Tuesday, November 13, 2007

Ultra High Vacuum

I've written very little about what I actually do in the lab, so I'm going to try to put up a few posts that explain some of what I do. This serves 2 purposes:

1) To give some relevant information about some topics that may not have clear explanations easily found elsewhere.

2) (more importantly) So that my father can reference it multiple times when he tries to remember the acronyms related to my work.

Today's acronym: Ultra-high Vacuum (UHV). Since there's a lot that goes into UHV technology, this post will focus specifically on copper gaskets.

While the UHV part of what I do takes up a lot of my time, it's not directly related to my actual research, but we cannot do the research without the vacuum. Ultra-high vacuum is usually defined as pressures that are lower than 10-10 torr. One atmosphere of pressure is 760 torr and this is anything below 0.0000000001 torr. In simple terms, there is very little air inside the chambers in which we work and even the tiniest leak ruins our ability to be at UHV. So the question arises, how do you seal all of the separate pieces so that there are no leaks? The answer is conflat flanges and copper gaskets. The first picture shows a 2 3/4" conflat flange setup. On the right is the STM (scanning tunneling microscope...the part that is related to our research) chamber and the left is a small window built into a flange. The inner ring of the flange is what is known as a "knife edge." The knife edge is an angled part of the stainless steel with a sharp edge that will cut into the copper gasket.
The second image shows a copper gasket that fits perfectly into the knife edges on both flanges. It's just a flat copper disk that comes clean and sealed inside its own plastic pouch that we open just before we install it between the flanges. Since copper is relatively soft, the steel knife edges will bite into the copper gasket as the flanges are tightened towards each other, leaving an impermeable metal-metal seal, which prevents even single gas molecules from entering the high vacuum side.
This picture shows a larger conflat flange with the copper gasket stuck to the knife edge after being removed from the system. In this picture, you can see the ring where the knife edge on the other side bit into the copper gasket to make the seal. The consequence of this sealing mechanism is that gaskets can only be used once, so everytime a flange is opened (hopefully, you never have to open them, but that's wishful thinking), you have to put in a new copper gasket and discard the old one. This gasket doesn't show it, but often the edge outside the knife edge will be greenish where the copper oxidized on the atmospheric side of the seal, but the inside part of the knife edge will still have a pristine copper shine because it was on the UHV side and was not exposed to oxygen.
The last step is to actually tighten the flanges together, which we do by evenly tightening down a set of bolts to pull the flanges toward each other until the vacuum seal is achieved. It doesn't always work the first time and can fail if there is a tiny scratch in the knife edge or a piece of grit gets in the knife edge, leaving a small passageway for gas molecules to enter. The only way we know that has happened is by leak testing, but that's another post!
Posted by Picasa

14 comments:

Ψ*Ψ said...

cool!!! (i know NOTHING about working under vacuum)

Anonymous said...

Greg, I'll have to come back and read the info later as it sounds interesting.

Wishing you a happy thanksgiving from Oregon!

Silke said...

Hello Greg, I'm desperately searching for the correct English term (if there is any) of something called "Spiesskantdichtung" in German. It's a special gasket for UHV systems and high pressure tubes made of soft metals, such as copper or aluminium. The profile is somewhat rhomboid or flat with a triangle poking out on both sides. Any idea what that is called?

Greg said...

Silke, as far as copper gaskets go, I'm really only familiar with conflat flanges and VCR fittings (from Swagelock). I looked through all the flange types and their seals at ancorp.com and see a couple other varieties of flanges and copper seals, but nothing like what you've described. I would suggest checking the product lines from the major vacuum manufacturers like ancorp, MDC, Kurt J. Lesker, Johnsen Ultravac, etc. If you don't see what you're looking for, contact the companies and talk to a technical person there; I've found most of these companies to be extremely helpful. Do you know what the type of flange you're using is since the gasket will have to match it?

Best of luck.

Norman Hill said...

Hi Silke
I think the gasket that you are looking for is a metal Kline flange gasket. These are available from Oerilikon vacuum, formerly Leybold vacuum. If you need other UHV gaskets we have a great selection at www.vgscienta.com
best regards Norman Hill

Anonymous said...
This comment has been removed by a blog administrator.
Avery Schlacter said...

Thanks for this I've been looking for copper gaskets and haven't had much luck finding any until now.

Alan Gross said...

This is neat, and you obviously know your stuff. I have referenced you on our blog post about tightening flanges correctly.

Perhaps we can interview you at some point.

Sam Chohan said...

Awesome post about vacuum flanges. A lot of this info I didn't know about it. Thanks. Do you mind if I reference this website in an article Im writing about?

Greg said...

Sam, go for it.

Yane Yang said...

Zhejiang Yaang Pipe Industry Co., Limited is a leading manufacturer and supplier of Stainless Steel Gaskets.

Anonymous said...

Another place to find great Copper Gasket is Accu-Glass Products, Inc. The name is misleading but they carry almost all the items in stock and can ship in 1 day.

Thanks,
Chris

Anthea Agius Anastasi said...

Hi, excellent post!
I have exactly the same kind of flanges and copper gaskets. My problem is that apparently I have a big leak. How can I perfectly align the gasket before tightening the bolts if the flange is vertical (gravity doesn't help!). Do you have any tips?
Thanks!!

Saini Flange Pvt. Ltd. said...

Hey there,

Firstly i would like to thank you for such an informative post.
Keep Sharing!!http://www.sainiflange.co.in