Olive compression fittings in High Vacuum

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Albert Mery
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Olive compression fittings in High Vacuum

Post by Albert Mery » Sun Jun 20, 2021 10:36 pm

Hello all,

So I am beginning to plan the construction a HV feedthrough and I was hoping to use regular olive compression fittings used in plumbing. I know that Swagelok fittings would be ideal but I don't have access to any such fittings ( due to the long shipping times from China or the US). So I was wondering whether anyone had any experience with using olive compression fittings in High Vacuum? I'm also planning to use them with quartz tubing and I'm not sure how tight the fit will be...

Thanks in advance!
Albert

John Futter
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Re: Olive compression fittings in High Vacuum

Post by John Futter » Mon Jun 21, 2021 10:01 am

Albert
Olive fittings they may or not may not work for high Vacuum --ie a very possible leak site
Swagelok have special weld on fittings that get rid of the olives for high vac work.
their other branch sells VCR fittings for high and ultra high vacuum work

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Joe Gayo
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Re: Olive compression fittings in High Vacuum

Post by Joe Gayo » Mon Jun 21, 2021 4:13 pm

Use orings.

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Richard Hull
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Re: Olive compression fittings in High Vacuum

Post by Richard Hull » Mon Jun 21, 2021 4:49 pm

Like already stated, Swagelok is more than suitable for the small gas lines in fusor work. VCR is vastly More expensive, but a must have for those working in very deep vacuum systems where larger lines are the norm.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Albert Mery
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Re: Olive compression fittings in High Vacuum

Post by Albert Mery » Wed Jun 23, 2021 2:16 pm

Thanks for the advice, I will look into purchasing some VCR fittings...

Joe, how would I make sure the o-rings maintain a tight seal? Alternatively do you think it is possible to braze alumina onto stainless steel?

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Mark Rowley
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Re: Olive compression fittings in High Vacuum

Post by Mark Rowley » Wed Jun 23, 2021 5:32 pm

Since you are going to use quartz tubing, perhaps this can help:

viewtopic.php?f=10&t=13782&p=89853&hili ... ect#p89853

I just made another a couple days ago for the tube furnace project and they easily allow for vacuums well below 1mTorr.

Mark Rowley

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Joe Gayo
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Re: Olive compression fittings in High Vacuum

Post by Joe Gayo » Fri Jul 09, 2021 1:24 am

Brazing is a delicate art/science. The alumina would need to be metalized with MoMn or TiW. I would look at older books on vacuum tubes, such as Materials and Processes of Electron Devices by Max Knoll (amazing book) on how to join insulators and conductors.

Orings are extremely versatile and could be implemented in a number of ways. A properly sized oring could replace the ferrule, but I don't recommend this. Something along the lines of a conical or radial seal would be best. I think Mark has a number of parts constructed this way and I would look at the post he suggested.

Albert Mery
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Re: Olive compression fittings in High Vacuum

Post by Albert Mery » Thu Jul 15, 2021 5:55 pm

Thanks for your helpful indications. I opted to use orings as you suggested in conjunction with some KF25 parts. The centerpiece of the feedthrough is a high voltage standoff through which I will bore a small hole for the conductor. The orings fits tightly around the ceramic stand off and seems to offer a fairly good seal. It was able to hold my vacuum pump's ultimate pressure with no problems however I have yet to test the setup with a sub-micron vacuum or a high voltage so I cannot say yet whether or not it is a success...
Does anyone have any tips for drilling through ceramic?
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Joe Gayo
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Re: Olive compression fittings in High Vacuum

Post by Joe Gayo » Thu Jul 15, 2021 6:10 pm

Looks like a good direction. You might want to add a metal ring that fits around the OD of the oring, but is thinner than the cross-section of the oring (an "overpressure" ring might work). This way the oring can't squish out too much and it's forced to seal to the flange and ceramic. That would be more of a proper radial seal (look at the Parker oring handbook)

Depending on the ceramic, a diamond drill bit might work. How do you plan on sealing the conductor to the ceramic? (a oring is an option here as well, but you will need a smooth surface on the ceramic to seal to) I would recommend a ceramic sealing cement (used with vacuum tubes and light bulbs). Again look at Materials and Processes of Electron Devices by Max Knoll for ideas.

I've not tried this but https://www.mcmaster.com/7482A11/ might work to seal the conductor to the ceramic.

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Richard Hull
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Re: Olive compression fittings in High Vacuum

Post by Richard Hull » Thu Jul 15, 2021 6:23 pm

Great for a demo fusor at 5-10kv, as would be a common spark plug of some special order type. I used a common ford spark plug for my early demo fusors. an a special formula one plug for my water arc explosion guns back in the 1990's.

A tour through the AC Champion special plug lineup would produce some interesting plug types, up to about 20kv or more.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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