Check this vintage ad out...

[Akimbo]

wo surprises (for me): (1) The fact that the bottom isn't substantially thicker than the sides.

In a fully optimum design, spherical ends can be significantly thinner than cylindrical walls on pressure vessels. This may be compromised by welds and forming processes, or just not enough of a difference to justify a different plate thickness.

As built, these 90s were pretty buoyant anyway so there wasn't much incentive to reduce the material weight.

 

[Luis H]

In a fully optimum design, spherical ends can be significantly thinner than cylindrical walls on pressure vessels. This may be compromised by welds and forming processes, or just not enough of a difference to justify a different plate thickness.

As built, these 90s were pretty buoyant anyway so there wasn't much incentive to reduce the material weight.

Hi @Akimbo
Your statements are correct (specially from a pure technical perspective).

Those statement are reflected in the design of ASME code cylinders. ASME code cylinders are heavier design for mostly stationary applications and do not require periodic requalification or hydro testing.

Welding is allowed in the design of ASME cylinders (with very specific and highly controlled procedures).

Also, ASME cylinders are required to have corrosion allowance, which normally means somewhat thicker material, just for corrosion.

ASME cylinders are often custom designed for a particular application, but they all have to be designed and stamped by a licensed Professional Engineer. In the past, I have designed and stamped ASME cylinders, but the codes are so extensive I always had to reference them. I would never rely on my memory...


DOT (Department of Transportation) cylinders on the other hand are designed to be transportable and are a lot lighter in design. They have to be seamless (no joints), and formed from one metal piece (a plate, a tube, or a metal block).

Welding is not allowed on DOT cylinders.

There is no corrosion allowance in the material thickness calculations of DOT cylinders.

Because of their light design (for transportation) and relatively small safety margins, the DOT cylinders require periodic requalification with hydrotesting (and the associated required VIP). The typical 5 year hydro test measures the material elasticity and part of the requalification is the required (associated) visual inspection.


BTW, that is also part of the reason why DOT cylinders cannot be repaired by welding, but ASME pressure vessels/ cylinders sometimes can be repaired (if approved by a Professional Engineer).

 

[John C. Ratliff]

Both of those pictures are mirrored image. You can tell that the regulators are mirrored. John has posted those pictures many, many times and I never noticed that before. Thanks for noticing that...

OK, I had to double check if they are mirror images. It could have been possible to assemble the regulators with the intake and the exhaust reversed, but the label in the exhaust can would have been wrong. The regulators are definitely mirror image.

One more look at those pictures, and now I am really confused. The numbers on the cylinders don't look to be mirrored imaged. But, the regulators definitely look like mirror image.
Are the numbers on the cylinders up-side-down? To be read from above on a horizontal tank rack?

The Mil spec for those cylinders is available to the public (as well as the mil spec for the steel 72). You can get a copy at VDH, but I will try to post it in a bit.
Both of these documents have been approved for public release.

Thanks Luis, I had not seen that over the years. I scanned these from negatives, I believe (not slides), and so it was more difficult to see that I had reversed them. I have corrected that below.

So far as weight, they were 84 pounds out-of-water, but neutral underwater when close to empty (from our pool sessions). We dove them, as you can see, in swim trunks. We also got pretty sunburnt, which I'm still paying for in the form of actinic keratosis on the top of my now bald scalp.

I've added two more photos from the Naval School for Underwater Swimmers, showing how well these tanks swam while underwater without any dive suit (wetsuit). I've also reversed these two, as they also were reversed. Now, imagine getting into and out of the water with these tanks on your back, and doing pushups with them on at the side of a pool. We also did "flutter kicks" while on these tanks on our backs, with our masks filled with water.

Now, about those bottoms of the tanks, they were never made to be hydroed; they were pulled out of service after a specific time period. They were stamped for 3000 psi but at the Underwater Swimmers School, they were restricted to 2150 psi. I don't know whether that was because of the cylinders, or a restriction of their compressor. At 3000 psi, the Navy taught us that they were 170 cubic feet. (This is from my notes from the USS.)

SeaRat

 

[Luis H]

Now, about those bottoms of the tanks, they were never made to be hydroed; they were pulled out of service after a specific time period. They were stamped for 3000 psi but at the Underwater Swimmers School, they were restricted to 2150 psi. I don't know whether that was because of the cylinders, or a restriction of their compressor. At 3000 psi, the Navy taught us that they were 170 cubic feet. (This is from my notes from the USS.)

SeaRat

That is not correct. I attached a copy of Mil-C -24316 (aluminum non-mag cylinders) above.

They were hydro tested per CGA Pamphlet C-1, just like any DOT cylinder. Read the Mil-spec.

All portable high pressure cylinders are hydro tested, no exceptions!

 

[John C. Ratliff]

John C. Ratliff:
Now, about those bottoms of the tanks, they were never made to be hydroed; they were pulled out of service after a specific time period. They were stamped for 3000 psi but at the Underwater Swimmers School, they were restricted to 2150 psi. I don't know whether that was because of the cylinders, or a restriction of their compressor. At 3000 psi, the Navy taught us that they were 170 cubic feet. (This is from my notes from the USS.)

SeaRat

That is not correct. I attached a copy of Mil-C -24316 (aluminum non-mag cylinders) above.

They were hydro tested per CGA Pamphlet C-1, just like any DOT cylinder. Read the Mil-spec.

All portable high pressure cylinders are hydro tested, no exceptions!

Luis,

I was going on the training I received in the U.S. Naval School for Underwater Swimmers. These are my notes from that training session in Key West in 1967. I have heard from multiple sources that, because of the plug in the bottom, these cylinders were never made to be subjected to a hydrostatic test.

I just went to the PSI-PCI website, and found this quote from Bill High in his discussion of "A Brief Scuba Cylinder History":

...By the mid 1950's, PST and one other company began making aluminum cylinders from 6061 alloy for the U.S. Navy. The fabrication process was very different from the way aluminum cylinders are made today. Those round bottom aluminum cylinders are illegal for most purposes U.S. because they have no DOT designation. They must not be hydrostatic retested and not filled at commercial air stations. Newly formed Luxfer USA, Ltd. (in 1997 the company name was changed to Luxfer Gas Cylinders), with DOT special permit SP6498, began producing aluminum cylinders in late 1971. Using 6351 alloy in a cold extrusion process, the cylinder did not require a bottom plug as did the former military type. The Luxfer approved cylinder had a flat bottom, as are all aluminum scuba cylinders made today. In order to be equivalent to the then popular steel cylinders, Luxfer made its cylinder 6.8 inches in diameter to fit existing non-adjustable backpacks and with a similar 2,475 psig service pressure. To achieve the 72 cuft capacity, the Luxfer cylinder was made longer but, as a consequence, it was about 11 pounds buoyant when near empty. Divers quickly named it the "floater". (emphasis added, jcr.)

The Mil-spec probably allowed the 3-year testing by Navy personnel, but these were not hydrostatic tests. They were filled to 5000 psi for the test, per my training in 1967.

These cylinders are unique, and historic, as they were the first anti-magnetic cylinders that allowed the UDT/SEAL teams and Navy divers to disarm magnetic-activated mines. These cylinders also are uniquely shaped, and can be used for displays today. But they are not good as an active diving cylinder; again my understanding is that the plug in the bottom may be unstable.

You posted two different military specs, one for these cylinders dated 1 September 1971 (superceding the MIL-C-24326)(SHIPS) 21 June 1968), and a second one dated 3 December 1971. That second military spec appears to represent the first commercially available aluminum tanks, the "floaters," which are a completely different tank and do have a DOT stamping, and are rated. Mine, which I dove in the 1970s, were 2475 tanks, but the spec states they are rated at 2250 psi, so I'm not sure. I have a photo of these 2474 psig tanks, the floaters (see the 8 pounds of lead in the center of my twin tanks). Anyway, neither of these specs were in effect when I received the training at the Naval School for Underwater Swimmers in 1967.

SeaRat

 

[Luis H]

John, please what you are saying is so incorrect it is incredible.
Your notes even says that they were tested every 3 years.

The only pressure test that any reasonable person would do in a high pressure cylinder is a hydro test. You would never do a pressure test using gas.

Yes, the Navy would only do hydro-static test on a high pressure cylinder! There is no way we would have used air! Do not try to tell me that they were tested to 5000 psi using air. I am sorry, that would have been unreasonable!

I just retired from NAVSEA (previously know as Bureau of SHIPS) as a civilian Navy engineer. Please do not try to tell me that we had unreasonable engineers in the Navy back then!

I am thinking that you do not understand what a hydro test means. Are you implying that they did a pneumatic test?

You don't seem to understand the danger of doing anything but a hydro test at that kind of pressure. Hydro test means pressurized with water. If there is a failure you are not storing a lot of energy.

BTW, Bill High is not an engineer. I have provided engineering assistance to PSI-PCI.

Read the Mil spec.

The test pressure is stamped on the cylinders (5000 PSI), with the hydro test Elastic Expansion (65.4 cc in the example below), as well as the hydro test Permanent Expansion (0.0 cc in this case).

 

[Luis H]

The Mil-spec probably allowed the 3-year testing by Navy personnel, but these were not hydrostatic tests. They were filled to 5000 psi for the test, per my training in 1967.

SeaRat

If not hydro-static test, then what, pneumatic? !

 

[Jennafeathers]

Does anyone remember diving without any training to 100 feet for an hour back in the day this all started?

Apparently with Aqualung’s self contained diving unit back in 1952 you could and they did. See attached advertisement.. dive to 100 feet for an hour without training and after 7 year no casualties.

Please share your stories if you started diving without training in the 40’s and 50’s and how you started.

View attachment 626574

Crazy!

 

[John C. Ratliff]

If not hydro-static test, then what, pneumatic? !

Luis,

I was a 21 year old when I took those notes. I’m only telling you what I was told by the instructors. I am pretty sure you are correct, that it was a hydrostatic test. But I’m also sure that these cylinders were not for civilian use, and should not be either hydroed or filled now.

SeaRat

 

[Luis H]

These Navy cylinders are in the same situation as foreign code cylinders in the US. They are not stamped or were originally qualified by the DOT. Therefore, they cannot be requalified under the DOT codes.

Requalification of DOT high pressure cylinders require hydro-testing, but hydro-testing can be physically performed on NAVY and foreign cylinders, without implying that they are going to be requalified under DOT codes.

Bill High and others have made the incorrect statement that they “cannot be hydro-tested”, when what they meant is that they “cannot be requalified under DOT codes”. The two statements are not the same and are not interchangeable. One statement is correct, the other one is not.

A cylinder that was never qualified under DOT codes cannot be requalified under DOT codes… that is a true statement.

Saying that a cylinder “cannot be hydro-tested” just because it cannot be “requalified under DOT code”, is incorrect statement, it is miss-information (and if it is not obvious, spreading that type of miss-information irritates me a lot).

In the US and all over the word hydro-tests of pressure vessels are conducted every day without being under the DOT codes. I have been involved in many non-DOT hydro-tests.

Another common piece of miss-information is that foreign cylinders (and these type of NAVY cylinders) cannot be hydro-tested in a DOT licensed facility. That is false and incorrect. The correct statement is that they can do the test and they can even stamp the test date, but they cannot stamp their DOT RIM number (license number) with the test date.

The testing of foreign pressure vessels happens frequently in international ports (like here in Portland, Maine), were foreign flagged ships have life-rafts with pressure vessels that require their periodic service. We have two companies that are qualified to service foreign life-rafts. They used to bring their foreign cylinders to Maine Divers Scuba center for hydro test. They were hydro-tested using the proper codes and stamped with only the date. This is all approved by the DOT, since these pressure vessels are not used in interstate commerce, they are in international use.

I personally have Swedish Poseidon cylinders and Drager German cylinders that I have hydro tested and stamped the date, but not the DOT license number, because they are not DOT cylinders.

Hydro testing non-DOT cylinders (including the NAVY cylinders) in a DOT facility is perfectly legal as long as they are not stamped with the DOT license number.

Here is a thread (from 2014) were Captain talked about hydro-testing his NAVY cylinders.
USN diver tanks, big about 80 or 90 cu. ft. with a welded plug in the bottom.

Again, let me make it clear: “DOT requalification requires hydro-testing”, but hydro-testing does not always mean automatic DOT requalification. You can physically hydro test anything!

Luis,
I was a 21 year old when I took those notes. I’m only telling you what I was told by the instructors. I am pretty sure you are correct, that it was a hydrostatic test. But I’m also sure that these cylinders were not for civilian use, and should not be either hydroed or filled now.
SeaRat

John, that (in bold) is your opinion and I totally respect that as your opinion, but it is an opinion...


I am sorry that this thread has been derailed, but I consider it important not to spread miss-information. Specially about vintage scuba cylinders.

I consider this type of miss-information in the same category as the dive shops that claim that 3AA cylinders can never be re-stamped with the + sign. The scuba industry is loaded with miss-information and I personally consider that a message board like this should try to correct that and present facts (not just opinions). It is OK to have opinions, but IMHO they need to be differentiated from the facts.

I am getting off my soap box…


I think it is time for more pictures of pretty girls wearing vintage or goofy scuba gear.
.