If I remember correctly over that broad pressure range I believe Boyle's law is not perfectly linear hence the discrepancy. This has come up before.
Pete
Pete
Help me with my tank math
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Charlie99
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At 4500psi/300 bar a tank will have about 8% less air than the ideal gas law predicts, but in the 200 bar/3000psi range the ideal gas law is still pretty close, so it is still a bit of a mystery.
http://http://www.babilim.co.uk/pages/gas_laws.html is a pretty good discussion of this.
The metric guys normally use bar for pressure and use the approximation of bar * internal volume = free volume. Since 1 atm is 1.01325bar, this overestimates the free volume by about 1.3%. For 3000psi tanks, the Van der Waals correction is in that same sort of size range. Both are negligible compared to normal variations in SAC, but it would still be an interesting intellectual exercise to figure out how the tank manufacturers come up with the numbers they do.
Here's a theory that accounts for a small portion of the difference...
Once the pressure in the cylinder equals ambient pressure, there's no way to create a vacuum to force the rest of that air out. That means there will always be at least 678 cu.in. of air left inside.
Like so many of you, I'm stumped as to the rest of it.
Once the pressure in the cylinder equals ambient pressure, there's no way to create a vacuum to force the rest of that air out. That means there will always be at least 678 cu.in. of air left inside.
Like so many of you, I'm stumped as to the rest of it.
I doubt the listed internal capacity is correct (678 cu in). You *could* fill it up with water. 678 cu in of water will weigh 24.49 lbs. Or you could fill it up with water and measure the amount of water in the tank. Should be 2.93 gallons of water.
I don't have any Luxfer 80's that need a vis or hydro right now, otherwise I'd be willing to measure internal volume for ya.
I don't have any Luxfer 80's that need a vis or hydro right now, otherwise I'd be willing to measure internal volume for ya.
OK...how about this?
The tanks are volume measured by how much water they displace externally - i.e. their total volume. The content will be a little less due to the fact that the metal walls don't hold air!
(OK....Luxfer describes it as "Minimum Internal Volume", but what's that? I bet it's more than the tank can contain at 4000 meters high with no internal pressure!!!)
The tanks are volume measured by how much water they displace externally - i.e. their total volume. The content will be a little less due to the fact that the metal walls don't hold air!
(OK....Luxfer describes it as "Minimum Internal Volume", but what's that? I bet it's more than the tank can contain at 4000 meters high with no internal pressure!!!
)
OK...how about this?
The tanks are volume measured by how much water they displace externally - i.e. their total volume. The content will be a little less due to the fact that the metal walls don't hold air!
No, that volume different is actually too high.
The volume of the bottom threaded section of the valve is only about 1 to 1.5 cu inches. Not enough to justify the discrepancy
If you do the math starting with the 77.4 cu ft it shows the actual inside volume of the tank is about 655 cu inches (0.379 cu ft).
Where did the 678 cu inches come from?
We are talking about a 3% difference (from 77.4 cu ft. to 80.1 cu ft).
My guess is that someone in marketing took the nominally advertised volume of 80 cu ft and did the math backwards (they use 204 atm and then just rounded the number). The answer comes to 678 cu in.
TaoH
Contributor
If I remember correctly, I read somewhere that each manufacturer has their own number for atmospheric/ambient pressure and maybe that's why the math seems off.
So if all numbers are correct, Luxfer is testing these tanks with an ambient pressure of 15.2078
So if all numbers are correct, Luxfer is testing these tanks with an ambient pressure of 15.2078
Charlie99
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Although the volume of the dip tube and valve are small, by the time you multiply by by 200atm, it could add up to a substantial number. Although most of the valve is external, the snorkel/dip tube and some portions of the valve will be filling up areas that are probably part of the "internal volume".
Of course, it could simply be that the numbers in the tables are bogus. I've seen several charts of tank specs that are obviously bogus in that the difference between empty and full buoyancy didn't match up with the weight of air that the tank holds.
Of course, it could simply be that the numbers in the tables are bogus. I've seen several charts of tank specs that are obviously bogus in that the difference between empty and full buoyancy didn't match up with the weight of air that the tank holds.
The valve volume inside the tank (about 1.5 cu inches) only accounts for about a 0.2 % discrepancy.
The total discrepancy of concern from 77.4 cu ft. to 80.1 cu ft. (or from an actual volume 655 cu in. to 678 cu in.) is in the order of 3%. That is an order of magnitude difference.
See post number 16.
The actual volume of just the aluminum is about 326 cu in.
Weight of tank with no valve = 32 lb.
Density of aluminum = 0.098 lb / (cu in).
32 lb / (0.098 lb / (cu in)) = 326 cu in
So the outside volume is about: 326 cu in + 655 cu in = 981 cu in
Give or take 23 cu in (the discrepancy in question).
The total discrepancy of concern from 77.4 cu ft. to 80.1 cu ft. (or from an actual volume 655 cu in. to 678 cu in.) is in the order of 3%. That is an order of magnitude difference.
See post number 16.
The actual volume of just the aluminum is about 326 cu in.
Weight of tank with no valve = 32 lb.
Density of aluminum = 0.098 lb / (cu in).
32 lb / (0.098 lb / (cu in)) = 326 cu in
So the outside volume is about: 326 cu in + 655 cu in = 981 cu in
Give or take 23 cu in (the discrepancy in question).
OP
scubabrains
New
I stumbled across this link which explains a lot. Once you start dealing with pressures above 20 bar (about 300psi), the molecules in air start to interact differently with eachother. I still haven't been able to make the formulas give me 77.4 cuft in an AL80, but what it does show me is that the volume of air in a tank isn't exactly proportional to the pressure of that tank. For example, at about 70 degrees F with a full tank reading 3000psi, half a tank of air (77.4/2=38.7 cuft) will read about 1450psi on the gauge, not 1500 psi.
Van der Waals and Nitrox
Van der Waals and Nitrox
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