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Temperature Sensors

Temperature Sensors. Objectives. Names and different methods to measure the temperature Resistance temperature detectors (RTD) Thermister NTC and PTC Thermocouples Pyrometers Smart temperature sensors Applications. Thermocouples.

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Temperature Sensors

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  1. Temperature Sensors

  2. Objectives • Names and different methods to measure the temperature • Resistance temperature detectors (RTD) • Thermister NTC and PTC • Thermocouples • Pyrometers • Smart temperature sensors • Applications

  3. Thermocouples • Thermocouples (T/C) are formed when two dissimilar metals are joined together to form a junction. • Joining together the other ends of the dissimilar metals to form a second junction completes an electrical circuit. • A current will flow in the circuit if the two junctions are at different temperatures. • The voltage difference between the two junctions is measured, and this difference is proportional to the temperature difference between the two junctions. • Three effects are associated with thermocouples. • Seebeck effect: It states that the voltage produced in a thermocouple is proportional to the temperature between the two junctions. • Peltier effect: It states that if a current flows through thermocouple one junction is heated (puts out energy) and the other junction is cooled (absorbs energy

  4. EMF generated by thermocouples • The emf generated can be approximately expressed by the relationship: • where T1 and T2 are hot and cold junction temperatures in K. C1 and C2 are constants depending upon the materials. For Copper/ Constantan thermocouple, C1=62.1 and C2=0.045 . • Thermocouples are extensively used for measurement of temperature in industrial situations. The major reasons behind their popularity are: • (i) they are rugged and readings are consistent, • (ii) they can measure over a wide range of temperature, and • (iii) their characteristics are almost linear with an accuracy of about 0.05%.

  5. Law of thermocouple • we have three laws of thermoelectric circuits that provide us useful practical tips for measurement of temperature . • law of homogeneous circuit (i): The first law can be explained using figure 1. It says that the net thermo-emf generated is dependent on the materials and the temperatures of two junctions only, not on any intermediate temperature. • law of intermediate metals: If a third material is introduced at any point (thus forming two additional junctions) it will not have any effect, if these two additional junctions remain at the same temperatures . This law makes it possible to insert a measuring device without altering the thermo-emf. • law of intermediate temperatures :The third law is related to the calibration of the thermocouple. It says, if a thermocouple produces emf e1, when its junctions are at T1and T2, and e2when its junctions are at T2and T3; then it will generate emf e1+e2when the junction temperatures are at T1and T3

  6. Applications • In the process control of chemical reactions, temperature control is of major importance, since chemical reactions are temperature dependent. • In an average household , at least a dozen temperature sensors can be found in various places, raging from coffee machine to heating system to a car. • Low cost • Small size • Robust • Wide range of operation • Reasonably stable • Accurate for large temperature changes • Provide fast response Advantages

  7. Resistance Temperature Detectors (RTD's)

  8. construction • RTD’s are built from selected metals (typically Platinum), which change resistance with temperature change. • The resistance temperature detector (RTD) measures the electrical conductivity as it varies with temperature. • The electrical resistance generally increases with temperature, and the device is defined as having a positive temperature coefficient. • The magnitude of the temperature coefficient determines the sensitivity of the RTD. • Apart from Platinum, other metals are used for RTD’s such as Copper and Nickel. • Platinum is the most common and has the best linear characteristics of the three, although Nickel has a higher temperature coefficient giving it greater sensitivity.

  9. Temperature coefficient • The temperature coefficient defines how much the resistance will change for a change in temperature, and has units of ohms/oC. • The greater the temperature coefficient, the more the resistance will change for a given change in temperature. • This ultimately defines how sensitive the device is. • RTD’s are generally quite linear, however the temperature coefficient does vary over the range of operation. • As an indication, the temperature coefficient for Platinum is averaged at 0.00385 over the range from 0oC to 100oC, but varies by about 2% over • this range.

  10. Resistance Temperature Detectors (RTD's) • Advantages • - Good sensitivity • - Uses standard copper wire • - Copper RTD’s minimize thermocouple effect • Disadvantages • - Bulky in size and fragile • - Slow thermal response time due to bulk • - Self heating problems • - More susceptible to electrical noise • - More expensive to test and diagnose

  11. Thermisters

  12. Basic of Thermisters • A thermistor is a semiconductor device formed from metal oxides. • The principle of temperature measurement with a thermistor is that its resistance changes with temperature. • Most thermistors differ from normal resistors in that they have a negative coefficient of resistance, this means that the resistance decreases with an increase in temperature. • Negative (NTC) thermistors are the more common although positive (PTC) are also available.

  13. Types of Thermisters • Types of thermistors vary in a number of ways and one change is their response to temperature changes. • Thermistors are not linear, and their response curves vary for the different types. • Some thermistors have a near linear temperature resistance relationship, others are available with a sharp change in slope (sensitivity) at a particular characteristic temperature.

  14. Advantages and disadvantages • Advantages • - Small size • - Fast response • - Very high sensitivity (Select range) • - No cold junction compensation • - Inexpensive • - Polarity insensitive • - Wide selection of sensors • Disadvantages • - Not easily interchangeable • - Non linear • - Narrow span • - Fragile • - High resistance, noise problems

  15. Pyrometers

  16. Pyrometers • Pyrometric methods of temperature measurement use the electromagnetic radiation that is emitted from a material. The emitted radiation is proportional to the temperature. • Any object with a temperature above absolute zero will radiate electromagnetic energy. • Infrared pyrometers measure the amount of energy radiated from an object in order to determine its temperature. • There are a number of different types of infrared pyrometers: • - Total radiation • - Single wavelength • - Dual wavelength

  17. Advantages and disadvantages • Advantages • - Non contact measurement • - High temperature sensing • - Remote sensing • - Fast response and can sense objects in motion • - Sense small or area targets • Disadvantages • - Expensive • - Non linear response • - Subject to emissivity of material • - Require wide range of operation

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