Thermocouples

Thermocouple Working

1. Thermocouple

2. Members Azeem Waqar 13-ME-3610 Tariq Aziz 13-ME-3617 Riaz - Ul -Islam 13-ME-3636 Sauz Zaman 13-ME-3648

3. Introduction A thermocouple is an electrical device or sensor which measures Temperature. It can measure a wide range of temperatures . I t consisting of two dissimilar conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, A nd this voltage can be interpreted to measure temperature.

4. Introduction The device consists of two dissimilar metals or semiconductors that contact each other in one are more points. One end is connected to the region where the temperature is to be measured. This end considered as the hot region or the measuring region. The end is connected to the body of known temperature. This end is considered as cold region or the reference junction . Thus it measure temperature with respect to reference junction.

5. Diagram

6. Working Principal The thermocouple works on Principal name Seebeck Effect Seebeck Effect In 1821, the German physicist Thomas Johann Seebeck discovered that when different metals are joined at the ends and there is a temperature difference between the joints, a magnetic field is observed.  The magnetic field he observed was later shown to be due to thermo-electric current . Or emf is generated. The amount of emf generated is different for different combinations of the metals.

7. Working Principal A Thermocouple comprises of two dissimilar metals. These are joined together to form two junctions which are maintained at different temperatures . Since the two junctions are maintained at different temperatures emf is generated within the circuit and it is the function of the temperatures of two junctions . If the temperature of both the junctions is same net emf will be zero. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions . The device for measuring the current or emf is connected within the circuit of the thermocouple. It measure the emf due to the two junctions of the two dissimilar metals maintained at different temperatures.

8. Working Principal As the temperature of the reference junctions is already known. While the temperature of measuring junction is unknown . The output current or voltage obtained from the thermocouple circuit is calibrated directly against the unknown temperature. Thus gives the value of unknown temperature directly . Formula is  

9. Types Of Thermocouples There are three main types of thermocouples based on the alloy combination. They are Nickel-alloy thermocouples Platinum/rhodium-alloy thermocouples Tungsten/rhenium-alloy thermocouples

10. Nickel-alloy thermocouples Type E, J, M, N and T are Nickel alloy thermocouples Type E It is chromel–constantan a non magnetic having range of −50 °C to +740 ° C Type J iron–constantan having range of −40 °C to +750 ° C Type K chromel– alumel most general purpose thermocouple its range is −200 °C to +1350 °C . Type M Molybdenum-constantan having range of up to 1400 ° C Type N Nicrosil– Nisil having range of −270 °C and +1300 ° C. Type T copper–constantan having range of −200 to 350 ° C.

11. Platinum/rhodium-alloy thermocouples Type B, R and S are the thermocouples Type B Platinum/rhodium-alloy thermocouple having range up to 1800 ° C Type R Platinum/rhodium-alloy thermocouple having range up to 1600 ° C Type S Platinum/rhodium-alloy thermocouple having range from 630 °C to 1064 ° C.

12. Tungsten/rhenium-alloy thermocouples Type C, D and G are Tungsten/rhenium-alloy thermocouples They have typical range of 0 °C to 2315 °C, which can be extended to 2760 °C in inert atmosphere.

13. Advantages It has rugged construction. Covers a wide range of temperature. Extension leads and wires can be connect to measure temperature at long ranges. Comparatively cheaper at cost. Calibration can be easily checked. High Speed of response. Satisfactory accuracy.

14. Limitations For accurate measurements Cold or reference junction is necessary. The Emf induced versus temperature is non linear. In many applications Amplification of signals are required. Stray voltage pick up is possibal .

15. Applications Type B, S, R and K are widely used in iron and steel industries to monitor the temperature through out any industrial operation. Gas-Fed heating devices such is Ovens and Water heaters. In testing of prototype of Electrical and Mechanical apparatus.

16. References https:// en.wikipedia.org/wiki/Thermocouple https://www.slideshare.net/anandbongir/thermocouple?qid=81339ae9-e2a1-4026-bc6d-b7eef52fb5a2&v=&b=& from_search=4 https:// www.slideshare.net/keshavakrishnan34/thermocouple-52341426

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