lecture-2 Sensors for automatic control systems.pptx
- 1. 1 Sensors & Actuators for Automatic Systems (S&AAS) Dr. Imtiaz Hussain Associate Professor email: imtiaz.hussain@faculty.muet.edu.pk URL :http://imtiazhussainkalwar.weebly.com/ Lecture-2 Thermoelectric Transducer
- 2. 2 Lecture Outline • Thermoelectric Effect • Introduction • Sensor & Transducer • Classification of Sensors • Thermal Sensors
- 3. 3 Thermoelectric Effect • There is a relationship between the temperature of a conductor and the kinetic energy of the free electrons. • Thus, when a metal is subjected to a temperature gradient, the free electrons will diffuse from the high temperature region to the low temperature region where they have a lower kinetic energy. • The electron concentration gradient creates a voltage gradient since the lattice atoms that constitute the positive charges are not free to move. • This voltage gradient will oppose the further diffusion of electrons in the wire and a stable equilibrium will be established with no current flow. 𝐾 . 𝐸= 3 2 𝑘𝐵 𝑇
- 4. 4 Thermoelectric Effect • The “thermal power” of a material relates the balance of thermal diffusion of the electrons to the electrical conductivity of the metal and is unique for every conductor and usually varies with temperature. • Thus, a SINGLE conductor with its ends at differing temperatures will have a voltage difference between the ends.
- 5. 5 Thermoelectric Effect • The trick is to be able to measure the voltage at both ends of the conductor and thus determine the temperature difference between those ends. • If we use the same type of wire to measure the voltage across the original wire, the second wire will develop exactly the same voltage difference when its ends are exposed to the same temperatures as the original wire. • Therefore, this effect cannot be measured with a pair of similar wires. • But because the voltage gradient is a function of the thermal power, which is different for each type of metal, a second conductor of a different type of wire can be used to measure the original voltage gradient.
- 6. 6 Seebeck Effect • Whenever two metals are joined together and the junction is at a different temperature than the free ends of the conductors, the free ends will have a potential difference between them. • This difference of potential is a function of the absolute temperature at the junction and the temperature of the free ends. • Where, – is emf produced in volts – junction temperatures in K – thermal transport constants for two metals 𝜀=∫ 𝑇1 𝑇2 (𝑄𝐴 −𝑄𝐵 )𝑑𝑇
- 7. 7 Seebeck Effect • Above equation describes the Seebeck effect. • In practice, it is found that two constants are nearly independent of temperature and that an approximate linear relationship exists as, • Where, – is constant in V/K – junction temperatures in K 𝜀=∫ 𝑇1 𝑇2 (𝑄𝐴 −𝑄𝐵 )𝑑𝑇 𝜀=𝛼(𝑇1 −𝑇 2)
- 8. 8 Example-1 • Find the Seebeck emf for a material with . If the junction temperatures are and . Solution: 𝜀=𝛼(𝑇1 −𝑇 2) 𝜀=50𝜇(100−20) 𝜀=40𝑚𝑉
- 9. 9 Peltier Effect • A useful extension of same thermoelectric properties occurs when the reverse of the Seebeck effect is considered. • An external voltage is applied to the system to cause a current to flow in the circuit. • Because of the different electro-thermal transportation properties of the metals, it is found that one of the junction will be heated and other cooled.
- 10. 10 Thermocouple • To use the Seebeck effect as the basis of temperature sensor, we need to establish definite relationship between the measured emf of the thermocouple and the unknown temperature. • We see first that one temperature must already be known because the Seebeck voltage is proportional to the difference between junction temperatures. • Further more every connection of different metals made in thermocouple loop for measuring devices (e.g. extension leads) will contribute and emf. 𝜀=𝛼(𝑇1 −𝑇2)
- 11. 11 Thermocouple • To provide an output that is definite with respect to the temperature to be measured an arrangement such as shown below is used. • This shows that the measurement junction is exposed to environment whose temperature is to be measured. • This junction is formed of Metal A and B.
- 12. 12 Thermocouple • To use the thermocouple to measure the temperature reference junction should be kept at constant temperature. • In most industrial environments this would be difficult to achieve if the measurement junction and reference junction were close. • Therefore the reference junction is moved to a remote location by use of extension wires as shown below.
- 14. 14 END OF LECTURE-2 To download this lecture visit http://imtiazhussainkalwar.weebly.com/