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Introduction: A thermocouple is a junction between two differentmetals that produce a voltage related to a temperaturedifference. Thermocouples are widely used types oftemperature sensors and measurement & control .It can alsobe used to a convert heat into electric power. Any junction of dissimilar metals will produce anelectric potential related to temperature. Thermocouplesof practical measurement of temperature are junction ofspecific alloys which have a predictable relationshipbetween temperature & voltage. Different alloys areused for different temperature ranges.
Principle of Operation: In 1821, the German–Estonian physicist Thomas JohannSeebeck discovered that when any conductor is subjected to athermal gradient, it will generate a voltage. This is now known asthe thermoelectric effect or Seebeck effect. Any attempt to measure this voltage necessarily involvesconnecting another conductor to the "hot" end. This additionalconductor will then also experience the temperature gradient, anddevelop a voltage of its own which will oppose the original.Fortunately, the magnitude of the effect depends on the metal inuse. Using a dissimilar metal to complete the circuit creates acircuit in which the two legs generate different voltages, leaving asmall difference in voltage available for measurement.
Types of Thermocouple: Thermocouple Metal Content in Positive lag Metal Content in Negative lag Temperature Range Types B 70.4% Platinum(Pt) 93.9% Pt, 6.1% Rh 1600-3100*F 29.6% Rhodium(Rh) (870-1700*c) E 90% Nickel(Ni) 50% Copper(Cu)45% Ni 32-1650*F(0-900*c) 10% Chromium(Cr) J 99.5% Iron(Fe) 50% Cu, 45%Ni 32-1380*F(0-750*c) K 90%Ni, 10% Cr 95% Ni, 5% Various Elements 32-2280*F(0-1250*c) N 84.4% Ni, 14.2% Cr, 1.4% 95.5% Ni, 4.4% Si 32-2280*F(0-1250*c) Silicon R 87% Pt, 13% Rh 100% Pt 32-2640*F(0-1450c) S 90% Pt, 10% Rh 100% Pt 32-2640*F(0-1450c) T 100% Cu 55% Cu,45% Ni -330-660*F(-200-350*c) C* 95% Turesten(W) 74% W, 26% Re 32-4200*F(0-2315*c) 5% Rhenium(Re) D* 97% W, 3% Re 75%W, 25% Re 32-4200*F(0-2315*c) G* 100% W 74%w, 26% Re 32-4200*F(0-2315*c)