Weight has everything to do with it. Everything. You must displace your weight in water in order to sink. As salt water weighs 102.5% as much as fresh water, and since you'll displace the same volume of water whether fresh or salt, you'll displace 102.5% more salt water by weight than you did fresh, and must compensate by adding weight - once you have your weighting for a particular set of gear figured out in the pool, you add 2½ pounds per hundred pounds total weight to get what's right for salt water. Every time. In this case Robz said he was wearing only a skin, so he's only having to sink his body, BC & tank - later he said it was an AL 80 - no exposure suit.sheck33 once bubbled...
i am assuming you mean an aluminium 80 for a tank?
this is very generalized. It depends on what BC you are using and what tank and exposure protection is used.
There is a big difference in bouyancy between a simple back inflate wing like Halcyon's and say a Zeagle ranger BC.
A regulator isnt even in the equation. Also someone's weight is NOT a very relevant factor. The MAIN things that are bouyant are the exposure protection, BC (if wearing something bulky like a Zeagle) and the difference full/empty tank one has to account for.
And of course 500 psi only equals 13 CF when using a 80 CF tank with a WP of 3000 psi
But you're making things entirely too complicated for yourself... just take each piece of gear to the pool and see how much weight it takes to sink it, or, using a balance, how much weight it takes to hold it up. Make sure the lead you're using for these calculations is submerged when you do the measuring. Measure the various tanks at "end of dive" pressure (typically 500 psi, but whatever suits you). I measure my tanks with regulator attached, as those two are unlikely to be used separately. Include your body on the list - with a nearly full breath. Write the weights down and keep the weighting list in your logbook. You only have to do this once for each piece of gear you use. (more than once for your body if you gain or lose a bunch of weight) Now, when you get ready for a dive, just add up the total weight needed (algebraicly) to sink you and what you're wearing, and if it's salt water, add 2½ pounds per hundred pounds total weight (your body weight + all your gear + your fresh water weights) and you'll have perfect weighting every time. Every time.
Rick