Metric and Imperial tanks.

[Kim]

No... Why? The tank would be the same size (still an "80"), just holding more pressure.

Why? Because a same volume tank at a higher pressure will surely have more air in it, no?

If it holds 80cf at 3000psi then how would you raise the pressure to 3500psi - adding air, right?

 

[Kim]

I agree... if determining the actual amount of gas is important, like in technical diving. For weenie rec diving (like I do), it's pretty esoteric. (*cringe!*)

This is another reason why I am attempting to work out more exact figures. I want to go further into tech and I know already that these finer details are rather important then! I have to admit that right now trying to work it out is still making my head hurt a lot though!

 

[Marek K]

Why? Because a same volume tank at a higher pressure will surely have more air in it, no?

If it holds 80cf at 3000psi then how would you raise the pressure to 3500psi - adding air, right?

Yeah, but what confused me was when you wrote "the second higher pressure tank will be physically smaller." Is that what you meant to write?

--Marek

 

[Kim]

Yes. It seems to me that 80cf of gas (a fixed amount), will occupy a larger space at 3000psi than at 3500psi. Therefore a tank at 3000psi will need to be larger than one at 3500psi to hold 80cf. Isn't that logical?

 

[grazie42]

I´m with you on that one Kim...
We have 12liter tanks here (200 bar) that would be a "80cft tank"...
I, personally, have an 8liter tank (300 bar) and while being physically smaller (though actually heavier because of the differences in material thickness required for higher pressure) that too would be an "80cft tank"...
Hence the pressure raiting on a tank is critical when trying to determine the "volume"...

although it seems to me that americans use volume, ie cft, in the same way that we use liters...as a means to describe the size of the tank rather than the ACTUAL volume it holds...when you think about it an "aluminium 80" (supposedly 80 cft) that is "cave filled" would no longer, ACTUALLY, be an "aluminium 80" as it would hold more than 80cft of gas...

maybe I´m way off base...lets kick this around some more so the americans will have something to laugh at when they wake up later tonight..."stupid euros" they´ll think...although most will propably/hopefully be to polite to say so...

 

[Kim]

I found one explanation searching the net. Metric tanks are measured by the volume of liquid that they will hold. Thus a 12 ltr tank holds 12 ltrs of liquid. (or thereabouts if you consider miketsp's post). American tanks are measured by the amount of gas they will hold filled to their rated pressure. Thus an 80 tank rated to 3000psi will hold 80cf (actually more like 77cf ) at it's rated pressure. This means that your 'cave filled' tank will still actually be an 80 because although it holds more gas it only does so because it has been filled to a higher pressure than the rated one.
Actually - now I am beginning to understand the difference (I think!:11 but it still seems an awful lot easier to work with liters and bars in the metric way.
For instance - if my 12ltr (real volume) tank is filled to 200bar then it contains 2400ltrs - 180bar/2160ltrs - 120bar/1440ltrs - all of which I can work out in my head while typing this post. Now take a AL80 filled to 3000psi and try to work out the cf volumes at 2600psi and 1700psi. Personally I'd need a pencil and paper to do that - although maybe it's mainly because I'm not used to thinking about it that way.

How does that all sound? (Gee the Yanks are going to have a field day with this! )

 

[Marek K]

Yes. It seems to me that 80cf of gas (a fixed amount), will occupy a larger space at 3000psi than at 3500psi. Therefore a tank at 3000psi will need to be larger than one at 3500psi to hold 80cf. Isn't that logical?

Nope. I mean, yes, your logic is correct. but nope.

Again as I understand it, an 80cf tank refers to the physical size of the tank, not how much gas is in it. The 80 cubic feet from which the nomenclature is derived assumes a pressure, I think, of 3,000 psi. But an 80cf tank that has, say, 3,500 psi in it (or, for that matter, 2,800 psi) will still be referred to as an 80cf tank... even though it now holds more (or less) actual gas. Does that make sense? :06:

Just like a 12-liter tank is still a 12-liter tank, no matter how much pressure it holds.

Maybe we do need to go to metric...

--Marek

 

[Marek K]

How does that all sound? (Gee the Yanks are going to have a field day with this! )

*shakes head ruefully...*

 

[Charlie99]

For instance - if my 12ltr (real volume) tank is filled to 200bar then it contains 2400ltrs - 180bar/2160ltrs - 120bar/1440ltrs - all of which I can work out in my head while typing this post. Now take a AL80 filled to 3000psi and try to work out the cf volumes at 2600psi and 1700psi. Personally I'd need a pencil and paper to do that - although maybe it's mainly because I'm not used to thinking about it that way.

Ahhh, but on the other hand we don't have to do any multipying to know that we have 77cu feet to start when we have a standard fill. When you tell someone that you have a 12 liter tank, you have to then specify whether it's a 200, 232, or 300 bar tank before they know how much gas you have. So it's 6 of one vs a half dozen of the other.

OTOH, figuring out that 50' is 2.5ata is a lot more difficult than figuring out that 15 meters is about 2.5ata.

The Luxfer Cylinders website give their product specs in both metric and imperial. The most common AL80 is the S080 ---- 11.1 liter, 207bar.

 

[Marek K]

I, personally, have an 8liter tank (300 bar) and while being physically smaller (though actually heavier because of the differences in material thickness required for higher pressure) that too would be an "80cft tank"...

Nope...

See my reply to Kim... your 8-liter tank would probably be referred to in the U.S. as a high-pressure 55cf, or so.

although it seems to me that americans use volume, ie cft, in the same way that we use liters...as a means to describe the size of the tank rather than the ACTUAL volume it holds...

Yes! Again, as far as I understand.

C'mon... where are all the U.S. experts? Probably laughing at my pitiful attempts to explain this?