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Temperature measurement, a vital part of most industrial operations, is typically accomplished by a temperature sensor- a thermocouple or a resistance temperature detector (RTD), in contact with a solid surface or immersed in a fluid. Although these sensors have overlapping temperature ranges, each has certain application- dependent advantages.

Several factors must be considered when selecting the type of sensor to be used in a specific application. Some of these factors are temperature range, accuracy, response time, stability, linearity, and sensitivity. An RTD is the sensor of choice when sensitivity and application flexibility are the most important criteria. When it comes to component cost, though, an RTD is more expensive than a thermocouple. Choosing the perfect sensor for a particula application therefore requires an understanding of the basic of RTDs and thermocouples.

           Resistance temperature detectors are available in 2-, 3-, and 4- wire configurations, but their basic components are identical. These sensors measure changes in resistance that result from changes in temperature.

RTDs operate by exhibiting an increase in resistivity with an increase in temperature. RTDs are most commonly made from platinum, nickel , or copper. Thermocouple is made up of two dissimilar metals, joined together at one end, that produce a voltage ( expressed in millivolts) with a change in temperature. Measurement errors can be easily introduced with thermocouples. Since the voltage created by the thermocouple is due to the bonding of two dissimilar metals, the introduction of the other junctions to the circuit results in voltage changes that are referred to as cold junction errors. If the temperature at the connections determined, this error can be corrected by a process called junction compensation.   Thermocouples are also available in three junction types, such as grounded, ungrounded, and exposed.

References:

https://www.sensormag.com/component/cchoosing-using-temperature-sensor