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Answer Posted / carl ellis
I disagree with answer #1
>other metals create junction (screw, conducting wire of
another metal)
This does NOT create an error according the Thermocouple Law
of Intermediate Junctions, which states that a 3rd metal
inserted between the two dissimilar metals of T/C junction
will have no effect upon the output voltage as long as the
connection junctions are isothermal, that is, both
connection junctions are at the same temperature.
So compensation is not needed for a 3rd metal because the
error in the positive and negative legs are of opposite
polarity, so the error is self correcting.
There is no practical method of compensating for
non-isothermal conditions so T/C compensation does not refer
to that.
>using ckt that may contain RTD at cold junction, eliminate
known emf at cold junction using electronic ckt
The original question is poorly worded because "thermocouple
compensation" refers to correction of non-linearities of
thermocouple's EMF with respect to the accepted temperature
value. That is done mathematically with a polynomial or in
some cases, with a look-up table, neither of which is an
electronic circuit.
The electronic circuit used with ALL thermocouples is a cold
junction circuit. ALL temperature measurements done with
thermocouples require cold junction compensation. The cold
junction measurement does not 'eliminate' anything, it
provides the required difference between the freezing point
of water and the temperature of the terminal block,
necessary to use standardized tables, all of which are
referenced to the freezing point of water. Without cold
junction compensation, the error of a thermocouple
measurement is the cold junction temperature difference.
That's not uncertainty, that's ERROR.
Industrial applications typically use a thermistor to
measure the temperature of the connection block at the
analog input. That temperature or its EMF equivalent is
'added' to the EMF measurement of the thermocouple.
Cold junction compensation can be done with a 2nd
thermocouple in series inserted into an icebath which still
done for laboratory and high accuracy measurements. A 2nd
T/C qualifies as a circuit, too.
Answer #2 is correct insofar as it refers to cold junction
compensation.
Carl Ellis
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