Thermocouples | Laws for thermocouple | Pettier effect

Here we learn about thermocouples, application Laws for thermocouples, Law of intermediate metals, Law of intermediate temperature and Pettier effect.

Thermocouples :

~ The basic principle of a thermocouple is that when two dissimilar metal wires are joined to form a completely electric circuit, and the two junctions are maintained at different temperatures, an electromotive force is setup.

 due to the algebraic sum of an emf developed between two dissimilar metals placed in contact which is known as Seebeck effect Or an emf developed between the two ends of a homogeneous wire when one end is heated, which is called the Thomson effect.

~ The above principle is used to form thermocouples and to measure the high temperatures as in furnaces. In this, the junction, which is kept at hot source is called hot junction and the junction, at whom the emf produced is called cold junction. he magnitude of the emf depends oi the temperature difference of the junctions and the metals used. A simple thermocouple is shown in Pig. 3.5.1.

~ Thermocouple measurement system consists of :
1. The thermocouple, made from two different metals or alloys,
2. The junction block outside the heater chamber.
3. The extension leads and the indicator or recorder.

Application Laws for Thermocouples :

Peltier effect :

When an electric current is passed through a junction between two dissimilar metals, it causes one junction to heat and other to cool. A peltier emf results from a temperature gradient in the metals. Por e.g. refrigerator.

(a) Law of intermediate metals :

~ If two dissimilar metals A and B with their junction temperatures at Ti and l are joined to a third metal C at one of its leg, if C is kept at a constant temperature along its entire length, the total emf in the circuit will be unaffected.
~ Application of this law is shown in Fig. 3.5.2. Here, the insertion of third metal C in the circuit has no effect on the emf, provided that both new junctions formed by third metal C, rands are at the same temperature T3. This permits insertion of a measuring device, or circuit without upsetting the temperature function of the thermocouple circuit.
~ The third metal can also be connected at the cold or reference junction as shown in Fig. 3.5.2. This also has no effect on the emf produced.

Law of Intermediate temperature :

It states that, if a simple thermocouple circuit develops an emf e when its junctions are at temperature T1 and T2, and an emf e2 when its junctions are at temperature T2 and T3, then it will develop an emf. ei+e2, when its junctions are at temperature T1 and T3.