Thermocouples

The working principle of a thermocouple depends on the thermo-electric effect. If two dissimilar metals are joined together so as to form a closed circuit, there will be two junctions where they meet each other. If one of these junctions is heated, then, a current flows in the circuit which can be detected by a galvanometer. The amount of the current produced depends on the difference in the temperature between the two junctions and on the characteristics of the two metals. This was first observed by Seebeck in 1821 and is known as Seebeck effect.

Instrument which records the variation in current flow are calibrated in terms of temperature and are know as thermocouple pyrometers. Thermocouples are made from two different kinds of metals. the wires are joined at the ends which forms two junctions, a measuring junction and a reference junction. Heating the measuring junction produces voltage greater that the voltages across the references junction. The difference between voltages is measured and voltmeter reading is converted to its corresponding temperatures. This conversion table is generally supplied by the thermocouple manufacturer. There are various types of thermocouples which are widely used in industries. Viz. copper constanant, iron constantan, chromel alumel, platinum rhodium and chromel-constantan.

Advantages

• They have rugged construction
• Thermocouple are inexpensive
• Theromcouples are simpler to use than resistance thermometers.
• There is no need of a bridge circuit.
  

Disadvantages

• Thermocouples have limited use in temperature spans of less than about 33°C because of the relatively small change in junction voltages with temperature.
• Thermocouples require much of an amplifier for many applications.
  
• Thermocouples need expensive accessories for control applications.