There are two factors that influence my perceived obsession with ideal buoyancy that may be foreign to many people on Scubaboard. First is Abalone, which can only be taken freediving from Northern Californias 50° ocean. Freediving too buoyant wastes a lot of energy, thus your bottom time, and too heavy is dangerous. There is a lot of displacement change to a 7mm freediving wetsuit with farmer, jacket, and attached hood. I need 22 Lbs at 33, 24 Lbs at 20, 26 Lbs at 12, and 28 Lbs at 8. Second, I learned to dive before BCs so you had to dial your weight in pretty close. I maintain that preference for perfect buoyancy, which is easier with minimum air in your BC.
How I adjust Buoyancy:
I use 1, 2, 3, and 5 Lb weights for adjustments so getting within 1 Lb is easy. I use two methods which are quick and accurate, but dont work worth spit shore diving. These methods do work in a swimming pool or off a pier. Add just under 3% to a fresh water test for salt water.
If I am solo, I lower a testing rig over the side of my kayak. The rig has a stainless O-ring with a several pieces of small line about 3 long with weights attached. The line from the surface is marked for different target depths. I lower the rig to the depth I want to be neutral, put on less lead than I estimate is needed, and pull myself down the line. At that point, I start picking up different weights until I am neutral. If in Scuba, I already know how much more lead to add to account for empty tanks (test are at the start of the dive with full tanks). I add those weights to pockets or string them onto a second belt (also attached to the O-ring) if it is more than a few pounds.
Off a boat is much easier. I have one of those digital fish scales around $10, accurate to 1 Oz/30 grams, and is also good for weighing luggage. I hang 20-30 Lbs over the side from the scale pull myself to the target depth. A friend reads the change in weight on the scale, which tells me how heavy or light I am. Again, I can almost always adjust for full/empty based on my buoyancy log.
I use 10/my shallowest decompression stop in Scuba, unless I am inspecting a hull or hunting just under a kelp canopy. Target depth for freediving is 33 or working depth, whichever is shallower.
One thing I have noticed is it is best to swim around a little at depth before testing, even in drysuits. Getting all the trapped air out of your wetsuit and pockets is one reason, but there is a surprising amount of air adhering to everything due to surface tension. Obviously, performing the test at different target depths is not necessary in a drysuit.
The OPs (Original Poster) question was about calculating weight change in cylinders. I use these conversions for this and many other diving related calculations in a spreadsheet. Since I am not burdened with remembering the conversions or performing the more laborious arithmetic, I see no good reason not to use the highest precision that might remotely be useful. If all you care about is estimating how much lead to use on a vacation dont bother. If you are carrying a pair of twin 130s plus two 80 Ft³ stage bottles you need to have a pretty good idea how much your buoyancy will change.
