Volume of air in tank at a given pressure/temperature

[fppf]

Ideal gas law is close enough for air and Nitrox mixes.
If you ever get into Helium Ideal gas laws will not be enough.

"Air", nitrogen, and oxygen all have simular compressabilities. However Helium is much more compressable than the later 3.

 

[--tom--]

So when I gear up on a nice 80F day, & go diving in refreshing 50F water, a drop of 200psi is pretty normal.

 

[mrlipis]

Yes, straight Boyles law. Nice to put some chemistry to work.

I believe that would be physics at work

 

[Xaryo]

PV=ZnRT

It says all.

Long live the Z, the compressibility factor.

 

[hudson]

jwalko:

I dive HP tanks and routinely get underfills. The excuse for that is they are used to fill AL tanks at 3000 psi, the banks can't really handle 3500 psi etc etc.

But if I was getting underfills with an LP85, I'd be SERIOUSLY pissed. There is no excuse for this. It is only 85CF at 2640psi btw, not 2400.

You should insist that they fill the tanks to 2900psi - this won't do any harm as this is pressure at increased temperature when it cools down will return to 2600 psi. If they cannot comply I'd seriously look for a fill elsewhere.

Be very matter-of-fact about this - you are not asking for a favor, they should fill the tanks to their rated pressure at the rated temperature. And it is costing you and your buddies bottom time since you don't have much air even with a proper fill.

 

[bfw]

See
Boyle's Law and Combined_gas_law for more info.

Basically P1V1 = P2V2. So, given a constant temperature ( 85/2640*2000 ) would be on the money. To check the pressure, just bring your own pressure gauge and check it yourself. Maybe they are right. Also, remember that after a tank is filled, it is hotter, so the pressure would be higher. Once it is cooled down, the pressure will go down. P1V1/T1 = P2V2/T2 (The temps are at absolute temps, so be sure to convert). You can see a decrease of 200 lbs easy from fill time until you dive.

By the way, this is covered in one of the first two modules of the PADI OW course, so no certified diver should need to ask.

 

[vixtor]

On a side note to the original question of the thread.. shoudn't this type of knowledge be explained (if not in school) at least in the most basic "open water" scuba training? Are there scuba classes which don't include gas laws and some gas problems in the theoretical exam?

 

[vladimir]

I will join in the general outrage and amazement that the OP forgot his gas laws. Shocking! Jwalko, you should be ashamed of yourself! You are an example of what is wrong with scuba training and, indeed, what is wrong with America!

 

[jwalko]

I will join in the general outrage and amazement that the OP forgot his gas laws. Shocking! Jwalko, you should be ashamed of yourself! You are an example of what is wrong with scuba training and, indeed, what is wrong with America!

Ah yes...but...if you re-read my original question, it had nothing to do with pressure or volume of gas with respect to temperature...I know Boyles Law...the question was if a formula exists to figure out how much volume my tanks contain at a given pressure. The only reason I threw the temperature portion in is because I knew that someone would write that it is all based on temperature.

What I was really looking for is something like take the volume of an empty cylinder and multiply it by the pressure/temperature to figure out exactly the volume of gas in your cylinder.

I realize the above comment is a bit tongue in cheek, but no one answered my original question. How do i figure out the volume of gas in a tank (assume that I don't know the rated volume/pressure)?

In this case, I am specifically asking about my 85's, but what if I didn't know the rated volume.

John

 

[icestac]

In this case, I am specifically asking about my 85's, but what if I didn't know the rated volume.

If you don't know the rated pressure, I can't think of an easy formula. You'd need to know the unpressurized volume of the tank. You could perhaps take the valve out and fill it with water and then measure how much water it held, but that would probably have quite a bit of error once you calculate high pressures (may not matter that much).

This is why all of the tanks have the specs stamped on the neck.

~Jeff