Consider the circuit below:
From the figure above, the sensing junction and reference junction are connected in series. When the junctions are at the same temperature the voltage output from the junctions is zero and the output from the amplifier is zero. When the junctions are at different temperatures, there is a differential voltage at the input to the amplifier that is then amplified and converted to a temperature reading. To make this absolute reading, the temperature of the reference junction is needed. This can be obtained by placing the junction in a constant temperature enclosure or the temperature of the reference junction can be measured and correction applied to the output reading of the thermocouple’s sensing amplifier.
The amount of signal conditioning required by a thermocouple will depend on its temperature range. The thermocouple accuracy is 1 to 2 % over a limited temperature range but requires conditioning i.e. linearization if used over its full operating range. The set zero conditioning is part of the reference temperature correction. Thermocouple voltages can also be sensed directly by the controller using an internal amplifier and then conditioned internally.
Related: Types of Sensors used in Measurement and Process Control
Thermocouple signals can be amplified with cold junction reference close to the amplifier and the signal transmitted in analog or digital format to a controller or the thermocouple can be directly connected to the controller for amplification and cold junction correction. The latter method is sometimes employed to eliminate the cost of remote amplifiers and power supplies. Controller peripheral modules are available for the amplification of several thermocouple inputs with cold junction correction.
Figure 1(b) shows a differential connection between amplifier and the thermocouple as twisted pair of wires that are shielded to minimize noise or electrical interference.
You can also read: Basics facts about Thermocouples
