K.ARUNKUMARAIT-07-002

Temperature Transducer

It is a temperature monitoring device.

The four most common temperature transducers:

the thermocouple the RTD(Resistance Temperature Detector),the thermistor and the IC (Integrated Circuit) sensor

What is?

Thermocouple

Two dissimilar metals connected to form two junctions is called thermocouple.

The emf developed in the circuit is thermo electric emf. The current through the circuit is called thermoelectric current this effect is called thermoelectric effect or Seebeck effect.

In a Cu-Fe thermocouple the direction of the current is from copper to iron at the hot junction.

If the hot and cold junctions are interchanged, the direction of current also reverses . Hence Seebeck effect is reversible

Applications

When one end (cold junction) is kept constant at a certain temperature, normally at 0°C, and the other end (measuring junction) is exposed to unknown temperature, the temperature at latter end can be determined by measurement of EMF so generated.

Three zones:Uniformly hot zone Varying zoneUniformly cold zone

Thermocouple device

One of the main application of thermocouple is to find out the unknown temperature.

Applications

Standard thermocouple types

They differ only by the way of pair of metals that forms the thermocouple.

Base metal type: T,J,E,K,NNobel metal types: S,R,B

Advantages

Self poweredSimplestrongInexpensiveWide variety of physical forms Wide temperature range

Disadvantages

Non linearLow voltageReference requiredLeast stableLeast sensitive

RTD(Resistance Temperature Detector)

• An RTD is basically a temperature sensitive resistor.

• It is a positive temperature coefficient device.

• As like thermocouple it doesn't produce any voltage by itself.

• It is used in electrical appliances such as motors, generators etc;

Tabulation • The RTD is made from

a pure material whose resistance at various temperatures has been documented.

• When voltage is passed to the RTD connected device there is a voltage drop across the RTD element.

• By that voltage drop the resistance of the RTD element can be easily determined.

• Then by tabulating the resistance value in the table, the temperature of the device can be measured.

Two wired RTD circuit Three wired RTD circuit

Common Resistance Materials for RTDs

Platinum (most popular and accurate) Nickel Copper Balco (rare) Tungsten (rare)

RTD elements

The RTD element is the simplest form of RTD.

It consists of a piece of wire wrapped around a ceramic or glass core.

Because of their compact size, RTD elements are commonly used when space is very limited.

RTD thin film element

A surface element is a special type of RTD element.

It is designed to be as thin as possible thus providing good contact for temperature measurement of flat surfaces.

RTD probe

This is the most rugged form of RTD.

A probe consists of an RTD element mounted inside a metal tube, also known as a sheath.

The sheath protects the element from the environment.

Advantages RTD

Most stableMost accurateMore linear than

thermocouple

Disadvantages

ExpensiveSlowCurrent source requiredSmall resistance changeFour-wire measurement

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