Vaccum Leak Testing / Technique and Power Supply Question

For posts specifically relating to fusor design, construction, and operation.
Taylor Shead
Posts: 15
Joined: Sat Oct 20, 2018 8:09 am
Real name: Taylor Shead

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Mon Mar 11, 2019 8:07 am

Okay, here's where we are at.

The TC gauge is good. We got it tested on another pump at school.

I believe the pump is either bad or needs some tuning (this is the first time I've operated a vacuum pump so I know not so much about that)

Video:
https://www.youtube.com/watch?v=ojFnM92hR9o

The hole in the top of the pump is where the air-oil separator should go but this one does not have one. I don't see it being a huge issue.


As far as power supply - I believe we have one that can do reversed polarity at proper power levels now. More later.


In other news the fusor body is coming together.

Test fit w/o insulator:
Image

John Futter
Posts: 1374
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by John Futter » Mon Mar 11, 2019 8:45 am

your short video shows that the pump is not reaching its base vacuum
ie you have a leak
the leak is so bad that using alcohol will not help
you hear that noise of gurgling at the end of your video
that should stop and the pump should become like between the gurgles

you need to fix this before going any further!!!!!

Taylor Shead
Posts: 15
Joined: Sat Oct 20, 2018 8:09 am
Real name: Taylor Shead

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Mon Mar 11, 2019 8:51 am

I don't think there is a leak as this same setup was tested on another pump without any error or leak.
I think it is the actual pump with the problem.

John Futter wrote:that should stop and the pump should become like between the gurgles
What do you mean by this exactly?

John Futter
Posts: 1374
Joined: Thu Apr 22, 2004 2:29 am
Real name:
Contact:

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by John Futter » Mon Mar 11, 2019 5:55 pm

Quiet
just a hum from the running motor

Peter Schmelcher
Posts: 163
Joined: Sat Nov 13, 2010 6:56 am
Real name: Peter Schmelcher

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Peter Schmelcher » Tue Mar 12, 2019 6:38 pm

I believe the gurgling sound in the video is cavitation in the oil which is normal but only when the gas ballast is open or if you have a large leak.
The oil reservoir screw cap probably has an o ring and should be closed when you are trouble shooting the vacuum pump.
https://vacaero.com/information-resourc ... pumps.html
If the pump was ever topped up with a clean good looking non-vacuum pump oil you will be measuring the vapor pressure of the added oil until the pump is flushed clean and filled with new vacuum pump oil.

Dow Corning industrial Molykote compressor and vacuum pump oils are locally available, shop industrial supply companies, but from memory a 5 gallon pail 5 years ago including delivery by UPS was about $150. The reason I dug deep into vacuum oils is because the oil specified for my Edwards vacuum pump was obsolete as was the recommended replacement oil. I contacted Dow for the additional unpublished technical information below.
MOLYKOTE L-0610 Vacuum Pump Oil Vapor Pressure:
mm Hg (ASTM D323)
100°F 4.0x10-5
150°F 1.3x10-4
200°F 1.5x10-4

These types of oils are vacuum distilled and you can purchase a lower vapor pressure grade, Dow just processes the oil more which adds to the cost.

Best of luck
-Peter

ian_krase
Posts: 508
Joined: Mon Nov 28, 2016 7:48 am
Real name: Ian Krase

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by ian_krase » Thu Mar 14, 2019 7:07 am

I'm a little worried about those conflat flanges -- it looks like there isn't much space between the weld bead and the knife edge part.


As with the other people: It looks like there is a big leak in your setup. Actually, I'm a bit surprised it got as low as 200 microns -- perhaps the gauge is wrong as others have said.

You may be overthinking the vacuum pump oil question. Car parts stores sell vacuum pump oil since rotary vane vacuum pumps are used to service air conditioners. Or you can buy a 1-gallon jug for a pretty reasonable price (about 25 bucks)

Bruce Meagher
Posts: 132
Joined: Fri May 13, 2011 3:25 am
Real name: Bruce Meagher
Location: San Diego

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Bruce Meagher » Thu Mar 14, 2019 10:13 pm

Ian,

I think you’re giving incorrect advice on vacuum pump oil. Look at the experiment Tyler ran to see why typical A/C vacuum oil is a poor choice

http://www.fusor.net/board/viewtopic.ph ... ler#p80054

I'd recommend ballasting the pump for a couple hours with the inlet blanked off. Then rerun a quick pump down test to see if there is any improvement. Also, I would not have the T/C gauge looking directly into the inlet port. Have a 90 degree bend at a minimum so you don't foul your gauge. You should also double check the inlet flange is tight the pump. I'd inspect the o-ring on the underside of the inlet flange too. If no improvement replace the oil and retest.

Bruce

Ben_Barnett
Posts: 17
Joined: Tue Sep 25, 2018 9:06 pm
Real name: Ben Barnett

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Ben_Barnett » Fri Mar 22, 2019 8:53 pm

UPDATE:
We procured another pump (short on time) and pulled a solid vacuum of around 10-15 mTorr w/ pump on on our whole system.
Pump off, valve closed, slow leak up to about 200 mTorr, but at least now we have vacuum.

User avatar
Dennis P Brown
Posts: 1673
Joined: Sun May 20, 2012 2:46 pm
Real name: Dennis P Brown
Location: Glen Arm, MD

Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Dennis P Brown » Sat Mar 23, 2019 8:10 pm

A 'leak' up to 200 microns (assuming that takes a few minutes) is mostly normal out gasing from the walls. Sounds like your system is fairly tight.

Post Reply