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Topic: Problem maintaining vacuum in schlenk line  (Read 1064 times)

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Offline sbarpen

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Problem maintaining vacuum in schlenk line
« on: September 19, 2019, 10:53:13 AM »
I'm struggling maintaining a stable vacuum in a new schlenk line I built and was hoping to get tips. My background is in materials science so my experience with chemistry labs is limited.

Currently I'm only able to a get the vacuum down to 200 mTorr but as soon as I close the valve to the pump, pressure goes up pretty quickly to a few torr and then the leak stabilizes to 1 torr/minute.  I have worm clamps at the tubing connection points and have tried both gum rubber and braided tubing. I also tried different vacuum pumps, tried switching where the connections  are and  I removed the cold trap flasks but the results are still the same.  I'm not sure what else I can try or where exactly i'm going wrong.

Offline OrganicDan96

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Re: Problem maintaining vacuum in schlenk line
« Reply #1 on: September 19, 2019, 12:29:30 PM »
i assume you have greased all your taps?

Offline sbarpen

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Re: Problem maintaining vacuum in schlenk line
« Reply #2 on: September 19, 2019, 01:14:50 PM »
Sorry should have mentioned that the manifold has teflon stopcocks

Offline wildfyr

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Re: Problem maintaining vacuum in schlenk line
« Reply #3 on: September 19, 2019, 09:57:38 PM »
Is the hosing pretty fresh? Older hosing will oxidize where it twists there are tiny holes that leak

Offline sbarpen

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Re: Problem maintaining vacuum in schlenk line
« Reply #4 on: September 19, 2019, 10:17:53 PM »
Yeah the gum rubber hosing is brand new

Online rolnor

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Re: Problem maintaining vacuum in schlenk line
« Reply #5 on: September 20, 2019, 02:40:06 PM »
I think glassstoppcocks with grease are better then teflon for highvacuum if they are not designed for highvacuum

Offline hollytara

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Re: Problem maintaining vacuum in schlenk line
« Reply #6 on: September 20, 2019, 11:17:34 PM »
Do you have access to a leak detector?

Offline Corribus

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Re: Problem maintaining vacuum in schlenk line
« Reply #7 on: September 20, 2019, 11:29:35 PM »
I think glassstoppcocks with grease are better then teflon for highvacuum if they are not designed for highvacuum
Agree with this.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline sbarpen

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Re: Problem maintaining vacuum in schlenk line
« Reply #8 on: September 23, 2019, 09:35:11 AM »
Thanks for the suggestions. I'll try to see if I can borrow a leak detector somewhere.

The valves are allegedly made for high vacuum but you never know. To test it out, I put pipette bulbs on each of the valves, open them to vacuum and then closed them. Then I shut off the vacuum and left the system alone for several days. When I checked back the pipette bulbs are all still evacuated but the line is back at atmospheric pressure. My guess is it's a leak in my gauge or in the actual line.

Offline phth

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Re: Problem maintaining vacuum in schlenk line
« Reply #9 on: September 25, 2019, 01:56:35 AM »
Thanks for the suggestions. I'll try to see if I can borrow a leak detector somewhere.

The valves are allegedly made for high vacuum but you never know. To test it out, I put pipette bulbs on each of the valves, open them to vacuum and then closed them. Then I shut off the vacuum and left the system alone for several days. When I checked back the pipette bulbs are all still evacuated but the line is back at atmospheric pressure. My guess is it's a leak in my gauge or in the actual line.

Gas is resilient; why not just leave the vacuum pump running all the time??? McLeod gauges do not give false readings. Use the process of elimination to narrow it down.

Offline clarkstill

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Re: Problem maintaining vacuum in schlenk line
« Reply #10 on: September 25, 2019, 05:52:14 AM »
I think glassstoppcocks with grease are better then teflon for highvacuum if they are not designed for highvacuum

Yes, as long as they are numbered and the taps are matched. But J. Young valves are best of all!

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