Foreline KF O-ring greasing

[ChristofferBraestrup]

Hi all!

My turbo high vacuum system is practically operational, but I'm encountering leaks in my foreline. It's not enough that I don't dare operate the turbo, since I still get dynamic foreline vacuums at 10^-2 torr, 3 orders of magnitude better than required by a turbo, but if I isolate the foreline from the rotary vane pump, it equilibrates to ~10 torr within a minute.

The fault is most likely stiff O-rings in the KF couplings, but I have so many of them I don't feel like scrapping them all. Getting spare O-rings for the metal centering rings costs almost the same as the entire assembly so that's not an option.

Some sources claim KF O-rings should be greased with apiezon or similar high vacuum grease. Is it worth trying, and more importantly: would it creep into the turbo?
Has anyone done this? One might argrue the foreline will never be completely free of oil mist anyway.

[Dennis P Brown]

Grease was and is a 'stop-gap' measure only for very minor leaks; and never to be used in place of a proper seal. Applying grease and then using a clean, lint free cloth to wipe it off is how and the only proper manner to 'grease' any o-ring. That all said, grease is for very small leaks - sealing very fine scratches on a metal surface or on the o-ring itself. A large leak will certainly cause grease to enter the vacuum system and using grease in those situations is unlikely, in any case, to work well. Further, grease will not fix a stiff o-ring; the very function of any o-ring is to readily deform allowing a seal. Once an o-ring can no longer deform, it is useless and needs to be replaced.

A 10^-3 torr to 10 torr for a small vacuum system in about a minute is certainly a serious leak and should be fixed. You can try grease (applied as I outlined) for a temporary fix but at some point that leak(s) will need to be addressed. Hopefully, cleaning your system afterwards isn't then added to that list.

By the way, I've seen set's of just the KF o-rings for sale on ebay in the past. They weren't expensive at all. The metal band is the biggest expense.

[ChristofferBraestrup]

Thanks for the info. The leak is not as bad as I remembered, now it goes from 10^-2 torr to 10^-1 torr in ~3 mins, and then stagnates around 1 torr for 15+ minutes.

Volume of system is probably around 150-200 ml. I might try greasing up the o-rings, but vacuum grease is the gift that keeps on giving: you never get rid of it. Hence my hesitation. I think I'll hold off for now.

[Jerry Biehler]

If you are seeing that slow of a leak that levels out you probably has something outgassing in your foreline.

[Dennis P Brown]

Agree with Jerry; to test that, simply run the pump twice a day for a few days and don't open the system to air. If out gassing, the rate and final pressure will be less.