Fusor Forums

I will use this post to document all issues with vacuum work on my fusor. I have a chamber but not all parts to seal off yet. Began testing the Welch 1400 today. Pump was purchased from Craigslist and the owner said it came from a local university auction. Had no motor or pulley. Added a 1/3 HP motor from old drill press and mounted to thick MDF board. 5" pulley on the pump from tractor supply. Vacuum measurement is kjl-6000 coupled to kjl -205bm. Duoseal oil. Kf O-rings are black, not sure of material.
First pump down was 1 hour and reached about 10 mTorr.
Changed oil. Brown with some darker flecks.
Second pump down was about 20 min. Reached 6mTorr.
Every time I turn off pump the pressure rises really fast. The needle just shoots rapidly to 200 then a more gradual climb. Tried coating all joints with oil which is what got me to the 6mTorr but did not seem to affect the rapid rise in pressure when pump is off.

Addendum: I think the "leak" may just be out-gassing since I have now made it to 4 mTorr and the rise time after pump turn off is getting longer. I'm going to change out the motor as the current one has trouble starting.

You have a fabulous pump there! It is doing better than I have ever heard of a little 1400 ever doing! Stop here! You are as low as you can go with most any forepump made in the world today.

I would have predicted an absolute bottom of no better than 15-20 mTorr from a 1400. Must be a nearly new, unused pump or one that was really well cared for. (rare for a little klunky 1400) Even at the head, you are doing great. Put it on a real chamber and you will not do so well, I bet.

Nonetheless you have done it all in the correct order. Get a pump, get a good gauge, check you vacuum at the head first! The chamber can come later. Right now, you know you can't fault your pump due to future vacuum issues. Very well down!

I wish others would read the FAQ's rather than blast out a million questions as to why their vacuum can't reach 6 mTorr when they have an old surplus pump with the oil it came with trying to pull down a 55 gallon oil drum volume.

Richard Hull

I attached the owner's manual for this pump for you. Welchs like this one will last forever with the slightest maintenance.

The rated ultimate pressure is about 3E-5 torr, but you will never get there in the real world. These pumps will usually get to the E-4's range when in good condition with no leaks and fresh oil. What you are seeing is exactly what you should expect for the first few runs on a newly refurbished pump; everything looks great, you needn't worry about this pump.

As a side note, one cheap trick to see if you have water offgassing inside your chamber is to warm the chamber with a heat gun or bakeout tape (or a blow dryer); making sure to stay well below the max temperature of any rubber seals you may have. If water is present on the chamber walls, your pressure will rise when they are heated. In a dry system, the pressure should not change with temperature.

One thing, the pump pulley is too small, it should be bigger and the motor pulley looks a little big. You are running the pump too fast and you will just wear things out.

Thanks for the advice guys and the PDF. I did go find the manual before I got the pump just to see what it could handle.

Jerry I misstated the pulley size, it is 6". The pulley on the motor is a step pulley so it looks a little large on the front end. Belt is placed on the 2" step. I assumed the motor speed was 1725. That gets me close enough to the 580rpm needed for the pump, I think.
I will be changing to a more robust DC pm motor with speed control at the end of the week.

Jerry is right. Welch duo-seal belt drives like to be slow winders. Speed does nothing for them expect wear them out.

To my ear, there is nothing more sweet to the ear than the lup-lup-lup of an old slow winding Welch and nothing more annoying than the continuous rasp of a direct drive pump.

I use both, of course, but I know what I like, and I like Welch.

Richard Hull

hmmmm, am I fudging the math here.

(Pulley Driven RPM)= (Drive Pulley Diameter * Drive Pulley RPM) / (Pulley Driven Size)

R=(2*1725)/6
R=575rpm

I think most observed what they thought was a large motor pully. I see that it is steped and you are using the smaller step. Anything from about 450 to 550 rpm is normal for the 1400. It will run and function at slower speed, though.

Richard Hull