The thermocouple is one of the least complex of all sensors and as such it can be both rugged and low cost making it ideally suited for use in industrial applications.
It consists of two wires of dissimilar metals that are joined at the measurement point; referred to as the hot junction. The output from the sensor is a voltage proportional to the temperature difference between the hot junction and the cold end of the wires. The voltage generated is both very small and nonlinear, this presents design challenges for manufacturers of thermocouple instrumentation. However if the instrumentation is designed correctly the thermocouple provides a very low cost sensor that can be easily replaced if damaged.