It looks like Bill beat me to it. What the hell, it took awhile to type it up - here it is anyway:
I think it's important to realize that the calculator mentioned in the original post gives an
estimation of how much lead a diver will need, based on certain criteria. The criteria that they use are valid, but the figures they provide are not hard and fast numbers that anyone should rely on. Consider any amount it recommends to be a ballpark figure that will need to be dialed in
for you.
My thoughts on the questions/points you posed:
• Going from Steel 100 to Steel 80 would lightens weights by 2 lbs (+ the weight of the tank)
Not necessarily. Tanks have different buoyancy characteristics. Check out this
tank specification chart, and take note of the Buoyancy Empty column. These are the numbers that you need to understand when changing from one tank to another. Notice that they can vary quite a bit, depending on the manufacturer of the tank.
• Going from novice to proficient is worth 6 lbs of lead!
A more experienced diver will generally be more in tune with his/her position in the water column, and will have a better "feel" as to if air is needing to be added to or released from their bcd. Less experienced divers may use more lead to help arrest runaway ascents. 6lbs seems to be an arbitrary amount, not sure who/what that number is based on.
• A body weight change of 10 lbs is worth 2 lbs
I can't agree with this. All other things being equal, a 200lb couch potato with a sizeable layer of "bioprene" should need quite a bit more lead than a 220lb weightlifter that has 5% bodyfat. Body densities vary, some divers sink like rocks, others float like corks.
• question: How much difference is there among cold-water, drysuit undergarments with respect to buoyancy?
Lots. There are those that do not compress as much as others, trap more air, and are therefore more inherently buoyant.
• Various other sources say that lung size changes buoyancy by 8 to 10 lbs. I assume this is the mechanism by which divers with more experience need less lead, right?
Wrong. Larger divers tend to have larger lungs than smaller divers. With the larger lungs, a diver may have an easier time adjusting their position in the water column (inhale deeply...ascend, exhale deeply...descend). It would be more accurate to say that the larger lung volume allows buoyancy change to be more responsive when compared to a smaller lung volume.
I don't see how divers would be able to gain lung volume with experience.
• According to http://scuba.about.com/od/scuba101/p/Bu ... Divers.htm (see quote below), overweighting makes it harder to control buoyancy.
True.
Your primary goal when calculating the amount of lead you need to carry is to be able to stay underwater until you choose to come up. As the air in your tank is depleted, it becomes lighter, right? So, ultimately you need to be able to stay underwater when your tank is "empty" at 500psi. And you need to be able to hold a safety stop at 15ft, with that same "empty" tank.
A very precise method of determining how much lead you need is to conduct a weight check at the end of a dive. With your tank at 500psi, dump all the air from your bcd and settle on the bottom in 15ft of water. Slowing start removing weight, one or two pounds at a time (this is easier with multiple soft weights instead of one big weight belt), then take a deep inhalation and see if you rise off the bottom. If you don't, remove another one or two pounds and take another inhalation. Repeat this until you rise off the bottom. Congrats, you've just determined your buoyancy for that suit, undergarment, bcd, tank, hood, etc. If you change your gear configuration, be aware that your buoyancy may also change.