Explain lift

[deepdiver47]

You will see number like 30, 40 and even 44 lbs of lift.
Exactly what are they talking about.

[3rdEye]

it's the amount of negative buoyancy the full bladder will offset, I believe.

[RonFrank]

Lift is what you do to bring a beer up to the lips!

Seriously, I think the idea is if you take a BC with a 40lbs lift bag, and pile 40lbs of lead on it, it should still float.

I know that different manufacture measure this differently, and many also round the numbers. I'm not sure if they factor in the buoyancy (pos or neg) of the BC or no.

Are you doing something where you NEED to know exactly how this is measured? If not, 30lbs of lift is enough for any singles tank (including steel). 45lbs of lift is enough for most double configurations.

[No Misses]

Here is the short answer...
Lift = Boyancy.
A BC with 30lbs of lift should be able to float 30 lbs of weight at or just below the surface.
Your BC's "Lift" should be slightly greater than your rigs weight. so that it will hold you at the surface (when needed).
Full AL 80 (~1.5 lbs in the water)
Lead (how much do you carry? for this exercize lets say 12 lbs.)
Any other gear that is not neutral boyancy. (lets just say another 4 lbs)
you should have at least 18 lbs of lift (1.5 + 12 + 4 = 17.5)

However, YMMV

[deepdiver47]

I am contemplating converting my 44 lift Zeagle Ranger to a 30 lb lift Brigade. I have never, nor do I ever want to do doubles. I want to travel lighter and hopefully be more streamlined UW as well.

Your definition makes sense to me

[deepdiver47]

Originally Posted by No Misses

Here is the short answer...
Lift = Boyancy.
A BC with 30lbs of lift should be able to float 30 lbs of weight at or just below the surface.
Your BC's "Lift" should be slightly greater than your rigs weight. so that it will hold you at the surface (when needed).
Full AL 80 (~1.5 lbs in the water)
Lead (how much do you carry? for this exercize lets say 12 lbs.)
Any other gear that is not neutral boyancy. (lets just say another 4 lbs)
you should have at least 18 lbs of lift (1.5 + 12 + 4 = 17.5)

However, YMMV

Understood, it is the weight of items in the water that has me confused

[No Misses]

Understood, it is the weight of items in the water that has me confused[/quote]

Now you have to look at displacement.
We all know that an AL 80 weighs roughly 36 lbs on the surface. If it weighs only 1.5 lbs when submerge in water, you can calculate that it displaced 34.5 lbs of water. A cubic foot of salt water weighs about 64 lbs. So, we could also state that an AL 80 displaces about .53 cubic feet of water. Now these are all just rough estimates. If you want to get technical, an AL 80 is 7.25 inches in diameter and 26 inches tall (but it is rounded at the top). Google an algebra site to get the cubic measurement :-) I hope this helped. While searching for weight of sea water, the first site that popped up was Scubatoys.com There is a lot of good information there.

Check out the Scubatoys education section https://www.scubatoys.com/education/archimedes.asp

[deepdiver47]

thanks no misses, I will check it out

[namabiru]

Is there a generality which states that you should have a BCD which will lift 10 pounds or so more than what you're wearing on your belt? So if you have, say, 18 pounds of lead on, your BCD should have 28 pounds of lift, at least? It kind of makes sense, as you figure you have your tanks and such to support.

[RonFrank]

Originally Posted by No Misses

Here is the short answer...
Lift = Boyancy.
A BC with 30lbs of lift should be able to float 30 lbs of weight at or just below the surface.
Your BC's "Lift" should be slightly greater than your rigs weight. so that it will hold you at the surface (when needed).
Full AL 80 (~1.5 lbs in the water)
Lead (how much do you carry? for this exercize lets say 12 lbs.)
Any other gear that is not neutral boyancy. (lets just say another 4 lbs)
you should have at least 18 lbs of lift (1.5 + 12 + 4 = 17.5)

However, YMMV

This is not quite right because people are positively buoyant, and that is based on a LOT of factors. In addition an AL80 with 500psi in it is positively buoyant, not negative depending on the tank (some are nuteral, but not most).

This is why there is no magical formula that works for everyone, and why agencies recommend a buoyancy check.

18lbs of lift is generally enough for most divers using AL80's, but one wants some wiggle room, especially in a situation where you may need to help haul a buddy to the surface.

I find I put VERY little air in my BC when diving. Even deep it's generally not more than a few squirts. I use a 30lbs wing.