Page 1 of 1

FAQ - A vacuum pump pecking order

Posted: Tue Jul 21, 2020 5:28 am
by Richard Hull
Their is a reasonable, established order of application in the highest level vacuum systems. Most high end vacuum systems want to operate continuously and are left on ,hopefully, forever.
Here is that order as a general rule.

1. Mechanical pump - to 10e-2 torr or better
2. Diff or turbo pump - to 10e-5 or 10e-6 torr - the deepest vacuum ever needed by the fusioneer for their fusor.
2a. Sorption pumps - These get only honorable mention. They work great, but demand a continuous source of liquid nitrogen every time you pump down a system. This aces the casual amateur out of the SP biz. They also require intelligent multi-valvings in operation. Big professional systems often use sorption pumps where hovering technicians mind the vacuum system almost 24-7.

After the above pumps, you are entering a realm of deep vacuum technology never used by most amateurs or even many professional systems that are not meant to run continuously over weeks. There are "follow-on" pumps that are used where the purity and depth of the vacuum must go deeper than the above pumps' final vacuum pressures.

3. Ion pump - 10e-7 to 10e-9 torr. - Very deep vacuum by most standards. It is rare to move beyond these depths of vacuum pressure... Ion pumps are heavy and expensive and their girds need to be replaced if started or constantly used at pressures above 10e-5 torr. They are not a "crud" pump for a mechanical pump!

There are some gases that must be removed no matter what. They may not be pumped well by the foregoing systems.

4. Titanium sublimation pump - This may or may not lower the pressure to 10e-10 torr, but will typically take out the trash of many specific gas species the others miss. These pumps may not run continuously once the gas purity is warranted after they are run to a level that a mass spectrometer verifies the offending gas or gases are no longer of significance. They may have to be run intermittently to keep the system cleaned of re-occurring offending gases.

Note: using either #3 or #4 pumps at 10-3 torr is never done by competent vacuum technicians. I have known folks who tell me they will not turn on an ion pump until they are at or very near 10e-6 torr!
Needless to say, few systems are sealed well enough to establish and hold 10e-8 torr vacuums without pumps running constantly.

No fusor builder has any need, whatsoever, for an ion pump or a Ti-sub pump!

Never use a ti-sub pump at 10e-4 vacuum! Even 10e-5 gives only a few days of filament life! Duniway's datasheet tells the story. Ti-subs are a last resort...Typically, the end of a pumping line.

Richard Hull

Re: FAQ - A vacuum pump pecking order

Posted: Tue Jul 21, 2020 10:01 am
by John Futter
You might want to add to the esoteric ultra vacuum pumps the Getter pumps that remove Hydrogen and other difficult to remove gasses at ultra high vacuum
We use Saes CapaciTorr ā€œZā€ series getter pumps on some of our detectors (multichannel plates) to maintain 10 to ^-9 to -10 millibar vacuum level
Seas also make the induction heated getters as used in vacuum tubes to maintain high vacuum during their lifetime

Re: FAQ - A vacuum pump pecking order

Posted: Tue Jul 21, 2020 8:45 pm
by Richard Hull
In your reply you have added the "getter" concept and pump and explained its value at the ultra high vacuum end of things. Thanks for the expansion of the FAQ

Richard Hull