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Edwards Diffstak 63/100

I'm refurbishing a Edwards Diffstak diffusion pump that I got on ebay. Turns out it was really gunked up on the inside and is going to take some cleaning. When I took the first look, I didn't have any specific knowledge about this design, which looks a bit different inside that most diffusion pumps.

manual pdf

I got these parts removed: cold cap, retainer strap, and spring that holds the jet assembly down against the boiler plate, revealing this:
Top view after cold cap removed

Unfortunately, this part, which I later verified is the top of the jet assembly, did not move or come out. I knew from other disassembly instructions and videos that this part usually is loose and will even fall out. The nice people on (a good DIY vacuum resource) were able to confirm that this was supposed to just pull out. I tried soaking acetone and mineral spirits in there; this did not loosen things up much, but flushed out a bunch of soot and carbon flakes.

I was able to get a manual pdf just by asking Edwards customer service nicely, though my saying I was from CMU on the contact form may have helped. This is for the “Mark 2” version; mine is slightly different, and is presumably older. Diffusion pumps were the first practical kind of high-vacuum pump, and many of the ones you see on ebay are 50 years old or more, made by companies that are long gone. The Diffstak is a late design, with refinements like a built-in cooled baffle to reduce backstreaming of oil vapor into the chamber, and also all stainless construction except for the heater mount.

Cutaway drawing from the manual:
cutaway drawing

It turns out that the tapped hole in the top of the jet assembly is for a puller, although the tool is not described in the manual. The threads are M5. I came up with this:

(I should have put a block under the lever to keep from marring the flange).

I decided to try heat to loosen things up. The base of the diffusion pump is designed to get hot, >200 C. Because I had already put solvents in, and who knows what was in there before, I did this outside.

I was prepared to escalate to a blowtorch if necessary, but did not have to. I connected up power (with no cooling water) and waited for it to heat up. Since this is a 220V heater, I used a variac to step up the 120 to 160. When it started sizzling and smoking and smelling like mineral spirits, I tried the puller again, and it came out with only moderate force.

This is the jet assembly and some loose carbon that fell off the bottom of the jet and out of the main tube when I turned it over. I was afraid this would be a hard carbon deposit, but it mostly seems more sooty. There is also some brown deposit that isn't completely carbonized and will require solvent removal.

This is looking down inside the empty tube:

I think the sticking was mostly due to the fairly close clearances between the bottom of the jets and the outer tube. This gap only needs to be wide enough for the trickle of condensing pump fluid to flow back down to the boiler, and should not be too wide or vapor will come up there rather than going out the jets. The manual says you are not supposed to use abrasives or mechanical cleaning because it is likely to damage the pump. I'll try scrub brush first.

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wiki/user/ram/vacuum/edwards_diffstak63.txt · Last modified: 2017/12/03 17:37 by ram