RTD stands for Resistance Temperature Detector, but are also known as PRT (Platinum Resistance Thermometers). A Platinum Resistance Thermometer is a type of RTD that uses platinum as a sensing element. The most common PRTs are PT100, PT500 & PT1000.
PT 100 sensors offer excellent accuracy over a wide temperature range. Unlike thermocouples, it is not required to use special cables to connect to the sensor. The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C.