A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the Seebeck effect, and this voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature sensors in gas turbine engines.
The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances.
When heat is applied to one of the two conductors or semiconductors, heated electrons flow toward the cooler conductor or semiconductor. If the pair is connected through an electrical circuit, direct current (DC) flows through that circuit.
Illustration of the thermoelectric effect with a simple thermopile made from iron and copper wires [Source: wiki commons. Author: CMG Lee]
Temperature measurement in gas turbines
The main reasons to measure the temperature in a gas turbine are:
Performance evaluation - to calculate the efficiency of compressors and turbines.
Engine control - the output thrust of an engine can be set by reference to a specific temperature e.g. Turbine Inlet Temperature. Temperature limits are implemented in the control of the engine to avoid component damage.
Health monitoring of high temperature components - temperature history of components can be used to estimate their residual life.