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Faulty temp readings on Allen Bradley j-type thermocouple
Thermocouples temperature fluctuation

All the thermocouples readings are changing with the control room temperature. Cold junction compensators are installed. There are no wiring issues that I can find and cold junction compensators are working properly. As I increase temperature in the control room all thermocuple readings are going up and with the AC on all readings are going down. Can you please help?

Thanks

Many T/C input cards have a more than one configuration choice of how and where the CJ is enabled:

- internal (or similar term) meaning the CJ device on the T/C card

- external 0C; for that rare lab that actually does a CJ ice bath

- external input reference, for an external dedicated CJ compensator

- external at specified temperature, for situation where a fixed temperature is available to reference CJ to.

I suspect your configuration of the T/C card overlooks how the CJ is executed and it isn't your CJ sensor on the card.

By curt wuollet on 15 July, 2010 - 1:01 am

Using compensators with inputs that already have compensators? Wrong type thermocouples for the compensators or vice versa? Wrong type of extension wires? Wrong connectors? Uncompensated junctions? Using regular din rail connectors is a favorite. For a test, I'd take the simplest possible case and hook up a short thermocouple locally and see how things behave. But, there are so many ways to screw up TC setups it would help to know a little bit more. But, unless your actual TC wire goes directly from the hot junction to the input, connections are always suspect. The setup measures _every_ junction in the wire.

Regards
cww

By Bob Gantt on 15 July, 2010 - 4:10 pm

> All the thermocouples readings are changing with the control room temperature. <

Are you by any chance using FlexIO 1794-IT8 thermocouple modules? Because they do have a recognized problem with cold junction compensation, we had to switch to 1794-IRT8 modules to fix the issue.

We are currently using 1794-IRT8 modules. Let me know if there is any fix for these modules.

Thank you

By Keith Muchnick on 23 September, 2010 - 3:05 pm

Hi,

We are having a similar issue. we are using 1794-IT8 thermocouple input cards. we have replaced all of the T/Cc with isolated probes, installed new cable with no terminations and only tied the shield to the I/O module end of cable (it is connected to a dedicated shield terminal strip that is tied to ground) and not to the module. I have also ensured that the module configuration is set up to accommodate the proper T/C (J).We do have 1 input that goes to 1 of the 3 cards that has 480kohms to ground but we cannot replace it at this time. When the problem first occurred our tempts were erratic and read higher than they should. After replacing the T/C's our temperatures are reading lower than they should. It seems once we start turning on AC loads the temperature drop off. Any help would be appreciated!!

Regards,
Keith

Hi,

We are having a similar issue. we are using 1794-IT8 thermocouple input cards.we found that it was the case due to low noise interference caused due to vfd signals running within the same panel,we applied peprl fusch signal isolators to sensitive lines.the problem was solved. also driving current of module from source was restored.

Regards,
pritesh

Run away from those flexIO TC modules. We switched from IT8s to IRT8s, much better but still not as reliable as the good old PLC5 IXHRs or the controllogix ones. I have also seen similar noisy readings in compactlogix TC readings also, but in that case I'm not totally sure if it is a module problem.

Allen Bradley should really advise everyone that those modules are not suitable for normal industrial environments.

1 out of 1 members thought this post was helpful...

We are also facing similar by using IT8 modules. Can our problem solved if we use IRT8? we have used isolator from P&F but again issue is same only in our case T type thermocouple is used.

>Run away from those flexIO TC modules. We switched from
>IT8s to IRT8s, much better but still not as reliable as the
>good old PLC5 IXHRs or the controllogix ones. I have also
>seen similar noisy readings in compactlogix TC readings
>also, but in that case I'm not totally sure if it is a
>module problem.

Hello,

When we took trials with same hardware including cable, connectors on different control system (Siemens/Honeywell/eurotherm controller/wika indicator), we find all readings are within acceptable limits but same is not getting on rockwell 1794-it8 module. If anybody faced similar issues and knows the solution, kindly guide. Here we have used belden shielded single strand cable. but if we use another non shielded multi strand cable rockwell hardware working fine.

Suggest you ditch the thermocouple modules for std analog and use locally mounted or head mounted 4-20 transmitters at the probe location. That usually works better for me.

Interesting similar situation....We found just the opposite...had a bunch of RTD's with 4-20ma converters in the heads which all failed one after another due to ingress of humidity. Switched to straight RTD's into an RTD module and never had any more problems. Maybe we just had bad modules? I would expect the same for TC's.

Gerald Beaudoin

Hmmm. [shrug] Horses for courses I guess.