Tank I.D. Question

[tlawler]

I just picked up a pair of older Pressed Steel tanks that I plan on refurbishing (stripping, repainting, hydro, O2 clean, VIP, new valves, etc.) and making into a matched set of Nitrox doubles. My question is: how can I determine the Cu. Ft. if they don't seem to match anything in a table. I printed out the cylinder spec. chart from Huron Scuba and tried to match them to the PST cylinders there. Have cyl. specs changed over the years? These tanks are pretty old. The physical dimensions are: 25.0" height (to the top of the threads) and 6.75" diameter. Working pressure is 2250 PSI. On Huron's chart, they are closest to the PST LP-80, but an inch taller and half inch less in diameter.
Is the tank thickness uniform? What I mean is, is the thickness in the neck at the threads the same throughout the length of the cylinder? If it is, I can calculate Cu. Ft.
They are both DOT 3AA, thus Chromoly steel and not really old carbon steel, so I think they should still be good, but if they fail visual or hydro, I won't cry because I did get them very cheap. Thanks for any info.

 

[teknitroxdiver]

They sound like 72s....just out of curiosity what color are they? If these are in fact the 'original' 72s, certain external colors signified that there was/is a coating on the inside of the tank which would best be removed by tumbling.

 

[Rick Murchison]

I just picked up a pair of older Pressed Steel tanks that I plan on refurbishing (stripping, repainting, hydro, O2 clean, VIP, new valves, etc.) and making into a matched set of Nitrox doubles. My question is: how can I determine the Cu. Ft. if they don't seem to match anything in a table. I printed out the cylinder spec. chart from Huron Scuba and tried to match them to the PST cylinders there. Have cyl. specs changed over the years? These tanks are pretty old. The physical dimensions are: 25.0" height (to the top of the threads) and 6.75" diameter. Working pressure is 2250 PSI. On Huron's chart, they are closest to the PST LP-80, but an inch taller and half inch less in diameter.
Is the tank thickness uniform? What I mean is, is the thickness in the neck at the threads the same throughout the length of the cylinder? If it is, I can calculate Cu. Ft.
They are both DOT 3AA, thus Chromoly steel and not really old carbon steel, so I think they should still be good, but if they fail visual or hydro, I won't cry because I did get them very cheap. Thanks for any info.

They sound like 72's
To my knowledge PST never painted any scuba tanks.
Can you copy all the stamped information from the tank shoulder here?
Rick

 

[tlawler]

Well, neither one has a lot of info stamped on them, but here it is. The first one is painted canary yellow with a Dacor sticker on it. 1st line: DOT3AA 2250 2nd line: 62879 3rd line: DACOR 4th line: 1C73+PST The "C" between the 1 and the 7 has a samll uppercase "L" stamped within it, similar to an engineers centerline mark. It has an old Dacor J valve installed in it.

The other tank is plain steel with a Scuba Pro sticker on it. 1st line: DOT3AA 2250 2nd line: K142095 3rd line: USD 4th line: 8C70+ The "C" between the 8 and the 7 also has a small uppercase "L" stamped within it. On the opposite side approx. even with the 2nd line, it is stamped "US DIVERS". It has an older Scuba Pro J valve with a built in fill indicator similar to a gas gauge E---1/2---F installed in it.

I made the assumption they were Pressed Steel tanks based on the PST stamp on the Dacor tank and I think I read somewhere that all US Diver tanks were made by Pressed Steel. The fact that both tanks have the same dimensions should point to the fact that they have the same Mfr. I would think. On the Huron Scuba spec. sheet, both the Faber and the PST 72 are just over 20" in length. I'm guessing that cylinder dimensions have changed over the years and the list from Huron covers more modern cylinders.

 

[teknitroxdiver]

And there are no other stamps like the 8C70+ ones? That's the original hydro date, the C with the L in it is the symbol of the place that hydro'd it, the 70 is the year, 1970.

 

[tlawler]

Oh yeah, they've both been hydroed since the mfr. hydro. I didn't include that info since I didn't think it pertinant to determining the pedigree of the tank. But if it helps, the Dacor tank has 8/78 and 4/91 as subsequent hydro dates and the SP tank has 8/85 as it's only other date.

 

[teknitroxdiver]

It's not really pertinant, I was just kinda concerned because if they haven't been hydro'd since manufacture, air pressure is not something you want in them.

 

[tlawler]

It's not really pertinant, I was just kinda concerned because if they haven't been hydro'd since manufacture, air pressure is not something you want in them.

Thanks for your concern, and you're absolutely right. That would be scary indeed. It's sobering enough that since they did have almost a full charge in them when I received them, they must have been full since the early 90's at least judging from the hydro dates and last vip. Anyhoo...so am I right with the PST manufacture? And have th dimensions of 72's changed over the years? There is a sticker near the neck of the unpainted (SP) tank that states the inner lining has been removed for safety.

 

[rgbmatt]

The first one is painted canary yellow with a Dacor sticker on it.

This is potentially bad! A lot of older painted tanks are not galvanized underneath, and will rust like crazy with the slighest nick. Often, it's an especially tenacious vinyl coating that's a total pain in the *** to remove, and doesn't blister when it rusts - completely obscuring the underlying damage. This is probably the case with yours, judging by the colour. If you plan on using this tank, make sure to inspect the surface very carefully for damage, remove any paint wherever you see rust or dings, and seal the damaged areas very well. Don't trust your local shop with it - they may not know what they're looking at.

I speak from experience on all of the above. However, as long as you take really good care of this tank, there's no reason not to use it.

But, you may not want to use these tanks as doubles. Their sizes probably match well enough, but having an old painted 72 in a twin set can be asking for trouble. You don't want to have to take the whole thing apart in order to touch up any rust spots! If I were you I'd just dive 'em as singles for now and look for another galvanized tank.

I made the assumption they were Pressed Steel tanks based on the PST stamp on the Dacor tank and I think I read somewhere that all US Diver tanks were made by Pressed Steel.

You are correct - both tanks are definitely PST. During the '70s, only two companies made steel 72s for the US market: PST and Norris. Norris tanks are easy to distinguish - they have different necks and more rounded shoulders.

It's sobering enough that since they did have almost a full charge in them when I received them, they must have been full since the early 90's at least judging from the hydro dates and last vip.

There's nothing wrong with this, as long as there was no moisture in the tanks when last filled! It's better for them to have been stored full than to be stuck in somebody's basement with no valves.

Just don't breathe the air (it may not have much oxygen left) and you'll be fine as long as the tanks have been properly cared for.

 

[tlawler]

Yep, you were right. It was some sort of vinyl coating. I tried my normal stripper (strypeze) and it didn't do anything. I then tried some Loctite Chisel Gasket Remover (Methylene Chloride) and it penetrated the vinyl and caused it to bubble up enough that I peeled the entire covering off as a single piece like peeling off a rubber glove. There is no corrosion on the tank and it seems to be painted with a very thin coat of what looks like beige paint, but really thin so that I can see bare steel through it.

Any thoughts on the best way to seal/preserve it? I was thinking I could get it hot dipped galvanized, but the heat could be a problem and it might not be cost effective.
I was also thinking cold galvanizing (spray on) and then painting or maybe something like zinc chromate as a primer and then painting it with appliance epoxy. I'm not really worried about appearance, just protecting it from corrosion. Also I like the idea of finding another galvanized tank for the doubles setup and diving this one as a single. Never can have too many tanks!

Thanks for positively I.D.ing the MFR. for me.

By the way, when you mentioned the possibility of the decreased O2 content, it got me to thinking: If I did an Oxychek on the gas and it did still have close to 21%, would that be a fair indication of the amount of oxidation/corrosion inside the tank, or lack thereof?

Thanks for your informative replies.