# UHV leak rate at pressure

## Main Question or Discussion Point

I'm trying to figure out some things regarding leak rates. I have a system which is constructed to adhere to UHV standards (total leak rate < 10^-10 Torr*L/s), baked out and evacuated to 10^-9 Torr, and then backfilled to about 600 Torr with N2 and Xe. So, there's a leak of air from the outside world to the lower pressure. I'm wondering how to calculate this leak rate. If I were at 10^-9 Torr, I'd just use the stated leak rate. I'd also be able to use a pressure rise measurement. But what about at higher pressure? I'd imagine that the leak rate is now less than 10^-10 Torr*L/s since there's a smaller pressure differential. Any way to calculate the leak rate at this pressure given the leak rate at UHV? I can't do a pressure rise measurement here because the leak rate is many orders of magnitude smaller than my base pressure.

## Answers and Replies

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UltrafastPED
Science Advisor
Gold Member
In my experience the loss of pressure in a good UHV system is due to outgassing from the interior surfaces, not exterior leaks. This is true even after you bake it for a few days.

Thus when I run at 10^-9 Torr I keep the ion pump running in order to maintain the pressure.

I'm not sure that this helps with your question, except that you probably don't have any actual leaks from outside; they are actually all inside.

In your case you can test the composition of your interior gas with a residual gas analyzer. Then for a well-baked UHV system the principle out gassing will be hydrogen. If you find oxygen then you may have a leak.