- If you want to be able to use all the air in your tank, you need to carry ballast for it. If you weight yourself based on getting out with 1000psi left, then you will become positively buoyant at your safety stop if you breathe your tank down below 1000psi.
I believe in weighting myself to be able to use all the air in my tank - just in case. As in, just in case something unforeseen happens and I stay down long enough to go into deco, for example. Or, even more likely, I used more gas than I expected. Maybe there turns out to be a current on the bottom and I suck more gas because of swimming harder than usual.
Assuming that you will have the weight of 1000psi (2# in an AL80) of your gas in your tank when you are doing your safety stop is, well, not what I would do.
- An AL80 has 6 pounds of gas. If you want to be able to use it all, you will be (at least) 6 pounds negative at the start of your dive. There is a slight difference in the weight needed, depending on whether you weight to be neutral at 15' or at the surface. Personally, I prefer to weight myself to be neutral at the surface. Weighting to be neutral at 15' means that if you accidentally ascend a few feet (and you're wearing a wetsuit that uncrushes a little bit), you will become positive and have to struggle to get back down to hold your safety stop. If that happens to me, I don't want to have to fight to get back down to 15'. Wearing a wetsuit and weighting to be neutral at the surface means that if I finish my safety stop with 500 psi in my tank, I will have a very tiny amount of air in my BCD. Which means if something happens before I'm ready to go the surface and I find myself at 7' and wanting to go back down, I can let that little bit of air out to get neutral, exhale deeply, and descend. And if I breathe my tank down to the last dregs, for some reason, I'll still be hovering comfortably at 15', not fighting to stay there.
- Corollary: If you are properly weighted with an AL80, you are 6# negative at the start of your dive. In that case, you absolutely should not have to fin down from the surface (assuming an empty BC). Particularly if you have thin or no exposure protection on. Thus, I agree with previous posters that you may be diving a bit underweighted. Or you may just need to pull open the neck of your suit, front and back, to burp it.
- A bigger tank holds more gas, so (if you're properly weighted), you will be more negative at the start of the dive. With a 100, you'll be a bit under 8 pounds negative. With a 120, you'll be 9. Roughly. It doesn't matter whether they are aluminum or steel, HP or LP. It's the weight of the gas that you will (possibly) use that matters. Again, assuming you want to be able to use all the gas you're carrying.
- So, once you have a baseline, changing tanks is just a matter of looking up the cylinder specs and comparing to your baseline, then adding or subtracting the appropriate amount of weight. If your baseline is an AL80 and you switch to a PST E7-100 HP100, the specs (for empty buoyancy) go from +4.4 (for the 80) to -1 (for the 100). The HP100 will be 5.5# more negative (than an AL80) when it's empty, so you can take 5.4# of weight off your rig to have the same buoyancy at the end of your dive as you would with your baseline. You can google scuba cylinder specifications to find various sites that have big lists of tanks and their specs. AL80s seem to be pretty much the same. Steel tanks vary quite a bit. E.g. the PST E7-100 is -1# empty, but the Faber HP100 (also a HP steel 100) is -7.26# when empty.
- If you take your wetsuits to a pool and use some weights to sink them, you can figure out what each one's buoyancy is. Then, as long as one suit is in your baseline, you can do the arithmetic to figure out weighting for each other suit. You could also do this experiment in a bathtub or plastic bin, if it's deep enough.
- For a drysuit, well, I took mine to a pool and put it on, then got in with nothing else on and kept adding more lead to a belt I was holding until it was enough to get me to sink below the surface. 10# wouldn't sink me, but 12# would.
- If you are diving in warm water with minimal exposure protection, you probably only need a BCD with around 20# (or less) of lift capacity. If the BCD you're using has significantly more lift than that (say, more than 30#), then it will make buoyancy control somewhat more difficult.
- I have had the experience before (first post-OW cert dive) where I got in for my first dive of the day and couldn't get down from the surface. My DM swam over and pulled the neck of my suit open, air burped out, and then I could get down. Now I know.
Bottom line: Tanks are a well known quantity. You just have to look up the specs. Your personal buoyancy and the buoyancy of your exposure protection are the only major variables. You can check those things in a pool (or maybe even your bathtub). You can even pretty easily find the info to convert those numbers from fresh water to salt water. I think it's something like things are 3 or 4% more buoyant in salt water (depending on the exact salinity of your location, of course) - but totally don't remember for sure. With the baseline info you already have, that will let you calculate estimates that are pretty close, for each configuration you want to use. If being a bit off and finding yourself overweighted by, say, 4 or even 6# makes your BCD the star of the dive, then your buoyancy control may not be as good as you think it is.
