seems there are always questions about how much weight someone needs or how much buoyancy is needed for some application, and then many answers that if you read them all you start to get an idea of what you need.
I thought it might be a good idea to give my opinion of the principals behind such decisions. (how I do it, YMMV)
(moto - disclaimer) some things are negative in water, like weight belts, knives, canister lights, regulators some BCs, some people.
and some things are buoyant like wet or dry suits, some fins,some BCs some people.
when added and subtracted the net result is very important to diving.
SO:
80CF of air weighs 6# an empty AL80 is 2# buoyant (most).
average lung capacity is 12-14# for men and 8-10# for women (tidal volume is less)
some people sink some float you know who you are but you may not know by how much.
for the rest, put each item in the water and find out if it sinks or floats. then if for instance you know you have the right kit weight in warm water and you are about to go diving with a new 7mm wetsuit, put it in the water and add weight to it till it sinks, then you know how much to add to your base. if you happen to be a fresh water diver and are about to go to salt, well you got some problems however on average I would bet most people only swing about 6# or less. (if you are one of those floaty people then maybe a little more). multiplying by a number will not work since you do not know what the biggest factor (you) weighs in salt water.
if you know each piece then you can add and subtract on the fly, ("gee it is too hot let me dump this 3mm, how much weight should I lose"?)
the goal would be to dive with no more air in your BC than the weight of the air you plan to breath. (5# for an AL80 assuming you will leave 500psi in the tank). if you are neutral at your safety stop with 0 air in your BC and on the boat with 500#(opinion) you are golden.
every dive unless it is the same kit same water same bloaty feeling in your belly should be preceded by a weight check.
Full lungs empty BC, empty tank, on the surface you float head just about 1/2 mask out of water, empty your lungs and you sink slowly. (simple version but please feel free to discuss doubles and dry suits if different).
Now how much buoyancy do you need in a BC? well if you had a buoyant Dry suit and you knew how much weight it took to sink it you would know how much lift you are going to need in your BC should your dry suit flood. similar with a wetsuit crushing at depth but at some point when it crushes enough you are going to get cold and want to go up. with doubles, the manufacturers specs on what the tanks will weigh in the water full compared to the specs of your single. knowing the parts lets you figure out a worst case scenario.
your body does not compress, the air in your BC does, every change in depth changes your buoyancy unless you have very little air in your BC, lots of problems can be avoided by proper weighting. the adage throw in a few extra pounds and don't worry about it is poor advice even and maybe especially for beginners.
and a last note, I am an average swimmer, I can swim at least 17#(try this yourself) up with no problem what is the most you would ever weigh in the water with a failed BC? do not exceed the amount you would be able to swim up.
I thought it might be a good idea to give my opinion of the principals behind such decisions. (how I do it, YMMV)
(moto - disclaimer) some things are negative in water, like weight belts, knives, canister lights, regulators some BCs, some people.
and some things are buoyant like wet or dry suits, some fins,some BCs some people.
when added and subtracted the net result is very important to diving.
SO:
80CF of air weighs 6# an empty AL80 is 2# buoyant (most).
average lung capacity is 12-14# for men and 8-10# for women (tidal volume is less)
some people sink some float you know who you are but you may not know by how much.
for the rest, put each item in the water and find out if it sinks or floats. then if for instance you know you have the right kit weight in warm water and you are about to go diving with a new 7mm wetsuit, put it in the water and add weight to it till it sinks, then you know how much to add to your base. if you happen to be a fresh water diver and are about to go to salt, well you got some problems however on average I would bet most people only swing about 6# or less. (if you are one of those floaty people then maybe a little more). multiplying by a number will not work since you do not know what the biggest factor (you) weighs in salt water.
if you know each piece then you can add and subtract on the fly, ("gee it is too hot let me dump this 3mm, how much weight should I lose"?)
the goal would be to dive with no more air in your BC than the weight of the air you plan to breath. (5# for an AL80 assuming you will leave 500psi in the tank). if you are neutral at your safety stop with 0 air in your BC and on the boat with 500#(opinion) you are golden.
every dive unless it is the same kit same water same bloaty feeling in your belly should be preceded by a weight check.
Full lungs empty BC, empty tank, on the surface you float head just about 1/2 mask out of water, empty your lungs and you sink slowly. (simple version but please feel free to discuss doubles and dry suits if different).
Now how much buoyancy do you need in a BC? well if you had a buoyant Dry suit and you knew how much weight it took to sink it you would know how much lift you are going to need in your BC should your dry suit flood. similar with a wetsuit crushing at depth but at some point when it crushes enough you are going to get cold and want to go up. with doubles, the manufacturers specs on what the tanks will weigh in the water full compared to the specs of your single. knowing the parts lets you figure out a worst case scenario.
your body does not compress, the air in your BC does, every change in depth changes your buoyancy unless you have very little air in your BC, lots of problems can be avoided by proper weighting. the adage throw in a few extra pounds and don't worry about it is poor advice even and maybe especially for beginners.
and a last note, I am an average swimmer, I can swim at least 17#(try this yourself) up with no problem what is the most you would ever weigh in the water with a failed BC? do not exceed the amount you would be able to swim up.
