Help me with my tank math

[scubabrains]

There's got to be an easy answer for this, I'm just not seeing it. An AL80 actually only holds 77.4 cu.ft, but I can't get the numbers to tell me that. What am I doing wrong?

Luxfer S080 = 678 cu.in = 0.392361111 cu.ft

3000psi / 14.7psi = 204.0816

204.0816 * .392361111 cu.ft = 80.0737 cu.ft

Either the tank volume is different (tank valve takes up room in tank?) or I'm wrong in using the 14.7psi value. Anybody?

 

[Kim]

I believe different tank brands have actually different internal volumes, even when they are both described as "AL80s"

For me your calculation above looks good (14.7psi is the correct figure for 1 ATA) so I'd imagine that this tank actually holds what it says it does. I could be wrong though....

Me? I use liters and bars.....it's all SO much easier!!!

 

[TaoH]

(Cylinder Service Pressure)(Cylinder Volume)=(Atmospheric Pressure)(Volume at Atmospheric Pressure)

From the variables I'm getting:

At 3000PSI:
(3000 PSI)(678 cu-in)=(14.7 PSI)(VOLUME ATMOSPHERIC)

((3000 PSI)(678 cu-in))/(14.7 PSI)=(VOLUME ATMOSPHERIC)
138367.34693877551020408163265306 cu-in=(VOLUME ATMOSPHERIC)
80.07369614512471655328798185941 cu-ft=(VOLUME ATMOSPHERIC)

Your math seems right, I've seen a few different values for atmospheric pressure that are smaller than 14.7 PSI, but that will only make the atmospheric volume larger. If someone told you an AL80 actually holds 77.4 cu-ft, they could be talking about the temperature change after a cylinder has been filled or maybe someone was testing these tanks at a different altitude.

Maybe their tanks are made by someone else.

 

[Scuba-Jay]

At 3000PSI:
(3000 PSI)(678 cu-in)=(14.7 PSI)(VOLUME ATMOSPHERIC)

((3000 PSI)(678 cu-in))/(14.7 PSI)=(VOLUME ATMOSPHERIC)
138367.34693877551020408163265306 cu-in=(VOLUME ATMOSPHERIC)
80.07369614512471655328798185941 cu-ft=(VOLUME ATMOSPHERIC)

Um square peg into the round hole...i mean, erm, left shoe goes on the right foot? Err no no hold on, 7 men with 8 wives and something about the town of VanIves?? Oh man im so confused111

J

 

[TaoH]

If I truncated those numbers, would it be any less confusing?

138367.3469 cu-in=(VOLUME ATMOSPHERIC)
80.0737 cu-ft=(VOLUME ATMOSPHERIC)

 

[Blox]

Interesting...

Well, the factor of 14.7 PSI = 1 ATA is correct, however, tank pressure is measured in BAR, which is not exactly the same as 1 ATA (1 ATA = 1.01325 bar).

3000 PSI = 207 Bar (abt).

That doesn't solve the riddle though, but instead increases the discrepancy.

I don't know what the reason for the discrepancy is - valve sounds like a logical explanation to me. Maybe somebody else knows and chimes in. If push comes to shove, and you really need to know, you may want to ask Luxfer.

 

[Blox]

Um square peg into the round hole...i mean, erm, left shoe goes on the right foot? Err no no hold on, 7 men with 8 wives and something about the town of VanIves?? Oh man im so confused111

J

HELPPPPPP!

One of the dreaded Dork Diver replies

 

[Kim]

however, tank pressure is measured in BAR,

Well it is some places!

Imperial supposedly gives you the volume in cu.ft when filled to rated service pressure - psi is a pressure measurement just like bar (pounds per square inch). This means that a tank called 80 cu feet and with a service pressure of 3000 psi should contain that amount of gas (80cuft) when filled to that pressure. However it's quite true that they normally don't and contain 77.4cu.ft. This can ONLY be because of the true internal volume AFAIK but the OP has a given internal volume of 678 cu.in, which should translate to more or less 80cu.ft at service pressure.

Bar is used with metric tanks that are already named by internal volume in liters. The internal volume is the amount of water in liters a tank will hold. I generally dive a 14ltr tank filled to 200bar. That's 2800ltrs of gas full. 2100ltrs@150bar, 1400ltrs@100bar, 700ltrs@50bar etc etc....it's easy to work out.

 

[Blox]

Well it is some places!

Imperial supposedly gives you the volume in cu.ft when filled to rated service pressure - psi is a pressure measurement just like bar (pounds per square inch). This means that a tank called 80 cu feet and with a service pressure of 3000 psi should contain that amount of gas (80cuft) when filled to that pressure. However it's quite true that they normally don't and contain 77.4cu.ft. This can ONLY be because of the true internal volume AFAIK but the OP has a given internal volume of 678 cu.in, which should translate to more or less 80cu.ft at service pressure.

The internal volume that the OP quotes as 678 cu.in, is the one stated by Luxfer's published tank specs. Converting that a fill pressure of 3000 PSI (or 207 bar) does indeed result in 80 cbft capacity. So that explains why it's usually called an "AL 80"

What it does not explain is why the same specs quote a "Capacity Air" of 77.4 cbft.
What happened to these 2.6 cbft

Bar is used with metric tanks that are already named by internal volume in liters. The internal volume is the amount of water in liters a tank will hold. I generally dive a 14ltr tank filled to 200bar. That's 2800ltrs of gas full. 2100ltrs@150bar, 1400ltrs@100bar, 700ltrs@50bar etc etc....it's easy to work out.

I know, and agree: It's easier to work out - I'm a "Metric Man" myself.

Only that your "14Ltr tank" probably only holds about 13.2 liters in reality

 

[bperrybap]

The numbers quoted originally seem to match the Luxfer web page.
Luxfer North America (Imperial)

Now to make it more interesting you can look at the metric numbers
for the same tanks:
Luxfer North America (Metric)

If you multiply the tank size of 11.1 Liters by a service pressure
of 207 BAR you don't get 2192 Liters.

Even if you account for 1 BAR not being exactly equal to 1 ATA
the numbers still don't line up with the values in the Luxfer table.

Seems like a good question to ask Luxfer.

--- bill