Vaccum Leak Testing / Technique and Power Supply Question

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Taylor Shead
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Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Tue Mar 05, 2019 4:10 am

Hello all,


ISSUE #1

Our power supply is not reverse polarity, however I have read with mixed consistency that we can use it by simply wiring it up in reverse for demo purposes. Is this true?

ISSUE #2
Ben and I's demo fusor project is coming together parts-wise but we hit a big bump and we are running out of time. Project is due to be presented on April 15th. We fitted a gauge to the pump today for testing and we either have a very weak pump, a serious leak, or a faulty gauge.

Pump: Fisher Maxima C Plus (rated for .01 mTorr) (w/ new KJLSS19 oil) (had been sitting for some time)
Gauge: VGT TC Type 500 w/ Varian display

The top of the pump has a KF25 fitting that a ball valve is attached to and on top of that another KF25 fitting that the gauge can attach to.

First run (valve on) it pumped down to 100 mTorr but leaked up (during pump-down) to ~200 mTorr. So I took the valve off and this time it didn't even get down to 100 mTorr. At this point I knew something was way off and it is my understanding that we need under 100 mTorr for any plasma action at all. I tested it further and got inconsistent results. No run hit under 100 mTorr and one wouldn't take it under 300 mTorr.

What is the best step to properly diagnose this (preferably with minimal wait/$$)? I feel like it is most likely the pump or a big leak. This is bothersome as this does not seem like a quick fix.

We also have a digital gauge to use but currenly cannot connect it as it has 1/4 NPT threads on it (we just found this gauge today).

Is there anyone in the DFW area that has a small vacuum tight system with gauge to lend for a day so we can rule out the pump?

Reference pictures:

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Digital Guage:

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Thank you all for your help.

John Futter
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by John Futter » Tue Mar 05, 2019 6:26 am

Taylor
good questions
1./ I never use teflon tape for vacuum connections
I used to but it let me down so many times (I didn't learn).
only use Loctite superwickin I cannot remember the 3 digit code but I have been saying this for many years and many on this site now seem to own this as their own idea!! --and or any other liquid setting type pipe sealant.
Unless your ball valve is an expensive bellows shft sealed valve ie aprox US$150 or more it is leaking down the actuating shaft --removeit from the equation.
so no teflon tape no ordinary ball valve now check vacuum

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Dennis P Brown
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Dennis P Brown » Tue Mar 05, 2019 2:20 pm

If the pump reaches 100 microns with the valve but maybe 200 microns without the valve (direct to the pump), this indicates that something is wrong relative to the gauge reader and/or the head (detector) or possibly the KF fittings on the detector/pump. I assume the pump oil is clean/clear looking and to proper level (?) - might need to "degas" the oil using the air leak valve - a sitting pump collects water in the oil and that can be an issue to get down to low pressure.)

Relative to the detector assembly I've certainly had those types of devices leak thru the electrical end (older ones.) Also, KF fittings o-rings can leak if they do not mate correctly or the surfaces are damaged. Inspect them closely. Have you tried checking for leaks via alcohol test on all assembled parts while running with the detector on?

I see you are using two detectors. Best to use them separately to be certain that one isn't leaking (that is, if you haven't done that already.)

Taylor Shead
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Tue Mar 05, 2019 7:58 pm

John Futter wrote:
Tue Mar 05, 2019 6:26 am
Taylor
good questions
1./ I never use teflon tape for vacuum connections
I used to but it let me down so many times (I didn't learn).
only use Loctite superwickin I cannot remember the 3 digit code but I have been saying this for many years and many on this site now seem to own this as their own idea!! --and or any other liquid setting type pipe sealant.
Unless your ball valve is an expensive bellows shft sealed valve ie aprox US$150 or more it is leaking down the actuating shaft --removeit from the equation.
so no teflon tape no ordinary ball valve now check vacuum
Do I need to put anything on the KF gaskets? Or just tighten them really well?

Taylor Shead
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Tue Mar 05, 2019 8:02 pm

Dennis P Brown wrote:
Tue Mar 05, 2019 2:20 pm
If the pump reaches 100 microns with the valve but maybe 200 microns without the valve (direct to the pump), this indicates that something is wrong relative to the gauge reader and/or the head (detector) or possibly the KF fittings on the detector/pump. I assume the pump oil is clean/clear looking and to proper level (?) - might need to "degas" the oil using the air leak valve - a sitting pump collects water in the oil and that can be an issue to get down to low pressure.)

Relative to the detector assembly I've certainly had those types of devices leak thru the electrical end (older ones.) Also, KF fittings o-rings can leak if they do not mate correctly or the surfaces are damaged. Inspect them closely. Have you tried checking for leaks via alcohol test on all assembled parts while running with the detector on?

I see you are using two detectors. Best to use them separately to be certain that one isn't leaking (that is, if you haven't done that already.)
The pump sat without oil for an unknown time period. I put fresh oil in it. The pump has a gas knob but I am not sure of its operation.

I am unaware of the alcohol test. The only test I know is the helium test which I cannot do.

I’m not using two, the digital one is an option to the TC one. I would like to use the TC gauge because I wont have to do extra work.

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Dennis P Brown
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Dennis P Brown » Wed Mar 06, 2019 12:01 am

Policy here is do not cut and paste passages here. It eats up and wastes storage space.

One sprays alcohol on all joints with the pump running and the TC gauge operating. This can cause the vacuum gauge to jump if that is the leak point.

The micron TC is your best first test but maybe that is leaking so try both separately.

If the pump was really dry, then unlikely that is the issue as long as you used good grade vacuum pump oil and it is at the correct level.

First step is to always use the TC gauge as directly as possible on the pump with as few connections as possible.

As I said, inspect all matting surfaces on the KF connectors - they should be scratch free. O-rings should be in good condition. Do not over tighten - as reasonably tight using your hand is or should be all that is needed on KF clamps. Don't use vacuum grease.

Read the FAQ on using the air leak valve on a pump. It tells when to use it as well as how. If that is open during operation, the pumps performance will be very bad. It is for removing water from pump oil. Be certain it is closed. Again, read the FAQ on that subject.

A picture of the TC mounted to the pump and what pressure that achieves would be your next step and what to show us here.

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Richard Hull
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Richard Hull » Wed Mar 06, 2019 8:37 am

Dennis is basically correct. If you can't get a good vacuum at the head. (pump inlet), with a known good gauge, on a clean pump with fresh oil, the pump is 100% the issue. If you have a questionable gauge, you don't know anything worth while about anything.

The valve issue that John brought up is only an issue once you have a good pump reading at the head. Fight your vacuum battles one at a time. A known good, quality vacuum gauge is far more valuable than any vacuum pump. For with a good gauge, you can tell what you have as you assemble your vacuum system, item by item added to the line.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Taylor Shead
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Taylor Shead » Wed Mar 06, 2019 9:08 am

I have already tested the TC gauge directly on top of the vacuum as I said before. This did not change anything.

Tomorrow we are meeting to leak check the gauge fitting. I will be getting rid of the teflon tape and using Loctite 290. I also acquired an adapter so we can switch between our gauges.

The pump oil I think is very old but it is NOT USED and is very clear.



Does anyone have an answer to my first question about the power supply?

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Richard Hull
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Richard Hull » Wed Mar 06, 2019 9:22 am

IN short...NO!!... Never,ever do this!!!

Only very special supplies can have their polarity reversed and it is always covered in the users manual for the supply.

You need to be very careful about hooking the supply up by just flipping wires around. Virtually all high voltage supplies that are not reversible polarity have the grounded case at zero potential! All suitable supplies that are positive HV out have the negative case grounded. when running a real fusor this would mean the HV insulator on the fusor would be at ground and the fusor body, gas lines and any touching metal would be at thousands of volts relative to electrical or house ground. (absolutely a no-no and a virtual death trap.)

Most folks power a demo system with a neon transformer with a HV diode in their home made supply. Switching polarity is simple, a matter of reversing the diode.

Again, this is in the FAQs. Never use a positive HV supply on a fusor or a demo fusor whose return (negative lead) is the grounded case by just swapping the output wires around!! Death will follow quickly!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Vaccum Leak Testing / Technique and Power Supply Question

Post by Dennis P Brown » Wed Mar 06, 2019 12:31 pm

Pay close attention to what Richard said about power supplies!

Using Teflon tape properly, like grease, isn't an automatic skill! It can be a life saver for many vacuum fittings like on the TC detector if used correctly - proper wrapping direction, pulled tightly as wrapped, and not using too much before threading into the connector. I've had great results with tape down to 10^-6 torr. But it has to be used correctly. For instance, it can't be used twice - if the TC is removed, then clean off the tape fully and apply new tape.

If I recall correctly, (and DO say otherwise if I'm wrong) you are using a dangerous MOT supply. If so, then negative/positive is NOT done by switching the AC lead outputs at/from the transformer. As Richard said, simply reverse the diodes (assuming their correct to start.) It is essential you understand diodes and what you are doing, because otherwise, a MOT can kill you.

If the old oil was in a seal container, then it should be fine. If it has been repeatedly exposed to air over the years, maybe not. In which case, using the air bleed on the pump for an hour would be in order to try.

Good luck - leak testing while one has an unproven vacuum gauge can be one of the most trying issues any newbie faces.

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