Thermocouple signal varies

H

Thread Starter

Hamad

Hi Guys;

We have centaur solar turbine with gas compressor set. We have control logix processor and flex I/O modules. we measure turbine engine temprature with K type thermocouples. The six numbers of thermocouple are used for that. T

he Tc module from AB have 8 channel. we used 6 channel. The temprature reading varies between 1350 to 1425 degree farenhite. We replaced all the thermocouple with new one but problem is still there. Thermocouple wires are connected first at the ISO THERMAL plate for cold jucntion ref. then signal of individual thermocouple connected to its respective channel.

we used twisted pair thermocouple wire for transmitt the signal.
please guide me what are the possible causes?

Thanking u, i will remain
 
Your question is not clear: do you mean that all 6 Thermocouples are reading the same exhaust gas temperature and the readings of the different Thermocouples varies between 1350 and 1425 degrees F?

The unconnected terminals of the AB input module will not affect the reading of the other six.

The purpose of the Isothermal Plate is to provide a cold junction reference. Make sure that neither of the thermocouple extension wires touches the plate. Each Thermocouple extension wire pair should be connected to a unique termination insulated from the Plate. However, you must make sure that the negative chromel (typically blue, or white) wire is connected to the AB negative termination, and the positive alumel wire is connected through the junction to the AB positive termination. I suspect that one or more of your thermocouples are mis-wired and you are experiencing a polarity problem. At least this is the first thing I would check.

See this reference:
http://en.wikipedia.org/wiki/Thermocouple

Dick Caro
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Thanks for reply.

Yes sir all the thermocouples are read the same temperature and installed circumferentianal periphery.

I confirm that the Al is connected -ve terminal and CR is connected at +ve terminal of the module to its respective channel and shield is connected to the IE ground.

The thermocouple connected at ISO THERMO PLATE are not touches the plate.

I want also learn that what is the cold junction temperature and how its effect on the temperature reading? If u have send me article on that i am very thankful to u.

Hamad
 
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