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
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Leak Troubles

Post by Tom McCarthy »

Turning to the more enlightened vacuum folks of the forum for help. Can't seem to get my foreline working.

I had my roughing pump (Edwards RV3) hooked up to a valve, couple of inches of hose and to my Pirani gauge. After some work and problems with the gauge ( http://fusor.net/board/viewtopic.php?f=10&t=11196 ) the small system pumped down to ~5 microns and I connected it up to my full, blanked-off chamber. Took a short few minutes longer to pump down (as expected), but got to single digit microns again and after more than 36 hours, the chamber was up to just about half a Torr, so it chamber was pretty leak tight.

Following this, I went to set up my turbo, bolted it onto the chamber, attached forepump through turbo exhaust and checked how the forepump was working. No joy, pump could barely get past atmosphere - turbo wasn't bolted properly (could hear the leak), so I tightened turbo conflat bolts and gauge now got down to single digit torr.

Wanting to make sure an issue hadn't appeared in the foreline, I disconnected it from the turbo so I could check it by itself and promptly ran into confusing results. Major discrepancy is between setups 3 and 4 below.

---------

Setup 1: pump -- valve -- two hoses -- gauge.

Takes only a few seconds to get down to 30 microns, slowing down as it goes and inching past 20 microns. If I turn the pump off with valve open, pressure shoots up to 2 Torr or so. Turn pump off with the valve closed there's no huge shoot up, but a quick, smooth leak of 3 - 4 microns/second. This is a problem. To try and diagnose where the issue is I took the line apart and added parts piece by piece to see if anything contributed dramatically to the leak.

Closing the valve, turning off the pump and then opening the valve still gives a shoot up, but less dramatic. Goes up to 800 microns - 2 torr and a lot less quick, but still quick.. 2 seconds as opposed to instantaneous. This isn't as much of an issue as I can just valve off the pump and there's no problem, also usually there's a small raise in pressure from atm when the pump starts and then it starts pumping down. Persists across all the setups described, so I won't repeat it below.

-----------

Setup 2: Gauge deadheaded directly on to pump inlet (with KF 10 -25 adapt)

As above, takes a few seconds, gradually slowing down as it goes to about 30 microns, then much slower inching down past 20 microns. Couldn't isolate gauge from pump, so couldn't check gauge for leak by itself as pressure just shoots up to 2 torr when I turn off the pump. When deadheading previously, it would get down quicker to single digit microns, seems to have slowed down, leak developed in the gauge connection perhaps.

-----------

Setup 3: pump -- valve -- gauge

Same pumpdown as above, gets to 30 microns, slowly gets past that. Closing the valve lets me check the leak rate, which I took some measurements as follows to do some real diagnostics.

40 -> 60 microns: 7 seconds
60 -> 80 microns: 7 seconds
80 -> 100 microns: 7 seconds
100 - 200 microns: ~45 seconds

Looks like a leak.
--------

setup 4: pump -- valve -- hose -- gauge

Here's a problem.

40 - 60 microns: 30 seconds
60 - 80 microns: 30 seconds
80 - 100 microns: 35 seconds

Looks like a leak. But it's nearly 4 times slower than in setup 3. This goes against my intuition, as all the offending fittings from setup 3 are still present along with the hose and an extra KF connection. Can anybody explain this?

---------

One thing I did notice is some grime/oily dirt on the pump inlet (pictured). From this post ( http://fusor.net/board/viewtopic.php?f= ... 4&start=30 ), could oil contamination be causing a problem?

Also, I need to find the exact locations of the leaks, I tried painting the rings with acetone one by one, but there was no noticeable shoot up on the pirani gauge. Will dust-off work better? I find KF connections hard to get at with this sort of spray-on/paint method as the clamp around them is a big obstacle. To get around that, I just turned on the pump and one by one removed a clamp from a connection, painted the o-ring circumference with acetone and put the clamp back on. No luck with that. From tests above, it looks like the gauge connection (gauge + adaptor) is leaking. What's the best way for me to deal with this? And once I've found the leak, should I just put a new o-ring in, or lather it up with some sort of grease, how to actually stop the leak?

Once I sort the foreline out I may have issues with the chamber, but need to get over this hump first!

-- Picture previews are showing up sideways, click on them and you'll get a full size image the right way up.
Setup 3
Setup 3
Setup 4
Setup 4
dirt on inside of pump inlet
dirt on inside of pump inlet
two hoses, not described in setups
two hoses, not described in setups
Tom
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Leak Troubles

Post by Richard Hull »

It looks like your pump is belching crud into your system and gauge!! This garbage looks like it is coming from your pump for sure! your gauge may now be fouled. Is that a bead of water I see in the photo??!! A big, big, no-no.

Never ever run with a gauge on the line when flushing!!! Blank off the pump inlet!

I once bought a used pump and dumped the oil which was filthy beyond belief. I ran a charge of fresh flushing oil for a few minutes and dumped it. The oil was still kind off cloudy. I ran another load of flusihing oil, but this time for two hours. This second flush of oil was more filthy than the first!!! A third flush for two hours of run time finally yielded acceptably clean oil. A charge of good pump oil and a T/C gauge showed the pump went down to about 15 microns in under one minute. I filled the T/C gauge tube with acetone and shook it vigously. I, then, dumped it out. after an hour of air drying, it went down to about 10 microns. T/C gauges can, over time, get fouled.

Just putting new oil in a pump of unknown origin is a losing situation. Run a first flush on a used, blanked off pump for 1 hour, minimum. Continue until additional flush oil drains clean and rather clear. Only then, put in decent oil and hook you gauge up to see what you have.

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
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Thanks Richard. I had decontaminated the oil a few weeks ago when trying to get my gauge working (ran for a couple hours with gas ballast open), but didn't replace it.

Today I cleaned out the sight glass, inlet and filter. I also drained the old oil, which was clear, but a kind of yellowy colour, see the picture below and flushed some new oil through. I then put a full fresh charge of oil in, and am currently waiting on the pump now, running it for 2 hours or so to see how the oil turns out. Note that in the picture of oil below, the solid particles you see (vast majority of them anyway) were already in the bucket, and didn't come out from the oil.

Hope this fixes things up, if not I'll give the gauge an acetone clean. It's an Edwards PR10K, Pirani gauge. Looks like I'll need to dismantle the gauge, as there seems to be a danger of damaging a filament. Is this true, or can I just immerse the thing in acetone as Richard's described and shake it around and leave to dry?

http://mfc.engr.arizona.edu/documents/E ... 157880.pdf

Page 11:
The interior of the gauge tube must not be cleaned as damage to the
filament may occur. If contamination is suspected the gauge should
be returned to BOC Edwards for exchange/replacement.
Will update ye when I've checked how the RV3 is performing with new oil change. May not have to go near the gauge at all, but from Richard's advice it looks like it will need to be done.

UPDATE: emptied the new oil out after > 2 hours running and about an hour to cool, it came out clear as water. Filled up with new oil and attached the gauge. Bit quicker on the pumpdown now, but not much difference. I guess the gauge needs cleaning. There's also definitely a leak in the KF fittings (KF10 - 15 centring ring, KF15 - 25 adapter) attached to the gauge, as isolating it leads to a rapid rise in pressure, as described in the original post.

Tom
Pump oil, first change
Pump oil, first change
User avatar
Jim Kovalchick
Posts: 717
Joined: Wed Apr 13, 2011 8:00 pm
Real name:

Re: Leak Troubles

Post by Jim Kovalchick »

Tom,
Your KF seals are leaking. In setup 3 with the valve closed you should see very little rise in pressure. Did you try a little grease on each viton seal ring?
My son and I have an RV-3 in our lab too. It's pretty reliable, but we did have a vane failure once that locked the shaft and shattered the motor coupling fingers. A rebuild kit fixed it up. I don't ever rely on that spring loaded back flow valve on the pump inlet. It will always leak back when the pump is off. This can lead to streaming of oil vapor into your turbo.

Good luck
User avatar
Steven Sesselmann
Posts: 2127
Joined: Wed Aug 10, 2005 9:50 pm
Real name: Steven Sesselmann
Location: Sydney - Australia
Contact:

Re: Leak Troubles

Post by Steven Sesselmann »

Tom,

Trust your gut feelings, you probably know where the leak is but hope it isn't because its hard to get to :)

Pull it apart, clean every part with acetone, check every KF fitting with a loupe, and if you see even a tiny scratch, wrap a cotton rag on a clean sanding block and polish the fitting with a bit of chrome polishing paste, until it's so shiny you can see yourself. Then clean the part and the O'ring with acetone, and apply a smidgeon of silicone vacuum grease to the O ring before you put it back together.

Rebuilding a vacuum system takes less time than messing around trying to find a tiny leak.

For comparison my unorthodox chamber now drops only 3 micron in 1 hour, and I have several KF fittings and a chamber made of 20 discs with 40 surfaces glued together with Latex. I bet some chambers made with Conflat copper seals aren't even that good.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Jim, thanks for the confirmation - I suspected leaks, but hoped it was something with the pump instead.

I've ordered a tube of Dow Corning vacuum grease and picked up some chrome paste as you advised Steven, cleaning and inspecting all parts today. As mentioned I did have leaks in mind and was avoiding them, so you're right about that gut feeling.

One question - I've read docs advising against using acetone, spirits, cleaning solutions etc with o-rings. Is it ok to wipe them down with some acetone, or should I use something else for the o-rings?

Will report back when the work is done.

Tom
User avatar
Jim Kovalchick
Posts: 717
Joined: Wed Apr 13, 2011 8:00 pm
Real name:

Re: Leak Troubles

Post by Jim Kovalchick »

Don't use acetone except on metal. If for some reason your o-rings are dirty use a little alcohol.
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Some progress:

Dow Corning grease arrived yesterday. Had already cleaned KF fittings with acetone and wiped down o rings a few days ago (played around with setting up MFC in the meantime). There's no detailed guide on applying vacuum grease online more than 'put a tiny bit on the o ring, less is more.'

I only got it tested with the valve and Pirani gauge fittings connected to the pump, but I've gone from 3 microns/s leak to 1micron / 3 seconds leak. Better, but still a leak. That may improve with runtime, as on first run with the newly cleaned and greased fittings, they leaked up much quicker, but have slowed down with more time in vacuum. Residues gassing off I guess?

I'll do some more runs and see how things are, if a noticeable leak persists I'll have to go at the system again.

Meanwhile, any advice on applying vacuum grease, in case I'm going wrong? What I'm doing at the moment is oozing a tiny bit out of the tube (like a quarter of a fat drop of water on a tap) wiping that on my glove finger and just massaging all around the o ring with it. There's a slight sheen of the o-ring, but no noticeable build up of residue.

Thanks,
Tom
User avatar
Steven Sesselmann
Posts: 2127
Joined: Wed Aug 10, 2005 9:50 pm
Real name: Steven Sesselmann
Location: Sydney - Australia
Contact:

Re: Leak Troubles

Post by Steven Sesselmann »

Tom,

If you keep pumping down the chamber for a week or so it will improve significantly as water comes out and if there is a leak you will see that pressure falls linearly.

If it is outgassing only you will usually see the chamber stabilise between 500 and 1000 micron and stop falling, but if it leaks it just keeps going.

Once again trust to your gut feeling and fix the leak.

Good flanges and new O'rings - Unlikely
Professional welds - Unlikely
Tapered threads with teflon tape - Very likely
Conflat flanges fitted properly - Unlikely
Shut off valves bought on eBay - Likely

If you are still having problems take a few pictures and post them here, maybe we can spot the weak point.

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

Using any vacuum grease to seal a leak means that either the o-ring or its mating surface have a minor defect. To use grease, follow the following: lightly apply a tiny amount of grease using proper gloves (to avoid lint and contamination of the grease) and spread as thin a layer as possible over the entire o-ring. Then using a lint free wipe, remove all the grease you can (a film will remain.) The grease should cover all the o-ring but should not be visible to the eye or get another wipe and wipe again. Then install the o-ring and test. If it still leaks, then either the leak is somewhere else or the 0-ring/mating surface is too damaged. Either case grease is not the answer.

Out gassing can easily go above 1 torr (1000 microns) but will never bring the system to atmosphere and will definitely slow as pressure increases.
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

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
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

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.
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Leak Troubles

Post by Jerry Biehler »

You are not trying to reuse conflats, right?
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

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
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Leak Troubles

Post by Richard Hull »

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
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
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

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.
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

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.
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

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.
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Leak Troubles

Post by Richard Hull »

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
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
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

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.
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Leak Troubles

Post by John Futter »

Tom
your penning gauge is dirty if it stops like that and reads an impossibly good vacuum.
I looked at your system and could not see the gauge head unless it is a philips Granville unit
the Edwards ones we regulary clean @ work every 3-5 years on a clean system or twice a year on a sputter system or ion implanter.
cleaning consists of of dissassembly and all parts polished with increasing grades of carborundum paper using a lathe to quicken the process. A wash
of the parts with acetone then ethanol and reassembly. takes about half a hour per gauge head
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

John, pictures attached of head. It's a LEISK unit.

Looks like there's another, smaller conflat fitting on the head, halfway along the length, behind black part. The port is a CF 2.75" fitting, I guess the inner one would be a 1 1/3" CF, though I can't get any manual so I'm not sure. I have a second unused one, just have to find it, it's lying around somewhere. Otherwise I'd rather hold off disassembling as I'd have to replace internal gasket and I don't have any gaskets less than 2.75" at the minute. If I can confirm size I'll order gaskets and disassemble.
CF 2.75 Port
CF 2.75 Port
Full head
Full head
Cable plug
Cable plug
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

I am both surprised and concerned by your turbo indicating the pressure of your system is too high or gas load too high. Either your gauge is not accurate for the leak rate you indicated or else the turbo pump has issues (remember, high load can also be caused by bearing's having issues ... ); I see you are addressing the gauge issue. Nothing can be done if one's vacuum gauge is in extreme error; that 10^-7 torr is in error for a unbaked system with that leak rate.
Ten minutes to go from 20 microns to 80 microns and your turbo indicates stalling after running!? That does not follow. My fusor far exceeds that leak rate after exposure to air and my DP pumps it into the low 10^-5 torr in under ten minutes; my turbo system has a similar leak rate and bottom outs 10^-5 torr in under five minutes (it is both an old and very small turbo.) So, definitely clean or check your gauge.
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Wary that there's a small CF gasket inside the heads, I was reluctant to dismantle until I got said gaskets for replacing, but I took apart the gauge head today. Looks like the internal CF gasket is a DN25/ 2 1/8" fitting. OD of gasket is 30mm , ID 20mm, DN25 gaskets are the closest I could find anyways. Though I don't have them here yet.

I was surprised on dismantling to find the whole thing quite clean to my eyes. No discolouring coatings or dirt buildups. The sensor itself is almost immaculate on the outside. I haven't tried to get at the inside, and I'm not sure if I should, seems delicate and better left alone. The dirtiest part is a streak along the inside of the CF connect between port and sensor.

Does this look clean to you guys? I've to wait for a set of DN25 gasket replacements before trying it out again, and I can compare to the other gauge head, which I found yesterday and dismantled too. The unused head is actually a lot dirtier, with discolouration, so I'll have to give it a clean.

John, you mentioned sanding the parts with high grit paper, I'm not sure I should go near the sensor though. What do you think, just give it a good wipe on the outside with acetone and ethanol, or should I even try and get inside the sensor? Doesn't seem like a good idea to me as I'd have to break and resolder parts.
Penning head disassembled
Penning head disassembled
Notice browny streak 3/4 of way along inside of tube
Notice browny streak 3/4 of way along inside of tube
Side 1 of Sensor
Side 1 of Sensor
Side 2 of sensor
Side 2 of sensor
Roberto Ferrari
Posts: 364
Joined: Tue Mar 02, 2004 12:21 pm
Real name: Roberto Ferrari
Location: Argentina
Contact:

Re: Leak Troubles

Post by Roberto Ferrari »

Hi Tom
This kind of gauges rely on measurement of very low currents.
So you need to pay attention to cleaniness on insulators. Look inside the conflat.
Roberto
Post Reply

Return to “Vacuum Technology (& FAQs)”