J Type Thermocouple

>on dcs system

A DCS will definitely have a 4-20mA analog input for a temperature transmitter and it might have an analog input for direct connection of a thermocouple; it depends on make and model.

A temperature transmitter handles cold junction compensation and linear conversion to temperature units. So on the DCS side, one does not calculate temperature values from mV values, one scales the 4-20mA input from the transmitter to the same range as the temperature transmitter is configured for, for instance, 4-20mA = 0 - 250 DegC.

If the DCS has a direct analog input for thermocouple, then the DCS analog input card provides cold junction compensation and linearization over some configured range.

One does not need to program the conversion of mV from a J type thermocouple on a DCS, the average installed cost of which is $5 million USD. It is a configuration task.

A DCS is a high-end platform.

>Cold junction compensation is usually valid for a range around room temperature.

Good cold junction needs to be valid for whatever the cold junction temperature is, whether that temperature is near to or far from room temperature.

Temperature transmitters are frequently located in hot ambient areas, sometimes too hot to touch. The cold junction needs to work at elevated or sub-freezing temperatures as well. In commercial instruments, the cold junctions works over the range of the whatever ambient exposure temperature the instrument is rated for.

>I think you subtract the cold junction mV value from the measured value in mV

The measured Cold junction value is added to thermocouple reading.

Take the example of a thermocouple on a desk (isothermal - both ends at the same temperature).

Both ends of a thermocouple lying on a work bench or desk at room temperature (say 25 Deg C) will have the same temperature, therefore a measurement at the cold end with a voltmeter will read 0.0mV.

But the Type J thermocouple table says that at 25 Deg C, the thermocouple generates an EMF of 1.277mV. Why does the voltmeter read 0.0mV?

The voltmeter reads 0.0mV because there is no temperature gradient across the thermocouple. It's the gradient, the difference in temperatures, that creates an EMF, not just being at an isothermal temperature. No gradient, no EMF.

Yet if you were to connect that thermocouple to an instrument, you would expect the instrument to read the room temp of 25 Deg C, right? That happens because of the cold junction measurement.

The cold junction measurement measures the temperature of the connection (terminals, terminal block) on the instrument.

In the example, the connection temperature is also at 25 Deg C, room temperature.

The cold junction measurement (the temperature of the connection terminals/block) of 25 Deg C is then added to the thermocouple measurement, 0 Deg C (no temperature gradient, EMF = zero), for a final reading of 25 Deg C.