Leak rate conversion with different gases 2

[cavva78]

Hello,

I have a requirement for a max leak rate with He in a pressure vessel (1e-4 mbar*l/s). I want to test it with pressure decay method, but using Nitrogen instead of He. What is the correct calculation of leak rate using Nitrogen, and the translation in max pressure decay (mbar/s)? The internal volume is 0.2 l.

Thanks
Marco

 

[JDmagnet]

Although you could do a leak test as such, It may not be an acceptable test to do. Helium is a smaller molecule and will therefore fit through a leak that a nitrogen molecule will not. That is why they are specifying helium as the leak standard instead of nitrogen. In mass spectrometer leak checking, when a leak larger than the leak detector can pump is present, they use this fact by placing a material porous to helium in the stream which reduces the pressure in the leak detector but still seeing the helium tracer gas in the flow stream. This could be a consideration depending on intended use although the leak rate specified is not real low.

 

[unclesyd]

Here is all you need to know to get started or utterly confuse you. The first reference is about as concise as one could get and still tell the story.

http://www.lacotech.com/products/997300658_leaktechref.pdf

http://www.mat-vac.com/tech-notes/he-leak-rate.shtml

 

[vpl]

The wording of the original post is what caught my attention "I have a requirement..."

The excellent post by unclesyd succinctly gives the information on how to convert between the two gases. The equally excellent post by JDmagnet provides an explanation of why making such conversion may not be allowable (or even wise, depending on the service.) Hopefully cavva78 will not ignore the warning that just because s/he can convert the leak rate doesn't mean that s/he should.

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[JR97]

vpl,

Good point. As long as the l/d is in or near the viscous flow range, there is no problem with molecule size. If you are in or near the molecular flow range than caution is warranted.