x 1.1 [E]
= BC lift capacity needed.

[A] At the start of your dive this is how much "overweight" you are; your BC needs to compensate for this. E.g. 80 cu. ft. of air at 2500 psi (=3000-500) weights 4.8 lbs. 120 cu. ft. at 2500 psi weights 7.2 lbs.
At 132 ft. your wetsuit will lose approx. 70% of its buoyancy. If you know how much weight you need wearing this wetsuit in the ocean and how much weight you need wearing just swimming trunks in freshwater (e.g. in a pool), your wetsuit buoyancy is the difference. At 132 ft. your wetsuit will loose about 70% of it's buoyancy due to compression. Your BC needs to compensate for this.
[C] Camera's, reels, lights etc.
[D] A safety margin of at least a few lbs is recommended, just in case you need to help your buddy carry something, go a little deeper or whatever.
[E] BC's typically have a 10% tolerance in their lift capacity. So a BC advertised with a 40 lb. capacity, may in reality give you anywhere from 36 to 44 lbs. To be on the safe side we assume the worst.

So a diver with a wetsuit with 20 lbs buoyancy, diving 120 cu. ft. tanks with a max. pressure of 3000 psi, carrying a 1 lbs camera and using a 5 lbs safety margin will need ( 7.2 + (20 x 0.7) + 1 + 5 ) x 1.1 = 30 lbs of buoyancy.
ScubaRon

 

[SeaJay]

Nice post, ScubaRon.

"What he said."

 

[DA Aquamaster]

That covers the under water requirements. But you will still need to add 15 lbs if you actually want to get your head above water, plus a little more if you actually want to be comfortable and have a bit more than your head above water. So 45 lbs minimum if you are diving a 7mm wet suit. Tropical divers can get away with 30 lbs but not a cold water diver.

 

[ScubaRon]

Not really, because at the surface you have the full buoyancy of the wetsuit and the safety margin. So in my example you have 19 lbs at the beginning of your dive (26 lbs at the end) of surface buoyancy. Unless you're very heavyheaded, this should keep your face well clear of the water.

However the calculation shows the minimum amount of lift. There is nothing wrong with having a little (say up to 10-15 lbs) more.
ScubaRon

 

[Karl_in_Calif]

DA Aquamaster once bubbled...
That covers the under water requirements. But you will still need to add 15 lbs if you actually want to get your head above water, plus a little more if you actually want to be comfortable and have a bit more than your head above water. So 45 lbs minimum if you are diving a 7mm wet suit. Tropical divers can get away with 30 lbs but not a cold water diver.

I have seen many divers who are extremely tall and "girthy" who need a whole lot more than 20 lbs of counter-buoyancy for their wetsuits. Although Ron's calculations look good, the underlying assumption of 20 lbs (which is what I use for freedving in a single piece 7mm wetsuit) leads to a false generalization.

DA's 45 lbs is a good general figure.

As long as you bear in mind that there are really tall divers out there who would need more possibly.

 

[cornfed]

Karl_in_Calif once bubbled...

As long as you bear in mind that there are really tall divers out there who would need more possibly.

What does one's height have to do with lift requirements?

 

[jonnythan]

cornfed once bubbled...


What does one's height have to do with lift requirements?

More neoprene = more buoyancy shift

 

[ScubaRon]

Karl_in_Calif once bubbled...
DA's 45 lbs is a good general figure.

Karl, I am purposely not trying to give a general figure, because they can be very misleading: should divers just bypass the on this board so immensely popular Pioneer 36 wing because it has 20% less lift that the "general" figure? The reason I posted the calculation is so people can really decide what the minimum lift is that they need.
Tall or round, heavy bones of layers of fat: it makes no difference if you use the calculation method I provided.
ScubaRon

 

[Karl_in_Calif]

ScubaRon once bubbled...

Karl, I am purposely not trying to give a general figure, because they can be very misleading: should divers just bypass the on this board so immensely popular Pioneer 36 wing because it has 20% less lift that the "general" figure? The reason I posted the calculation is so people can really decide what the minimum lift is that they need.
Tall or round, heavy bones of layers of fat: it makes no difference if you use the calculation method I provided.
ScubaRon

I know you did, Ron. And you gave a great math example.

But the example was misleading, since there was only one.

Its just the way reader-psychology works.

Three examples would have been better. A skinny person, a typical person, and a really tall and large person.

Nice job on detailing the math, by the way!

 

[ChrisM]

That covers the under water requirements. But you will still need to add 15 lbs if you actually want to get your head above water, plus a little more if you actually want to be comfortable and have a bit more than your head above water. So 45 lbs minimum if you are diving a 7mm wet suit. Tropical divers can get away with 30 lbs but not a cold water diver.

Have to take issue with this. For single tank diving? I dive California, 7mil wet off the beach and dry off a boat (sometimes wet). None of the wetsuit divers I know use more than the 36# or equivalent,

I find the 36 floats me nicely at the surface, even in an old ratty pre-compressed 7 mil

Chris