Used tanks

[2airishuman]

many hydro shops won't hydro them, there are some, they are out there, but many won't. Many dive shops won't VIP them either

This varies regionally.

I've never run into the least bit of complaining from dive shops or hydro shops in Minnesota, other than some good-natured grumbling about having to fiddle with tapered threads during a VIP, very similar in tone to the good-natured grumbling I get whenever I bring in HP120s (because they're heavy and take the shop half an hour to fill). As noted above, hydro shops vary in which cylinders they'll consider for a + rating, but that's different.

 

[JamesBon92007]

Hey I am looking at buying some old steel tanks last hydro was in 84. Both are full of air. Is it a bad idea to buy tanks made in the 70s. Any help would be appreciated. Thanks

Just about everything seems to have been covered already but I just wanted to add my experiences:

I own seven steel tanks and all of them are from the 60s and 70s. They all pass hydro and VIP however, since I got my own compressor, I do my own inspections and internal cleaning. I still get them hydroed every five years (or close to it). I downloaded one manufacturers instructions for doing tank inspections which included both aluminum and steel and have de-comissioned two aluminum tanks that were made in the 80s. I would be very wary of buying any aluminum tanks from that era but you said steel.

Valves need to be serviced too so I would look for signs of corrosion and I generally stick to 3/4" valves but that is not a deal breaker for me. If the tanks look OK from the outside and they are steel 71.2 tanks I probably would not hesitate to buy them but I would probably not pay $50 each unless they had been recently hydroed.

I also have a small CO2 tank (1800 psi) with a 1/2" thread which I was going to have hydroed but my fire extinguisher guy considered hydro testing on such tanks a complete waste of time and money. He's never seen one that failed. Unfortunately the threads are different so my 1/2" J-Valve won't fit

 

[Caseyd1971]

Hey I was also wondering when figuring cubic ft on a streel 72 do you use the service pressure of 2250 or the + pressure of 2475. I seem to be getting conflicting explanations

 

[pinshootr]

should be able to pick up 72's for around 20-30 dollars

 

[JamesBon92007]

Hey I was also wondering when figuring cubic ft on a streel 72 do you use the service pressure of 2250 or the + pressure of 2475. I seem to be getting conflicting explanations

That would be about 71.1 to 71.3 cu/ft at 2475 psi. At 2250 psi it's closer to about 65 cu/ft.

A typical aluminum 80 tank is (usually) about 77.4 cu/ft at 3000 psi. They need the extra 10% (3300 psi) to get up to 80 cu/ft.

Some tanks may vary.


In my experience most dive shops (at least they used to) fill the steel tanks to 2475 psi and the aluminum 80s to 3000 psi so the difference between the two, for practical purposes, is about 6 cu/ft. or about 8% more air with the aluminum 80.

Here's a chart that lists the capacity for many tanks:

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

I have seen it stated that a typical aluminum 80 holds 80 cu/ft at 3000 psi but that would seem to be a misconception generated by dive shops trying to sell tanks.

 

[rx7diver]

A typical aluminum 80 tank is (usually) about 77.4 cu/ft at 3000 psi. They need the extra 10% (3300 psi) to get up to 80 cu/ft.

Be careful! No aluminum 80 cylinder that I am aware of should ever be overfilled. So far as I am aware, there is no "10% overfill" that applies to aluminum cylinders. Do NOT overfill an aluminum cylinder!

Safe Diving,

rx7diver