ocpaul
Contributor
Scubaroo:I don't know why suit compression calculations come into play when determining lift.
I think you answered your own question when you said "You need enough lift so that at the beginning of a dive, you can counter your full tanks and your negative weight at depth"
I still believe that the MINIMUM lift you need is your own personal Factor of Safety (F.O.S.) x (weight of air in tank + loss of bouyancy due to suit compression at depth + excess weight beyond neutral at surface at end of dive).
I agree compression of 7mm suit at depth will be more than 4# - I was just using the figure as an example (might be representative of a 3mm suit).
It all comes back to how you are going to set yourself up to begin with. If you weight yourself so you are neutral at the surface when your tank is empty (or near empty), then when you begin your dive and go to depth you would need to have enough lift for the weight of the air + loss of bouyancy due to suit compression. If your overweighted at the beginning of the dive then you would need to add lift for that overweighting also. Remember I also suggested a F.O.S. of 2.0 - this covers a lot of variables and gives you a little added help at the surface. I would also always recommend a secondary lift source (bag, safety marker, etc. just in case).