Formula for figuring out volume from pressure

[UaVaj]

Searching for a compressor so until that happens.

I been having a tough time finding a dive shop that can successfully fill my HP120 tank to the rated spec 3442psi.

Almost all the time I get ~3000psi. How much air does this translate too? How do I converted pressure (psi) to volume (cf).

 

[ianr33]

(3000/3442) X 120 = 105 cubic feet.

 

[cool_hardware52]

Searching for a compressor so until that happens.

I been having a tough time finding a dive shop that can successfully fill my HP120 tank to the rated spec 3442psi.

Almost all the time I get ~3000psi. How much air does this translate too? How do I converted pressure (psi) to volume (cf).

(Actual pressure/3442) x 120 = actual volume in your tank.

If you take (120 / 3442) x 100 you get about 3.5 cuft / 100 psi tank pressure. That "Tank Factor" allows quick and dirty adjustments.

Say your tank is at 3200 or about 250 psi below full. 2.5 x 3.5 = roughly 9 120-9 = 111. Check your answer (3200/3442) x 120 = 111.56. Close enough.

Tobin

 

[Scared Silly]

Look in your OW book - there is probably a mention of a guy named Boyle.

Otherwise:

Boyle's law - Wikipedia, the free encyclopedia

 

[spectrum]

There are many ways to calculate the volume you have. Another method that leaves you with a number for some future mental math is as follows.

On a perfect fill....
3442/120 = 28.68 PSI per cubic foot

3000/28.68 = 104.6 cubic feet

You can see that that even mentally dividing the actual pressure by 30 gets you within 5% and on the conservative side. Breaking the calculation into steps also gets you closer to understanding the physics.

Pete

 

[UaVaj]

My thoughts were: Volume is constant since the tank don't change size. So as pressure increases. You would need put more air. Afterall it is why harder for the compressor to pump those last few psi.

Are you 100% sure it is linear??

 

[prskate2]

I just studied this is Chemistry... HAHA!

 

[spectrum]

I didn't realize it was linear as what you guys posted. That is a no brainer.

I thought that as pressure increase. The volume also increases. After all it is harder to pump those last few psi.

Are you absolutely 100% sure it is linear??

It's not perfectly linear but close enough for this exercise. The extra work of pumping those higher pressures does not represent the nonlinearity, that's just higher resistance or work. The slight nonlinearity is in the realm of the "ideal gas law'.

Pete

 

[Couv]

There are many ways to calculate the volume you have.

Here is another:

Volume when filled to rated pressure divided by rated pressure = constant.
Constant X remaining pressure = available volume

Therefore in your case:

120 (volume when full)
3442 (pressure when full) = .034 (it is actually .034863 etc but I rounded down toward the conservative side)

So .034 is the constant that you can multiply by any psi in that tank to get your volume.

.034 X 100psi = 3.4 cubic feet
.034 X 1000psi = 34 cubic feet

With a calculator you will see that it is 104.59035 etc

couv

 

[Charlie99]

Are you 100% sure it is linear??

It's linear to within about 5% for air below 3500psi. See ATOMOX Inc. - HeLIUM ANALYZERS for more info.

On the graph below, the center blue line at 3442psi shows a compressibility factor of 1.05. This means that there is about 5% less air in the tank than you would have if air followed the ideal gas law curve (yeah, I know that a 1.05 factor would seem to mean that there is more air than expected in the tank, but the z-factors are kind of upside down).