tmassey
Contributor
To me, there are four separate factors to consider with steel tanks:
1) Coating. There are two types: hot-dipped galvanized and epoxy-coated. HDG used to be the "old" tanks: they stopped being made something like mid-late 2000's (decade). They are much more durable: they're much less susceptible to corrosion, for example. The epoxy tends to chip, and creates crevices for water to stay, which creates corrosion.
One item that is on the plus side for the epoxy tanks: Faber (who made the epoxies) *also* passivated the *inside* of the tank. It makes things like flash rust a non-issue on those tanks.
But now we have the best of both worlds: Faber HDG. HDG outside, passivated inside. Sweet. The problem is: those are 2 years old or newer: you'll have to buy them new.
2) HP vs LP. HP tanks are very likely to be filled to a useful pressure by any dive shop. LP's, at least outside of N. Florida, will be filled to a somewhat un-useful pressure: the one they're actually rated for, which is *minus* 10% of its stated capacity. Unless you live in N. Florida or own your own compressor, I'd stay away from LP tanks. There are nearly identical versions of any common LP tank -- except LP85's. Those are unique, and very nice, but only if you can fill them to a useful pressure (beyond their rated pressure).
3) Neck size. The original "true" HP tanks are rated for 3500 PSI. For lots of historical reasons, it was decided to make these incompatible with the standard SCUBA infrastructure. So they changed the neck size of the tanks to a smaller 7/8 UNF thread (instead of the*larger* 3/4" NPS thread). Trick to identify: quarters can go into the larger 3/4" NPS, and can't the smaller 7/8 UNF.
Note that there are late-model PST and Worthington HP tanks with the 3/4" (larger) neck. They're rated at 3442 PSI instead of 3500 PSI to avoid the regulations that forced the different-sized neck. From a practical standpoint, they're identical tanks, just altered ever so slightly for legal reasons.
The only time this matters is if you want to double them. Getting an isolation manifold for the tanks either means using the then-current Genesis-style manifold and bands (with slanted DIN openings and slightly non-standard spacing), or hunting around for a set of Thermo or Dive Rite manifold that can then use standard bands. If you're not going to double them, then getting valves is not at all difficult if you don't mind used.
Given the small hassle this presents, the smaller necked tanks should sell a bit cheaper, but as a rule, not *that* much cheaper. They're just much easier to get, so I've ended up with quite a few -- and I've managed to double up a total of four sets. It's really not that difficult.
4) Tank construction. There are two ways of making them: deep drawn and spun. Google if you want details. The biggest thing to know: spun tanks are *extremely* heavy. Like 50% heavier than deep-drawn, and with substantially negative buoyancy. Some people consider this a "benefit" if you're diving single-tank with a drysuit: less lead! And I think they would simply be wrong. There is *zero* reason to dive a spun tank.
Spun tanks are made by Heiser and Asahi. Sometimes the were resold by others (very late Genesis tanks were spun). The Asahi tanks are marked "SPUN". Avoid them. Which shouldn't be hard: they're not very common because nobody bought them. And you can tell because they weigh noticeably more.
I know that's more than you asked for. What can I say: I've spent a lot of time on tanks...
And on manifolds, too, it seems...
1) Coating. There are two types: hot-dipped galvanized and epoxy-coated. HDG used to be the "old" tanks: they stopped being made something like mid-late 2000's (decade). They are much more durable: they're much less susceptible to corrosion, for example. The epoxy tends to chip, and creates crevices for water to stay, which creates corrosion.
One item that is on the plus side for the epoxy tanks: Faber (who made the epoxies) *also* passivated the *inside* of the tank. It makes things like flash rust a non-issue on those tanks.
But now we have the best of both worlds: Faber HDG. HDG outside, passivated inside. Sweet. The problem is: those are 2 years old or newer: you'll have to buy them new.
2) HP vs LP. HP tanks are very likely to be filled to a useful pressure by any dive shop. LP's, at least outside of N. Florida, will be filled to a somewhat un-useful pressure: the one they're actually rated for, which is *minus* 10% of its stated capacity. Unless you live in N. Florida or own your own compressor, I'd stay away from LP tanks. There are nearly identical versions of any common LP tank -- except LP85's. Those are unique, and very nice, but only if you can fill them to a useful pressure (beyond their rated pressure).
3) Neck size. The original "true" HP tanks are rated for 3500 PSI. For lots of historical reasons, it was decided to make these incompatible with the standard SCUBA infrastructure. So they changed the neck size of the tanks to a smaller 7/8 UNF thread (instead of the*larger* 3/4" NPS thread). Trick to identify: quarters can go into the larger 3/4" NPS, and can't the smaller 7/8 UNF.
Note that there are late-model PST and Worthington HP tanks with the 3/4" (larger) neck. They're rated at 3442 PSI instead of 3500 PSI to avoid the regulations that forced the different-sized neck. From a practical standpoint, they're identical tanks, just altered ever so slightly for legal reasons.
The only time this matters is if you want to double them. Getting an isolation manifold for the tanks either means using the then-current Genesis-style manifold and bands (with slanted DIN openings and slightly non-standard spacing), or hunting around for a set of Thermo or Dive Rite manifold that can then use standard bands. If you're not going to double them, then getting valves is not at all difficult if you don't mind used.
Given the small hassle this presents, the smaller necked tanks should sell a bit cheaper, but as a rule, not *that* much cheaper. They're just much easier to get, so I've ended up with quite a few -- and I've managed to double up a total of four sets. It's really not that difficult.
4) Tank construction. There are two ways of making them: deep drawn and spun. Google if you want details. The biggest thing to know: spun tanks are *extremely* heavy. Like 50% heavier than deep-drawn, and with substantially negative buoyancy. Some people consider this a "benefit" if you're diving single-tank with a drysuit: less lead! And I think they would simply be wrong. There is *zero* reason to dive a spun tank.
Spun tanks are made by Heiser and Asahi. Sometimes the were resold by others (very late Genesis tanks were spun). The Asahi tanks are marked "SPUN". Avoid them. Which shouldn't be hard: they're not very common because nobody bought them. And you can tell because they weigh noticeably more.
I know that's more than you asked for. What can I say: I've spent a lot of time on tanks...
And on manifolds, too, it seems...
