1. Is it worth buying an old used AL80 air tank, near it's lifetime but still passing, to use in maintenance,
No. Buy a cylinder, used or new, on the basis of it being something with which you can dive. And, if you happen to have two cylinders, and use a bit of air from one at times, so be it. You can still do a nice dive with 2500psi in an AL80.
maverickNH:
2. I'll eventually buy a few tanks for my local diving. What factors to consider in cf volume? I'm leaning toward steel as a long-term investment and maybe a 120cf HP tank. Why? On the upside, cost/cf appears to be lower than 80 & 100 (not much lower for 140cf) and empty weight/cf appears lowest of 80, 100, 120, 140 tanks.
Acquisition cost / cf is probably not among the top ten considerations in cylinder selection. In contrast, a) volume and b) bouyancy (and trim) characteristics would be at the top of that list. I also don't know if I would pursue steel as a better 'long term investment'. I would, instead, buy cylinders that 'fit' me, both in terms of bouyancy and trim, and available gas supply. Generally, people looking for more gas volume go with steel cylinders (LP or HP) instead of AL - if you want a cylinder larger than 80 cf (or 77cf) you are generally looking at steel. Bouyancy changes, full vs empty, are entirely a function of the volume of gas, as noted, and the bigger the cylinder, the bigger the difference. Most steels start out negative (full) and end up negative (empty). That may not be the case with AL. Trim characteristics, full vs empty, may vary, however, according to metal and size (physical dimensions, not just volume), as well as manufacturer.
Cylinder selection is in many ways a matter of personal preference. Two divers of the same size, with similar gas consumption, may dive the same cylinder, and one will love the trim while the other will hate it. In general, the bigger the cylinder, the more likely the divergence of opinion. Some divers, particularly those under 5' 10" in height, really don't care for a HP120, because it is a longer cylinder, and they feel like their trim is 'foot-down' when they dive it. Taller divers may have a different experience. However, the same, shorter divers may find a HP119 to be ideal. It is shorter, albeit with a larger diameter, but offers an equivalent volume of gas. There are divers who don't care for the HP130 simply because it is heavier, and exceeds the size they are comfortable with. You will hear some divers say that the 130 is large enough to give them problems with rotation in the water - the weight causes them to want to 'turn turtle'. I haven't had that experience with a single 130, and I really like the cylinder as a single, but I have had some divers report it, hence my comment on personal preference.
I agree with Lynne, that you would be well-advised, as a newer diver, to purchase cylinders for single cylinder diving on the basis of what you need for single cylinder diving, rather than on the basis of what will double up nicely. I have used AL80s, HP100s, HP119s, HP120s, and HP130s as singles and as doubles, and they all work. I happen to prefer HP100s for single cylinder diving, and for (most) doubles diving, but that is my personal preference. When I need A LOT of gas for deeper, longer diving, I use my double 130s. But, I have to work a bit harder to maintain optimal trim with them, so I don't use them unless I have to, or unless I simply want to keep in practice.
MaverickNH:
I know I'm not considering all those many factors like what type of diving (shore, boat, wreck, cave, etc) and depths, but am looking for a flexible system, initially for shore & boat to 130ft, some wrecks with no overhead/penetration.
Given your location (New England), with the likelihood of colder water and associated need for thicker exposure protection, and your stated interest in deeper diving, you are most likely to want steel cylinders. If you want a cylinder that affords suficient gas for deeper dives, you are looking at something that will fill to at least 100cf, and possibly to 120 - 130 cf. (Note that I mentioned 'fill to' rather than a specific nominal size, because you will possibly get recommendations on LP as well as HP cylinders. I happen to be a HP cylinder diver, so my recommendations are necessarily limited to HP cylinders.) Depending on your size, strength and inherent trim characteristics - not noted in your post - there are several cylinders that might suit you, starting with the HP100 (a very popular cylinder, albeit at the low end of the volume scale), and going up to the 130. The 100 offers a lot of flexibility -enough gas to go to recreational limits, but still small enough (length and diameter) and light enough, to be versatile in the water. The HP119 would be a second option, followed by a 130. Although there are a few divers using the HP149, it appears to be a bear of a tank. I can't comment on it from personal experience (and can't imagine why I would want to dive it - I would rather go with my double AL80s).
Although it is frequently not possible, it would be best to try these cylinders out before buying. If there are other divers in your area who dive one or more of these cylinders, it is with trying to arrange to use them if at all possible.
