Thermocouples

Thermocouples - Thermocouples and Convective Heat Transfer...

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Thermocouples and Convective Heat Transfer A thermocouple is a simple device for measuring temperature. Two metals (wires) of different composition are joined (welded or soldered together) at one end and that's it! They produce a small voltage between the other, open ends that is proportional to the temperature that the joint is exposed to. The operation of a thermocouple can be understood by remembering two basic principles: 1. When two dissimilar metals are in contact, their different affinities for free electrons produces a small current across the junction. A B T = T1 T2 > T1 2. The magnitude of the difference varies with temperature. A B The voltage produced is small, on the order of a few millivolts, so it requires amplification. The metals used are alloys that are specially formulated to maximize the emf output, i.e., maximize sensitivity.
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The standard configuration is often denoted this way: T1 T2 A B p q i Note that if T 1 = T 2 , both thermocouples will produce the same emf but with opposite sign, so the total emf will be zero. If the circuit is opened, and T 1 T 2 , there will be an emf across the terminals. T1 T2 A B p q The second thermocouple is not strictly necessary, but is useful as a reference junction. It is usually kept at 0°C. 2
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Further useful properties: Law of Intermediate Metals The introduction of a third metal into the circuit does not affect the emf, provided T is uniform across all the junctions. Practical consequences of LIM Consider how the thermocouple is constructed. Solder sits between the two metals. Essentially, this creates 2 thermocouples, one of Metal A and the solder, and another of Metal B and the solder. The output emf then is E AS + E SB = E AB Metal A Metal B Solder S Another pair of thermocouple junctions are created when the thermocouple wires are attached to electrical wires (Cu). It is important that those junctions are kept at the same temperature so that the net emf they produce is zero.
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Thermocouples - Thermocouples and Convective Heat Transfer...

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