# calculate air tank capacity?

• posted on April 4, 2004, 1:19 pm
I have 2 air tanks containing compressed air. One tank has an internal capacity of 7 1/2 gallons, and the air in it is at a pressure of 100 psi.
The other tank is a 7 gallon tank, and the air in it is at a pressure of 135 psi.
Which tank has the "most" air in it? I guess what I mean is............if each tank was to release all of its air, into the atmosphere, the air from which tank would occupy the most space?
I hope I worded this with enough clarity..........
Thank you..... Lee Carkenord
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• posted on April 4, 2004, 2:38 pm

Air density is propotional to absolute pressure and inversely proportional to absolute temperature (air heats up as it is compressed). But assuming they are at that pressure at same ambient temperature, and figuring relative density, makes it fairly easy to figure out which one holds more air.
7.5 * (100 + 14.7)/14.7 = 58.5 (gallons of atmospheric air) 7 * (135 + 14.7)/14.7 = 71.3 (22% more)
If you use those tanks to fill a tire to 32 psi (subtract same calculation for eash at 32 psi), the difference is a higher percentage (smaller, higher pressure tank would supply 41% more air at 32 psi). But don't exceed a tank's pressure rating, because that 7 gallons of air suddenly expanding to 71.3 gallons can go bang.
--
David Efflandt - All spam ignored http://www.de-srv.com /

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• posted on April 4, 2004, 2:42 pm
Carkenord) wrote:

Just multiply the volume times the pressure. There's more air in the smaller tank.
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• posted on April 5, 2004, 2:51 am
Lee Carkenord wrote:

Who says "air" isn't perfect?
Remember the old PV = nRT or, more useful in this case, RT = PV/n
Where
P = Pressure (in appropriate units) V = Volume (in appropriate units) n = "Amount" of gas (in appropriate units, e.g. moles) T = Temperature (in appropriate units) R = the "Gas Constant" (in units appropriate to the other units)
Let n1 = the "amount" of air in the 7.5 gal. tank n2 = the "amount" of air in the 7.0 gal. tank
Therefore:
(100*7.5)/n1 = RT = (135*7.0)/n2
n2/n1 = [(135*7.0)/(100*7.5)]
n2 = (1.26)n1
So, the 7.0 gal tank at 135 psi contains 1.26 times the amount of gas as the 7.5 gal tank at 100 psi.