25-02-2012, 04:10 PM
Thermocouple
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Thermocouple operates on the phenomenon of Thermoelectric or Seebeck effect.
Thermoelectric effect states that “when one junction is heated while the other is kept cool, the galvanometer shows deflection indicating the flow of electric current through the circuit.
Thus, emf is produced in the circuit due to temperature difference between the two junctions. This emf is called as thermo-emf.
The junction consists of the joint of two different metals. Some of the metals are: Sb, Fe, Zn, Ag, Au, Sn, Cu, Pt, Co, Ni, Bi.
Characteristics of Thermoelectric Effect
When a temperature difference is maintained between two junctions of a thermocouple, a thermo-emf is generated in the thermocouple circuit.
The magnitude of thermo-emf depends upon the temperature difference between the two junctions and the two metals forming the thermocouple.
For a given thermocouple, the thermo-emf increases with the increase in the difference in temperature difference between the two junctions.
For the same temperature difference between the two junctions, the thermo-emf produced in thermocouples of different metals is different.
Explanation of Seebeck Effect
Any metal contains free electrons. These electrons forms “electron gas” which possesses the same properties as those of ordinary gases.
The electron gas exerts pressure which increases with increase in temperature. The pressure is different for different metals.
Therefore, when a junction is formed by joining two dissimilar metals, there is a migration of some electrons across the junction from a metal of higher electron pressure to the metal of lower electron pressure.
As a result of migration of electrons, a contact emf is developed at the junction.
First Law: Electric current cannot be sustained in a circuit of a single homogeneous metal, however varying in section, by the application of heat alone. What this means for thermocouples, is that if there is a temperature distribution along the wires between the hot and cold junctions, the total thermal EMF will be unaffected. Only the temperature at the junction between the two dissimilar metals will have an effect on the EMF produced.
Second law: Law of Intermediate Metals If two dissimilar metals A and B with their junctions at T1 and T2 are joined to a third metal C at one leg, if C is kept at a uniform temperature along its entire length, the total EMF in the circuit will be unaffected.
