Vapor/liquid permeation dependence on pressure differential?
Vapor/liquid permeation dependence on pressure differential?
(OP)
Hi,
I am new to vacuum technology and recently confronted with a problem of polymer seal design for a vacuum application. I am calculating the permeation rates of various gases through polymers.
After going through some handbooks for permeability coefficient values of gases for polymers I am confused about the units used. Could someone please clarify the theory behind these two units...
The units of Gas permeability, P = (cmm3 mm)/(cm2 day atm) or (gm mm)/(cm2 day atm)
where as, the units of Vapor permeability, P = (gm mm)/(cm2 day)
When it comes to vapor, why the pressure unit in the denominator disappeared?
Doesn't the vapor/liquid permeation depend on pressure differential between the surfaces (in and out) of polymer membrane?
Thanks,
Nodal
I am new to vacuum technology and recently confronted with a problem of polymer seal design for a vacuum application. I am calculating the permeation rates of various gases through polymers.
After going through some handbooks for permeability coefficient values of gases for polymers I am confused about the units used. Could someone please clarify the theory behind these two units...
The units of Gas permeability, P = (cmm3 mm)/(cm2 day atm) or (gm mm)/(cm2 day atm)
where as, the units of Vapor permeability, P = (gm mm)/(cm2 day)
When it comes to vapor, why the pressure unit in the denominator disappeared?
Doesn't the vapor/liquid permeation depend on pressure differential between the surfaces (in and out) of polymer membrane?
Thanks,
Nodal





RE: Vapor/liquid permeation dependence on pressure differential?
RE: Vapor/liquid permeation dependence on pressure differential?
http://www.astm.org/Standards/F1249.htm
Do read the scope, where this appears:
RE: Vapor/liquid permeation dependence on pressure differential?
On the the other hand, I am still not clear when "the inverse relationship of WVTR to thickness and the direct relationship of WVTR to the partial pressure differential of water vapor" applies. I have been thinking these relations always apply for gases that do not interact with the material they are diffusing through..any ideas?
In my calculations i am assuming air (either dry or humid) as an ideal gas and caclulating the leak rates based on the partial pressures of each in the atmosphere. Now when vapor transmission rate of H2O is given instead of permeability, should I just ignore the pressure differential? I mean, according to the equation for transmission rate,P in my post
H2O Leak amount (gm)= P x Transmission Area x time/Thickness
assuming P value and my claculations are at STP and same RH.
Thanks,
Nodal
RE: Vapor/liquid permeation dependence on pressure differential?
Um, I think the caution is two-fold. First, water vapor does interact with some materials (nylon for instance), and might cause slight swelling of the material as it absorbs water, which might then change the permeability. Second thing is that temperature may have more to do with permeability of some membranes to water, and since partial pressure of H2O and temperature are hard to seperate, the permeability may be hard to interpret.
Past those cautions, I'd say permeability rates really depend more on the particular method of manufacture of the parts, i.e. molded PVC will have a different permeability than sheet extruded form, which may differ from a blown film; and one factory's formulation may have a markedly different rate than another's. It really does pay, when vapor permeability is critical, to have an independent test conducted.
I've done seal permeability testing on my own before, but I had the advantage that I was trying to seal a thermoelectric device whose cold face was running below 0 deg. C, so I could simply weigh the sealed unit before and after operating for a fixed time in a humid environment (the cold face would just collect ice over time, and hold the cold side humidity very constant). It did make a very nifty test setup, and wouldn't be very hard to rig...
RE: Vapor/liquid permeation dependence on pressure differential?