MARK VIe Thermocouples

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Thread Starter

Yassar

Hi everyone.

At our site we have a GE 9FA gas turbine with MARKVIe control system. I have a few questions regarding thermocouples connected to the MARKVIe.

Thermocouples work on the principle of Seeback effect and they produce output in mV.
Now my question is

1) the thermocouple junction at the field end (hot junction) is supposed to be referenced to zero so that it can be measured with respect to the reference table for say K-type thermocouple.

But the cold junction in reality is at the terminal end of the JB or at the MARKVIe card which is exposed to ambient and not zero degrees. Now how is the temperature compensated or cold junction compensated for the ambient temperature?

2) How is the ambient temperature measured for cold junction compensation at GE MARKVIe?

3) If the erection team by mistake wire the thermocouple cable from JB to MARKVIe using a different material other than the material of K-type how do we identify? (irrespective of the color code of the wire)

Please reply so that it would be very useful for everyone.
 
Yassar,

One of the better manuals produced by GE, the Mark VIe System Guide, GEH-6721, has some very useful information about the hardware of the Mark VIe and how it interfaces and works with inputs and outputs (I/O).

Check the PTCC section ('P' for IO<b>P</b>ack; 'TC' for <b>T</b>hermo<b>C</b>ouple; 'C' for '<b>C</b>ompensated) for more details.

You will find that cold junction compensation is (or at least it was) covered very well. The Mark VIe even has remote cold junction compensation capability!--which is what would be used if someone planned to use POCW (Plain Old Copper Wire) for connecting a remotely-located thermocouple to the Mark VIe and needed to compensate for the cold junction at the terminal between the TC or thermocouple extension wire and the plain old copper wire.

Hope this--and the available documentation in the Mark VIe System Guide, GEH-6721--helps!

(By the way, most digital control systems that are capable of handling thermocouple inputs all do it in a very similar fashion--though many do not have the remote cold junction capability the Mark VIe has.)
 
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