Thermocouple- Metrology& Instrumentation
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The thermocouple is a device which converts thermal energy to electrical energy. It is particularly used as a thermosensor. It has wider applications in instrumentation and measurements. This PPT gathered many relevant topics relating to thermocouple like its working, principle, laws and different types. Thank you!
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Thermocouple- Metrology& Instrumentation
- 1. Thermocouples MODULE 6 Abin Basil Shaji Btech/ME MITS, Varikoli,Kochi
- 2. Content. Principle Application laws for Thermocouples Thermocouple materials& Construction Measurement of Thermocouple EMF Thermocouples 2
- 3. Thermocouple. Thermocouples 3 A thermocouple is a device used to measure temperature. It is made up of two different wire joined at ends called junction end. The two wires are called thermo elements. One wire will be positive and other will be negative. The junction end will be placed in the environment whose temperature has to be measured and other end called tail (reference) end will be in ambient temperature. The temperature difference between ends will create a emf which is linearly proportional to the temperature difference. This is how a thermocouple works.
- 4. Principle of Thermocouples. Thermocouples 4 The working principle of thermocouples is based on 3 principles namely:- Seebeck Effect Peltier Effect Thomson Effect The Seebeck effect states that when two metals or unlike metals are joined together at junctions, an emf is generated at the two junctions. The amount of emf will be different for different combinations. As per Peltier effect, when two dissimilar metals are joined together forming two junctions, emf will be generated in circuit due to different temperatures at the two junctions.
- 5. Contd. Thermocouples 5 As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit. In many cases the emf calculated by Thomson effect is small, but emf suggested by Peltier effect has significant role in selection of metals. When the temperatures of junctions are same, there will be two opposite currents of same magnitude flowing making the resultant emf zero. Otherwise there will be net current causing thermo emf.
- 6. Laws of Thermocouple Thermocouples 6 There are 3 laws used for practical measurement of temperature. They are:- Law of Homogeneous Circuit Law of Intermediate Metals Law of Intermediate Temperatures Law of Homogeneous Circuit says that the net thermal emf generated is dependent on the materials and temperature of two junctions only and not on any intermediate temperatures. Law of Intermediate Metals: If a third material is introduced at any point forming additional two junctions will not have any effect, if these additional junctions remain at same temperature.
- 8. Contd. Thermocouples 8 Law of Intermediate Temperatures says that, if a thermocouple produces emf e1, when its junctions are at T1 and T2 , and e2 when its junctions are at T2 and T3 , then it will generate emf e1+e2 when the junction temperatures are at T1 and T3. The significance of the second& third laws make it possible to insert a measuring device without altering thermo emf and calibration respectively. The 3rd law also helps us to calculate the actual temperature using calibration chart. The third law also helps to create the calibration chart.
- 9. Thermocouple Materials. Thermocouples 9 The thermocouples can be specified as Types E,J,K,T as base metal couples (1000 °C) and S,R,B as noble-metal couples (2000 °C) The choice of materials for thermocouple is influenced by several factors like: Sensitivity Stability in Calibration Inertness and Reproducibility Some pairs and their features are given in the table:-
- 11. Construction. Thermocouples 11 The main parts of a thermocouple are:- Thermocouple wires Metallic Sheath Magnesium Oxide Layer The thermocouple wires are normally seen at the bottom of the device. It will be protected by a Magnesium Oxide layer against vibration and also to enhance heat transfer rate to junction. The leads will be enclosed in a metallic sheath which protects it from physical damages.
- 12. Contd. Thermocouples 12 The probes are available as:- Grounded thermocouples Ungrounded thermocouples Exposed thermocouples In grounded couples, the couple wire and sheath will be welded together and have good response time. But in ungrounded, the metallic wires will not be welded to sheath. In the exposed couple, the wire junction will be out of sheath which gives high response time than others, but prone to physical and corrosion than others.
- 13. Measurement of EMF. Thermocouples 13 The devices used commonly to measure emf of thermocouples are galvanometer and voltage balancing potentiometer. One junction can be connected to the body to be measured and other will be connected to a reference which can be ice temperature. The potentiometer will be connected in middle. Other way is by using voltmeters. Take the case of Type T thermocouple (Cu-Constantan).
- 14. Contd. Thermocouples 14 To find the voltage e1,the voltmeter is connected to the couple which creates two more junction, J3& J2. As J3 is Cu-Cu junction the net emf produced will be zero. So it can be neglected. But J2 is Cu-Constantan, which creates another emf V2 (opp. to V1) which creates a net emf and is measured by voltmeter. ie, net V ∝ Tj1 –Tj2 where Tj2 is the reference temp.
- 15. Conclusion Thermocouples 15 Merits of thermocouples Cheap Simple construction Temperature range upto 1000°C Demerits Corrosion Relationship between process temp and thermocouple signal is not linear. Requires high temperature difference