CompactLogix L24ER thermocouple issues

[IamCool]

Most of the time don't need to post anything because I always find an answer I need here. But, this problem is driving me crazy and I need help. I have a L24ER with programable analog inputs. I can't get the PLC to read the thermocouple type J correctly, 40 or 20 degrees off. I did try every setting and the best and closest results I have gotten is when I ground the thermocouple housing 20 degrees off. I am in testing phase of a rebuild project and my wire is only 3 feet long, I can't think I am getting that much noise from it.

I was thinking of putting an offset in the HMI to be programed then do an add in ladder code. It controls a PID for my burner modulator in an oven.

Any ideas or a better way of fixing this?

1769-L24ER-QBFC1B
v.24.13

 

[widelto]

Cold Junction Compensation
The CompactLogix 5370 L2 controllers require the use of the 1769-CJC CompactLogix CJC Sensor when the embedded analog input of the controller is configured for Thermocouple mode.

 

[IamCool]

I am using a

1769-L24ER-QBFC1B
Firm. v.24.13

 

[MikeyN]

What widelto has said is correct for that processor. It's almost as if he took it straight from the specification sheet: https://literature.rockwellautomation.com/idc/groups/literature/documents/td/1769-td005_-en-p.pdf. The manual goes into more detail about this, including wiring diagrams on page 188: https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1769-um021_-en-p.pdf. Pay attention to the important note underneath of figure 29.

 

[widelto]

You´ve got to get this part:
1769-CJC CompactLogix CJC Sensor

 

[IamCool]

Thank you guys, I will have to order this part. It just frustrates me this was not on the parts list, on the print, or ever installed on this panel. Now I know why they were having cooking temperature issues for the last 5 years.

 

[IamCool]

I have installed 1769-CJC and now I am 25 Degrees above. I have tried every combination of option some of them raise the temperature but the closest I can get is 25 degrees above.

 

[danw]

I was thinking of putting an offset in the HMI to be programed then do an add in ladder code.

Did you do that and the offset is still resident?

 

[IamCool]

I put an adjustable offset on the HMI and I can get it in range not sure why I am so high now with the CJC in it now.

 

[Steve Bailey]

Are you using the correct thermocouple wire from the device all the way to the PLC module? Incorrect wire would introduce an offset.

 

[Robb B]

Any splices in the thermocouple wires must be rated for those thermocouples. If you use a terminal block that is not thermocouple rated, put both wires twisted together in the same side of the terminal.

 

[IamCool]

Using a type J and I have the wires straight into the PLC no splices. I am in testing phase right now everything is mounted in the enclosure.

 

[arpus4KM]

Any splices in the thermocouple wires must be rated for those thermocouples. If you use a terminal block that is not thermocouple rated, put both wires twisted together in the same side of the terminal.

This, and don't forget to clean the wires up before putting them together.

Had an issue where a customer had a heating unit that wouldn't heat, and then would runaway with the heat. Everything checked out while using alligator clamps with a meter(the teeth dug in), etc, except the wires had a thin film of something from the process coating it and everything which was messing with the expected mV reading from it.

a quick swipe with a screwdriver on the exposed wire and it was reading good again.

 

[Mickey]

Polarity? Red is always negative.

 

[IamCool]

I'm going to see when it gets installed and see what I get for reading. I put an offset into the program so I can fine tune it.