More Air.....Once More

[Rev. Blade]

:doctor: I think the term that would normally be used instead of "equivalent volume" would be "volume at Standard Temperature Pressure"... and the term commonly used instead of "actual volume" is "liquid volume", because liquids do not compress (within the standards of commonly used for measurement... and if we were going to get SUPER persnickity, the SCUBA cylinder does actually expand, to an extremely small degree, when it is pressurized).

That said, I actually agree with the previous posts: An "80 cubic foot tank" would actually only hold 80 cubic feet of air STP at it's rated working pressure. A higher pressure tank with a working volume of 80 cubic feet, has a smaller internal volume than a lower pressure tank of "80 cubic feet".

...and then we can go on again about external volumes, and the strengths of steel vs. aluminum....

(New diver, but daddy was a Physics teacher)

 

[JT2]

Hi Rev. Blade, let me be the first one to say,"Howdy" and welcome you to the board. I am sure with your background you will be a valuable contributor. Be sure and drop by the "Introductions and Greets" section and let everyone know you're here.

 

[MikeS]

Rev,

Thanks for the terminology. I’ve always found it strange that tanks are sold in Europe based on liquid volume in liters while sold in the U.S. based on volume at standard pressure in cubic feet. See I've already incorporated the terminology! If we can learn by it, by all means be persnickety. Can you give us an example? For example how much does the liquid volume of a HP120 increase at rated pressure of 3500 psi?

Mike

 

[Rev. Blade]

I don't have any appropriate engineering books around at work (and hope I've thrown out the ones at my house). My off the top of my head guess is that the volume would only increase a few cubic centimeters. If I was trying to find out for sure, I'd do a search for specs on a hydro test. If I heard correctly from someone, a hydro test is checking to see how much a tank swells when pumped up to very high pressure. I'm guessing it's a small fraction of one percent.

As far as tank size measurement:
I'm glad we use the STP measure in the US. It is a handier number to work with, assuming the fill station fills the tank to it's rated service pressure... not more or less.

I rented a ScubaPro steel 95 last night, and found the shop had 3000psi in it. My guess is that the "+" pressure for this tank is closer to 2600. I'll need to look it up if I'm going to calculate my surface air consumption.