Anyone have experience troubleshooting a thermocouple ground loop. I installed a surface mount thermocouple on a motor surface to monitor overall temperature. I coupled it to a basic Red Lion readout and it worked fine.
About two weeks into its life, it started acting strange. The temperature reading started going from positive to negative and back again. I replaced the thermocouple with a new one and it did the same thing.
I then replaced the Red Lion readout with another one I had in stock. The problem remained. So i don't understand but it seems as though a ground loop has been created or something. I never heard of thermocouples getting noise.
Anyone have any thoughts on how to troubleshoot a thermocouple ground loop?
I think a thermocouple ground loop is more possible than noise. A thermocouple generates a small DC voltage as it heats up and I'm not sure how it could be a noisy signal. I'm assuming the thermocouple is grounded to the surface?
To be more specific, I am using a Red Lion CUB5TC Thermocouple Meter with an optional single relay card inserted to trigger a fault on the plc. The thermocouple is a Type J with a #10 eye hole that I screwed to the surface of the motor. I am trying to create a redundant temperature monitor for this motor since it is located in an out-of-the-way place.
The thermocouple is located about 50 feet from the Red Lion meter and this setup worked for a while before i started having problems. Not sure where this is going but I just thought it might be a grounding issue. ???
OK, here's an update... If I unscrew the thermocouple from the motor and hang it in mid-air, it reads the air temperature just fine. As soon as I touch it back to the metal surface of the motor, the reading goes from positive to negative and then bounces back and forth like it's confused.
Try isloating the thermocouple from ground. Get creative with some kind of insulator but still contacting the surface of the motor and see what you get.
Cheller,
That did it, it is reading temperature accurately now. Here's what i came up with:
(http://www.obsoleteindustrial.com/roundtable/gallery/45_04_02_16_4_24_04.jpeg)
I used a magnet with a metric threaded hole that i got from McMaster for another application. I screwed the thermocouple to the magnet and I coated the bottom of the magnet with electrical tape. I also covered the braided wire with heat shrink tubing and it works great.
I'm still not sure why it worked for a while, but the probelm seems to be resolved. Thanks!
In closing, here's a couple of parts numbers if you were looking to duplicate this deign: I got the Red Lion CUB5T from our local Red Lion distributor but the rest of the parts came from mcmaster.com.
- Magnet with M4 mounting hole: 5819K52
- Type J thermocouple with a 3/16 hole: 3648K22
- Clear Heat shrink: 7586K14
Good work on the thermocouple tests...
(https://www.obsoleteindustrial.com/i24/thermocouple-fault-test.png)