†Thermocouple• It is based on the principle of “ seeback effect “.• Based on generation of thermoelectricity.• When two dissimilar metals are connected together at two different temperatures, an emf is generated.• The magnitude of thermal emf depends on the wire material and in temperature difference between the junctions.• It can be used for both ac and dc applications.
Types of thermocouples• “ J “ type – iron and constantan.• “ K “ type – chromel and alumel.• “E “ type – chromel and constantant.• “ T “ type – copper and constantan. copper constantanSensing Referencejunction junction
• Thermocouples may be connected either Thermopile in series or in parallel. B • This arrangement is called thermopile. A • In this, all measuring junctions are at same B temperature while all reference functions A are at another temperature. B • Used to measure temperature of A furnance, temperature of moving masses B of metal.B are reference junctionsA are heated junctions
Applications of thermocouple Advantages• Thermocouples are suitable for • Simple. wide range of temperature, upto • Rugged. 1800⁰C. • No battery required. • Measure wide range of• Widely used in steel temperature. industries, heating furnances etc. • Cheap.
• Stands for Resistance Temperature†RTD Detector. • It is positive temperature device. • Material used- Pt, Ni, Copper, Tungesten. • Pt is preferred because it can withstand high temp., while maintaining excellent stability. • Range: -250 to 1ooo C.
Applications Advantages• It is used in electrical appliances • Most stable. such as motor, generator etc. • Most accurate.• It is used in engine testing beds. • More linear than thermocouple.
†Bimetallic thermometers• Based on the principle:i. All metals expand or contract with change in temperature.ii. The temperature coefficient of expansion is different for all metals.• It consist 2 strips of different metals welded together.• When welded strip is heated, length of metal changes.• This causes movement of bimetallic strip.• This movement deflects the pointer over calibrated scale.• Range: -70 to 540 C.
Application Advantages• Refineries • Simple• Oil burners. • Robust.• Hot solder tanks. • Inexpensive.• Hot wire heaters. • Easy to install and maintain.