A Quick Question about tank capacity

[Mav1290]

Im in the process of getting my Open water certification. The other day they gave us our gear. Well since i was taller i got a taller tank and my friend got a shorter one. Well i figured a shorter tank would hold less air, unless of course it was just wider. Well she said that they were both the same width but they held the same. Well i didnt think this was right so I asked the lady at the dive shop. But all she told me was that they both hold the same amount of air ,3000psi. I justa kinda nodded at her b/c i didnt think that made any sense at all. SImply enough pressure is the force exerted by the air on the tank b/c it is compressed. Thus it would take more air to get the same pressure on a bigger tank and less air to get the same pressure on the smaller tank.


Am I right here, or am I wrong. I just wanted to clear this up b/c it has been bothering me.

 

[bwerb]

You are likely correct.

It is likely the smaller tank was a 65 cubic foot tank (or something similar) vs. the 80 (77 ) you were diving.

The pressure may have been the exact same in both tanks but you are correct in your assertion that the smaller tank would actually hold less gas at the same pressure.

But...

The other possibility was that one of the tanks was steel and one was aluminum. In that case the shorter steel tank may have contained very close to the same amount of air.

I'd suggest that you ask your instructor (or someone else from the shop) to show you how to read the markings on the tank which will tell you what the tanks are made of and some other interesting stuff...have fun!

Welcome to the board

 

[rdriver]

By God I think you have it..... moreover it's a function of volume at a given PSI...... for example you can have two tanks one is 13 cubic ft. and the other is 80 cubic ft. now they are both filled to 3000 PSI but just because they are filled to 3000 PSI doesn't mean they will last the same time under water.
Rob

 

[Mav1290]

Thanks

I didnt think of the possibility of the steel vs alluminum and I didnt think one would hold more. So btw two tanks the same size one steel the other alluminum the steel one may hold more air?

 

[WileEDiver]

Mav1290 once bubbled...
So btw two tanks the same size one steel the other alluminum the steel one may hold more air?

Yes, a tank holds a given (rated) volume of gas when it is pressurized at its rated pressure. A physically smaller high pressure steel tank can hold more air than a physically larger aluminum (lower pressure) tank.

 

[Mav1290]

WEll heres what i dont get then

If one can hold more air then the other b/c it is high pressure then why would I have been told they both have the same psi in them. WOuldnt the high pressure have a higher pressure allowing it to hold more air.

 

[WileEDiver]

Mav1290 once bubbled...
If one can hold more air then the other b/c it is high pressure then why would I have been told they both have the same psi in them. WOuldnt the high pressure have a higher pressure allowing it to hold more air.

You were right in your original message when you said that "I didnt think this was right ". It wasn't.

 

[jonnythan]

The walls of an Al tank are like an inch or more thick. The walls of a steel tank are more like 1/4 inch thick.

A steel 72 is a bit smaller than an Al 80. I don't know of any steel 72's rated to 3000 psi, but 72 is pretty close to 77.4 (the real capacity of an Al 80 at 3000 psi).

A steel HP 100 is a few inches shorter than an Al 80 and it holds 85 cf at 3000 psi.

I figure that chances are the smaller tank was either a steel 72, an Al 63, or a steel HP 80. In any of these cases, they were lying about something

 

[bwerb]

Mav1290 once bubbled...
WEll heres what i dont get then

If one can hold more air then the other b/c it is high pressure then why would I have been told they both have the same psi in them. WOuldnt the high pressure have a higher pressure allowing it to hold more air.

It was a low pressure steel...in which case at 3000 psi, it would hold MORE air than it's rated capacity...

As I stated originally, you are likely 100% correct, I just think it would be worth a discussion at the shop to have someone knowlegeable use this as a teaching tool to show you how to read tanks (and perhaps show your instructor that they don't know how the ideal gas laws work too ).

 

[Mav1290]

A little chemistry and physics. Which is why I dont understand why they ever made me take any other classes then math and science in hs. lol