2. ‡Introduction
• Temperature is an important parameter in many control systems.
• Temperature is not measured directly like displacement, pressure or
flow.
4. †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.
5. Types of thermocouples
• “ J “ type – iron and constantan.
• “ K “ type – chromel and alumel.
• “E “ type – chromel and constantant.
• “ T “ type – copper and constantan.
copper
constantan
Sensing Reference
junction junction
6. • 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 junctions
A are heated junctions
7. 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.
8. • 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.
9. 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.
10. †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.
11.
12. Application Advantages
• Refineries • Simple
• Oil burners. • Robust.
• Hot solder tanks. • Inexpensive.
• Hot wire heaters. • Easy to install and maintain.
