Thermocouples and RTDs (Resistance Temperature Detectors) are two common types of temperature sensors used in various industrial and commercial applications. They both measure temperature by converting it into an electrical signal, but they work on different principles.   Thermocouples Principle: Generates a voltage proportional to the temperature difference between two dissimilar metals joined at a junction. RTDs Principle: Measures temperature by measuring the change in electrical resistance of a metal wire or film.