Weird thermocouple reading behavior

[hagaib]

Hey all,

i have a friend with an application where he has a "black box" which is comprised of a heater and a thermocouple (4 wires coming out in total).

he's using a B&R digital output module to turn the heater on (i.e. give it 24V) and a thermocouple module to read the temperature from the TC sensor.

with one of these "black boxes" he has encountered a strange phenomenon - once the reading from the thermocouple reaches a certain temperature (and the digital output giving power to the heater from that same "black box" is still on), instead of climbing, the temperature reading begins to drop. Once the output from that same "black box" is turned off - the temperature reading jumps to what it was supposed to show if it hadn't dropped.

other than a defective sensor ("black box" was changed and the phenomenon stayed), do you have any other explanation for why this would happen?



Thanks.

 

[keithkyll]

Back circuit. The 24DC powering the output is also powering the analog input electronics.
Use an isolated tip thermocouple. Isolate the heater. Use a dedicated supply for the output.

 

[hagaib]

Solution!

saw it today first hand and solved it.
quite weird - it's a model built of an IR heater and a thermocouple.
one was working fine and the other showed the phenomenon i described before.
turns out that B&R help recommend connecting the thermocouple's COMMON to the PLC's COMMON, once i did that, issue was solved!