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TC outputs all reading ZERO
"J" Thermocouples all reading 0.0C. for process variable.
By David A. Nickey on 28 April, 2004 - 5:32 pm

TCs connected to ADU257 module on Modicom PLC all are reading Zero. Software is IN Touch - WindowViewer; Observations: TCs pluged in; All LED lights indicate normal on PLC, processor, etc.; power supplies okay; fuses okay on PLC; Re-booted PC; If you disconnect a TC, it still reads 0.0C. (should read 409.5 indicating an open TC); Loop controlers work okay on PLC.

Any ideas what is going on?

(Note: I'm out of my field - mine is chemistry!)

Thank you,
David A. Nickey

I ask you, Have you checked the thermocouple itself ?. It seems the thermocouple already broken (inner circuit was disconected). The characteristic of thermocouple is if its circuit already disconected, indication will zero because it give no voltage signal output. 0 mV is converted as 0.0 degC.

If this case occured, the indication in controller (or PLC) will not always zero. It depends a mode selected in controller, Burn Up or Burn Down mode. In mode is Burn Up, if thermocouple broken, indication will change to highest scale in controller, but if mode selected is Burn Down, indication will change to lowest scale (0.0 degC). My suspect are your thermocouple already broken and the mode selected in PLS is Burn Down, so indication is zero.

So, if you are sure that your PLC still ok:
a. please check and calibrate first your thermocouple.
b. please check your mode selected in PLC, Burn Up or Burn Down.

I hope it will useful.

Best regards from Indonesia


By Matthew Hyatt on 30 April, 2004 - 11:17 pm

Do you need a consultant? I would get a simulator (TC type) and see if that provides a indication at the readout - which is where?

I would check the software, the database, the data being sent from the PLC to the InTouch software - look i n the database, you should be able to see the scaled TC data in the InTouch database, if it is not there, it is not getting sent by the PLC, check the PLC, is the data correct in the code for the analog input?