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Diffusion pump project

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plazmatron

Sat Oct 10 2009, 07:46PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

I have wanted to get into high vacuum for some time, so I can make homemade x-ray tubes etc. However I was always under the impression it was expensive.
I picked up this small vintage Edwards EO1 diffusion pump about a month or so off of eBay, for just Â£5.50!

It is a water cooled affair, and had to have manifolds made for it, plus I wanted a closed loop cooling system for it, since I didn't want to be tied to the sink.
I made all the manifolds out of soldered copper and brass, and built a cooling system based around a 120mm PC radiator, the kind enjoyed by the watercoled PC crowd. Water is pumped round the system by a small 12v window washer pump, and the whole lot is mounted on a 5mm thick Aluminium plate

Front view.

Rear view.

Hooked up to the Javac.

The backing pump is a Javac CC-141 rotary vane pump, that by itself, is quoted as pumping down to 15 microns, though I estimate 20Âµ.
In the above photo, note there are two vacuum lines to the backing connection. The reason is that the diffusion pump requires a backing pump speed of at least 30 l/min, and whilst the Javac is rated for 140 l/min, the small diameter of the provided inlets chokes the pump somewhat, so two of the three inlets were used, so I would have reasonable pumping speed.

A discharge tube (made from a Pyrex rolling pin!) was hooked up to the assembly, to which 30kV DC was applied The cathode is the filament from a 12v lamp, and the anode is a copper rod. The above picture was taken with just the Javac running, and shows the striated positive column.

Once the Javac has reached its base pressure, the positive column is much shorter, and the luminous disks, have become thicker, and more diffuse.

Once the Diffusion pump gets upto operating temperature, the column fluctuates in length, then shrinks until.......

.....we get into the soft x-ray region shown above. The walls fluoresce a pale blue from the impact of electrons. Note the luminous vertical mark. This is some diffusion pump oil that ran down the inside of the tube, after I applied too much to the top bung, it is brilliantly fluorescent under the impact of electrons.

Seconds after this photo was taken, the vacuum had become so hard, that the tube became dark, and ceased x-ray emission. So now a hot cathode is needed.

Once again HV is applied, except the filament is now lit. You can't tell in the photo as the light from the filament has washed it out, but the glass surface of the tube is glowing pale blue again, and the tube is once again emitting x-rays.

I spent a lot of time and effort before finally acquiring a diffusion pump, trying to squeeze an extra micron out of the Javac, by using molecular sieves and cold traps, and can honestly say that this project was much easier, in terms of cost time and effort, in reaching a repeatable high vacuum.
I highly recommend that anyone wanting to build a high vacuum system should go down this route, its not that expensive.

The Javac as I recall cost Â£120 new, the Diffusion pump was Â£5.50, and the Radiator was about Â£6 new (in a "brown box") everything else was out of the junk pile.

The diffusion pump is tiny (only 1 inch inside diameter) I also have a 3 inch pump (these show up on eBay all the time, out of Spectron leak detectors), and this will be radiator cooled as well. This pump was also cheap at just 50p!

Les

Proud Mary

Sat Oct 10 2009, 08:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A marvellous effort!

Scott Fusare

Sat Oct 10 2009, 08:04PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Lovely work Plazmatron! The venerable diff pump lacks the convenience of a turbo but more than makes up for it when it comes to price and durability. Out of curiosity, what are you using for pumping fluid?

Scott

plazmatron

Sat Oct 10 2009, 08:15PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

scott fusare wrote ...

Lovely work Plazmatron! The venerable diff pump lacks the convenience of a turbo but more than makes up for it when it comes to price and durability. Out of curiosity, what are you using for pumping fluid?

Thanks everyone ! I'm using Dow corning DC-704 as the pump fluid. The manual specifies DC-702, DC-704 is a superior oil, but requires a lower backing pressure. DC-704 is claimed to be a hardier oil too, being able to resist the pump being opened up to atmosphere whilst boiling, and other accidents!

The pump isn't to inconvenient to run either. This one warms up in around 10 minutes, and cools down in around the same amount of time. Plus if I had a turbopump I would always be worrying about a washer falling in there!

Les

Scott Fusare

Sat Oct 10 2009, 08:23PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

DC-704 is still around, wonderful. I have not used it since the 1980s but it was a great improvement over the easily cracked hydrocarbon based oils. It used to be horribly expensive, has that changed?

I suppose my "inconvenient" statement is an old bias that stems from working with large, water cooled diff pumps. The little guys like you have are indeed much friendlier!

Again, great work!

Scott

plazmatron

Sat Oct 10 2009, 08:39PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

scott fusare wrote ...

DC-704 is still around, wonderful. I have not used it since the 1980s but it was a great improvement over the easily cracked hydrocarbon based oils. It used to be horribly expensive, has that changed?

I suppose my "inconvenient" statement is an old bias that stems from working with large, water cooled diff pumps. The little guys like you have are indeed much friendlier!

Again, great work!

Scott

In the US, svp-neon does 50ml of DC-704 for around $30 USD. (just search eBay for DC-704)
They wouldn't ship abroad. so in the UK, I found a neon supplier that would do 25ml for Â£9 UKP
Its still horribly expensive, but this tiny pump only needs 11ml, and my 3 inch pump only needs 50ml,so its not a fortune to charge a pump.

Oddly it turns out that the old oils like DC-702 are still available, but are now more expensive than DC-704, and approaching the price bracket of Santovac-5!

Les

Hazmatt_(The Underdog)

Sat Oct 10 2009, 10:20PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Lemme know when you need a bigger pump

thedatastream

Mon Oct 12 2009, 07:16AM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

How good is soldered copper pipe with a vacuum? I had considered it for a vacuum project (that never went anywhere)

plazmatron

Mon Oct 12 2009, 04:02PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

thedatastream wrote ...

How good is soldered copper pipe with a vacuum? I had considered it for a vacuum project (that never went anywhere)

Personally I find it OK, there seem to be a consensus that the vapor pressure of lead in a concern, however the vapor pressure is quoted as 10E-4mm Hg at 548 C, so as long as the parts are cool, it poses no problem.

Ideally one would use silver solder, to join copper, but I have found ordinary solder to be quite alright. You must flush all the pipework with acetone, to wash out the fluxes used in soldering, and I try to keep the exposed solder on the inside of the joins to a minimum, and also the number of joints to a minimum.

There are only two soldered joints on the high vacuum side of this pump.

Les

Steve Conner

Mon Oct 12 2009, 05:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I recently found an Edwards vacuum coating machine in the uni's junk pile, and it looked to me as if it just used a length of plastic hose between the diffusion pump and the backing pump. The clear stuff with the reinforcing braid.

I never figured out a way to dismantle it or get it off campus without getting in trouble, and it's now been rained on.

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