Erratic Thermocouple Behavior When Activated Under Vacuum

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Posts: 48
Joined: Mon Jul 19, 2021 11:08 pm
Real name: Maxwell Epstein
Location: North Carolina

Erratic Thermocouple Behavior When Activated Under Vacuum

Post by Maxwell_Epstein »

Hello All!

I apologize if this is common knowledge to those with more experience, however, I've encountered a really strange issue with my Varian 531 thermocouple and 804-a controller that I was hoping someone with more experience operating them could shed some light on.

I have recently rebuilt my system with a new turbopump and am currently in the process of leak chasing. Last night, I went to bed with my foreline vented, but the chamber at about 200 microns of pressure. As part of shutting down the system, I deactivated the 804-a thermocouple controller. My leak rate at the moment is enough that I anticipated leaking back to atmosphere or very close to atmosphere overnight. This morning, when I began another test, I turned on the controller and saw that it indeed showed atmosphere (although the needle was closer to the edge of the atmosphere range than usual). Without venting the chamber back to atmosphere (to avoid introducing unwanted moisture) I turned on my Precision D-25 roughing pump to keep working. However, after five or so minutes of pumping, my gauge plateaued at about 300 microns, which is unusually high.

On a whim, I turned the thermocouple controller off and back on again (using the dial on the front, not doing a hard cutoff of its power from the wall). When I did this, the gauge very quickly dropped below 100 microns. At this point, I tentatively activated my turbopump and the gauge immediately began dropping further as expected before going haywire and rapidly swinging from well below 0 microns to atmosphere and back and forth. The turbopump also sounded like it usually does at 200 microns rather than how it sounds when running at 10 microns (blades spinning slower and with more resistance sound slightly different). I immediately shut everything off, unplugged the thermocouple tube from the gauge, and vented every part of my system to atmosphere. I powered the gauge back on at atmosphere and it has behaved normally since.

I am very confused, as I have not seen anything here or elsewhere to suggest that thermocouple tubes must be activated at atmosphere before operation. Is this common knowledge and I am just uninformed? What part of the way thermocouples work makes this the case? I imagine it must have something to do with filament heating, but that's really just a guess. Any guidance or explanation would be appreciated!

Max E.
I'm a high school student and aspiring fusioneer
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Liam David
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Joined: Sat Jan 25, 2014 5:30 pm
Real name: Liam David
Location: Arizona

Re: Erratic Thermocouple Behavior When Activated Under Vacuum

Post by Liam David »

I've never worked with thermocouple gauges directly, but my first guess is contamination. Second is a problem with the electronics. Have you flushed the gauge with IPA?
John Futter
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Real name: John Futter

Re: Erratic Thermocouple Behavior When Activated Under Vacuum

Post by John Futter »

Why go back to atmosphere
the longer under vacuum the cleaner the gauge
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Richard Hull
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Real name: Richard Hull

Re: Erratic Thermocouple Behavior When Activated Under Vacuum

Post by Richard Hull »

The TC gauge is typically used rather close to the head of the mechanical pump. Many amateur TC gauges have no isolation valve from the pump to the foreline. Amateur fusioneers are not use to valving a system properly with quality bellows valves. If the foreline is not kept under vacuum where the only proper valve is an isolation valve just at the secondary pump outlet to allow the secondary pump and the chamber to be kept under vacuum, the TC gauge can collect hot oil vapor over time. Often, the foreline, the TC gauge and the mechanical pump are let up to air at every shutdown.

In these latter cases, even though the rest of the system is held under vacuum, the TC gauge can get contamination and is regularly brought up to air.
Professional systems at the high end are pretty much 24 hour 7 day a week running with isolation valves festooned all over the system. Such systems will not suffer long down times due to failure to supply a bunch of $600 bellows valves all over the system to allow for servicing sections. This avoids long bake outs and leaves critical parts of the system under vacuum.

In short, if you can isolate a TC gauge with a good valve in your system, do so. Also, if possible, move the TC gauge tube as far away from the mechanical pump as possible while still trying to keep the foreline as short as possible.

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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