Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: New/used vacuum pumps- advice.
Date: Jan 18, 11:34 am
Poster: Richard Hull

On Jan 18, 11:34 am, Richard Hull wrote:



I just purchased two new large two stage pumps for a super low price. Both were 5.4 CFM or more. When I connected them, I figured they would both zero out the TC gauge. As it turned out the best pump would go down to only 30 microns and just sit there. I have seen this before and knew exactly what to do.

Most folks who own vacuum pumps are not necessarily vacuum technologists. Consequently, they often, unknowingly abuse their pumps. They seem to do everything wrong. These two pumps were classic examples of unintentional abuse and neglect.

To solve the problem, I ran the pumps for over 2 hours with the input ports valved off. (max load)

After shutdown, I immediately dumped the hot oil. The oil was visibly dark, dirty and fouled with water. I then refilled with good looking, but old oil which I have taken from my good pumps on routine changes. I ran the pumps for another hour and then changed the hot oil with fresh new oil.
The pumps now zeroed the TC gauge in a minute!

A novice might have thought the pumps bad, in need of major overhaul or rebuild, and his purchase unwise. The $2000 pumps were not bad just the $6.00 oil!

Here are some tips................

When buying a used pump, don't bother initially testing it as it will most likely appear defective. Run it for 1-2 hours with the input or intake port valved or blanked off. Note the pressure. If above 10 microns after 30 minutes, you absolutely must change the oil. I personally change oil on any and all new pump acquistions, regardless.

Dump the hot oil and examine it carefully during the entire draining. If just dark and dirty you are probably OK. Refill with fresh oil and run a pressure test. If the pressure is ok for the pump size, you are ready to use it.

If the oil at first dump was filthy, slow moving and or filled with black or brown goo or particles, your pump has been abused horribly.
Here is where saving old oil from well maintained pumps can help out. It would be a waste to put in new oil now as you must run two or more new oil fills through the pump to flush the system of particulates and goo that is fouling it.

Refill and run again for 1/2 hour. The pressure should hopefully be better and deeper than on the first pass. Keep doing this refilling/ dumping operation until the pump appears to bottom out. If the best bottoming occurs before 10 microns is reached, the pump is worn out and will require rebuilding.

When you have a good pump, PROTECT IT!

1. change oil frequently!!!

It is just too cheap not to do this instead of letting your good pump wear out prematurely!

I change oil at the slightest provocation or tendency of the pump to not go as "deep" as in the past, even if only a few microns. SAVE this oil! Carefully label this container "good, clean, old oil". You can use this to purge filthy new/used pump acquistions.

2. Never run a pump with an open-to-air inlet port! Always pump against a sealed chamber. I cringe when novices do this to show how good their pump "sucks".

Letting outside air into your pump, especially while it is running, fouls the oil in seconds with moisture! Just don't do it! If the pump sits for even a few minutes unconnected, blank off the inlet to avoid condensation within the pump over time. When pros bring their stuff back up to atmosphere, they backfill the vacuum lines with dry nitrogen of carbon dioxide! For our purposes, this is overkill, but keep this in mind as you go beyond mechanical pump vacuums. The key is to valve off the entire system to avoid air getting back into pump oil.

3. Schedule PM (preventative maintenance)sessions for oil changings.

After a lot of pump downs from atmosphere, especially on large chamber volumes, your oil will contain moisture, but will remain very clean.
This happens more when you are trying to locate leaks in bringling up a new system or array as you are opening and closing your system a lot and re-pumping often. The best indicator is a slight reduction in bottoming pressure. This doesn't necessarily mean you have a leak. The test is does the pressure continue to rise after valve off? If so then a leak or slow outgass is the problem. If the pressure stays fairly high, but constant after valve off, then you have successfully sealed the system, but have fouled oil.

It is normal for a system pressure to jog around up and down at first pumping. (slugs of traped air and outgassings) Finally, the pressure falls to some low value. (few minutes)

It is also normal for the pressure to very slowly fall after this by a 5 or more microns as the oil warms and pump efficiency increases. (5-20 minutes) Most of this gain goes away and the pressure rises slightly after many minutes of pump time as the oil becomes very hot (oil vapor presssure)and the pump efficiency falls off slightly.


This is as much a part of fusor construction as any thing in the chamber!!! It is one of the many technologies, filled with "double secret hand shakes" known only to the annointed, which must be mastered by the would be fusorite.

Richard Hull