F2-04THM thermocouple card

I have a AD F2-02THM thermocouple card and I'm getting a 12 to 15 degree temp. rise after power up. When I remove power and immediately power up, the temp. stays high. If I let the controller remain off for say an hour, The temps are at the correct temp and then rise again. The temp. rise is gradual. I compare the temp. to a regular mercury thermometer to use as a reference. I'm using type J ungrounded thermocoples connected to chan.1. The jumper settings on the card have been triple checked and are correct. I am reading the temp.in Data view in the memory location assigned to that channel. All unused inputs are grounded. There is no potential differance between neutral and ground. Can't figure this out. Any input would be appreciated. Thanks.

The grounded T/C may be a problem. Can you remove the T/C from the vessel, and test the response with it floating - not touching any metal?

It's an ungrounded T.C. (floating).

Did you splice the TC wire at any point?
Can you twist the TC wire together and stick it where the TC was to test that its not somehow a bad TC?
Does the PLC rack have adequate ventilation?
Is the 24v to the TC card really 24v, and does it stay at 24v?

The T.C. is connected to a terminal mounted under the PLC.I am gonna connect the T.C. directly to the T.C.card term. strip. I duplicated this setup at home and it does the same thing. The PLC is mounted on a board in open air so ventilation isn't an issue. The PLC, T.C. card and T.C. are spares so it's not the components. The temp. does drop to the correct level when I blow on the T.C. card with compressed air. The voltage is at 24.01vdc.

Sounds like a bad TC card to me.

rubber ken said:

The T.C. is connected to a terminal mounted under the PLC.

Click to expand...

Unless the terminal is made specifically for the type of thermocouple that you are using, then that is where the problem lies. A differing type of metal will create another thermocouple junction causing all sorts of sporadic readings as the temperature of the terminal changes.

That is what the problem was. I thought I could get a good reliable connection but apparently not. I used this same setup before and not had any problems so I overlooked the connection initially. The deeper I looked the more I realized that was the problem. Thanks guys.