An RTD sensor is a type of temperature sensor that changes resistance in response to temperature changes. Usually a 100 ohm platinum element is used where the relationship between resistance and temperature is standardised and repeatable. RTDs are passive, meaning they do not generate an output on their own but use electronic devices (indicators) to measure the sensor's resistance by passing a small current (typically 1mA) through the sensor, which in turn generates a voltage. The resistance of the sensor increases as the temperature increases and are provided in our resistance vs temperature tables.