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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I have a 1 liter bottle at one atmosphere (15psi for simplicity). The gauge reads 0psi. I fill it until the gauge reads 45psi. Am I correct to say that if I want to know how many liters of gas are in the container I need to account for atmospheric pressure as such:
(45psi + 15psi of the atmosphere)/15psi(of the atmosphere) = 4
Most calculations I've seen for high pressure tanks (2000psi) seem to ignore the addition of atmospheric pressure. If I vent one of these tanks the gauge reads zero when it should really read 15psi. This isn't a problem unless you are dealing with minimal pressures above atmospheric.
Registered Member #3271
Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159
PV=nRT assuming your container does not change shape (volume) with pressure and the gas is well behaved and the temperature (adiabatic compression) of the gas constant....
Gauge is relative vs absolute. usual mechanical gauges read atm on one side and will display the difference between input and local atm pressure. Absolute gauges will reference to a vacuum (independent of local atm pressure) and thus read about 15psi depending on your altitude and weather. You are right that for a gauge reading the cyl pressure of 2000psi it will not matter much as 15psi is probably beyond its intrinsic precision.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It really depends on what use you want to make of the information "how much gas is in the container?"
For a 1 litre bottle reading 45psi, 3 bar above atmospheric pressure ...
If you want to know how much gas you can get out of it venting it to ambient pressure, so leaving it full of 1 litre of gas at 1 bar when it's "empty", then it's 3 litres.
If you want to know how much gas is actually in it, say you want to displace it all out by filling it with water, then it's 4 litres.
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proper way to measure volumes under pressure
