Help me with my tank math

[The Anchor]

I am just as confused as the rest of you. I have dome my own set of calculations, and I too get 80.1 cu-ft.

If we assume that the manufacturers volume of 77.4 cu-ft is correct this gives a diference of 2.67 cu-ft at pressure. Working backwards, this gives a difference in volume of 22.6 cu-in at atmo pressure. I.e. to get a volume of 77.4 cu-ft, the tank must have an atmo volume of 655 cu-in.

22.6 cu-in translates into a cube with each side measuring 2.8 in or a cyliner 3.06 in dia x 3.06 in tall.

That seems like more than what would be occupied by the valve.

 

[spectrum]

I am just as confused as the rest of you. I have dome my own set of calculations, and I too get 80.1 cu-ft.

If we assume that the manufacturers volume of 77.4 cu-ft is correct this gives a diference of 2.67 cu-ft at pressure. Working backwards, this gives a difference in volume of 22.6 cu-in at atmo pressure. I.e. to get a volume of 77.4 cu-ft, the tank must have an atmo volume of 655 cu-in.

22.6 cu-in translates into a cube with each side measuring 2.8 in or a cyliner 3.86 in dia x 3.86 in tall.

That seems like more than what would be occupied by the valve.

Keep in mind that the basline volume you are refering to came from the same folks that call a 77.4 cubic foot capacity cylnder an 80 cubic foot product.

I agree that as Luis suggested the number was cooked and nobody expected anyone to use it to plan a moonshot.

It's also a possibility that the number is sound and the discrepancy comes from several data points including but not exclusively the nonlinear prssure/volume relationship.

Pete

 

[nereas]

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?

You must be doing this for a D/M or instructor exam. In the real world, this approach accomplishes nothing.

In the real world, you simply multiply capacity at rated pressure by 0.08 lbs per cu ft. to get negative "buoyancy" of tank gas.

On further thought, gas blending is the only other time when such precision might be useful. I have not done any blending myself recently. I just let the dive store do that, and then I will double check the oxygen and helium fractions with an analyzer.

 

[SparticleBrane]

You must be doing this for a D/M or instructor exam. In the real world, this approach accomplishes nothing.

In the real world, you simply multiply capacity at rated pressure by 0.08 lbs per cu ft. to get negative "buoyancy" of tank gas.

That has absolutely nothing to do with the question.


I suppose the real question here is: why does anyone care if there's a 3cf discrepancy? If you're planning dives that have a margin of error of 3cf, there are other issues at stake.

 

[nereas]

...I suppose the real question here is: why does anyone care if there's a 3cf discrepancy? If you're planning dives that have a margin of error of 3cf, there are other issues at stake.

Gas blending would be the only other issue that comes to mind for such precision.

Otherwise, 3 cu ft x 0.08 lbs per cu ft = 0.24 lbs insignificant.

 

[Blox]

That has absolutely nothing to do with the question.


I suppose the real question here is: why does anyone care if there's a 3cf discrepancy? If you're planning dives that have a margin of error of 3cf, there are other issues at stake.

I suppose, that there are some people in this world, who wonder why an "AL 80" is called just that, although it only holds 77.4 cbft at the rated pressure, while the internal volume as per supplier's specs seems to support more, and are looking for a logical explanation for that. Can't blame them - I do too.

 

[Colliam7]

Seems like a good question to ask Luxfer.

Hear, hear! We await the outcome of a call to Luxfer, so we can all be enlightened.

 

[The Anchor]

I am having flash backs of High School Chemistry here.

I seem to recall the Ideal gas formula being PV=nRT.

P=pressure (increasing)
V=Volume (constant)
n= mols of gas (increasing)
R=gas constant (8.314)
T=Temperature (increasing or decreasing)

The only explanation I can come up with is that becasue they are compressing the gas from a lower pressure (supply) to a higher pressure (your tank), it heats up and the warmer gas is less dense. When it cools to room temperature the density increases but at a loss of volume.