Erratic Thermocouple Behavior When Activated Under Vacuum
Posted: Sun Jan 15, 2023 11:39 pm
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!
Thanks!
Max E.
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!
Thanks!
Max E.