We are using IC200630ALG Thermocouple input module to measure process temperatures at our plant. I want to create the thermocouple cold junction away from the IC200630ALG module. I have two questions regarding this implementation.

1) How can I exactly implement a remote compensation when I select the "Remote" option on the module for Cold Junction Compensation?

2) What if I select "local" option on the module for Cold Junction Compensation and connect thermistor (physically located near the remote thermocouple cold junction) to the IC200AlG630 via a single pair copper cable?

 

Ahsan Ejaz,

What do the manufacturer's instructions for configuring the module say?

 

Ahsan Ejaz,

Are you certain the thermocouple extension wire is properly terminated along the entire length of the circuit between the PLC module and the thermocouple? Have you observed the proper T/C termination practices?

Because, if you have, it's hard to understand (without a lot more information about the application and the site) how it's possible to have a situation where remote CJ compensation would be required.

Improper terminations in a T/C circuit can cause a lot of error in the signal--and simply correcting them removes almost all error.

omega.com has some very good information about T/C termination, and this topic has been covered many times before on control.com. It would be helpful if you could describe the terminal boards/blocks in use and how the wires are terminated. Proper T/C terminations virtually remove the effects of cold junctions, and, again, without understanding your application and need for accuracy and the error you are experiencing and how long the error has been existing (is this something that just started, or has it been ongoing for some time?) there's not a lot we can do. And, again--to answer your specific question I'm sure the configuration information in the instructions provided with/for the module will explain what's required or possible for remote CJ compensation for the module.

If you're not 100% certain the wiring is correct, that's the first place to start. (And if you are certain--you've personally been over all the terminations and know they are good and correct--then that would also have been helpful information. Along with the error being experienced, and the application and the need for accuracy, etc.)

There is a 'Search' field cleverly hidden at the far right of the Menu bar at the to of every control.com page. The context of search terms isn't intuitive (use the Search 'Help') but it is a fast search, and if you're patient and try several combinations of words and terms you can usually find what you're looking for. Again, the topic of proper T/C terminations has been covered before on control.com.

So, if you can provide more information about the situation at your site and what the needs are and what you've done to try to resolve the problem and what the results were--perhaps we can be of more help.

 

The spec sheet for the GE Versamax IC200ALG630 T/C card here,

http://tinyurl.com/hptj9vh

on page 5 under cold junction compensation says,

Cold Junction Compensation: The Thermocouple module provides four choices for Cold Junction Compensation.

- No Cold Junction Compensation: This is used for millivolt inputs or if cold junction is maintained at 0 degrees C.

- Remote Cold Junction Compensation: With this option, cold junction is measured externally and provided to the module from the application, via the module's analog output (word output) data. If the module has multiple thermocouples that are configured for remote compensation, the same compensation value must be used by each.

- Fixed Cold Junction Compensation: This option uses a fixed compensation value provided by the user in the configuration parameter, "Reference Junction Value." The units of this fixed value are defined in configuration parameter "Engineering Units."

- Local Cold Junction Compensation: The best way to provide local compensation is with an Interposing Thermocouple Carrier, which has a builtin thermistor. Using an Interposing Carrier allows the thermocouple connections to be placed farther away from the I/O modules in the system, which helps shield thermocouple connections from module heat.

If Local Cold Junction Compensation is configured and an Interposing Thermocouple Carrier is not used, a separate thermistor must be installed directly at the module's I/O Carrier, using the Thermistor (+) and Thermistor (-) terminals. The thermistor must be the type specified in the I/O Modules User's Manual. Note: If Local Compensation is selected but an Interposing Thermocouple Carrier or local thermistor is not used, erroneous temperatures may be reported and a thermistor error will be reported in the fault table
----- end -----

That seems pretty clear. You can

- select "No Cold Junction" and maintain an ice bath, put a 2nd T/C in th icebath, wire it in series with the process T/C to the T/C terminals or

- select "Remote Cold Junction Compensation" and use a thermistor wired with copper wire to the thermistor connections on the T/C terminal block and and hope that the placement of your thermistor accurately reflects the temperature of T/C termination connections.

 

CSA,

I want to connect the thermocouple to a copper wire pair in a JB and connect that copper wire to the IC200ALG630 thermocouple module.
The reference junction is thus on JB side.

I want to use this module to measure the correct temperature with this scheme.

Hope this clarifies the situation.

 

Dear David,

Thanks for your reply.

Well I hope this works.
But I'm confused by the statement mentioned in the quotes below:

Remote Cold Junction Compensation: With this option, cold junction is measured externally and provided to the module from the application, "via the module's analog output (word output) data". If the module has multiple thermocouples that are configured for remote compensation, the same compensation value must be used by each.

The module is a thermocouple input module. How come it can give output?

 

I only quoted GE's spec. I'm not a GE guy, so the interpretation partially mystifies me, too.

However, I've done remote thermocouple CJ with a Honeywell HC-900 PAC. Honeywell's method of using a remote sensor might have some similarity to GE's method, although Honeywell uses a 100 ohm Pt RTD as the cold junction sensor, wired to its own AI, not a thermistor.

In the case I was involved with, the T/C's were terminated in a remote junction box, wired with 50' of STP copper wire cable to the rack mounted T/C input module inputs. A very thin RTD element (1/32"?) was fastened into a terminal block screw connection, to sense the terminal block temperature.

Each field thermocouple (rack/module/point) was mapped to an RAI function block for thermocouple remote cold junction sensing and compensation. The RAI block reads the T/C value from its mapped AI, and uses a separate analog input tag (RCJ) value from the 100ohm Pt RTD wired to a conventional, separate analog input (AI's are universal, T/C, RTD, 4-20mA). The RAI function block calculates and outputs the process temperature from the analog input value and the RTD CJ value, doing 'remote CJ compensation'.

The details on the RAI block are here:
https://s32.postimg.org/ovp2f24ph/RAI_Analog_Input_with_remote_T_C_CJ_FB.jpg
The setup for the RAI block is here:
https://s32.postimg.org/m7lq6fv7p/RAI_setup_selections.jpg

The point being that any T/C measurement requires 2 temperature measurements, the process and the cold junction measurement. It wouldn't surprise me if the GE's built-in thermistor temperature value was available in the 'system' since the thermistor signal has to be converted and then used in the temperature calculation. So the 'system' might well report the CJ value for each thermistor which covers any given T/C module. Since the thermistor is an 'input' to the T/C module, its reported CJ value would be an 'output' from the T/C module.

If you use the T/C module where the thermistor connects, I would think that you could remote mount and wire a remote CJ thermistor to that module (assuming T/Cs are remote mounted, too). But I don't know how 'free-standing' these modules are.

The 4th choice in the GE spec sheet (re-quoted below) infers that some kind of different hardware, an "interposing carrier", allows for the use of a built-in CJ thermistor, which sounds like GE's way of remote T/C termination. But I'm not familiar with the differences between "interposing carriers" and "modular I/O carriers".

4th choice or CJC:
Local Cold Junction Compensation: The best way to provide local compensation is with an Interposing thermocouple Carrier, which has a built-in thermistor. Using an Interposing Carrier allows the thermocouple connections to be placed farther away from the I/O modules in the system, which helps shield thermocouple connections from module heat.