brass, vacuum and degassing

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
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Alberto Sanchez
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brass, vacuum and degassing

Post by Alberto Sanchez »

Hello.

I have a small homemade oil difussion pump. I want to replace one part of it with brass. I have coated the brass whith tin+silver. But the coating is not perfect. Small spots are not coated and the total surface is, lets say 1 square centimetre. Is that something to worry about in the range of 0,001 microbar?

Thank you

best regards
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Dennis P Brown
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Re: brass, vacuum and degassing

Post by Dennis P Brown »

What type of application? If a fusor, no concern; a high vac system aiming for lower levels or sealed system, or one that requires high purity, then likely major issues could arise. Since you are achieving 7*10^-7 torr (during active pumping?) and the system has reached that level, then is that what you wanted to reached?
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Richard Hull
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Re: brass, vacuum and degassing

Post by Richard Hull »

Dennis brings up a good point. These forums are fusor forums. Vacuum is just one small goal and one of the easier ones to achieve on the way to doing and proving fusion.

We need to know what your vacuum goal is! If a fusor, then 10e-6 is fabulous! You need go no deeper.

We have folks occasionally on some other mission with vacuum to far greater depths. Many of us here with fusors are very happy with 10e-5 torr.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Alberto Sanchez
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Re: brass, vacuum and degassing

Post by Alberto Sanchez »

Hi, thanks for your answers.

A fusor is in my to do list. I need to learn to much yet. Now I´m making triodes. But the oil difussion pump is not reaching a good vaccum and I don´t know to blame on the brass. But now I know is not the brass because I replace it with aluminium and the results are the same. It is something wrong with my oil difussion pump. I only reach 0,02 Microbar
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Dennis P Brown
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Re: brass, vacuum and degassing

Post by Dennis P Brown »

For most here, I'll convert your units (yes, yours are SI.) Getting to only 15 microns is poor and not very good for a mechanical pump (assuming it is a std two stage oil based pump - for cheaper single stage pumps that is ok.) With a diffusion pump (DP) running, that is extremely bad. Even w/o a baffle a DP should do far better than that value.

Have you checked for a major leak(s)? Or is your vacuum oil bad (contaminated) - and/or for the mech pump & diffusion pump? Have you measured the pressure with just with the mechanical pump running? Is your DP being properly cooled? Is the base of the DP even hot (be careful and don't directly touch it - they can cause bad burns when operating.)

I could add other things to check but these are the ones that need to be looked into, first.
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Rich Gorski
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Re: brass, vacuum and degassing

Post by Rich Gorski »

After reading your post the first question that comes to mind is about your "homemade" diffusion pump. Have you used it before and achieved a reasonable vacuum? If you know it works then the next things to check is as Dennis described above.

Rich G.
Alberto Sanchez
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Re: brass, vacuum and degassing

Post by Alberto Sanchez »

Hello. Thanks for your answers, and remeber I don´t speak english very well, so excuse me if I make mistakes.

About the problem with brass, it wasn´t the brass, I have change this part with one made of aluminun and the problem is the same.

When I connect the mechanical pump (2 stages, 21 microbar or 15 microns) it makes some vacuum, and 3 or 4 minutes later, when the oil star boiling, it start making some vaccum, and around 20-30 minutes later the vaccum reach 0,02 microbar or 0,014 microns, but it doesn´t go further. It doesn´t matter if I wait 3 hours.

I have check the leads. I have drained all the oil, wash the DP and put inside a water tankt and put pressure in the DP looking for leaks. That is dangerous, so I applied 4 bars. No bubbles, so I think no leaks

The DP was air cooled initially, and thinking that wasn´t enought, I put copper tube around brazed with silver (the pump is made of stainless)

And to monitoring everythinh, I put a thermal type K probe to check the temperature of the heating element. The oil start boiling between 290 and 295 ºC
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Dennis P Brown
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Re: brass, vacuum and degassing

Post by Dennis P Brown »

Alberto, your written English is certainly fine.

Rich raised an excellent point - a 'home made' diffusion pump has a lot of potential issues - designing proper jets is not something many even advanced machinist could do readily. Your unit (the DP) might not be designed properly and is simply incapable of achieving any high vacuum. I am assuming your using proper grade, clean diffusion pump oil. If not, that alone would harm even a perfect DP.
Alberto Sanchez
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Re: brass, vacuum and degassing

Post by Alberto Sanchez »

Yes I´m using dow corning 704
Alberto Sanchez
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Re: brass, vacuum and degassing

Post by Alberto Sanchez »

The pump is based on an Edwards EO 50/60. I have read the manual and for that pump is recomended a backing pump of around 3 CFM or 1.7 m3/h
and 0,5 microbar or 0,4 microns.

Mi backing pump is 3 CFM or 1.7 m3/h buuuut, the final vacuum is 20 microbar or 15 microns. I don´t know if this coul be the problem. Of course Edwards in its manual recomend one of their pumps, but I´m not sure if you need so high vacuum for backing in an oil difussion pump

Another strange thing is I check the backing vacuum of the mechanical pump with a HVAC gauge and the vacuum of the oil difussion pump with a cold cathode probe. The needel never stays quiet!! Sometime stays fixex, but usually it is movin a lot. That doesn´t happen if I measure the vacuum in a turbomolecular pump. The needle is steady and movingslowly measuring higher and higher vacuum
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Dennis P Brown
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Re: brass, vacuum and degassing

Post by Dennis P Brown »

Any typical small diffusion pump (DP) can easily start lowering a system when the roughing pump gets below 100 microns. No way 0.4 microns is needed - that is not really required. However, back pressure - issues like a very large chamber, leaks, out gassing, contaminates - all can overwhelm a small DP. You need to test the DP when it is plated/gated off so it is pumping only on your vacuum gauges. See if the system almost immediately gets below 0.005 microns or better. If not, likely the DP is failing due to bad oil, contaminated oil, burned oil, to low a temp heater, and the lists go on. But first test the DP alone.
Alberto Sanchez
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Re: brass, vacuum and degassing

Post by Alberto Sanchez »

Thanks for your answer.

All the test is with the DP alone.

Is not the oil, is new, dow corning 704, and not the temperature. I have been controling it using a variac,
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