Need help identifying VOIT Steel tank size

[John C. Ratliff]

Yes, 25" tall (a little more with a plastic boot) is the right size for a steel 72

What's the most recent hydro date?

The steel 72s are popular for such things as shore diving because they weigh less than an AL80, hold almost as much air, and are neutral buoyancy (more or less, depending upon the tank) when empty. That means about seven or eight pounds you don't have to carry and wear. I overfill mine a tad so they are equal to a typical AL80 filled to 3000 psi. As long as they are not really rusty and pitted inside they seem to last just about forever. My oldest is a '62 and I expect my grandkids to be using it when I'm gone.

It doesn't say where you are located but I've heard rumors that some people in Florida and thereabouts overfill them by a lot for cave diving and they could be less likely to pass a hydro test, at least somewhere down the road. Mine were all bought in California and they always pass hydro. I bought two recently on Craig's List and they just got their second hydro test! They were made in the late 60s so they probably just sat there all that time.

I agree that this is probably a steel "72." I put that is quotes because it is not really 72 cubic feet of air, but rather 71.2 cubic feet at a 10% overfill pressure of 2475 psig (allowable with the "+" sign on the hydro).

Now, in order to get the equivalent of 80 cubic feet of air in a steel "72" you have to use the ratio:

80/71.2 = 1.1236

Then multiply that times 71.2.

1.1236 x 71.2 = 2780.89, or ~2780 psig

Most steel 72s are not rated with a "+," so filling beyond 2250 psig is not a good idea. This gives an effective volume of 64.7 cubic feet of air. (71.2 x [2250/2475] = 64.7273)

Filling these tanks to effectively 2800 psig to gain a volume of 80 cubic feet would require a different Overpressure relief disc, which would be illegal to transport between states too. So, my recommendation is not to do this.

SeaRat

 

[JamesBon92007]

I agree that this is probably a steel "72." I put that is quotes because it is not really 72 cubic feet of air, but rather 71.2 cubic feet at a 10% overfill pressure of 2475 psig (allowable with the "+" sign on the hydro).

Now, in order to get the equivalent of 80 cubic feet of air in a steel "72" you have to use the ratio:

80/71.2 = 1.1236

Then multiply that times 71.2.

1.1236 x 71.2 = 2780.89, or ~2780 psig

Most steel 72s are not rated with a "+," so filling beyond 2250 psig is not a good idea. This gives an effective volume of 64.7 cubic feet of air. (71.2 x [2250/2475] = 64.7273)

Filling these tanks to effectively 2800 psig to gain a volume of 80 cubic feet would require a different Overpressure relief disc, which would be illegal to transport between states too. So, my recommendation is not to do this.

SeaRat

I disagree with you on one point--a "typical" AL80 does not hold 80 cu/ft of air at 3000 psi. Most need to be filled +10% to achieve that volume. They are usually about 77.4 cu/ft at 3000 psi.

Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan

If I fill my "steel 72" to just under 2700 psi it equals approximately 77.6 cu/ft. Since the 71.2 is really an average then this number may vary a little, one way or the other. I fill my tanks to about 2650 which should give me a little over 76 cu/ft. Not much difference between that and a "typical fill" on a AL80 which, in my experience, is usually just under 3000 psi.

It is my understanding that the "+" rating is usually absent on the steel 72s because the specific technical information needed by the hydro shop is not available much or most of the time. Also, knowing that my tanks have been hydro tested to 3750 psi I do not feel uncomfortable filling them to 2650. And, of course, I have replaced all of my burst discs with the next higher rating so they won't blow if I leave my tank in the sun for a little while. I did not know that this makes it illegal to transport the tank across state lines. If I ever decide to do that I'll be sure and keep an eye out for the DOT police

They way I look at it is that most people consider 500 psi to be out of air so if I overfill my tanks by 175 psi that is fairly negligible. And it's a long way from 3750 and a long way from the burst pressure of my burst discs. I have read about many people in "cave country" filling these things to 3300 psi or more. I would expect that to cause early metal fatigue and failed hydro tests. Another thing I do not know is the actual burst pressure of a steel 72 in good condition but hopefully it is quite a bit more than the 3750 that they are tested at. In any case if the OP is able to get the tank filled (some shops won't fill old tanks) then my guess is that they will probably fill it to 2475. I can only recall one time out of thousands that a shop filled my steel 72s to 2250, with or without the "+." I can recall only a few times that any AL80s that I've used were filled past 3000.

 

[runsongas]

personal tanks and transport are not bound by dot regulations as long as the quantity is lower than 1000 pounds.

Removing the Confusion Over Scuba Cylinder Transportation | SDI | TDI | ERDI

the prd for a steel 72 should at a minimum fail at 3375 psi or higher (90% hydro pressure), so no need for a different burst disc.

 

[2airishuman]

The correct burst disc is 3750 PSI. Since these are no longer widely available, many divers substitute a 4000 PSI one.

I have maybe 8 LP72s and they are all + rated. It is just a matter of finding a hydro shop that is willing to do it given the iffy state of documentation for these older cylinders.

I do not fill my LP72s beyond 2475 PSI at 72 degrees. If I need more air, I use a bigger cylinder.

 

[Caveeagle]

Of the cave divers I know.. none would advocate "cave filling" a old steel 72 to over 3,000psi.

I own several, and normally fill to around ~2,900 (warm), that will typically cool down to around 2,700.

I do make sure that by 72s all have the lower "normal" rated burst disks in them. As opposed to my other LP95/108 and hp100s that all get the highest rated burst disk I can find.

 

[2airishuman]

personal tanks and transport are not bound by dot regulations as long as the quantity is lower than 1000 pounds.

Removing the Confusion Over Scuba Cylinder Transportation | SDI | TDI | ERDI

I don't think the SDI/TDI/ERDI article is an accurate summary.

Under federal law, there is no mention of a 1000 pound threshold for personal equipment. Federal law would apply, in any case, only to transportation across state lines. Most (maybe all) states have incorporated the federal hazmat regulations into state law, but each has their own exceptions and interpretations.

The "materials of trade" exemption is for paperwork only. The safety rules and marking rules still apply.

Whether and how all of this is actually enforced is another matter.

 

[Caveeagle]

I deal with transportation of co2 tanks in many of my projects at work and DOT rules (and mythology) come up a lot.

I will just say this... chances of being pulled over by the DOT are practically zero, and chances of them inspecting your burst disks is even less than that..

 

[JamesBon92007]

Of the cave divers I know.. none would advocate "cave filling" a old steel 72 to over 3,000psi.

I own several, and normally fill to around ~2,900 (warm), that will typically cool down to around 2,700.

I do make sure that by 72s all have the lower "normal" rated burst disks in them. As opposed to my other LP95/108 and hp100s that all get the highest rated burst disk I can find.

I fill mine to 2700 and they cool down to about 2650. My compressor will only go to 3000 anyway.

The discs I'm using are rated for 3000 psi "service pressure." They didn't specify the actual burst pressure but I couldn't find another option that sounded like it would work.

 

[runsongas]

3000 psi discs are for 5000 psi hydro, so should fail between 4500 and 5000 psi

 

[2airishuman]

With a 5000 psi burst disc you'll usually reach the yield strength of the cylinder wall before the burst disc blows.