\▐thәr-mә-▐kә-pәl\ n (1890) (TC) A pair of connected, welded junctions formed by two wires of dissimilar metals such as iron and Constantan. If a temperature difference exists between the two junctions, a weak emf, 40–50 μV/K, is generated in nearly linear proportion to the ∆T. Originally, one junction was placed in an ice bath to serve as a reference. Then, the emf developed in the circuit was simply convertible to temperature. Modern thermocouples employ a single junction and the instrument to which the TC is connected senses its own temperature and compensates the incoming signal for the difference between that temperature and 0°C. Plain wire TCs respond very rapidly to temperature changes but those used in plastics-processing equipment are always sheathed in a sturdy protective tube, so are slower. Metal compositions of commercial TC wires are so carefully controlled that, except for exacting laboratory work, it is usually unnecessary to calibrate thermocouples. In one type of hand-held instrument, called a Pyrometer, the TC is integral with a microammeter whose needle moves across a temperature scale. Thermocouples are the most used temperature sensors in plastics processing because they are sturdy, simple, reliable, readily available, and cheap (Weast RC (ed) (1971) Handbook of chemistry and physics, 52nd edn. The Chemical Rubber Co., Boca Raton).
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