1794-IT8 for termocouple K

matatata

Member
Join Date
Aug 2017
Location
Croatia
Posts
32
Hello


Im using module 1794-IT8 to read values from termocouple type K. I configured the module to read this type of termocouple and i still get different temperatures than i should get, i get -5 for around 30 degrees.



Im not using cold junction and i would like to know, how can i compensate this with software? Is it even possible to compensate cold junction using software?


Regards
 
If the T/C is connected backwards a hot junction 'hot' voltage output is seen as 'negative' and will indicated temperature will be drive downwards as the T/C is heated. If the T/C hot junction is at the same temperature as the terminal connections then the indicated temperature is whatever the cold junction is.


Generally not have CJ enable indicates a lower temperature than expected, the difference being the difference between the AI terminal temperature and the freezing point of water.



Cold junction should be measured because any difference between a software 'offset' and the true cold junction (CJ) (difference between the temperature of the input connection terminals and the freezing point of water) is a direct error.


I've only run into one AI card that did not have a factory cold junction installed on it in the last 30 years, so chances are if your input is designed for thermocouple inputs, then it has a CJ sensor and circuitry on it. But it has to be enabled.



Sometimes there is a choice internal or external CJ. The factory CJ is always 'internal'.
 
If the T/C is connected backwards a hot junction 'hot' voltage output is seen as 'negative' and will indicated temperature will be drive downwards as the T/C is heated. If the T/C hot junction is at the same temperature as the terminal connections then the indicated temperature is whatever the cold junction is.


Generally not have CJ enable indicates a lower temperature than expected, the difference being the difference between the AI terminal temperature and the freezing point of water.



Cold junction should be measured because any difference between a software 'offset' and the true cold junction (CJ) (difference between the temperature of the input connection terminals and the freezing point of water) is a direct error.


I've only run into one AI card that did not have a factory cold junction installed on it in the last 30 years, so chances are if your input is designed for thermocouple inputs, then it has a CJ sensor and circuitry on it. But it has to be enabled.



Sometimes there is a choice internal or external CJ. The factory CJ is always 'internal'.


can i add temperature measured at ai module to measured temperature to compensate it that way? using software
 
The right way to compensate the CJ temperature is adding the millivolt corresponding to the CJ temperature to the millivolts of the hot junction.
Adding degrees instead voltage produces some error, a small one for K thermocouple due it has a mV/temperature curve pretty linear, but a big one for other TC types.
 
hey dan, i know that it comes with CJ and i should use it, but i simply dont have it and i must work with what i've got...
 
I am not convinced the lack of a CJC sensor will give you that big of an error. But if so you might try just adding 35 deg to the value the card gives you (add instruction in logic) and see if that works as you. Test it through your operating range.

As others have said check polarity.

Edit:You need to order the CJC sensor if you want accuracy.
 
Last edited:

Similar Topics

Just commissioned two new skids for a customer. Each has a 1794 chassis over Ethernet back to an L81. The second module of each chassis is a...
Replies
8
Views
2,334
So this is a first time I've had no choice but to use Flex I/O with anything RSLogix 5000. I've got everything up and running, but the values I...
Replies
2
Views
1,956
We are getting ready to do a small project with AB Flex I/O and intend on using a 1794-IT8 module to measure some temperatures. We are using a...
Replies
1
Views
2,341
Installing a IR T/C onto a DeviceNet Flex IO drop. Card is a thermocouple card 1794-IT8/A. According to the manufacturer, I'm getting leakage...
Replies
0
Views
1,873
I have two 1794-IT8/A modules on ethernet. They are not reading the correct temperature. According to the user manual, modules on a Flex I/O...
Replies
6
Views
8,339
Back
Top Bottom