Leak Troubles

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Tom McCarthy
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Re: Leak Troubles

Post by Tom McCarthy » Thu Mar 09, 2017 9:22 pm

Confident it's still leaking, much more slowly, but still a leak. About 1 micron every 8 seconds. Up from 20 microns to 2 Torr over the course of 3 hours. This compares to 3 microns a second before greasing.

I'll reapply grease properly as per Dennis' directions (didn't wipe the layer the first time) and see how things go. I actually have some new o-rings unopened, so will see how they work out too.

The pressure rise seems linear based on my observations, so I'm leaning away from outgassing. Following the gut Steven!

Tom

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Dennis P Brown
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Re: Leak Troubles

Post by Dennis P Brown » Fri Mar 10, 2017 1:10 pm

Your leak rate is high for such a tiny system but may be out gassing. Your pump isn't back flowing gas? You have a valve between the pump and gauge? Remember, without a anti-backflow device, pump oil will out-gas into your system and you may be seeing that.

You changed the oil but often if a pump has a lot of bad stuff, one needs to open the pump and clean it out manually using solvents and lint free rags (I've done that to pumps before that needed it; such a job isn't easy and one can also easily damage o-rings/seals so it isn't a job done unless one is ready to deal with those issues and is careful. Of course such seals can be very bad and will fall apart when the pump case is opened; in which case they needed to be replaced.

In any case, I would carefully check the surfaces of all flanges and look for tiny scratches - remember, grease is like a sponge to water and other things that can out gas; that is why one wants as little as possible.

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Re: Leak Troubles

Post by Jerry Biehler » Sat Mar 11, 2017 3:02 am

You are not trying to reuse conflats, right?

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Re: Leak Troubles

Post by Tom McCarthy » Sun Mar 12, 2017 9:23 pm

Nope, not reusing any conflats... As it stands I actually have 0 conflat connections for testing. Chamber is all conflat but I don't have it hooked up, just trying to get these KF fittings under control.

Dennis, I do have a valve on the pump inlet, so I don't think there should be any oil outgassing. I test the leak rate with the pump off and valve closed, so in theory the gauge is completely isolated from the pump. Hopefully the pump doesn't have badness inside... It's producing much less vapour now, no fog in the shed air after running for a while (I know, I'm getting a longer exhaust hose to run out the window), though it's a little noisy. The oil remains clear after ~6 hours operation. 3 x 2 hour runs, a little more maybe.

I'm pretty sure it's leaks in the KF fittings rather than the pump. I'll keep at them for a few days and see if things get better.

Tom

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Richard Hull
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Re: Leak Troubles

Post by Richard Hull » Mon Mar 13, 2017 4:36 am

KFs with smooth faces rarely leak or outgass. The key factor is no fine sleeks should be visible on the faces of the mating flat surfaces. When properly tightened, the elastomer ring really spreads out over the flat, but micro sleeks and ultra fine scratches of some length or depth can defeat the nice broad area of contact.

Richard Hull
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Re: Leak Troubles

Post by Tom McCarthy » Tue Mar 21, 2017 9:53 am

Fairly certain I'm only dealing with out-gassing now, and possibly vanishingly small leaks. Pressure rose from 20 microns to 1.5 torr in about 3 hours, and 12 hours later it's only at 2 torr, so the rate is definitely sloping off. It's still a fairly large 'leak' - 1micron/5 seconds at the start, but I hope that'll reduce with some pump downs.

To fix, I followed Steven and Jim's advice - Sanded each KF face and polished to a mirror finish. Cleaned all centering rings and KF faces with Acetone, and greased all o-rings. I'll post some pictures of them later. Peter Schmelcher's post on polishing up KF faces was helpful: viewtopic.php?f=10&t=4175. For the actual process of sanding + polishing I first asked a man I know who works in a metal shop to sand + polish the faces on one piece for me and then I worked from them, trying to copy his work on everything else.

I started with 120 grit, then went 240, 320, 600, 1200, 1500, all manually, followed by Peek polish with a rag. I did everything manually, placing the sandpaper on a cloth on the bench surface and rubbing the piece against that. Each face took about 30 minutes to do from start to finish. I followed Dennis' advice for greasing the o-rings.

Now, I haven't checked all the parts to see if they're leaking, but the gauge, KF nipple and valve, which accounted for most of the leak (if not all) seem leak free.
If I can get the out-gassing dealt with I should be good to get the turbo pump up and running.

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Re: Leak Troubles

Post by Dennis P Brown » Tue Mar 21, 2017 10:35 am

If your "leak" rate is just one micron in five seconds (if that includes your chamber volume, that isn't bad at all), why aren't you running your turbo!? That "leak" rate should easily allow your system to get in the very low 10^-5 torr range and possibly the 10^-6 torr range.

Remember, once air is allowed in, water vapor will cover all vacuum surfaces and easily create the leak rate you are measuring. That is why I built an air dryer system for my vent air source. Improves things a great deal; however, a dryer is not needed for a fusor - I use the air dryer only for my accelerator.

More to the point of what you are doing, if your system gets to the mid/low 10^-5 torr range with the turbo, then it is fusion grade! That means your system, running in the micron range with deuterium being leaked in (against the turbo that is being gated) will be more than sufficient to allow fusion. But again, if your system reaches the mid/low 10^-5 torr than it is doing very well and you don't need to pursue leak methods; even 10^-4 torr will mean the system can do fusion; however, with the gate valve (or what ever valve system) fully open, 10^-4 torr means there is real leaking (not just out gassing.) At that level of vacuum, using 'dust off' or alcohol with your high vacuum gauge on will locate such leaks easily!

My large fusor had to have its chamber run with a plasma before my out gassing drops from a micron every few seconds to one micron every minute (water vapor.) A plasma is a great way to clean a fusor! I always pre-clean my fusor in that manner before making a fusion run.

Aside: any HV power supply can be used to "plasma ash" your fusor; that is, a simple NST (AC is fine) will 'strike' a plasma that will clean your system. That is a simple way to improve the chamber and reduce out gassing.

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Re: Leak Troubles

Post by Tom McCarthy » Sat Mar 25, 2017 12:28 am

Today I ran the turbo pump - thanks for the prompt Dennis.

The RV3 pumps the chamber down to 40 microns in 10-15 minutes and in another 10 minutes down to 20 microns. I find isolating the system from the RV3, letting the pressure rise a little (to 60-80 microns) with outgassing and then open up the pump again, it gets the pressure a bit lower. The RV3 is already running through the Leybold Oerlikon Turbovac 150, so once the system was down to 20 microns, I started the turbo pump. Pressure in the fore line quickly rose to 80 microns, a graph in the pump manual shows a rise in forepressure, so this is to be expected. The Pirani gauge is located between the RV3 and turbo pump, not on the chamber. I have a Penning gauge on the chamber, part of a LEISK VC15 unit, good for 10^-5 to 10^-10 mbar. The readout for this with the turbo running (after brief acceleration phase) was a steady 2 x 10^-7 mbar // 1.5 x 10^-7 torr, this was after 5 minutes max. I find this suspect, it seems quite low for a Fusor system with no bake-out or rigorous cleaning, though maybe I'm lucky. It just seems a little too good to be true, though comparing to Andrew Seltzman's post (http://fusor.net/board/viewtopic.php?f=10&t=11373) he's getting a small volume with a small pump down to 10^-5 quickly. Does 10^-7 sound sane here? Pumping speed is 115 l/sec with a 14l chamber.

However, the Penning gauge operates based on ionisation currents and I read that above 1 micron, the response is not reliable. I.e: Instead of reading at the top of the 10^-5 - 10^-10 scale if it were at 20 microns, the gauge could be giving some wonky reading anywhere in the range with no useful relation to what the actual pressure inside is. I'm suspicious the turbo didn't actually get really pumping and the

Anyways, after 10-15 minutes with the turbo on normal operation, the failure light lit up on the controller and the turbo spun down. The controller attributes this to too low a rotational speed (below 80%) in the turbo. The turbo pump manual advises checking that fore vacuum pressure is not too high, or else gas volume is too great (leak) for the pump. I'm going to try running again tomorrow to see if the turbo doesn't shutdown and check if the Penning gauge pumps down through the 10^-5 and 10^-6 ranges before getting to 10^-7, as opposed to jumping straight there with a wonky reading. I can start identifying problems then.

Maybe I have a tight system that's performing exceptionally well. Here's hoping.

Also, I finally got an extended exhaust hose for the RV3, no more smelly oil mists in the shed.

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Re: Leak Troubles

Post by Richard Hull » Sat Mar 25, 2017 6:19 am

Puzzling report. The turbo shutdown due to speed fall out sounds like gas drag due to high foreline pressure like the manual said.

Was your slow leak rate stated in the past for the foreline only or the foreline+ shut off turbo and the fusor chamber?

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Leak Troubles

Post by Tom McCarthy » Sat Mar 25, 2017 7:53 pm

The leak rate for the foreline + shut off turbo + chamber averages at 1 micron / 10 seconds from 20 microns up to 80 microns. This is a little faster at the start and slower with higher pressure (as expected, outgassing), but it takes 10 minutes to rise from 20 microns to 80 microns.

I did another run today and the turbo ran fine for 15 minutes without any failure, I switched it off then. I think yesterday's failure may have been a once-off I'll run again tomorrow or tonight and see if anything goes wrong again.

Regarding the Penning gauge - When I started the turbo, the gauge moved down the range through the e-4, e-5 and e-6 readings and sat at 2 x 10^-6 after 3 minutes with the turbo running. A few minutes later, it was down at 3 x 10 ^-10, which seems absurd for a system like this - no bakeout, no rigorous cleaning. I'll see if this happens again with next run, the 2x10^-6 reading seems plausible though.

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