Foreline Pressure Measurement Issues

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Brianna Gopaul
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Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

Hi everyone,
I've recently assembled my foreline consisting of the following:
- JB DV142N Roughing Pump
- Varian Block Valve connected to an Edwards 63MM Diffusion Pump
- CPS VG200 Micron Gauge
- 6" Hose, T fitting, adaptors, and fittings

I've tested the roughing pump by attaching the micron gauge on directly and got a pressure of 4 microns in 4 minutes. I believe the gauge and roughing pump are in good condition.

Issue:
When I connected the gauge to the T-fitting and pumped down for 15 minutes, my gauge gives me no reading. I assumed there was a blockage so I disassembled every piece, cleaned everything, and put it back together. I found no blockage. After another 15 minute pump down, I once again have no reading on my gauge. It reads "- - -" as shown in the photo below. I read some HVAC forums about this micron gauge that recommended closing the roughing pump valve and then turning it off to get a pressure reading after pumping down. I did this and had no luck.

Are there any visible problems with my setup? Does anyone have any recommendations about what else I should do to debug this issue?
I'd appreciate any insights.

Thanks,
Brianna
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Brianna Gopaul
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Re: Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

I believe I've isolated the issue by removing parts. I assume from this point it's just a really bad leak caused by my KF adaptors and hose fittings. Every thread is wrapped in teflon and the clamps are tight.
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John Futter
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Re: Foreline Pressure Measurement Issues

Post by John Futter »

Brianna
pressure is too high for your gauge to read ie leaks somewhere
most of your problems will be with those plumbing fittings especially the mac union these are ok for water or gas use but are not good for vacuum service.
You need to get bottle of loctite 290 and use it on all those brass and galvanised iron fittings.

and as a general rule I do not like teflon thread sealing tape it causes more problems than it fixes --use the loctite 290 instead
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Richard Hull
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Re: Foreline Pressure Measurement Issues

Post by Richard Hull »

You did a great job of isolating issues. If your pump ever read 4 microns, directly connected to the gauge, your pump is NOT the issue. As you note there is serious leak beyond the pump entrance. That is a complex garbled tree of fittings... Pump to KF "T". The size of the fittings will forever have a strangle hold on the conductance of your future operations. Ideally, you should never exit the pump orifice with any plumbing smaller than that opening size. It chokes the pump off. Likewise, even if you you do not choke the pump off with reduced size plumbing, really long hoses and paths in the plumbing also serve to reduce conductance in the vacuum system.

What I see looks rather good and short headed to the diff pump. I can't imagine a real leak at the properly clamped KF "T". Make sure the varian valve is closed and take a measurement. Keep your effort right now focused on the pump to "T" fitting. I worry the varian valve has a faulty bellows or bellows seal. When working to test and seal a system you should have one of two "blank-offs" on hand. If you have a KF blank-off remove the varian valve, blank off the "T" and see what happens.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Mark Rowley
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Re: Foreline Pressure Measurement Issues

Post by Mark Rowley »

One other consideration....
I have one of those CPS digital vacuum gauges which was leaky as a sieve right out of the box. On Amazon there are no shortage of HVAC folks indicating it’s leaky tendencies in the reviews section.

Rather than return mine, I elected to remove the snap-fit o-ring assembly and replace it with a custom machined NPT fitting; problem fixed. Custom machining is not in everyone’s bag of tools so it’s a remedy that may not be available. And of course that’s providing your meter is in fact leaky. You may be one of the lucky ones where it’s not.

I know you said it read 4 microns at the pump head. However, so did mine. The nature of the snap-fit is wonky because if it’s slightly tilted in one direction it’s a tight seal. A slight tilt in the other direction it leaks.

Mark Rowley
Last edited by Mark Rowley on Thu Apr 29, 2021 3:58 pm, edited 1 time in total.
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Richard Hull
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Re: Foreline Pressure Measurement Issues

Post by Richard Hull »

Assuming you read 4 microns on it before, the hope is that it has not suddenly developed a magical leak. I'm not familiar with the "wonky fit" related to the snap gasket. Wow! Thanks for noting your issues with the gauge, Mark.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Brianna Gopaul
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Re: Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

Thank you, Richard and John. I'm going to get some better fittings rated for vacuum. I had a feeling they would be the cause of my leaks.

Mark, I unfortunately don't have the equipment to machine my own parts right now. Thank you for the insight though! I hope my gauge isn't leaky, I was planning on using another VG200 for main chamber measurement.

I'll keep working at it.
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Mark Rowley
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Re: Foreline Pressure Measurement Issues

Post by Mark Rowley »

Richard, it’s totally an issue with that particular gauge and whoever they sourced the snap gasket fitting from. It seemed very “harbor freight” like in quality. I’m sure normal HVAC fittings from Grainger and such are of better quality.

Brianna,
Keep up the good work and am looking forward to your progress!

Mark Rowley
Brianna Gopaul
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Re: Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

Update:
Got some new fittings, removed the Teflon, and applied orange Permatex threadlocker (I'll apply red Loctite threadlocker when I'm not disassembling it frequently). I measured 53 microns after 4 minutes of pump down.

There are clearly leaks but at least an improvement has been made!

Next steps will be to locate which fittings are leaking the most. I'm thinking of spraying each fitting with pressurized air and looking for the pressure to increase. I'd be interested in hearing if anyone else has recommendations on next steps to decrease the leaks once I've located them.

Thanks,
Brianna
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Re: Foreline Pressure Measurement Issues

Post by John Futter »

Brianna
a spray bottle with alcohol or acetone sparingly spray at the joints starting with the lowest to highest points of your system can make a brief blip in the gauge reading
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Richard Hull
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Re: Foreline Pressure Measurement Issues

Post by Richard Hull »

Note if you bought your pump brand new from Harbor Freight or Northern Tool, these are less than top flight pumps (ChiCom) and 53 microns may be their bottom. Top Brand 5 CFM, direct drive, dual stage refrigeration pumps like Yellow jacket have little trouble hitting 10 microns.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Brianna Gopaul
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Joined: Tue Dec 22, 2020 7:03 am
Real name: Brianna Gopaul
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Re: Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

Thanks John, I just sprayed IPA on each fitting from bottom to top and I saw no increase in pressure at each fitting, the pressure remained the same. This pump down I measured 42 microns compared to 53 microns last time, likely because the threadlocker cured for 24h. I'm a bit stuck on next steps right now. Clearly there is a leak somewhere given that I've measured 4 microns at intake. I was thinking of removing the KF adaptors but I doubt the o-ring and clamp is a leak source. I'll spend some time thinking about this and then test for vacuum leaks again to check.

Richard, the JB DV142N roughing pump is rated for 15 microns. I also measured 4 microns at the pump's intake so I don't believe this is the problem but thanks for the note.

Thanks,
Brianna
John Futter
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Re: Foreline Pressure Measurement Issues

Post by John Futter »

I note you have not used the loctite 290 and have used another loctite product
the 290 is also called super wickin and is very fluid and designed to seal hydraulic fittings
once cured 290 does not appear to have any significant vapour pressure
Brianna Gopaul
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Re: Foreline Pressure Measurement Issues

Post by Brianna Gopaul »

Took a 2 week break for finals but I'm back:

Thanks John, I applied Loctite 290 to my fittings and let it cure for 2 weeks.
After 5 min, I got a foreline pressure of 17 microns.
I then opened the varian valve, the pressure skyrocketed and then went down to 28 microns (foreline + diffpump body)
From the FAQs, I know I should be aiming for a pressure of 12-15 microns for the foreline + diffpump body.

I'm thinking next steps are to check if the varian valve seals are faulty.
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Re: Foreline Pressure Measurement Issues

Post by Richard Hull »

Wow! I think you are sealed. That diff pump body is chamber sized! I would turn on the heater until the body warms just a little. (not too long though without oil) If the pressure goes up a lot, you have absorbed water that will leave once the oil is in the pump and reaches over 100 deg c.
Water in the pump in my imperfectly sealed system had to boil off after months of inactivity The system would hit 25 microns mechanical pump only with diff pump also evacuated. As the boilier got to work pressure would rise to 50 to 60 microns and then go down to 10 microns in the foreline.
Run the tests before going nuts only to find out you are chasing absorbed water in a chamber sized diff pump.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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