Presented By:HIMANI HARBOLAEIE( 4th year )

TEMPERATURE SENSORS

‡Introduction • Temperature is an important parameter in many control systems.• Temperature is not measured directly like displacement, pressure or flow.

Temperature measuring devices

Electrical instrumen

ts

Non electrical instrumen

ts

‡Sensors covered• Thermocouple.

• RTD.

• Bimetallic thermometer.

†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

constantan

Reference junction

Sensing junction

“ T “ Type Thermocouple

Thermopile• Thermocouples may be connected

either in series or in parallel.

• This arrangement is called thermopile.

• In this, all measuring junctions are at same temperature while all reference functions are at another temperature.

• Used to measure temperature of furnance, temperature of moving masses of metal.

A

A

A

B

B

B

B

B are reference junctionsA are heated junctions

• Thermocouples are suitable for wide range of temperature, upto 1800⁰C.

• Widely used in steel industries, heating furnances etc.

• Simple.• Rugged.• No battery required.• Measure wide range of

temperature.• Cheap.

Applications of thermocouple

Advantages

†RTD• Stands for Resistance Temperature

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 1oooC.

• It is used in electrical appliances such as motor, generator etc.

• It is used in engine testing beds.

• Most stable.

• Most accurate.

• More linear than thermocouple.

Applications Advantages

†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 540C.

• Refineries• Oil burners.• Hot solder tanks.• Hot wire heaters.

• Simple• Robust.• Inexpensive.• Easy to install and maintain.

Application Advantages

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