by : by Kaushik sanghani
Pin : 361161
Mo : 91 9909401178 Thermocouples : Thermocouples Most frequently used method to measure temperatures with an electrical output signal. What are thermocouples? : What are thermocouples? Thermocouples operate under the principle that a circuit made by connecting two dissimilar metals produces a measurable voltage (emf-electromotive force) when a temperature gradient is imposed between one end and the other.
They are inexpensive, small, rugged and accurate when used with an understanding of their peculiarities. Thermocouples Principle of Operation : Thermocouples Principle of Operation In, 1821 T. J. Seebeck observed the existence of an electromotive force (EMF) at the junction formed between two dissimilar metals (Seebeck effect).
Seebeck effect is actually the combined result of two other phenomena, Thomson and Peltier effects.
Thomson observed the existence of an EMF due to the contact of two dissimilar metals at the junction temperature.
Peltier discovered that temperature gradients along conductors in a circuit generate an EMF.
The Thomson effect is normally much smaller than the Peltier effect. Slide 5: Let’s take a look at this circuit How thermocouples work : How thermocouples work It is generally reasonable to assume that the emf is generated in the wires, not in the junction. The signal is generated when dT/dx is not zero.
When the materials are homogeneous, e, the thermoelectric power, is a function of temperature only.
Two wires begin and end at the same two temperatures. Generally, a second order Eqn. is used. Material EMF versus Temperature : Material EMF versus Temperature With reference to
of pure Platinum emf Temperature Chromel Iron Copper Platinum-Rhodium Alumel Constantan Thermocouple Effect : Thermocouple Effect Any time a pair of dissimilar wires is joined to make a circuit and a thermal gradient is imposed, an emf voltage will be generated.
Twisted, soldered or welded junctions are acceptable. Welding is most common.
Keep weld bead or solder bead diameter within 10-15% of wire diameter
Welding is generally quicker than soldering but both are equally acceptable
Voltage or EMF produced depends on:
Types of materials used
Temperature difference between the measuring junction and the reference junction Thermocouple Tables (EMF-Temperature) : Thermocouple Tables (EMF-Temperature) Thermocouple tables correlate temperature to emf voltage.
Need to keep in mind that the thermocouple tables provide a voltage value with respect to a reference temperature. Usually the reference temperature is 0°C. If your reference junction is not at 0°C, a correction must be applied using the law of intermediate temperatures. Reference Temperature Systems and Zone Boxes : Reference Temperature Systems and Zone Boxes Ice Baths
Accurate and inexpensive
Electronically Controlled References
Require periodic calibration and are generally not as stable as ice baths, but are more convenient. Zone boxes : Zone boxes A zone of uniform temperature
that insures all connections made
within the zone are at the same temperature. What thermocouple materials should be used? : What thermocouple materials should be used? Depends on requirements:
Chemical resistance issues
Abrasion or vibration resistance
Installation requirements (size of wire)
Thermal conduction requirements Thermocouple Material Vs EMF : Thermocouple Material Vs EMF Types T, J, and K are most commonly used thermocouples (see Table 16.8 of the “Handbook”). Simple TC Model “EMF-Temperature Sketch” : Simple TC Model “EMF-Temperature Sketch” Two materials
Material A (+)
Material B (-)
Plus and minus refers
to how the emf changes
Number junctions around circuit and draw emf Temperature 3 1 2 3 1 2 T meter T junction Measured Emf A B Law of Intermediate Metals : Law of Intermediate Metals 2) Insertion of an intermediate metal into a thermocouple circuit will not affect the emf voltage output so long as the two junctions are at the same temperature and the material is homogeneous.
Permits soldered and welded joints. A Demonstration of the Law of Intermediate Metals : A Demonstration of the Law of Intermediate Metals emf Temperature 3 1 2 T ref T 2 and 4 Measured Emf Fe C Fe (+)
P (+) 5 4 6 Tcandle 1 2 T measured 3 5 4 6 Signs of the
materials used Law of Intermediate Temperatures : Law of Intermediate Temperatures If a thermocouple circuit develops a net emf1-2 for measuring junction temperatures T1 and T2, and a net emf2-3 for temperatures T2 and T3, then it will develop a net voltage of emf1-3 = emf1-2 + emf2-3 when the junctions are at temperatures T1 and T3.
emf1-2+ emf2-3= emf1-3 T2 T3 T1 T3 T2 T1 A Demonstration of the Law of Intermediate Temperatures : A Demonstration of the Law of Intermediate Temperatures emf T 1 T 2 Fe C T 3 emf23 emf1-2+ emf2-3= emf1-3 emf13 emf12 A Demonstration of the Law of Intermediate Temperatures : emf T ref T hot Measured Emf Fe C 1 2 T measured 3 5 4 Hot Zone 1 2 3 4 A Demonstration of the Law of Intermediate Temperatures Slide 20: If a thermocouple circuit of materials A and C generates a net emfA-C when exposed to temperatures T1 and T2, and a thermocouple of materials C and B generates a net emfC-B for the same two temperatures T1 and T2, then a thermocouple made from materials A and B will develop a net voltage of
emfA-B = emfA-C + emfC-B
between temperatures T1 and T2.
Sometimes useful in the calibration of different thermocouple wires. Single and multiplexing : Single and multiplexing Temperature Measurement Errors : Temperature Measurement Errors Conduction
Grounding issues and shorts, especially on metal surfaces