O-ring Permeation

[dgallup]

I'm looking at the figures in

https://www.marcorubber.com/o-ring-permeation.htm

The greater the thickness in the permeation direction the smaller the permeation rate, right?
I think the D dimension should be at right angles (horizontal) to what is shown.

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[hydtools]

The greater permeation rate would be through the smaller dimension D, or nearly so. Would that not make D the dimension of interest?

Ted

 

[3DDave]

I think it is the ratio of exposed surface area to permeation width. If the depth of the o-ring is narrow the gradient will be higher, so maybe that is what drives the rate, which is a random walk rather than a dynamic flow condition.

 

[dgallup]

I'm inserting the graphic from the link just to make it simpler to discuss.

The D that's shown in the graphic is at right angles to the direction of permeation/leakage. Any hydrogen that moves in that direction is not a leak. Also, there is no pressure differential in that direction to force permeation. But delta P is in the numerator of the equation.

I agree it's not a dynamic flow condition but if I double the distance the hydrogen has to travel shouldn't that reduce the flow rate? So the D in the denominator should increase the distance the hydrogen has to travel. The D that's shown is already factored into the area that's in the numerator.

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[3DDave]

I think if one doubles the distance it doubles the time before the first molecule gets through, but the velocity of any molecule through the material will be a constant. Reducing the width reduces the ability for the molecule to leave the main path.

Consider this as a drunkard's walk; if the alley narrows and the drunk maintains their speed, it is likely they will go faster in the direction of the alley. And since this isn't concerned with the individual drunk, but the entire number of them, making the path longer changes the time of the transient rate.

 

[hydtools]

What is confusing is the indication of area. What area? Exposed surface area or cross-section area? I suspect the indicated area is exposed surface area.

Ted

 

[dgallup]

That's what I took it to mean hydtools.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.