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Thermistors and Resistance Temperature Detectors

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PRESENTED TO

Dr.Ing. Naveed Ramzan

PRESENTED BY Hina Muryam

2007-chem-07

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Contents Introduction RTD,s Platinum RTD,s Selection criteria of RTD,s Advantages of RTD,s Thermistors Characteristics of thermistors Applications

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Introduction

Thermistors and the resistance temperature detectors are the electrical temperature sensors.

Their function depends upon the change in resistance with the change in temperature.

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Electrical Temperature Instruments

Electrically temperature measuring sensors use any electrical circuit.

Three types of electrical temperature sensors which are generally used,

1. Resistance temperature detectors

2. Thermistor

3. Thermocouple

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Resistance Temperature Detectors

Principle Resistivity of most of the metals increase with

the increase in temperature, and on cooling to the original temperature, resistivity starts to decreased.

Different types of materials which are used for such type of detectors are platinum, copper and nickel in industries

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Platinum RTD’sPlatinum is a metal with a very good stability

and almost linear proportion of the resistance versus temperature.

Most of the time platinum is used for RTD’s due to its stability and reproducibility temperature characteristics.

Resistance versus temperature function of platinum is accurately determined by the callendar-van dusen equation.

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Callendar-Van Dusen EquationIt describes the relationship between resistance

and temperature.For the 0 -100 °C temperature range this

equation will be used, 

R(t) =  R(0) [1+αT] For the 0-650 °C temperature range this

equation will be used,

R(t) =  R(0) [1+ αt + K1T2]

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Common Resistance Materials For RTD’s

Platinum (most popular and accurate)NickelCopperBalco (rare)Tungsten (rare)

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Selection Criteria Of RTD’s

The temperature range of the media of which we measure the temperature .

What is the immersion length is required for our application.

How can we insert the RTD in the process, and how can we support the RTD.

A thermowell is required If pressure or vacuum has to be maintained.

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Advantages Of RTD’s

High accuracy of measurementWide temperature range from -200 to 650 degcSmall size Fast responseGood reproducibilityStable and accurate performance over many

yearsTemperature compensation not required

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Limitations Of RTD’s

Cost is very high.It needs power supply for bridge circuit.It cause mechanical abuse and vibration.It has large bulb size than thermocouple.Self heating.

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Thermistors

Resistors with the negative temperature coefficient are called thermistors.

The word thermistor is a contraction of words thermal and resistor

The word that best describes the thermistors is “sensitive”

Resistance of them is decreasing with temperature.

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Characteristics Of Thermistors 

Their temperature resistance coefficient is about 10 times the other common metals.

The resistance temperature coefficient is not constant of the thermistors but with the inverse function of temperature

Resistance of them is decreasing with temperature, where the resistance of most of the metals is increasing with the increasing temperature

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Thermistor Resistance

As temperature increases, its resistance gets decreasing according to the R vs. T equation

  R = Ro exp( Beta/T - Beta/To)

And coefficient beta can bf found by this relation

αT = -β / T2

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Applications Of Thermistors

Thermistors are used in industries not only for the measurement of temperature of the process it is also used for such kind of applications.

Liquid Level IndicatorsTemperature CompensationFlow Measurement

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Advantages of Thermistors

Thermistor has the small size and low cost and easily available.

These are having comparatively a large change in resistance with a given change in temperature.

These sensors give a fast response over a narrow temperature range.

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Limitations Of Thermistors

The temperature vs resistance characteristics are highly non-linear.

These are not suitable for the wide range of temperature.

It requires the electrically supply for working and also a Wheatstone bridge circuit.

More susceptible to permanent decalibration at high temperatures.

Respond quickly to temperature changes, thus, especially susceptible to self-heating errors.

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References

1- Jacob, Fraden; “Handbook of Modern Sensors: Physics, Designs, and Applications”; 4th edition; page 141-142.

2- R. S. Sirohi, H. C. Radha Krishna; “Mechanical measurements”; 3rd edition; page 184-185.

3- Tattamangalam R. Padmanabhan; “Industrial instrumentation: principles and design”; page 422-427.

4-www.ecircuitcenter.com/circuits/therm_model1/therm_model1. html

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