Why Steel hold More Gas than AL

[Jason McK]

OK So I'm still researching tanks
I was wondering if someone could explain why @ 3000psi. A Steel tank has more air than an AL tank

I was looking at 120 Steel verses AL80 is it just the 40 cu. ft. of space that gives you more air. So there fore if you could have a steel 80 and a AL 80 filled to 3000psi they would then be holding the same amount of air?

Thanks
Jason

 

[Charlie99]

The capacity of the tank is simply the internal volume of the tank, times the rated pressure in atmospheres. Steel and aluminum tanks of the same internal volume (also called "water capacity" since it is most easily measured by measuring how much water it takes to fill the tank) will have the same amount of air stored at any given pressure.

Aluminum is weaker and more brittle than steel, so the wall of an aluminum tank is thicker. This means that, for the same weight and external size, the internal volume of a steel tank is larger.

 

[SDAnderson]

But let me try this for an answer: if you have an 80cf steel tank rated at 3000psi and an 80cf aluminum tank rated at 3000psi, both tanks will have exactly the same interior volume. The steel tank will, however, be smaller on the outside because it's walls can be thinner - steel being much stronger than aluminum.

The capacity of a tank, expressed in cubic feet, is a function of the interior volume (best measured in liters of water) and the pressure that the tank can safely hold.

Did I come close to being helpful?

 

[H2Andy]

So there fore if you could have a steel 80 and a AL 80 filled to 3000psi they would then be holding the same amount of air?

yes, assuming (1) they are both truly 80cf tanks (the advertised amount and the "real"
amount don't match all the time), but they'll come within 5 cf or so (for example,
my 85cf tank is really a 82.7cf tank); and (2) assuming they both hold 80cf at 3000 psi (for example, my tanks hold 82.7 at 2640 psi -- if i fill them to 3000 psi, they'll
hold more than that)

also, the steel tank would be smaller and denser, so you would need less weight
to stay neutral at the end of the dive. the aluminum will be larger and will
be positive at the end of the dive, so you'll have to wear more weight

hope we're all helping answer your question

 

[Tom Winters]

Without getting onto a whole discussion about tank metallurgy, due the difference in the two metals, you can overpump LP steel tanks. I can take my steel LP 85s, already smaller than an aluminum 80 and put in 120 cubic feet of gas.
If you do this with aluminum tanks, i.e., overpump them, sooner or later, parts of the person next to the aluminum tanks being overpressurized will be separated from the host organism due to the tank cataclysmically exploding.
I know guys who religiously pumped their own aluminum 80's to 4000psi for years and years with no problems. I liked it when they pumped my tanks up that high, but I was never around while they did it.

 

[spectrum]

OK So I'm still researching tanks
I was wondering if someone could explain why @ 3000psi. A Steel tank has more air than an AL tank

I was looking at 120 Steel verses AL80 is it just the 40 cu. ft. of space that gives you more air.
Jason

As mentioned the steel cylinders have thinner walls due to steel being a stonger material. That creates additional internal volume since most common cylinders stick with several outside dimeters. Steel cylinders may also be found to be stouter.Also remember that in your example it is room for 40CF at 1 ATM, due to the compression the cylinder volume required for that 40 CF is much less. roughly 1/204th of 40 CF

So there fore if you could have a steel 80 and a AL 80 filled to 3000psi they would then be holding the same amount of air?
Jason

The twist is that a 3000PSI steel tank is an odity. Most are 2400/2640 or 3442. However the fact that they are called 80CF tanks means that when filled to the rated pressure they hold 80CF of air at 1ATM.

To confuse things just a bit the ubiquitous AL80 really only holds about 77.4CF at 3000 PSI but that's just a quirk.

Thanks
Jason

 

[ScubaSixString]

And noone has mentioned HP tanks (5000 psi, i think?)

 

[SDAnderson]

And noone has mentioned HP tanks (5000 psi, i think?)

I don't know of any scuba tanks rated for 5000psi - do you?

 

[teknitroxdiver]

HP is 3500psi. Only thing up around 5k is HP bank bottles.

 

[spectrum]

I don't know of any scuba tanks rated for 5000psi - do you?

Maybe in Europe? I know I've heard of higher than our 3443/230 BAR stuff.

Either 5000 PSI or 4500 PSI/300 BAR

Makes you wonder what those cave divers pump them up to!

In any case for a discussion that seemed more geared towards tank capacity physics I didn't want to dig a deeper hole.

Pete