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Gas Law Calculation

  1. Jul 22, 2015 #1
    [Moderator's note: Recategorized thread to "Basic".]

    I'm having an issue at work that I think may be down to temperature.

    I have a sealed chamber that is 1m X 1m X 1.5m. The chamber has to be pressure tested but the results are all over the show. The only variable is temperature.

    So my question to you guys is can you calculate how temperature will effect the pressure? The pressure I'm trying to maintain in 200Pa. The temperature varies from 17'C to 22'C. How much does each 1'C effect that pressure if we are starting at 200Pa and 22'C?

    Any help would be very much appreciated.
     
    Last edited by a moderator: Jul 22, 2015
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  3. Jul 22, 2015 #2

    PeterDonis

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    Staff: Mentor

    You haven't given enough information. What is inside the chamber?
     
  4. Jul 22, 2015 #3
    What do you mean by a sealed chamber ? Does it exchange matter or energy with the surroundings ?
    If not , temperature will not be the only variable ( pressure ) and they will follow a simple relation , T/P = constant . Change in pressure would however be negligible .

    Same for the case in which heat can be exchanged - Process would be isochoric and follow same relation as the previous case .

    If matter can be exchanged , n*T = constant c- change in pressure zero .

    * I was not sure which case you meant and so I mentioned all three .

    If matter is not exchanged and volume is constant , how are you going to maintain a constant pressure - Say if the change in temperature would be 50 K and not 5 ?
     
  5. Jul 23, 2015 #4
    Sorry, I probably should have been a bit more detailed and specific.

    The 1m X 1m X 1.5m chamber is empty (other than the air) and positioned in a room that is currently 17'C. The room is also positive 30Pa to atmospheric. The chamber is temperature/humidity controlled and sits at 21'C and when the door to the chamber is open equalizes with the room at 30Pa. What I'm doing is shutting the door to the chamber and pressurizing it internally to 200Pa using air, and then watching that pressure for any decay after all valves are shut. At this point the temperature/humidity control is turned off, and the temperature in the chamber naturally equalizes with the room around it, dropping from 21'C to 17'C. The chamber is pressure sealed so the 200Pa of air cannot leak out into the room.

    So what I'm wondering is if that temperature drops from 21'C to 17'C in the chamber, would this cause the internal air pressure of my chamber to drop with it?
     
  6. Jul 23, 2015 #5
     
  7. Jul 23, 2015 #6
    So I should be able to calculate my constant and work backwards from that to see how pressure changes as temperature does.
     
  8. Jul 23, 2015 #7
    No - the constant depends on a lot of things - just do T1/P1 = T2/P2 .
     
  9. Jul 23, 2015 #8
    Okay thanks.

    One last question and sorry to be a pain, what units should I be using? I'm assuming temperature in kelvins right?
     
  10. Jul 23, 2015 #9
  11. Jul 23, 2015 #10
    What about pressure? Do I have to use absolute... e.g. 200Pa gauge is 101525Pa absolute (assuming atmospheric pressure is 101325Pa)?
     
  12. Jul 23, 2015 #11
    You need to use absolute pressure.
     
  13. Jul 23, 2015 #12
    Change of unit for pressure is not required , but obviously , absolute pressure is required .
     
  14. Jul 23, 2015 #13
    So this appears to make a fairly big difference, unless I'm doing something wrong.

    Results:

    200Pa @ 22'C
    -144Pa @ 21'C
    -488 @ 20'C
    -831 @ 19'C

    So fairly substantial changes there.
     
  15. Jul 23, 2015 #14
    Yes you've gone wrong somewhere .
    Look at it this way - P2 = P1*T2/T1 . T2 ≅ T1 (If one is say 295 , other is 294) .

    So P2 ≅ P1 . So you can see your answer is wrong .

    Why don't you post your working ?
     
  16. Jul 23, 2015 #15
    So the pressure is 200Pa gauge or 101525 absolute @ 22'C or 295Kelvin.
    I'm trying to find P2 after a 1'C pressure drop so 21'C.

    P2 = P1*T2/T1 therefore P2 = 101525*294/295. The answer for that is coming out as 101180.85 absolute or -144.15Pa gauge.
     
  17. Jul 23, 2015 #16
    Yeah sorry my bad . But if you could correct this :

    I think you mean 200kPa .
     
  18. Jul 23, 2015 #17

    PeterDonis

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    Not in an absolute sense; these are gauge pressures, so all this is telling you is that the pressure inside the sealed chamber goes from a little bit above atmospheric to a little bit below atmospheric.
     
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