GE PLC temp loop programming help

Hi all,

We currently have GE PLC's for some machinery that uses either Versapro or ME. In the programming we have some temperature PID loops that controll burners for our gas ovens. The process temps are displayed on an HMI screen on the machine. MY question is this. We used a calibrated thermocouple simulator the other day and plugged in to see how far off the HMI temp showed compared to the simulator. we dialed to readings in one at 100 deg. f and then did at 500 deg. f.. We noticed after stabilizing that the display was off anywhere from 2 degrees to as much as 20 degrees on some of the machines. Question is why would the readings vary so greatly. Second, how would you correct this within the PLC programming. Not sure where you would put the offset in. And third, if you do put an offset in to correct for the display, is that going to throw all your PID loops out of whack therefore needing to retune the loops again. Thanks for the help.

I would start by checking to make sure your thermocouple module is configured to match the thermocouple type you're simulating.

Presumably the same meter you're using to simulate the thermocouple signal can also read the signal from the actual thermocouple. Try injecting the same temperature you read from the field device into the PLC module to see if the HMI displays the same number. Any difference there would point to scaling issues in the PLC program or HMI. Thermocouple modules in GE PLCs report data in fractions of a degree, so the scaling is pretty simple. For example, if the module reports tenths of a degree, then the raw data only gets divided by ten.

What model PLC and thermocouple module?

Rather than try to do anything in programming, I would focus on figuring out the real problem and trying to correct that.

We are using the 90-30 with cpu363. The thermo module is a Horner HE 693THM888. This is new to me so not sure how to check the config. for the module. When I go into ME and go to this module and click on it it only shows reference adressing and nothing else. Does this config. reside on the module itself that I have to access and if so how. Also, our simulator doesn't read the thermocouple at all it just out puts the mV signal depending what temp you dial it to. That is what we did and now we are reading up to the 20 deg. difference and wondering why. These machines have been in use for years and we never conducted this test so this is where the flag was raised when we did it

Do you get to select what thermocouple type the simulator is set for? If not, do you know what type it is supposed to simulate?

In hardware configuration in ME, you show the module as "3rd Party". If I remember correctly, bytes 5 - 12 define the thermocouple type for each channel. I think default setting is for Type J. You can get the data sheet from Horner's website, www.heapg.com