Pressure Gauge Problems

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.
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Jeremy Adams
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Pressure Gauge Problems

Post by Jeremy Adams » Mon Jan 02, 2017 3:13 pm

Happy New Years Everyone!

So today I gave my vacuum system a run to see what pressure I could get out of it. It is an oil diffusion pump (a reliable edwards diffstack model 63) and roughing pump. They are both in great condition and the roughing pump has been consistently working properly. The diffusion pump is off eBay, but I made sure it was working and in good condition. Today was the first time I ran the diffusion pump, and it seemed to be heating up properly. I had water circulation to cool it down and made sure to find an outlet with the right power (since I live in the U.S. and Edwards pumps need a British power supply). I made sure to use the right oil suggested in the manual (I’m pretty sure it is called ECO730 but I will double check).

However, the problem was that my pressure gauge gave off some weird readings that were completely unrealistic. It read something like 1.8x10^88 torr… I bought my hot cathode gauge and controller as calibrated, although this is my first time using it in almost a year. Unfortunately, I broke the original cathode gauge that came with the MKS controller and had to buy a new hot cathode gauge. The new gauge I bought uses a tungsten filament instead of an iridium filament. The manual for these gauges said both should be compatible with the same controller. Do you think this is solely a problem of calibration of the controller (has not been recalibrated in at least a year) or does it have to do with oil vapors from the diffusion pump coming into my cathode gauge and messing with my pressure measurements?

Does anyone know how to calibrate an MKS controller? It is common that diffusion pump oil (specifically ECO730) can produce vapors that effect the function of a hot cathode gauge placed directly above the diffusion pump?
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Granville Phillups Gauge.jpg
Pressure Gauge with Stain.jpg
Vacuum System.jpg

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Garrett Young
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Re: Pressure Gauge Problems

Post by Garrett Young » Mon Jan 02, 2017 4:53 pm

Are your pictures supposed to showing the gauge on or off, because the filament is definitely not conducting current.
- Garrett

John Futter
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Re: Pressure Gauge Problems

Post by John Futter » Mon Jan 02, 2017 7:31 pm

Jeremy
you have not connected the pump to the backing port of the diff pump.
the backing port is the one lower down on the diff pump. You seem to have the rotary pump connected to the roughing port.
secondly you have a lot of tunsten oxide in your new gauge head from not having a good enough vacuum before turning on the gauge, I hope the gauge head is still alright.

prestonbarrows
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Re: Pressure Gauge Problems

Post by prestonbarrows » Tue Jan 03, 2017 2:04 am

The grayish white on the inside of the gauge is a bad sign. That is basically the material of the filament being sputtered off and depositing on the inside of the tube. This will always happen at a very minuscule level under normal operation, but because greately acceleratred if the filament is run at high pressure (over 1E-3 Torr or so). The deposits in your pictures suggest the gauge is either very old with thousands of hours of use, or you tried to run it before the pressure was sufficiently low.

If those pictures were taken while the gauge was on, there definitely is a problem. A hot cathode gauge like that will be incandescent and look quite like a lightbulb when operating normally. Typically, the controller regulates the filament heating current to a level where the electron current being 'boiled off' is some calibrated value on the order of a few mA. So if you started up the controller and were getting strange readings and no visible glow, it is again likely that the pressure is too high and too much electron current was being drawn before the filament got up to full power. Alternatively, the filament could be broken; that can be checked by ohming out the filament pins with a meter.
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It's a bit tricky to see in your pictures, but it looks like the rough pump is not connected correctly. Diffusion pumps work by creating a fountain of sorts which physically pushes the gas back towards the bottom reservior. This means the roughing pump removing that gas needs to be physically connected here. Diff pumps also often have an additional elbow on the foreline holding a trap to minimize oil leaks and backstreaming. That is, the outlet port connecting to the rough pump is almost always oriented vertically not horizontally. The rough pump should be connected to the lower port which looks to be currently blanked off with a KF.

It is common to have a secondary port on the top inlet of a diff pump. Unfortunately this is sometimes called a 'rough port' or similar even though it is NOT meant to be attached to the rough pump. The idea is that a valve is fitted here that would be open when the system is first being pumped down from atmosphere (or 'roughed out'). This way the relatively large amount of gas in the system bypasses the diff pump instead of chugging through the oil. This is especially a problem for very large volume systems that have 100's or 1000's of liters of volume to pump out. Once the system pressure approaches the rough pump's ultimate pressure, this valve is closed and the diff pump is no longer bypassed.
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VAC035.gif (5.52 KiB) Viewed 2759 times
So to wrap up a long winded response. It looks like you connected the rough pump to the intake of the diff pump while its foreline was blanked off. This means your hot ion gauge was at whatever pressure your rough pump can achieve, almost certainly higher than 1E-3 Torr, when you turned it on. Connect the vacuum lines correctly and everything should work. Best case, the gauge's lifetime is shortened some but otherwise alright. Worst case, you burnt out the gauge filament and need to replace it.

Jeremy Adams
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Re: Pressure Gauge Problems

Post by Jeremy Adams » Tue Jan 03, 2017 2:45 am

Thank you for the advice, I was confused on where to connect the roughing pump and connected it to the "rough port" instead of where it should've been connected. The pictures of the pressure gauge were taken while it was turned off. It was running at to high of a pressure because of my incorrect roughing pump connection which must've caused it to get the sputtering on the side. Hopefully it has a lot of life left still

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Richard Hull
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Re: Pressure Gauge Problems

Post by Richard Hull » Wed Jan 04, 2017 5:08 am

Vacuum systems are simple, yet they do require careful and knowledgable gauging as gauges can be ruined in seconds if not under a specific vacuum when turned on. The hot cathode gauge is really great for pressures 10e-5 and deeper. You need to know you are there before turning it on. This means proper connection of the fore-ump to the diff pump with a good TC gauge in the foreline to assure that the system is sealed throughout, first.

To do this, open all valves all the way to the hot cathode gauge tube. Start the forepump. Watch the TC gauge plunge to well below 50 microns. At about 30 microns and hopefully well below, you should hold well and stable if the foreline and the diff pump and the hot cathode tube are all sealed well. If not, you have a major leak. FIX IT! never turn on the the diff pump or the hot cathode gauge until the foreline TC gauge is stable below 30 microns.

Once stable, you can turn on only the diff pump heater! Do not turn on the hot cathode gauge! Only after maybe 20-30 minutes of the diff pump heater working should you turn on the hot cathode gauge system.... PROVIDED....The TC gauge is still below 50 microns.

This should have been researched via reading and internet searches on how to operated a high vacuum system using a hot cathode gauge. If you don't do the research and go heavily pro-active in the area of self-directed learning you will burn through a lot of money and pay through grave disappointment.

The upshot is to never attempt to use a hot cathode gauge without first having a good TC gauge in the foreline.

Richrd 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: Pressure Gauge Problems

Post by Dennis P Brown » Sat Jan 14, 2017 4:32 pm

I would strongly urge you to study vacuum systems before operating them - not knowing how to operate a diffusion pump is not exactly a good start. These units are tough but burning the oil into a carbon based goo isn't helpful and that can occur if one is not careful .... . Vacuum technology looks easy but has many complexities and a learning curve like most devices in physics. Read the FAQ's here and ask questions when you are in doubt and about to start up a system. Could save you money and maybe even protect your health.

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