Theory

The conversion of temperature difference to electric current and vice-versa is termed as thermoelectric effect. In 1821, Thomas Johann Seebeck discovered that a circuit made of two dissimilar metals, when subjected to a temperature difference at their junctions, could cause a nearby compass needle to deflect. He realized that an electric current was induced in the circuit, which, according to Ampère's law, produced a magnetic field that deflected the magnet. This phenomenon demonstrated that a temperature difference can generate an electric potential (voltage), which in turn can drive an electric current in a closed circuit. This effect is now known as the Seebeck effect, a fundamental component of the thermoelectric effect.

To measure this voltage, one must use a second conductor material which generates a different voltage under the same temperature gradient. Otherwise, if the same material is used for measurement, the voltage generated by the measuring conductor would simply cancel that of the first conductor. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. It is thus clear that, based on Seebeck's principle; thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings.

The principle behind it states that

$$V=a(T_{h}-T_{c})$$

V- Voltage difference between two dissimilar metals

a- Seebeck coefficient

T_h - T_c - Temperature difference between hot and cold junctions

There are three major effects involved in a thermocouple circuit: the Seebeck, Peltier, and Thomson effects.

The Seebeck effect describes the voltage or electromotive force (EMF) induced by the temperature difference (gradient) along the wire. The change in material EMF with respect to a change in temperature is called the Seebeck coefficient or thermoelectric sensitivity. This coefficient is usually a nonlinear function of temperature.

Peltier effect describes the temperature difference generated by EMF and is the reverse of Seebeck effect. Finally, the Thomson effect relates the reversible thermal gradient and EMF in a homogeneous conductor. Thermocouples generate an open-circuit voltage, called the Seebeck voltage that is proportional to the temperature difference between the hot and reference junctions

$$E=\alpha(T-T_{0})+\beta(T-T_{0})^{2}$$

Since thermocouple voltage is a function of the temperature difference between junctions, it is necessary to know both voltage and reference junction temperature in order to determine the temperature at the hot junction. Consequently, a thermocouple measurement system must either measure the reference junction temperature or control it to maintain it at a fixed, known temperature.