The physics of compressed air...
Discussion
Okay... so you know when you're sat on the can, and you have a thought... ?
... No ?
.... Okay, just me then ?
If you have an air compressor, with a 24 litre tank, and it's filled with air at a pressure of 8bar, how much air is there in there at normal room temperature and pressure ?
This thought came from filling Alex's inflatable ball pool in the middle of the living room, where the compressor cut in 3 times to refil the tank ! Also from staring at the air compressor in the bathroom, (don't ask !) (and needless to say WD* is very understanding)
So there you have it... the challenge! - And bonus points to someone who can tell me how much that volume of compressed air weighs ;-)
... No ?
.... Okay, just me then ?
If you have an air compressor, with a 24 litre tank, and it's filled with air at a pressure of 8bar, how much air is there in there at normal room temperature and pressure ?
This thought came from filling Alex's inflatable ball pool in the middle of the living room, where the compressor cut in 3 times to refil the tank ! Also from staring at the air compressor in the bathroom, (don't ask !) (and needless to say WD* is very understanding)
So there you have it... the challenge! - And bonus points to someone who can tell me how much that volume of compressed air weighs ;-)
?thebluemonkey said:
192 litres and 248g at 273K. I think.
i got 192 litres and 230 gramms, but is only an approximation.
( because at uni atmospheric pressure is always taken as 1.013 bar and not 1 bar that i used to work it out, we also say that 1 cubic metre of air weighs 1.2 kg.)
>> Edited by danielgray on Monday 6th February 21:18
danielgray said:
192 litres and 248g at 273K. I think.
i got 192 litres and 230 gramms, but is only an approximation.
( because at uni atmospheric pressure is always taken as 1.013 bar and not 1 bar that i used to work it out, we also say that 1 cubic metre of air weighs 1.2 kg.)
>> Edited by danielgray on Monday 6th February 21:18
So is the calculation simply (volume of tank)*(pressure in bar)
Also, can anyone explain then, if atmospheric pressure is 1bar (or thereabouts) that the needle starts at 0bar .... is is simply "pressure above atmospheric" ?
One Bar is one atmosphere, more or less, I think.
That would tend to suggest that you have eight atmospheres, or eight times the volume.
Someone else said 192, and it makes sense to me.
Edited to add;
It is pressure above atmospheric, and that's why you don't count the 9th atmosphere that was in the tank before you started, and is still there after you finish.
>> Edited by dilbert on Monday 6th February 21:27
That would tend to suggest that you have eight atmospheres, or eight times the volume.
Someone else said 192, and it makes sense to me.
Edited to add;
It is pressure above atmospheric, and that's why you don't count the 9th atmosphere that was in the tank before you started, and is still there after you finish.
>> Edited by dilbert on Monday 6th February 21:27
Well yes, 24 ltr times 8 bar does equal 192 litres right enough, but you have to take in account the compressability factor of air, and what the standard litre is based upon, for example, in diving, Helium gas is usually based on a temperature of 58 degrees, (I seem to remember) but it depends on which gas company one uses.
For all intense and purposes, volume times pressure is good enough, so your answer will be 192 litres. Same as in SCUBA diving, pressure times volume is the amount of air available.
Incidently, 1 atmosphere in imperial units is 14.7 psi, if you convert 1 bar to psi, it is 14.51, not worth bothering about in this application, but when you deal in thousands if not millions of cuft of Helium, which is a very complex gas as far as compreesabilty goes, thats a different matter.
For example a bog standard 45 bottle gas pack of Helium at 196 bars contains 14400 cuft of gas, a mixture of Helium and Oxygen is billed at 14750, wheras the same container of Oxygen would be 17100, the formulas for Helium also takes into account the stretch coefficent of steel containers, much like the rubber hot water bottle, you fill it, it expands, and thus will hold more water. You fill a steel bottle, it will expand and will therefore contain more gas.
Sorry to go on, but with 30 years experience in the diving industry, just happen to know these things, the acutal formulae for calculating volumes of helium in steel containers, is quite complex, not that anyone would want it, but if they do, I will try to put it on here for you
Sorry, seem to have gone on a bit here.
For all intense and purposes, volume times pressure is good enough, so your answer will be 192 litres. Same as in SCUBA diving, pressure times volume is the amount of air available.
Incidently, 1 atmosphere in imperial units is 14.7 psi, if you convert 1 bar to psi, it is 14.51, not worth bothering about in this application, but when you deal in thousands if not millions of cuft of Helium, which is a very complex gas as far as compreesabilty goes, thats a different matter.
For example a bog standard 45 bottle gas pack of Helium at 196 bars contains 14400 cuft of gas, a mixture of Helium and Oxygen is billed at 14750, wheras the same container of Oxygen would be 17100, the formulas for Helium also takes into account the stretch coefficent of steel containers, much like the rubber hot water bottle, you fill it, it expands, and thus will hold more water. You fill a steel bottle, it will expand and will therefore contain more gas.
Sorry to go on, but with 30 years experience in the diving industry, just happen to know these things, the acutal formulae for calculating volumes of helium in steel containers, is quite complex, not that anyone would want it, but if they do, I will try to put it on here for you
Sorry, seem to have gone on a bit here.
thebluemonkey said:
Also, can anyone explain then, if atmospheric pressure is 1bar (or thereabouts) that the needle starts at 0bar .... is is simply "pressure above atmospheric" ?
Yes that sounds right, which would throughout the numbers above.
Any pressure you measure will always be in gauge pressure which is the pressure measured.
Calculations nearly always done in absolute pressure which is gague + atmospheric pressure.
I assume the 8 bar is gauge pressure, so use 9 bar in the calculations.
I always wondered if Gas cylinders are evacuated before they are filled with whatever it is they're going to be filled with. Also I wonder if there is an obvious way to get the last volume of gas (at atmospheric pressure) out of a nearly dead bottle.
Knowledge like that could come in handy when the gas (natural) finally stops flowing!
Knowledge like that could come in handy when the gas (natural) finally stops flowing!
Gassing Station | The Pie & Piston Archive | Top of Page | What's New | My Stuff