Diagnosing pressure rise in bell jar

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Arun Luthra
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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra »

Here is a longer run that is a lot more promising, in the sense that it had a good fit to a two-outgassing-component model with a fast component and a slow component. The fit suggests that leakage is low.
double-outgassing.png
Not sure if this is enough information to diagnose, but does anyone think some H2O microfilm is the main source of the outgassing? I've heard that chamber bakeouts are a common method for fixing this?

I added a foreline trap. It does not seem to have electrical connections. The flow conductance on the trap seems really low (pumpdown is a lot slower). It has been sitting in open air for probably 2 years or more, maybe soaking up water vapor, so, I imagine it would need a multi-hour pumpdown to help dry it out. I might have to ditch the trap.

At the end of the day, I'm not doing a fusor experiment, my operating pressure is more like 1 torr. So I may not need to figure out all of these issues.
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Dennis P Brown
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Re: Diagnosing pressure rise in bell jar

Post by Dennis P Brown »

So it looks like about one micron per three minutes - that is classical outgassing and not an issue. One the system is clean (been under vaccuum and not exposed to moisture) it will hold a lot longer. An old trap has a lot of contaminates and it too will outgas so, again, not to be an issue.

Some traps use an active agent that can be removed for replacement or separate cleaning. If yours can be removed, a quick heat in an oven might help - not too hot (100 - 150 C). Then place in (still hot) and pump it down for awhile.
Arun Luthra
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Re: Diagnosing pressure rise in bell jar

Post by Arun Luthra »

It is actually about 500 microns over the first 2000 seconds (even faster initially), so 15 microns per minute. With high vacuum grease on the gasket it is about the same.
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Dennis P Brown
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Re: Diagnosing pressure rise in bell jar

Post by Dennis P Brown »

Ever since I learned calculus, my simple math skills are terrible ... ;) ( I saw the top line as brown but now realize its two other colors that appear brown on my screen.)

Fifteen microns per minute is a bit fast for simple out-gassing unless you have a huge surface volume, a very small, real leak or a very dirty system.

For leaks like this I generally break down all my KF's (if you use those), clean the 0-rings (a lint free cloth; maybe a hint of Isopropyl alcohol), bake anything that is all metal, and reassemble after an over night wait under vacuum. I check the pump oil (should look very clean even after being run for a long time), and also purge my vacuum pump (air inlet valve on the pump.) In extreme cases, clean my oil trap (for the pump - again, an alcohol rinse, allow to dry, then a bake out followed immediately by a pump out).)

When I pump the system down, I close the pump's cut off valve even if there is a oil trap (I have the valve above the vapor trap, of course). Then I test again. If issues, I spray every connection with a little alcohol - this can often close a very tiny leak or possibly, allow a short improvement in the system in which case you found a possible problem area.

If you have the equipment to get down to under 10^-4 torr, and an ion gauge, the alcohol test is ideal.
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