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Vavuum Chamber Design - Air Permeation Calcs

Vavuum Chamber Design - Air Permeation Calcs

Vavuum Chamber Design - Air Permeation Calcs

(OP)
Hi,

I am trying to solve an air permeation problem through a polymer seal but i don't have any experience in vacuum technology. I worked on this problem a bit but not sure if my approach is correct.....

    Let's say I have a cubical Glass vacuum chamber (volume V) with initial high vacuum about 10-5 torr. (No more pumping to maintain this constant).  The chamber has a rectangular seal of area 'A' on one of its sides which is exposed to atmosphere. I would like to calculate the reduction in vacuum pressure with time because of air permeation through seal alone.

I know the permeability coefficients of each of the species in air. The question I have is about the pressure difference in the following equation.

According to the hand book of polymers, Permeate leak amount (of any species in CC),

Qi=  Pi x t x A x (P_Out - P_In)/L  
where,
   Pi - Permeability  Coefficient of N2,O2,Ar, etc ( in cm3-cm/cm2-sec-Pa)
   t –  time increment  in seconds
   A – Seal Area (exposed to air)
   P_Out – atmospheric  pressure? ( or is it partial pressure of each species in air)
   P_in  –  instantaneous  vacuum pressure ( again is this the total vacuum or partial pressure of each gas inside the chamber?)
   L - Seal thickness

    Currently I am using the atmospheric pressure (101325Pa) for P_Out and using the instantaneous vacuum pressure for P_in,  but multiplying the diffrerence (P_Out-P_In) with the volume fraction of each species as follows.

For a small time increment of dt,  Q_N2 = P_N2 x dt x A x (P_Out - P_In) x 0.78/L

Q_Air (in CC)= Q_N2 + Q_O2 + Q_Ar + Q_CO2 +....


  Also, once I get Q_Air, I am calculating the reduction in vacuum assuming Air obeys Ideal Gas Law under vacuum:

  P_reduction  =  nRT/V where n=Q_Air/22400 , R-Gas cont, T-ambient Temp in K, V-chamber vol
 
Am I doing it correct or it's not this simple? Any help is greatly appreciated.

Thanks,
Nodal
  

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