Some of the more common types of temperature sensors used in our trade are thermocouples, resistance temperature detectors (RTD), negative temperature coefficient (NTC) thermistors, and transistors. Some may look similar when encased in a particular housing, but they all are quite different and cannot be interchanged.
Thermocouples are sensors that generate a very low dc voltage. They are made of two dissimilar metals in contact with each other. A voltage is created across this junction which is proportional to the junction’s temperature. The “type” of thermocouple describes the metals used to make the junction, e.g., a J-type thermocouple uses iron in one wire and a copper/nickel alloy in the other. The higher the temperature, the higher the voltage produced by the thermocouple.