Troubleshooting a Thermocouple Ground Loop

[mike1999]

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?

[Cheller]

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?

[mike1999]

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.

[mike1999]

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.

[Cheller]

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.

[mike1999]

Cheller,
That did it, it is reading temperature accurately now. Here's what i came up with:



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!

[mike1999]

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

[benklassne]

Good work on the thermocouple tests...