#3 FAQ - Operating a Fusor - startup - run - shutdown

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Hector
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Re: FAQ - Operating a Fusor - startup - run - shutdown

Post by Hector » Fri Dec 11, 2009 4:45 pm

I know this is going to sound ignorant, but I just want to make sure that I'm on the same page.
When you speak of "microns" you are referring to milliTorr, correct?

I ask because I never use microns I keep all my measurements in either Torr or milliTorr, so I just want to make sure I'm in the same page as everyone else here.

Sorry for the ignorance.


Hector

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Richard Hull
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Re: FAQ - Operating a Fusor - startup - run - shutdown

Post by Richard Hull » Fri Dec 11, 2009 5:01 pm

No ignorance there, just inquistiveness, which we love. Micron and millitorr are the same.

I talk technical vacuums in microns and scientific vacuums in Torr

I use microns because that is where the fusor operates (technical vacuums - single digit microns - no need for notation). Plus, most TC gauges that the average amateur is likely to pick up surplus, are marked in microns, most often.

When I talk sub micron, deep vacuum, I talk in scientific notationed Torr. This is what most surplus ion gauges are marked in. (analog metered)

I, personally, refuse to speak in or use bars or pascals. If I ever write a peer reviewed paper for publication, (never happen, now), I will adhere to SI units. Most SI units are crap save for the sensibly derived becquerel.

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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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Carl Willis
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Re: FAQ - Operating a Fusor - startup - run - shutdown

Post by Carl Willis » Tue Dec 15, 2009 10:19 pm

>if a pump with a smaller volume of oil, ie a smaller pump will reduce backstreaming, then I want as small a pump as possible.

While it's true that the backstreaming rate is proportional to the exposed surface area of oil--namely, the surface area of the outer wall of the pump opposite the jet(s)--choosing "as small a pump as possible" on account of backstreaming strikes me as a case of absurdly-misplaced priorities and engineering direction. There's no justification in that conclusion.

The size of pump is dictated by speed and throughput requirements. The gas load comes from outgassing, permeation, diffusion, virtual leaks, and intentional sources like D2 being bled in. If backstreaming is considered an issue in a particular application, the solution is not to then reduce the size of the pump, but to use a cold trap or baffles. (If there's any consequence upon the pump size, it might be to make the pump bigger in order to offset the reduced conductance of said trap or baffles).

I recommend John F. O'Hanlon's "User's Guide to Vacuum Technology" for a good guide to vacuum system design. Page 201, Table 8.3 shows backstreaming rates for DC-705 under a variety of circumstances (baffles, no baffles, cold trap, no cold trap, etc.) For hobby fusion, the high-vacuum requirement has typically been about modestly improving pumping speed in the millitorr regime beyond the capabilities of mechanical pumps. No need as of yet has been identified for UHV conditions, for silica-free conditions, or for any other circumstances that would indicate a need for anti-backstreaming traps or baffles in this line of work. Of course, using them will not kill you, just will put a dent in the wallet.

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