I posted this earlier on a different forum but Ill post it here for you as well..
Here is some info i thought id pass along:
Step One: Calculate for your body (Typical range 1- 10lbs)
How much weight would you need to make your body alone neutral? There are two methods, one pretty accurate, the other a guesstimate.
Method 1: Take a few weights to a swimming pool. You will be perfectly weighted when you can hang motionless with half a breath and sing when you exhale. Using snorkel can make this easier. Assuming this is a freshwater pool, you can then make the saltwater conversion below. Remember to correct your total weight, body plus lead.
Method 2: Most lean adults are 1 to 4 pounds positive, although musclar, big-boned divers may be a few pounds negative. Remeber your weight when you were in your best physical condition as an adult? How much have you gained since then? How much of the gain is fat? For ever 10 pounds of fat you've gained add two pounds of lead. For ever 10 pounds of muscle , subtract 1 pound of lead.
Step Two: Calculate for your tank (Typical Range -7 to +5lbs)
Buoyancy characteristics of tanks vary. Find your tank below. Using the tanks weight when empty, add lead for positively buoyant tanks, subtract it for negative ones.
Step Three: Calculate for your exposure suit (Typical Range 2 to 20lbs)
There are several methods to estimate how much weight you'll need to compensate for the buoyancy of your weightsuit. Remember, its the buoyancy in shallow water (at your safety stop) that counts. Shell drysuits with full-thickness underwear normally require a few pounds more then a 7mm weightsuit.
Method 1: Take it to a swimming pool. Wearing your exposure suit and weight belt, figure your buoyancy as above in step one.
Method 2: Weigh it. Neoprene is 2 to 3 pounds buoyant for every pound it weighs in air, depending on the quality of the neoprene and age of the suit. (1mm neoprene skins, being proportionally more nylon, are less buoyant)
Method 3: How thick is it? A mans large fullsuit has 2 to 3 pounds of buoyancy per millimeter of thickness (a 3mm fullsuit has about 6 to 9 pounds of buoyancy; a 7mm fullsuit about 14 to 20 pounds). If you wear a differnt size, estimate an adjustment. Keep in mind that thinner suits and neoprene skins will have less buoyancy.
Step Four: Calculate for other gear (Typical Range 2 to 4lbs)
Regulators, gauges, knives, most fins and BCs are slightly negative. The total of your mandatory gear is probably 2 to 4 pounds. Optional equipment can change the equation by a pound or so. Most large lights are slightly negative. Cameras may be either negative or positive by about 1 pound.
Step Five: Total it up (Typical Range 2 to 40lbs)
This is your target weight and should be accurate to within 4 or 5 pounds.
Freshwater/Saltwater Conversion:
By how much do you have to change your weight belt when going from fresh to saltwater? To be accurate, you have to consider the whole package: diver plus equipment.
Stand on the bathroom scale with the equipment and weight that makes you neutral in one medium or other. Or you can estimate. A stand aluminum 80 tank weighs 32 pounds, a 7mm wetsuit is about 8 pounds. For regulator, mask, fins, etc, figure about 15 pounds. Include your weights.
Going from fresh to salt.. multiply by 0.025 and add that amount.
Going from salt water to fresh water.. multiply the total by 0.025 and subtract that amount from your weight belt or integrated-weight BC.
Tank Table: The Weight of Air
Type of Tank Full Empty Change
aluminum 50 @ 3000 psi -3.0 +1.0 +4.0
aluminum 63 @ 3000 psi -2.0 +3.0 +5.0
aluminum 67 @ 3000 psi -5.0 +0.5 +5.5
Steel 72 @ 2475 psi -5.0 +0.5 +5.5
Steel 76 @ 2640 psi -6.0 0.0 +6.0
aluminum 80 @ 3000 psi -2.0 +4.5 +6.5
aluminum 80 @ 3000 psi* -4.0 +2.5 +6.5
Steel 95 @ 3300 psi -15.0 -7.5 +7.5
Steel 95 @ 2640 psi -7.5 0.0 +7.5
* Super 80
This data is based on actual test and differs from mfg. specs, mfgs calculate specs for an ideal tank in fresh water with no valve.
