Type J Thermocouples through a 1756-IT6

I have 14 Type J thermocouples wired to 3 Controllogix 1756-IT6 modules. Originally all modules using the default configuration. All wired channels appeared to be approx. 40 Degrees low. This is based on the heaters being off and the extruder at room temperature for weeks (there are no manual temp gauges).

I've worked for SI's since the earlier 90's but I'm relatively new at the compact/controllogix platform and thermocouple modules in general. With my experience I've always seemed to get the projects on the legacy controllers as the youngsters don't know them. And when it has came to temperature I've mainly used RTD modules, which when setup right report a temp with one implied decimal. I would think the thermo module would be the same.

I can change the numbers in the scaling and get it close to what I think it should be. But, I don't know if it is linear or right when heated up. But I don't think this is a proper solution to a perceived module setup issue.

Anyway, I know this is a lot of babble. But any help in getting this right would be appreciated. Again, the only info the customer is giving me on these thermocouples is they are type J.

Check polarity. In the USA red is negative.

I'll check it. However, I know I've swapped it at the module terminal strip in the panel and the value was way off the estimated temp. I don't remember if it went way high or way low, but it made me think they must be wired right.

It would be very usual for the module to be off by that much.

Cold junction compensation installed?


I am not a Contrologix user but if you can post the program ( zip it first) someone can double check the configuration.

I don't own RSLogix 5000 standard, only the mini version which works only for CompactLogix not ControlLogix. So I'm using the customers laptop until I can afford to purchase standard. I'm not currently onsite. When I'm over there (either later today or tomorrow) I'll post a print screen of the module config.

Thanks for your help. I've viewed this site numerous times over the years after a google search and found the advice to questions helpful. Glad I signed up.

I don't know anything about CJC or if it is installed. I've checked the module config checkbox for CJC and I don't remember how that affected the temp reading for the channel but it didn't make me think that's what is wrong.

Are there connections between the thermocouple and the module. If so are they matched to the thermocouple type. This can give a different reading.
Can you connect the thermocouple directly to the module to see if the reading becomes closer to say ambient.

From the module t-strip there is thermocouple cable to 2 1492-W? in the PLC control panel. Then field wiring approx. 25 to 50 feet to the thermocouples on the extruder. I personally didn't do the field wiring and I would have to trace one out to see it there are any other terminals on thermocouples at the extruder. I'll verify this as well when I'm onsite.

OneManShow said:

I'll check it. However, I know I've swapped it at the module terminal strip in the panel and the value was way off the estimated temp. I don't remember if it went way high or way low, but it made me think they must be wired right.

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Check the polarity on both ends. It is a very common problem for field electricians to assume that the red wire is positive. The wire must be connected with the correct polarity on both ends, or you'll see the kind of erroneous readings that you are describing.

Thanks, I'll check a couple of them in the morning.

I agree that it has to be in the wiring or the module config. It has been my experience with temperature devices and PLC modules that if the module is configured right and the device is wired properly you get the correct temp in the PLC data. If not you don't. However, since it seems all channels are off approx. the same amount I thought maybe it was something with thermocouples devices I don't know about.

OneManShow said:

Thanks, I'll check a couple of them in the morning.

I agree that it has to be in the wiring or the module config. It has been my experience with temperature devices and PLC modules that if the module is configured right and the device is wired properly you get the correct temp in the PLC data. If not you don't. However, since it seems all channels are off approx. the same amount I thought maybe it was something with thermocouples devices I don't know about.

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You should know that thermocouples can fail over time and return erroneous data, especially if they are exposed to very high temperatures or corrosive environments. The fact that all of your TCs appear to have the same margin of error leads me to believe that it is the result of either a wiring or configuration error.

I don't believe there are any 1492-W thermocouple terminals. For J type thermocouples you would need 1492-JTC3J terminals, one for each thermocouple.

Also be sure the compensation thermistors are attached to the RTB terminals on the module. If they are not there, the module will have to be reset after you add them. You can reset it from the module properties dialog box, or pull the module out of the chassis and reseat it.

Listen to Mickey - 40 degrees F is about what a cold junction sensor would add to the thermocouple reading at 72 Deg F ambient.

It's easy to check if the thermocouple wires are backwards, apply a little heat and the indicated temperature will go down, not up. At ambient, a backwards connected thermocouple would give the same signal as a properly connected thermocouple.

But 40 degrees low at ambient temperature is a VERY likely a cold junction issue.

There might be a separate cold junction sensor that needs to be installed, or a configuration check mark to enable 'internal CJ' or something similar.

Check the CJC sensor on the terminal strip first. Then hook a calibrator up to the module and stop guessing about the thermocouple/wire condition for now.