How much BC lift do I require?

[Starfish]

Thank you for such an informative posting on BC and lift capacity. I have a jacket style BC which I bought 2nd hand (for $75) when I learned to dive. I now have 29 dives to my name and want to purchse gear more to my needs. I want to move to a BP and wing and was having difficulty deciphering the lift I would need. This has been most informative. --Starfish

 

[JamesK]

This is easily the most informative thread I have read on BC lift capacity. Thank you.

 

[donacheson]

Originally posted by DivingDoc


WOW! What an articulate and knowledgable answer!

My fins are (unfortunately) slightly positive. I really don't know what types of exposure suits I may need in the future. So far, I've only needed a skin. I, myself alone in fresh water -- and even more in salt water - float in just a swimming suit -- probably normal for an overweight middle aged female.

I read in Rodale's that water temperature is the main determinant of the amount of bouyant capacity one needs in a BC -- is that true or not?

I never plan to dive deeper than 130 feet at the most. What is a 7' AJ? A type of fish? 800 feet??? I didn't know it was possible to dive that deep with SCUBA equipment -- do you do this for sport or for a living?

Since I really can't predict what type of diving I will be doing a couple years from now, how do I decide how much lift capacity to go for? Right now, I don't visualize going deeper than 100 feet or so, and in tropical waters. Usually my problem is more getting down than getting up. I think I'd have even more problem if I wore a thicker wet suit -- which would mean LESS lift capacity needed, right? I suppose that someone might talk me into diving a wreck off of N. Carolina at 130' in temperate water, but that would be about my max. limit.

I never plan to buy my own tank. I always will be renting one -- another unknown.

Is there a downside to getting a BC with too MUCH lift capacity? Is something like a Seaquest DIVA with 24 lbs lift capacity going to be too little if I normally need 20 lbs of lead with just a diveskin in salt water?

Ugh -- too many unknowns.

Thanks for your help, though.

Another (and IMHO simpler) way to look at it for recreational diving is as follows.

One should be weighted so that at the end of a dive with a near-empty tank and empty BC, one is neutral at 10 or 15 feet. Use this as a starting point.

At the beginning of a dive, one is heavier by the weight of gas in a full tank (about 5 pounds for an Al80). One is also heavier at depth because of wet (or dry) suit compression. So the BC must be have enough capacity to overcome these two factors.

If one dives with a T-shirt, skin, or otherwise without a compressible suit, the BC needs only to provide about 5 pounds of buoyancy. I view this as an absolute minimal capacity; however, 10 or 15 pounds should suffice.

If one dives with a compressible suit, an additional amount of BC capacity is needed. However, I can't quantify this factor. It obviously depends upon suit material, size, and thickness. Anybody know?

 

[Se7en]

Wet suit boyancy

This is my experience of boyancy changes with different wetsuits, may be useful as a guide, but don't take it as definative.

For a benchmark, I'm 5' 10" and 85 kgs (approx 190 lbs) - so imagine the size wetsuit I'll be wearing...

Exposure Protection Weight carried
Naked 6 lbs + Ali 88
3/2 short 9 lbs + Ali 88
3/2 long 12 lbs + Ali 88
4/3 long 15 lbs + 100cuft Steel tank
5 semidry 18 lbs
7mm farmer John 24 lbs + 100cuft Steel tank
Dry 24 lbs + 100cuft Steel tank

So, if we assume that 6lbs is what is required to sink me, if you take this away from the weight carried with relative suits, it should give an indication of how much boyancy each suit has...

BTW - my 100 cuft tank is about 3 lbs neg when empty from memory, and I have no idea what the ali 88 is...
I also have been diving for a while, so these tend to indicate the minimum weights for the config - ie if I remove a weight, I will be positive at the end of the dive. Also consider that with the 7mm or drysuit, I am most likely carrying a torch, plus reel and bag etc, which all together are slightly negative. I also tend to duckdive and swim down at the beginning of a dive, until all trapped air is replaced by water. (often dive in current, so waiting around on the surface can mean that the bottom is now at 600 ft instead of 60...)

Mike

 

[DivingDoc]

Originally posted by Se7en
Wet suit boyancy

This is my experience of boyancy changes with different wetsuits, may be useful as a guide, but don't take it as definative.

For a benchmark, I'm 5' 10" and 85 kgs (approx 190 lbs) - so imagine the size wetsuit I'll be wearing...

Exposure Protection Weight carried
Naked 6 lbs + Ali 88
3/2 short 9 lbs + Ali 88
3/2 long 12 lbs + Ali 88
4/3 long 15 lbs + 100cuft Steel tank
5 semidry 18 lbs
7mm farmer John 24 lbs + 100cuft Steel tank
Dry 24 lbs + 100cuft Steel tank

So, if we assume that 6lbs is what is required to sink me, if you take this away from the weight carried with relative suits, it should give an indication of how much boyancy each suit has...

Mike

Thanks -- this looks very helpful. But pardon the stupid newbie question -- what's an "Ali 88" -- is that an aluminum 88 cu ft tank? How much do the various tanks weigh, and how much difference is there in the weight of the gas fully loaded in the different sized tanks?

 

[Dee]

Diverlink has a really good article on tank specifications.

Check it out here.

 

[donacheson]

Originally posted by Se7en
Wet suit boyancy

This is my experience of boyancy changes with different wetsuits, may be useful as a guide, but don't take it as definative.

For a benchmark, I'm 5' 10" and 85 kgs (approx 190 lbs) - so imagine the size wetsuit I'll be wearing...

Exposure Protection Weight carried
Naked 6 lbs + Ali 88
3/2 short 9 lbs + Ali 88
3/2 long 12 lbs + Ali 88
4/3 long 15 lbs + 100cuft Steel tank
5 semidry 18 lbs
7mm farmer John 24 lbs + 100cuft Steel tank
Dry 24 lbs + 100cuft Steel tank

So, if we assume that 6lbs is what is required to sink me, if you take this away from the weight carried with relative suits, it should give an indication of how much boyancy each suit has...

BTW - my 100 cuft tank is about 3 lbs neg when empty from memory, and I have no idea what the ali 88 is...
I also have been diving for a while, so these tend to indicate the minimum weights for the config - ie if I remove a weight, I will be positive at the end of the dive. Also consider that with the 7mm or drysuit, I am most likely carrying a torch, plus reel and bag etc, which all together are slightly negative. I also tend to duckdive and swim down at the beginning of a dive, until all trapped air is replaced by water. (often dive in current, so waiting around on the surface can mean that the bottom is now at 600 ft instead of 60...)

Mike

Interesting information. Thanks.

 

[Uncle Pug]

Hi Doc,
I'm not sure how to figure for you because I can't predict just exactly what your future confifuration is going to be....

So just my expample and you take it for what it is worth...

Drysuit (trilam shell) in cold water (47F)....
6# SS backplate, 12# soft weight belt, LP steel 104 single.
27# lift pioneer wing

6# SS backplate, 2# argon inflation, Double 104s, deco bottles, ect.
70# lift explorer wing

 

[donacheson]

Originally posted by FredT
diving with an 80 cubic foot tank this buoyancy change is in the range of 6.5 to 8 pounds, assuming you plan on coming up with around 500psi.
FT

Nice discussion.

However, a minor pick: Your rangle of 6.5 to 8 pounds is a bit high. Dry air at 1 atmosphere (14.7 psia) and 68F "weighs" 1.205 grams per liter. An aluminum 80 is an 11 liter tank, so at 3000 psig it holds 1.205x11x3015/14.7 = 2719 grams = 5.99 pounds of air. After breathing the tank down to 500 psi, it's almost exactly 5 pounds lighter than when full.

 

[FredT]

That's what I get for writing "off the top of my head" without checking references.

FT