Diffusion Pumps 101

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.
George Schmermund
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Diffusion Pumps 101

Post by George Schmermund » Tue Jun 21, 2011 9:45 pm

Well, Jon R has thrown down the gauntlet and I suppose that I've got no choice at this point but to try to pull the rabbit out of the hat. Yesterday I went shopping at Home Depot, Target, and the hobby shop for parts. I suppose that all the stuff is obtainable from Amazon.com, too, but I like to look at the items in real life if I can. Rotating the parts in my hands makes it easy to figure out how well they'll fit together.

My mission will be to build a small, high performance, metal diffusion pump that requires only simple tools, easily acquired parts, and takes no more than a day to assemble. Easy construction doesn't account for much if the parts aren't readily available.

Also, the glass pump projects that I've posted about in the past aren't in the running because they're inaccessible to the average Fusor builder due to the special talent needed to do the glassblowing and the high cost.

The most difficult part about building a diffusion pump is understanding how they work. I mean REALLY work. There are no moving parts in a DP except for the high velocity motive vapor which is working in a very low pressure environment. There are areas of pumping operation that are very unintuitive even to those who would claim to know everything about how they work. I've had experienced vacuum system builders check out one of my glass pumps in operation and they confessed to being stunned at what they were able to observe. It changed their whole mental model of DP's.

So, rather than belabor you guys with another one of my protracted tales, I'll try to march a straight course and just show some pictures of the parts and how simple it is to put them together. I'll start with what I've got on hand now and progress to the finished device in a later post. The parts should be self explanatory. The total out of pocket cost sofar is less than $60.

For scale, the cocktail shaker is 9" tall
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Anything obvious in high vacuum is probably wrong.

Sawtoothfusion
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Re: Diffusion Pumps 101

Post by Sawtoothfusion » Tue Jun 21, 2011 11:33 pm

Hi George,

Have you made a glass diffusion pump before? I'd be very interested to hear your results and experience as I'd like to go the glass direction myself.

I also appreciate you posting this thread. I will be following it closely. Thanks again.

-James

Sawtoothfusion
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Re: Diffusion Pumps 101

Post by Sawtoothfusion » Tue Jun 21, 2011 11:37 pm

Hi again,

I'd actually be curious of where you're located and if you'd be up to collaborate on a glass diffusion project? Feel free to contact me anytime.

-James

db
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Re: Diffusion Pumps 101

Post by db » Wed Jun 22, 2011 3:03 pm

I will be very interested to see this built. From my own disassembly of a diffusion pump, all the parts make sense.

I did a quick search on youtube for any videos of glass diffusion pumps in operation and did not see any. It would be very cool if you could post a video of one of your glass pumps in operation.

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Richard Hull
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Re: Diffusion Pumps 101

Post by Richard Hull » Wed Jun 22, 2011 3:35 pm

All the best of luck on the DIY diff pump. Sounds like both fun and quite a challenge.

I bought my current 2.5" throat Veeco diff pump currently used on Fusor IV at my HEAS gathering fleamarket about 10 years ago from an old friend selling out of his trunk for $30.00! It is air cooled with high CFM blower. Doesn't get much cheaper than that.

FYI for the newbs..............

If you are building a fusor and the chamber is less than 10 liters, (most always are), there is little need for a diff pump having much over a 3" throat. Air cooled is always the best if you can find it.

Used, air cooled diff pumps of this size command a slight premium due to their being among the most desirable of all diff pumps. Expect to pay about $100.00 on e-bay or the internet as a reasonable sum.

You may, however, locate a 6" throated, water cooled diff pump for free or even $10.00, but remember the adapter might cost $100.00 and then there is that flowing water plumbing.

No need paying $20.00 for an enitre, functional 8" throat diff pump, then, $250.00 for the adapter network, $70.00 for water plumbing hookups and $75.00 for the diff pump oil to mechanize such a big monster, drawing down a 3 liter chamber.

caveat emptor!

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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Carl Willis
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Re: Diffusion Pumps 101

Post by Carl Willis » Wed Jun 22, 2011 3:57 pm

Hi George,

Some creative shopping there.

I wonder if Mike Kan, our memorable vacuum-gear Marxist (go read his old posts for a laugh, and I mean that in a good-natured way), will be around to witness your effort's success.

Are you planning to go the solder or brazing route with all this stuff?

-Carl
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George Schmermund
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Re: Diffusion Pumps 101

Post by George Schmermund » Wed Jun 22, 2011 6:23 pm

Carl - The plan so far is to braze the foreline joints and epoxy the top colder joints. This will be another chance to play with some JB Weld.

The jet assembly will be compressed together using the threaded rod and nuts & washers to form a single unit.

The top most piece will be the KF 40 half nipple which allows direct connection to standard manifold hardware.

A small string heater of a couple hundred watts will heat the boiler.

The cooling will be done with a miniature muffin fan attached to a simple cowling made from aluminum flashing.
Anything obvious in high vacuum is probably wrong.

DaveC
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Re: Diffusion Pumps 101

Post by DaveC » Wed Jun 22, 2011 10:18 pm

I, for one, have no doubt that George's effort will be successful.

And the liberating effect of the DIY effort..... is reward enough.

Thanks for picking up the challenge, George...


Dave Cooper

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John Taylor
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Re: Diffusion Pumps 101

Post by John Taylor » Thu Jun 23, 2011 4:06 pm

George, I have a new 4" muffin fan with connector and matching guard I'll give you if you promise to keep us informed of the details. I, for one, would rather build my own equipment than buy it if at all possible. This seems like it would be a rather fun project.

George Schmermund
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Re: Diffusion Pumps 101: Jet Assembly

Post by George Schmermund » Sun Jul 03, 2011 6:25 pm

I had some time this week to play with the DP project and decided to do some brazing. The setup and results are in the photos. The tools needed for the project to this point are: tin snips, hacksaw, file, sand paper, Scotch-Brite, torch, beer.

The brazing alloy of choice was Harris Safety-Silv 45 in a 1/16" wire and Stay-Silv paste flux. This will give a good match for the copper to stainless joints and has the advantage of a reasonably low melting point. The only torch needed is a propane / air type which should be easy for all to use. The parts are small and light weight and won't over heat with this approach. Some practice on scrap pieces is recommended.

For prep and post braze clean-up, I have not found anything easier to work with and more effective than Lysol Power Toilet Bowl Cleaner and Scotch-Brite.

The pictures should be self explanatory as to how the jet assembly goes together. What's pictured is a fit test and the final alignment will be made when the assembly is compressed with the threaded rod.

More to follow.
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Anything obvious in high vacuum is probably wrong.

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