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  #1  
Old 02-03-2009, 02:39 PM
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physics/thermodynamics question

Not a trick question -

What's the ratio of air mass between a fixed volume of air at STP and an identical fixed volume of air pressurized to 2 bar gauge, stabilized to the same ambient temperature? I figure there's 3 times as much air by mass in the pressurized container based on the ideal gas law. What's the proper way to go about this?

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  #2  
Old 02-03-2009, 02:44 PM
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(p1 v1)/t1=(p2 v2)/t2
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Old 02-03-2009, 02:48 PM
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Yeah, that's what I was thinking too


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  #4  
Old 02-03-2009, 03:16 PM
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So wouldn't it be twice as much mass in the pressurized container? Assuming 1 bar is standard pressure. As I remember PV = nRT.
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  #5  
Old 02-03-2009, 03:23 PM
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Those are variations of the ideal gas law. Can ideal gas laws apply to typical air at STP and STP + 2 bar?

Volume doesn't change, temperature doesn't chage (after equalizing) and the ideal gas constant doesn't change (if it even applies) so we're down to P1/n1 = P2/n2. P1 = 1 bar absolute, P2 = 3 bar absolute. If P2/P1 = 3, n2/n1 = 3. That's how I came up with 3:1 air mass ratio. But I don't know if all the stuff before getting to the P/n ratios is valid for a non ideal gas. Nor if a mole is directly proportional to mass in this situation.

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Old 02-03-2009, 03:36 PM
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Quote:
Originally Posted by sixto View Post
Those are variations of the ideal gas law. Can ideal gas laws apply to typical air at STP and STP + 2 bar?

Volume doesn't change, temperature doesn't chage (after equalizing) and the ideal gas constant doesn't change (if it even applies) so we're down to P1/n1 = P2/n2. P1 = 1 bar absolute, P2 = 3 bar absolute. If P2/P1 = 3, n2/n1 = 3. That's how I came up with 3:1 air mass ratio. But I don't know if all the stuff before getting to the P/n ratios is valid for a non ideal gas. Nor if a mole is directly proportional to mass in this situation.

Sixto
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Ideal gas law is a good first approximation. It breaks down when the gas in the sample is near a phase change.

The 'n' is the sum of the number of moles [6.02 * 10E24 molecules] for each different kind of molecule in the gas mixture [O2 + N2 + Ar + H2O + CO2 +....]
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Old 02-03-2009, 03:38 PM
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Originally Posted by sixto View Post
and an identical fixed volume of air pressurized to 2 bar gauge
In your OP, you said 2 bar, not 3 bar.

I think air is reasonably close to being an ideal gas. Are you trying to get an exact answer or approximate? Mole is indeed directly proportional to mass.
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Old 02-03-2009, 03:40 PM
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Oh wait, never mind, now I see you meant 1 bar + 2 bar. So yes, there would be 3 times as much mass in the pressurized container.
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Old 02-03-2009, 03:41 PM
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Quote:
Originally Posted by sixto View Post
Those are variations of the ideal gas law. Can ideal gas laws apply to typical air at STP and STP + 2 bar?


Sixto
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Is this valid?

In your original statement you were comparing STP and at 2 bar.
Is that not 2X, not STP +2?
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Old 02-03-2009, 03:56 PM
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My assumption is 2 bar on a gauge is 3 bar absolute.

The reason I ask is the prevalence of "nitrogen filled" tires. How much of the air in the tire isn't nitrogen by virtue of the tire mounting process? My guess is the tire is full of regular air even if they use nitrogen to seat the beads. Then inflating to a typical 30 psi means the tire contains 2 parts by weight pure nitrogen for each part of regular air. So what they claim is a nitrogen fill is at best 89% nitrogen (66% pure nitrogen + 70% nitrogen in the third by mass that's regular air). Is there a better way to calculate this?

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  #11  
Old 02-03-2009, 04:05 PM
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Quote:
Originally Posted by sixto View Post
My assumption is 2 bar on a gauge is 3 bar absolute.

The reason I ask is the prevalence of "nitrogen filled" tires. How much of the air in the tire isn't nitrogen by virtue of the tire mounting process? My guess is the tire is full of regular air even if they use nitrogen to seat the beads. Then inflating to a typical 30 psi means the tire contains 2 parts by weight pure nitrogen for each part of regular air. So what they claim is a nitrogen fill is at best 89% nitrogen (66% pure nitrogen + 70% nitrogen in the third by mass that's regular air). Is there a better way to calculate this?

Sixto
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That sounds right to me, unless they evacuate the air from the tire somehow.
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  #12  
Old 02-03-2009, 04:06 PM
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The green caps mean "another sucker rolls by."

When they say 89% nitrogen, I assume that they mean that they have a supply of 89% nitrogen, rather than the 80% nitrogen supply that I have in my garage. There are "nitrogen generators" that are used. These tire shops are not buying cylinders of pure N2.
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Old 02-03-2009, 10:36 PM
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There is way tocalculate it because of mixing at the rim. All they really care about is getting rid of water vapor, the amount of N2 is not really what they seek.

Quote:
Originally Posted by sixto View Post
My assumption is 2 bar on a gauge is 3 bar absolute.

The reason I ask is the prevalence of "nitrogen filled" tires. How much of the air in the tire isn't nitrogen by virtue of the tire mounting process? My guess is the tire is full of regular air even if they use nitrogen to seat the beads. Then inflating to a typical 30 psi means the tire contains 2 parts by weight pure nitrogen for each part of regular air. So what they claim is a nitrogen fill is at best 89% nitrogen (66% pure nitrogen + 70% nitrogen in the third by mass that's regular air). Is there a better way to calculate this?

Sixto
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  #14  
Old 02-04-2009, 08:28 AM
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Originally Posted by Emmerich View Post
There is way tocalculate it because of mixing at the rim. All they really care about is getting rid of water vapor, the amount of N2 is not really what they seek.
That and the larger nitrogen molecules don't seep through the rubber as fast as the smaller oxygen molecules will.

http://en.wikipedia.org/wiki/Boyle's_law

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  #15  
Old 02-04-2009, 08:28 AM
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Originally Posted by DieselAddict View Post
So wouldn't it be twice as much mass in the pressurized container? Assuming 1 bar is standard pressure. As I remember PV = nRT.
I think Kuan's post is the right set-up for solution. It's either Boyle's or Charle's Law (I never have gotten the names straight). It can be derived from the Ideal Gas Law. I just don't remember how anymore!

B

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