ECE5320 Mechatronics Assignment#01: Literature Survey on Sensors and Actuators Topic: Thermocouples

1. ECE5320 MechatronicsAssignment#01: Literature Survey on Sensors and Actuators Topic: Thermocouples Prepared by: Jacob Brown Dept. of Chemistry and Biochemistry Utah State University E: jacob.brown@usu.edu ; T: (435)797-1314 W: http://www.chem.usu.edu/faculty_staff/webpages/jacobbrown.php 3/7/2008

2. Outline • Reference list • To probe further • What is a Thermocouple • Major applications • Basic working principle illustrated • Intermediate Metals and Extension Wires • Thermocouple Construction • Major specifications • Limitations • where to buy • how to choose ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

3. Reference list • Beckwith, T. J, Marangoni, R. D. and Lienhard, J. H., Mechanical Measurements, 5th ed. Addison-Wesley, 2003, pp. 676-691. • Omega, Temperature Measurement Handbook and Encyclopedia, vol. MMX 6th ed., pp xx-xx, 2000. • Seebeck, T. J. Evidence of Thermal Current of the Combination of Bi-Cu by Its Action on Magnetic Needle. Berlin: Abt. D. Königle, Akad. D. Wiss. 1822-23, p. 265. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

4. To explore further (survival pointers of web references etc) • Omega, Thermocouple Selection Guide • http://www.omega.com/toc_asp/frameset.html?book=Temperature&file=SEL_GUIDE_THERMOCOUPLE • A Wikipedia entry for thermocouple • http://en.wikipedia.org/wiki/Thermocouple • Temputures.com • http://www.temperatures.com/tcs.html ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

5. What is a thermocouple? • A thermocouple is a temperature measurement sensor that consists of two dissimilar metals joined together at one end (a junction) that produces a small thermoelectric voltage when the junction is heated.[2] • The thermoelectric effect was discovered by T. J. Seebeck in 1821.[1] • The thermoelectric voltage occurs when a wire experiences a temperature gradient. • Two dissimilar wires experiencing the same temperature gradient will produce dissimilar potentials and thus create a very small µV-mV thermoelectric voltage. • The change in thermoelectric voltage is interpreted by the thermocouple thermometers as a change in temperature.[2] • Thermocouples are often abbreviated as TC ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

6. Major applications • Measuring temperature of • Surface of a solid or semi-solid • Core of a Liquid or semi-solid • Gas or vacuum • Objects with small mass or small area • Oven Temperatures up to 2000°F/1100°C • Molds, dies and platens ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

7. Basic working principle • In 1821, T. J. Seebeck discovered that an electromotive force exists across a junction formed by two dissimilar metals [3] • The coupling of two temperatures via two dissimilar wires A, B is the simplest of thermocouples (see figure 16.9 , [1]) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

8. Basic working principle • Note that at minimum two junctions are required. • one is generally the desired sensing spot • and the other is at known preferably stable temperature • Typically referred to as the cold junction or hot junction • The thermoelectric voltage (emf) is generated by to two separate sources[1] • Peltier Effect • Created by the joining of the two metals at the junction • Temperature gradient within the junction of the Thermocouple • Major contributor • Thomson effect • Created by the temperature gradient along the wires. • Usually ignored with proper selection of wires/insulation ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

9. Intermediate Metals and Extension Wires • The inclusion of a third metal has no effect on the total emf so long as: • The two ends of the wire are at the same temperature • The wire completes the simple thermocouple circuit • Figure 16.10 from [1] • Third wire added in line • Third wire added at junction, typically done in order to measure the emf with a miliVolt meter ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

10. Intermediate Metals and Extension Wires • It may be desirable to add copper extension leads to reduce cost. • Thermocouple wire is relatively more expensive than general copper wire • Figure 16.14(a) [1] is an appropriate use of copper extension • Temperature Tr must be known. • Figure 16.15(b) is an appropriate use of copper to extend a Copper-Constantan thermocouple. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

11. Intermediate Metals and Extension Wires • Typically the Reference is a cold ice water bath • Ice water having the nice properties of • Consistent Temp = 0°C • stable until all ice is melted • Other References may be used • Constant voltage supply • Heater element temperature • Hot junction ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

12. Thermocouple Construction • Thermocouples may be prepared by twisting the two wires together and brazing, or preferably welding, as shown in Fig. 16.11 [1] • Point j in Fig. 16.11 is the location of the effective junction. • Notice it is on the inside between the two wires at the first point of contact • This point is where the effective temperature is being measured ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

13. No cold Junction • If no cold junction is used. • The voltmeter terminal temperature must be known. • Preferably held constant • Allow system to run for an hour before operating • Once temperature of terminals is known it can be added/subtracted from the measurement. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

14. example non-cold junction • Example • Temp of non-cold junction = 20°C • Emf read by milivolt meter = 2.887mV • Thermocouple Type=T • Using TC tables or equations(that are assuming 0C cold junction) • 20°C would translate to an effective emf of 0.790mV • Add this to measured emf • 2.887mV + 0.790mV = 3.677mV (corrected emf) • 3.666mV translates to 87°C (correct measured temperature) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

15. Type T Thermocouple Table Non cold junction temperature Corrected emf Example Using a standard NIST Table From Pyromation Inc's http://www.temperatures.com/tctables.html ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

16. Major specifications • Classified by type (J,K,T,E,R,S,B) • Refers to the metals used • Defines what color lead wire insulation is to be used. • Size of junction • Size of lead wires • Junction mounting style • Insulation or Leads leads • Certified calibrations against standard calibration • Class one • Higher accuracy • Class two • Lower accuracy ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

17. Thermocouple Types • Below shown Thermocouple Type • Temperature range • Tolerance • Insulation Color code • Table taken from Wikipedia ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

18. Limitations • Each Thermocouple type has a finite temperature range which varies with Type. • Thermocouples are most suitable for measuring over a large temperature range. • They are less suitable for applications where smaller temperature differences need to be measured with high accuracy. • The response time decreases as the size of the thermocouple junction increases. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

19. How to choose • Select Temperature Range • Narrows down which Type can be used • Select Probe Type • Selected by media to be measured • i.e. Surface mount or internal pipe, etc • Select Probe Size • Larger size decreases response time but more robust. • Select lead wire • Factory mounted plug or bare wire. • Shielding for both noise and harsh environment • Select Calibration • Higher accuracy comes at a price. ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators

20. Where to buy • Omega (www.omega.com) is probably the largest supplier • Nordic Sensors Inc. (www.nordicsensors.com) • Omron Electronics (available though Digi-Key) ECE5320 Mechatronics. Assignment#1 Survey on sensors and actuators