Introduction:: Introduction: A thermocouple is a junction between two different metals that produce a voltage related to a temperature difference. Thermocouples are widely used types of temperature sensors and measurement & control .It can also be used to a convert heat into electric power. Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples of practical measurement of temperature are junction of specific alloys which have a predictable relationship between temperature & voltage. Different alloys are used for different temperature ranges.
Principle of Operation:: Principle of Operation:
Principle of Operation:: Principle of Operation: In 1821, the German–Estonian physicist Thomas Johann Seebeck discovered that when any conductor is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect . Any attempt to measure this voltage necessarily involves connecting another conductor to the "hot" end. This additional conductor will then also experience the temperature gradient, and develop a voltage of its own which will oppose the original. Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement.
Types of Thermocouple: : Types of Thermocouple: Thermocouple Types Metal Content in Positive lag Metal Content in Negative lag Temperature Range B 70.4% Platinum(Pt) 29.6% Rhodium(Rh) 93.9% Pt, 6.1% Rh 1600-3100*F (870-1700*c) E 90% Nickel(Ni) 10% Chromium(Cr) 50% Copper(Cu)45% Ni 32-1650*F(0-900*c) 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% Silicon 95.5% Ni, 4.4% Si 32-2280*F(0-1250*c) 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) 5% Rhenium(Re) 74% W, 26% Re 32-4200*F(0-2315*c) 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)
Thermocouple Advantages & Disadvantages:: Thermocouple Advantages & Disadvantages: