Pirani gauge vs plasma

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Maciek Szymanski
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Pirani gauge vs plasma

Post by Maciek Szymanski »

I’ve a question to those with pirani gauges experience. For the demo fusor at work I’ve used the thermoelectric gauges but in my garage setup (viewtopic.php?f=10&t=13708) I’m using the double head pirani. Today I’ve made some test discharges at low vacuum (1e-2 to 5e-5 Torr) and noticed that during the discharge, especially at currents above 20mA the pressure indication gradually rises. It is not visible in the visual character of the discharge nor in the discharge current. Turning of the HV causes the pressure indication to fall to the previous value. A slight opening of the control valve immediately rises the current and changes the discharge appearance. So it seems to be the problem with indication, not the vacuum itself. Additional grounding of the gauge controller didn’t changed anything.
Has anybody experienced this behavior with piranis? Is it normal? The TC gauges do not show this kind of behavior.
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Richard Hull
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Re: Pirani gauge vs plasma

Post by Richard Hull »

I could see this on the low end of the scale (10e-5) but not on the high end (10e-2). At any current over zero ma you are heating the gas. More current means more pressure. Gas pressure is sensitive to temperature and will increase as more current heats any gas at any pressure. I am not an expert on pirani gauges. I have noted that at a fixed current and pressure, the pressure will drop as the wall tends to absorb some gas. This is normal over time as I have had to up the gas flow to maintain a fixed, established pressure.

Richard Hull
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John Futter
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Re: Pirani gauge vs plasma

Post by John Futter »

Maciek
It is important that the pirani cannot see the plasma.
active ions arriving will cause heat. The actual exposed thermocouple is extremely small with very low heat capacity.
a couple of righthand bends to isolate the Pirani from the main chamber should sort it
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Richard Hull
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Re: Pirani gauge vs plasma

Post by Richard Hull »

John is correct. Regardless of gauge type I have always allowed for a right angle in my connection of a gauge in a device where a plasma is present. Ionized gas environments should not have a straight path into a gauge opening. What's worse, beaming into gauge tubes can really kill the tube, much less just give an incorrect reading. I attach a fusor V image during assembly and testing showing my right angle to protect my $1000 capacitive barotron style gauge. I lucked onto this rare 1 torr gauge at a hamfest for $50. I don't think I could even hope to be so lucky again so I protect it well.

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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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Maciek Szymanski
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Re: Pirani gauge vs plasma

Post by Maciek Szymanski »

I don’t think that plasma can directly enter the pirani gauge. My setup looks roughly like this:


A5EDC260-CD0A-42CB-BDD9-BE62788C8C49.jpeg

The pirani gauge is mounted quite far form the grid on the KF25 side port. The gauge itself has quite small and log aperture. I’ve the second pirani mounted on the vacuum canister at the roughing one of the diff pump. I’ve tried pumping with the gate valve open through the diff pump and the second gauge behaved almost the same like the first one (a bit smaller rise of the indication). So I think it’s the mater of gas heating by the discharge.
Maybe I should put a grounded metal mesh on the chamber - gate valve interface?
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Richard Hull
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Re: Pirani gauge vs plasma

Post by Richard Hull »

I agree it is right angle mounted. It looks a bit close to the action area. A 50mm KF to KF extension fitting would move the pirani a bit more out of the environment, perhaps.

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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