An RTD sensor, also known as a Resistance Temperature Detector, is a type of sensor used to measure temperature by correlating the resistance of the RTD element with temperature.

RTD sensors are commonly made of materials like platinum, nickel, or copper, which exhibit a predictable change in electrical resistance with changes in temperature. The most popular type of RTD is made from platinum, often referred to as a Pt100 sensor, where 100 represents the nominal resistance of the sensor at 0 degrees Celsius.

As the temperature changes, the resistance of the platinum element changes in a predictable and repeatable manner, allowing for accurate temperature measurement.