Comment by Composite Analytica on 11 Oct 2010 at 11:29:49

responses: 1
Dear Martin,
In line with the thermodynamics and diffusion theory, the Helium solubility (S) should be in the order of 1E2 1E3 m3/m3 atm and the diffusivity (D) in the order of 1E9  1E10 m2/s at room temperature (25 degrees Celsius) and 1 atm pressure. This gives for the permeation rate (Pstp=DxS): 1E11  1E13 [m3/m3 atm] x [m2/s].
To obtain the permeation rate in mass, multiply this figure by the Helium density at room temperature and 1 atm pressure. Using ideal gas law: Helium 1.6 e2 gram/m3,
Pmass ~ 1 E11  1 E13 gram / m x s
Divide P by Oring thickness [m] and subsequently multiply by diffusion surface [m2] to obtain the emission in gram /s.
Measured figures far above the stated figure might be due to viscous flow along the Oring (a leak) or a fracture/microscopic void in the Oring. Increase in termperature and pressure, increase the above given permeation rate in mass.
Hope this helps.
Kind Regards,
Composite Analytica