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  for Metal Finishing since 1989
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Improve mounting of cold cathode

I am looking for a better way to mount our cold cathode and still get an accurate reading for vacuum we deoil and braze in the same oven but after about a month the cold cathode becomes contaminated and has to be rebuilt (cleaned and new insulator installed). We do use a valve that is closed untill we reach into the -2 torr. The mounting is up on the transition to the diffussion pump . Is there a better spot or some different baffles that anyone uses out there.

Rick Johnson
maint for aluminum vacuum brazing - Canton, SD, USA
2004

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If the cold cathode is directly hooked up close to diff pump and below high-vac valve (toward pump), even diff pump oil backstreaming will cause a probeem. So you need a cold trap between the cathode and the pump, and cathode mounted above the valve.

I can think of two options or combination thereof to prolong the life a little. 1) if the cathode is directly connected to the vacuum line, use a T or an L shaped, or oven a coil shaped tubing to isolate it from direct exposure to carbonaceous vapors. This will change the pressure reading and you may have to settle on a new reading. 2) Use a separate valve for the cathode to open and close so you only read it when needed. Close the valve most of the time to prevent exposure. This option could cause problem if you are using cold cathode as a sensor for automated vacuum sequensing.

Mandar Sunthankar
- Fort Collins, Colorado
2004