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Hi

I'm designing and building a vacuum system for telescope mirror coating. I have inherited a 12" bell jar that needs a new base, I have a Diffstak 63 and an Edwards EDM12 (that's really old but I just rebuilt it and it's working well). I have a Baratron 5 torr gauge and a Penning CP25K. I will be connecting the gauges to a laptop via a LabJack USB data acquisition device.

I am fortunate to have a lathe and small mill so I am going to machine a base out of 1" thick stainless steel. I have a couple of questions for the group:

My design (and I am happy to post a DXF file of the base) has the Diffstak mounted directly on the base (no cryo trap - I'm using 704 Diffpump oil & the Diffstak has a built in baffle). However, rather than mount it concentrically, I thought to offset it so that I have more room for gauges and power feeds.

Here's my concern: I will machine the base plate on the lathe and I will also run over with a fine emery so that its very flat. I understand that machining roughness should be minimized and that there should be no radial scratches where an O-ring sits. the problem will be that I can't machine concentrically around the offset Diffpump connection (i'll cut this on my mill), but will have to rely on the flatness of the finish of the main plate. I am going to rely on Fomblin high vacuum sealant on the Diffpump seal (expensive but good to 10exp-12 torr, IIRC). I hope this makes sense - does anyone have experience of this kind of configuration?

My second question regards high voltage feeds. I have seen some older designs use the base plate as an anode for the high voltage return and for the high current return on the heaters). This seems risky to me .. I think I would prefer to install separate power feeds for all connections and basically just earth the Diffpump (I have a thermocouple taped to the pump to measure it's temperature plugged into the Diffstak - not sure what 15Kv return would do to it!! Any one got an opinion?

thanks Dominic

Hi Dominic,

Sounds good, especially the bit about grounding the diff/baseplate!
The finish on the baseplate may be OK if there are no scratches. If neccesary, use the emery through the very fine grits, centered on the O ring diameter. If the surface undulates by a few thou of an inch, the O ring will accomodate it. Much better to have a blended surface than a narrow valley.
Maybe try to get a good vac without the Fomblin to keep the system clean. Even though it has a good vapor pressure it can get everywhere if you are not careful.

John