Dear Peter and Dave, >No need to increase the plexipipe diameter. We just have to think how >to mount it to the PM. 1.8" or 2" both are way larger than the >20mm OD of the plexi. How long is the current Alu pipe around the plexi? > > > The aluminium pipe around the plexi is 33cm.The plexi itself seems to be ~40cm long. >The o-ring size and shape of the grooves are essential. The PSI >vacuum expert recommended rectangular cross sections for my >NW40 adaption flange. However, for this case rounded grooves >might be better. Please discuss the vacuum group. > > I took the existing feedthrough and went over to the vacuum group. When I explained our need to Mr Ruettimann and asked about the o-rings, he immediately told me that o-rings aren't suited for such a dynamic feedthrough. The optimal solution would be a linear feedthrough. By chance, he has just recently designed that and the picture is attached. He's got the technical drawings in AutoDesk on his computer and he's willing to resize it to fit the standard NW40. I pointed out that this would need to be done tomorrow and if 'll tell him to do that in the morning he'll sit down and do it. Let me now explain a few things: - In the drawing, the two striped white parts are the bearings for the pipe. These are two rings made out of teflon or the harder vespel. - The blue parts are the sealings. These are specially suited for such a dynamic feedthrough and they work without grease. The top one in the picture could be equipped with an additional ring or device to strip off any dust. - The silver parts next to the sealings are two special metal rings (need to be milled) which hold the sealings. They are not absolutely necessary but it's better with them. In addition they have small holes for a differential pumping. - The side flange is optional and enables to pump differentially. So in case of a leak we can just mount a pump and that's it. The entire system is good for 1E-7-1E-8 mbar so far below our needs. The additional pumping port could be attached with a thread so that it is easy to remove it and close the port with a threaded cap. He just phoned the company providing the blue gaskets. For a 25mm diameter pipe they have enough in stock. The cost per feedthrough is ~30 CHF. For a 20mm pipe they only have two left. In principal since the feedthrough is made out of plastic we could also insert the plexi directly. However, an aluminium protection might be still better. The entire fitting is very good but one can still move in and out. The strength for that depends on the fitting of the white bearings. The only question remaining would be a set screw to prevent the scintillator from being sucked in. Maybe we could just prolong the upper part of the side walls and drill some holes for the screws. Greetings Peter PS: After you had a look at this email and reviewed the idea we could maybe chat on that after the Mulan meeting if necessary.
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