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Thermistors and Resistance Temperature Detectors

Thermistors and Resistance Temperature Detectors. PRESENTED TO Dr.Ing . Naveed Ramzan PRESEN TED BY Hina Muryam 2007-chem-07. Contents. Introduction RTD,s Platinum RTD,s Selection criteria of RTD,s Advantages of RTD,s Thermistors

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Thermistors and Resistance Temperature Detectors

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  1. Thermistors and Resistance Temperature Detectors

  2. PRESENTED TO Dr.Ing. NaveedRamzan PRESENTED BY HinaMuryam 2007-chem-07

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

  4. 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.

  5. Electrical Temperature Instruments • Electrically temperature measuring sensors use any electrical circuit. • Three types of electrical temperature sensors which are generally used, • Resistance temperature detectors • Thermistor • Thermocouple

  6. 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

  7. Platinum RTD’s • Platinum 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.

  8. Callendar-Van Dusen Equation • It 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]

  9. Common Resistance Materials For RTD’s • Platinum (most popular and accurate) • Nickel • Copper • Balco (rare) • Tungsten (rare)

  10. 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.

  11. Advantages Of RTD’s • High accuracy of measurement • Wide temperature range from -200 to 650 degc • Small size • Fast response • Good reproducibility • Stable and accurate performance over many years • Temperature compensation not required

  12. 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.

  13. 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.

  14. 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

  15. 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

  16. 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 Indicators • Temperature Compensation • Flow Measurement

  17. 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.

  18. 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.

  19. 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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