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FAQ - Never turn on a turbo or diff pump until....!!!

Posted: Fri Jan 11, 2019 11:13 pm
by Richard Hull
OK, you have it all hooked up. Your vacuum system is finally ready to pull that chamber down.


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

Re: FAQ - Never turn on a turbo or diff pump until....!!!

Posted: Sat Jan 12, 2019 2:05 am
by Jerry Biehler
If you are direct pumping to a chamber with a turbo you can start the roughing pump and turbo at the same time with newer turbos. Even says so in the manual, they are power limited and will not come up to anywhere close to speed until they have a good enough vacuum to run. I would still pump down a chamber just to check for leaks first but once you know you have no major leaks you are good to go.

My big turbo needs at least 2 torr on the fore line, some others can take considerable more, this is because almost all modern turbos are turbo-drag.

Interesting tidbit of info, the mars rover with the mass spec on it has a tiny turbo pump and the foreline is just vented to atmosphere since it is mostly a vacuum on mars.

Re: FAQ - Never turn on a turbo or diff pump until....!!!

Posted: Sat Jan 12, 2019 9:05 am
by Richard Hull
Thanks Jerry for the info on modern turbos with self-limiting drives under light torr loadings.

Once I am conditioned, Fusor IV can have the Diff pump turned on at the same time as the roughing pump as the roughing is down below 40 microns in 1-2 minutes, long before the boiler temp gets up to pumping speed, ~ 8-10 minutes.

I wrote the FAQ for those who are totally new to assembling a vacuum system it is especially true for the diff pump folks. there is nothing worse than to have a diff pump vomit all over the inside of one's vacuum system.

Most newbies here are befuddled and often stalled at the mechanical pump stage. They all seem to get a terrible pump that won't quickly yank an 8-inch length of tubing to the TC gauge down below 100 microns. I feel for them. In 9 cases out of 10, their gauge may be at fault or they haven't sealed the test setup well enough. They will learn though.

Richard Hull