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Generally in which location ( JB or IO card or DCS) the
thermocouple compensation is done.Also where they measure
the atmosphere temp ( for compensation) in JB OR DCS panel?
How they compensating that atmosphere temp? is any circuit
for that ( like in rtd wheatstone bridge)?
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Answer Posted / carl_ellis
Cold junction compensation (CJC) is done at the point where
the thermocouple wire or thermocouple extension wire
connects to the thermocouple input, either an I/O card, a
temperature transmitter, or a thermocouple input on a
recorder/controller/indicator/alarm. RTD's do not need
CJC. Thermistors do not need CJC.
The purpose of CJC is to measure the temperature of the
termainal connection block with respect to the freezing
point of water (because that is the 'cold junction' and the
national standard thermocouple tables are referenced to) a
cold junction at the freezing point of water) and add that
value to the temperature value determined by the EMF/mV
output of the thermocouple.
The temperature of the terminal connection block is affected
by ambient, room, or atmospheric temperature but is also
affected by the heat generated by the device's electronics,
other electronics in close proximity, and whatever cooling
the installation provides. Because all of those factors can
vary in temperature, the CJC involves measuring the
tempeature of the (terminal) connections.
Temperature-sensing ICs, thermistors, and RTDs are used for
CJC measurements. Thermocouples cannot be used for CJ
measurement.
So the answer is that the CJ measurement is made at the
thermocouple input, presumably the I/O card or temperature
transmitter, never at a junction box.
The actual temperature calculation, measured T/C EMF plus
CJC, can be done on the I/O card or in the 'system', a PLC,
temperature transmitter, controller, DCS, whatever,
depending on design.
CJ should not measure room temperature or atmospheric
temperature (although some poorly designed thermocouple
devices do so); CJ should measure the terminal temperature
where the thermocouple wires connect to the I/O. A
temperature transmitter measures the CJC in order to do the
calculation for temperature.
There are special techniques for CJC for high accuracy
calibration involving an ice bath, but industrial devices
use integrated CJC.
Tell 'em Carl sent you.
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