FAQ - Never turn on a turbo or diff pump until....!!!
Posted: Fri Jan 11, 2019 6:13 pm
OK, you have it all hooked up. Your vacuum system is finally ready to pull that chamber down.
WAIT!!!
While, as mentioned in early FAQs, you should have been testing your vacuum system with your mechanical pump from the get go, piece by assembled piece, you must now protect your investment in your secondary pumping system. I have two valves in my system. One valve to isolate the mechanical pump and another to isolate the fusor reactor vessel from the diff pump. I have two gauges in my system. One is a TC gauge placed just before the diff pump and one monitoring only the reactor vessel.
Turning on and gas ballasting the mechanical pump for a couple of minutes, I note the TC gauge pressure in the foreline drop to about 10 microns. I open the valve to the diff pump and the pressure sinks a bit, but in a moment the TC gauge is now measuring the combined foreline and diff pump volume pressure and it is close to 12-15 microns. When it is stabilized I open the reactor vessel valve and the pressure really drops into the low torr range! Within seconds, the TC gauge is plunging again to 100 microns, then 50 microns and finally in a minute or less I am below 30 microns, but really slowly dropping now. At about 20 microns I now know I can safely start the diff pump.
The upshot of the above is not that my system is nicely sealed...........It is that it is nicely sealed ENOUGH! This may sound cavalier, but it is a fact that while perhaps still leaky, a diff or turbo pump will be safe as my mechanical pump and all my leaks, wherever they may be, are both in sufficiently good order to not damage my secondary pump or mess up the vacuum system.
****The Bottom Line******
Never turn on a turbo or a diff pump if you can't hit 20-30 microns in the fusor with all valves wide open against your running mechanical pump!
****************************************
Richard Hull
WAIT!!!
While, as mentioned in early FAQs, you should have been testing your vacuum system with your mechanical pump from the get go, piece by assembled piece, you must now protect your investment in your secondary pumping system. I have two valves in my system. One valve to isolate the mechanical pump and another to isolate the fusor reactor vessel from the diff pump. I have two gauges in my system. One is a TC gauge placed just before the diff pump and one monitoring only the reactor vessel.
Turning on and gas ballasting the mechanical pump for a couple of minutes, I note the TC gauge pressure in the foreline drop to about 10 microns. I open the valve to the diff pump and the pressure sinks a bit, but in a moment the TC gauge is now measuring the combined foreline and diff pump volume pressure and it is close to 12-15 microns. When it is stabilized I open the reactor vessel valve and the pressure really drops into the low torr range! Within seconds, the TC gauge is plunging again to 100 microns, then 50 microns and finally in a minute or less I am below 30 microns, but really slowly dropping now. At about 20 microns I now know I can safely start the diff pump.
The upshot of the above is not that my system is nicely sealed...........It is that it is nicely sealed ENOUGH! This may sound cavalier, but it is a fact that while perhaps still leaky, a diff or turbo pump will be safe as my mechanical pump and all my leaks, wherever they may be, are both in sufficiently good order to not damage my secondary pump or mess up the vacuum system.
****The Bottom Line******
Never turn on a turbo or a diff pump if you can't hit 20-30 microns in the fusor with all valves wide open against your running mechanical pump!
****************************************
Richard Hull