Re: Alum vs Steel?

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Posted by Glenn on August 17, 2001 at 14:47:49:

In Reply to: Re: Alum vs Steel? posted by retrodiver on August 17, 2001 at 13:24:30:

It doesn't matter what the psi rating of a tank is when trying to comapre capacaties. What you are concerned with is the cu. ft. rating. A 80 cu. ft. tank at 3000 psi is the same as 80 cu. ft. at 2400 psi. At first it might seem that a 3000 psi tank would be smaller but that is not necessarily so since most 3000 psi tanks are aluminum and will have a thicker wall to accomidate the stress of the higher PSI.

Now then, one thing to realize to keep apples to apples, is that most Aluminum 80s are really rated at 77 ft^3 and not 80 ft^3.

Steel ratings are interresting too. A steel 95 is 95 ft^3 capacity based on a working pressure of 2640. But the tank is rated at 2400 psi. So how is this done? It's an old allowance that emerged in WWII called a + (plus) rating. When you look at the neck of a low pressure (LP) steel they typically state 2400 for the pressure. But when you look at the first hydrotest stamp there is usually a plus rating next to it. What the plus rating does is increase the working pressure 10%. So a 2400 psi tank is now rated at 2640 psi.

Now to really muddy the waters low pressure steel tanks are usually rated at their + fill capacity. So a steel 95 at 2640 is actually about 86 ft^3 when filled to 2400 psi.
There are some people who will argue that a + rating is only good for the first 5 years of a tank, however, DOT has a pending (at the time I referenced it) draft that basically ammounts to once a tank is plus rated it is forever plus rated. There's more about this at

As for the differences with the HP tank, there are a couple. First and least noticeable is the alloy used is of a different type. It's stronger and allows for a higher pressure fill. The overall tank size is reduced when compared with a standard low pressure tank. The second big and most noticeable difference is the use of a DIN vavle. The DIN valve can handle the higher pressures that a yoke can't. Because of this you will need to convert your first stage to a DIN valve. Conversion kits are available for most every modern 1st stage. If you do a lot of travelling where you use rental or provided tanks you will most likely need to convert back to a yoke valve.

Something to be aware of though is that most facilities are not able to fill to 3500 psi. 3000 is about the limit for most, and some boats even have a tough time with that in limited time they usually fill tanks. If a 3500 psi 120ft^3 tank is only filled to say 2800 psi, it basically has around 96 ft^3 of gas. Another problem you may run into is that you may need to bring your own yoke adaptor so the boat can fill your tank. It's a small block, typically brass, that screws into the DIN valve that provides a yoke mount surface for filling purposes.

Some tanks, such as ScubaPro, have a convertible valve that is both a DIN and a yoke. It basically starts out as a DIN valve but with the yoke dimple on the back. A piece is screwed in that then makes it a yoke. It's a nifty "feature" that my tank has, but I've never used it as nothing I own is DIN.

There's plenty to think about even for something as simple as a tank. Work with a shop that will let you try the tank out. Take it diving for a day. Don't forget to try it while it is low on air. Try hanging out at 15 ft with 300-500 psi in the tank.

And finally, for more references on tanks and their capacaties, boyancy characteristics, check out:


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