How much BC lift do I require?

[devjr]

I've never seen so much brain power brought to bear on this issue. Pug's first post was the right one. Get the 24 pound Diva. That is more than enough buoyancy for a 5'4" woman, size 18 or whatever and who will be diving rental tanks and a wetsuit. Figure out the weights you need to be slightly negative while on the bottom of the pool with a full aluminum tank and the suit of choice. Write down the number and list the gear. Later, with the same outfit, add four pounds to adjust for saltwater. Ensure you can dump at least half the lead when and if the time comes.

Get your buoyancy right and let the BC do its job. Don't worry about it, just add or subtract a little air and it will take care of you.

I've seen newbies trying to vent air while struggling to submerge feet first. In clear water, I just grab his/her hand, point down, and bottoms up. Once she/he is down about 15 feet the bouyancy decreases, the newbie calms down, and the dive unfolds. Later, there is plenty of time to figure it all out and decide if more weight is really needed or not.

Since you've had twelve dives, you've been through this. Guess I'm trying to get a handle on the problem, but I can't seem to find a problem. Oh well...

 

[DivingDoc]

Originally posted by Uncle Pug
Hi Doc,

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

This is just way over my head. I don't know what you're adding and what you're subtracting and I don't know what half of the items are that you are listing.

 

[DivingDoc]

Originally posted by devjr
I've never seen so much brain power brought to bear on this issue. Pug's first post was the right one. Get the 24 pound Diva. That is more than enough buoyancy for a 5'4" woman, size 18 or whatever and who will be diving rental tanks and a wetsuit. Figure out the weights you need to be slightly negative while on the bottom of the pool with a full aluminum tank and the suit of choice. Write down the number and list the gear. Later, with the same outfit, add four pounds to adjust for saltwater. Ensure you can dump at least half the lead when and if the time comes.

I guess what made me start to think about all this in the first place is two things:

1) My LDS insists that most women's BC's are way too low in BLC. That's why he wants to sell me a Zeagle Ranger as opposed to some woman's BC. The guy in the other LDS wants to sell me a Zeagle 911 -- not sure why except that that's what he uses.

2) On my last trip I needed 20 lbs of weight, wearing a dive skin in salt water. The Diva QD has a weight pouch capacity of 20 lbs and a buoyant lift capacity of 23 lbs (if I remember correctly).


I haven't tried it on yet, but the Zeagle Zena really appeals to me because it looks so minimalistic.

http://www.zeagle.com/bc2001_women.htm

But it only has a weight capacity of 20lbs, which might be too small for me. If I need 20lbs wearing a skin -- I'll need more if I wear 3mm or 6mm, right? It does have an optional larger bladder for 34 lbs BLC -- prob as much as I will ever need, right? I don't like the ripcord weight release system, though.

 

[Uncle Pug]

Originally posted by DivingDoc
This is just way over my head.

Sorry...
I should have left it at my first post...
Forget the second one...

 

[devjr]

Don't get hung up on the weight capacity of a BC. Those pockets are really only there to provide "trim". There is no design relationship between a BC's buoyancy and its lead capacity. Most of your lead weight should be on a belt.

I have no idea why anyone would try to convince a recreational diver, and female, that they need one of those humungous Ranger BC's.

I dived for 15 years with no BC at all. Did you ever see J. Cousteau wearing a BC? Now, think how big that 24 pounds of lift would have seemed to us.

 

[Zept]

Can you rent a BC and go for a dive? If you know the make and size of the rental BC, and it is a reasonably recent model, you should be able to look up the buoyant lift. That way you'll have some facts to work with, rather than trying to guess how you'll feel with xxlbs of lift.

The Diva QD is not the floatiest BC in the world, especially in the smaller sizes. The Diva LX offers considerably more lift, size for size. I have the QD in XS, with 18lb of lift, and although I like it a lot, it would be nice to have some lift capacity in reserve. As it is, I have to have it pretty much fully inflated to be comfortable at the surface. Underwater it is fine, because all my diving is tropical. That's just my experience, though...

As devjr says, the weight capacity is not the be-all and end-all, because you can always wear some of your weight on a belt -- gives you more flexibility if you need to dump it.


Zept

 

[donacheson]

Originally posted by DivingDoc


I guess what made me start to think about all this in the first place is two things:

1) My LDS insists that most women's BC's are way too low in BLC. That's why he wants to sell me a Zeagle Ranger as opposed to some woman's BC. The guy in the other LDS wants to sell me a Zeagle 911 -- not sure why except that that's what he uses.

2) On my last trip I needed 20 lbs of weight, wearing a dive skin in salt water. The Diva QD has a weight pouch capacity of 20 lbs and a buoyant lift capacity of 23 lbs (if I remember correctly).


I haven't tried it on yet, but the Zeagle Zena really appeals to me because it looks so minimalistic.

http://www.zeagle.com/bc2001_women.htm

But it only has a weight capacity of 20lbs, which might be too small for me. If I need 20lbs wearing a skin -- I'll need more if I wear 3mm or 6mm, right? It does have an optional larger bladder for 34 lbs BLC -- prob as much as I will ever need, right? I don't like the ripcord weight release system, though.

You seem to be missing a key point - your BC isn't there to lift lead weights. You should be wearing just enough lead so that you're neutral with a near-empty (say 500 psi) tank and an empty BC. Then all of the flotation capacity of the BC is available for other needs. As I pointed out in an earlier post, at the beginning of a dive, some of the BC capacity must be used to compensate for the additional weight of air in a full tank and wet or dry suit compression.

You seem to be diving in the tropics with only a skin and probably using an aluminum 80. If that's correct you must have at least 5 pounds of floatation capacity in your BC; 20 pounds is plenty.

On the other hand, if you're wreck diving and a brass hound who won't trust his treasure to a lift bag, you'd better get as much lift in your BC as possible 'cause someday you might get lucky and greedy and try to haul 75 pounds of brass up to the surface. <G>

 

[devjr]

>>Your BC isn't there to lift lead weights<<<

Graybeard, Zept, I agree.

Doc, you're hung up on the mechanics of buoyancy, and it seems to be related to the recent practice of cramming the weights inside the BC, and which leads the newbie(you?)to conclude there must be some kind of parity.

The purpose of the BC is to compensate for changes in buoyancy which, given your dive plans, should only be in the 5-10 pound range. Nevertheless, trying different stuff in the pool is good if you can arrange it.

One more thing, you said you need 20 lbs of lead to submerge while wearing a "skin" and Al tank. The skins that I've seen are neutrally buoyant, but you said yours is made of compressed neoprene? I'm not familiar with this except as a drysuit material. Are you sure there is no inherent buoyancy in the suit or other gear? No trapped air anywhere? It's just that I find it hard to believe that your native buoyancy is + 16 pounds. I'm not trying to embarrass you but this means that, wearing only a bathing suit, you could float with your shoulders above the water.(?) Some people say that adipose tissue is positively buoyant and that women have a naturally higher ratio. True, but 20 pounds of fat has a buoyancy of only about two pounds. Women have lighter bones and muscle but they also have smaller lungs and smaller surface area, not quite a wash but still not much difference between the two sexes. (A normally fit male would need about 5-6 pounds with an Al tank).

I have been on dives with women who were about your size, and wearing a full wetsuit it seems they were carrying about 20 pounds of lead. I'm trying to say that you should make sure that you are not carrying too much. Here's another spin on it; if you are carrying the right amount, you should not have to inflate your BC to more than 50 percent when on the bottom with a full tank and with a full 7mm suit. With a skin, that number should be more like 20%, just a short "swoosh".

 

[rcohn]

All true. however you should note the following. If someone puts 20 lb in a BC rated for 24 lb lift using an Al80 tank, the math isn't good. Note that most manufacturers overrate their lift capacities, as has been reported by Rodale's testing so I'll reduce the lift by 10%. If you remove the BC at the surface, which is often required when diving from small boat you may easily have a problem.

at the end of the dive we have:
-20 weights
+3 tank buoyancy at around 500 psi
-1 regulator
+24 lift from a fully inflated BC
-2.4 correction for overrated lift

+ 3.6 lb TOTAL

So if you manage to fully inflate the BC BEFORE removing it, and have burned of enough of your air, it should float. Not too comfortable, many people don't enjoy the squeeze from a maximum inflation with a jacket.

Donning a BC in the water with a full tank is much less common. Nevertheless, note that regulators and BCs are rather expensive, it would be nice if they didn't drop straight to the bottom. Also you might want to remove it when the tank contains more than 500 psi.

at the start of the dive we have:
-20 weights
-2 tank buoyancy at around 3000 psi
-1 regulator
+24 lift from a fully inflated BC
-2.4 correction for overrated lift

-1.4 lb TOTAL

Houston we have a problem here.

IMO if you insist on stuffing your weights into your BC (I use a weight belt) you should have sufficient lift to reliably support the rig with a full tank. Note using a tank with less positive buoyancy such as a steel or compact 80 would be even worse.

Ralph

 

[donacheson]

Originally posted by rcohn
All true. however you should note the following. If someone puts 20 lb in a BC rated for 24 lb lift using an Al80 tank, the math isn't good. Note that most manufacturers overrate their lift capacities, as has been reported by Rodale's testing so I'll reduce the lift by 10%. If you remove the BC at the surface, which is often required when diving from small boat you may easily have a problem.

at the end of the dive we have:
-20 weights
+3 tank buoyancy at around 500 psi
-1 regulator
+24 lift from a fully inflated BC
-2.4 correction for overrated lift

+ 3.6 lb TOTAL

So if you manage to fully inflate the BC BEFORE removing it, and have burned of enough of your air, it should float. Not too comfortable, many people don't enjoy the squeeze from a maximum inflation with a jacket.

Donning a BC in the water with a full tank is much less common. Nevertheless, note that regulators and BCs are rather expensive, it would be nice if they didn't drop straight to the bottom. Also you might want to remove it when the tank contains more than 500 psi.

at the start of the dive we have:
-20 weights
-2 tank buoyancy at around 3000 psi
-1 regulator
+24 lift from a fully inflated BC
-2.4 correction for overrated lift

-1.4 lb TOTAL

Houston we have a problem here.

IMO if you insist on stuffing your weights into your BC (I use a weight belt) you should have sufficient lift to reliably support the rig with a full tank. Note using a tank with less positive buoyancy such as a steel or compact 80 would be even worse.

Ralph

Point taken. Good thing he isn't diving double steel 120's. <G>

However, I suspect that Diving Doc doesn't really need 20 pounds of lead with a skin. He should go into a pool with about 500 psi in his tank and a weight belt with 20 pounds of lead on it, empty his BC, take off his weight belt, hold one end in his hand, and see how much lead has to be lifted off the bottom to keep him underwater. That weight lifted is what he needs in fresh water. Add 2.5% of his body weight for salt water.