Slowly piecing together doubles?

[Gombessa]

If you're diving a set of LP 95's or 108's you wouldn't need to "overfill" them to have damn near 100cuft as these tanks are 95 or 108 cuft at the rated pressure.

Well sure. I was comparing the HP100s to LP80/85s, which are very close in terms of dimensions and weight. You could go to LP120s as well and have even more gas without overfilling, but as with 95s and 108s, you have corresponding drawbacks by doing so. I wouldn't recommend any of those tanks for beginning doubles users.

I find it hard to believe that anyone could mistake an HP tank for an LP and short fill it. That would be like mistaking an AL tank for an LP steel. They are completely different in size, a 100 cuft HP steel is about the same size as an AL 80 where a LP 95 or 108 is the size of a small school bus.

Again, those are obviously different tanks and I wouldn't expect anyone to make that specific mistake either. But in reality, it's unfortunately not uncommon for shops to mistake HP100s and LP80s/85s; they are nearly identical in size, shape and weight. I haven't been diving that long and it's happened to me a few times (and not just at bush league shops, either).

To tell you the truth, I'm a bit confused by this "some HP and LP tanks are dissimilar, therefore all are dissimilar" kind of logic. I don't recall anybody saying that LP95s look like HP100s.

The older style HP tanks, like the ones I have, are 3500 psi and have a 300 bar valve which will not work with a yoke reg. In fact the thread size on these tanks is different and it's extremely difficult to even find a valve that will fit them any more.

I have no experience with the 7/8" UNF valve tanks, but I'd be wary about getting those if I were planning on doubling them up, for the reasons you mention. I do see them pop up on craigslist quite often though, and have to wonder how many people end up buying them without realizing the valves are different.

 

[DA Aquamaster]

I agree, the Sherwood/Genesis 3500 psi HP tanks use a 7/8" valve that offers very limited manifold options and most of those are the odd Genesis angled variety that I really despise.

232 bar DIN/K valves are standard on 3442 psi tanks and as a practical matter almost no one is requiring 300 bar DIN valves on 3500 psi tanks - limitations in available valves aside.

In terms of LP versus HP most LP tanks will have a high pressure counterpart that is similar in size and weight and both tanks will hold very similar volumes at the same fill pressures. So...if you live in one of the few areas where you can get 3600 psi fill in LP tanks, they can be a very good way to go. On the other hand, if you can't get cave fills, then getting an HP tank of similar weight and dimensions to the LP tank you desire gives you the same volume as the overfilled LP tank without the problems getting an overfill. In that regard, HP tanks offer more flexibility. The downside is that an HP 130 tends to be more expensive than an LP 95.