Sensors are used to detect certain quantities such as temperature and irradiation. Platinum sensors, which change their electrical resistance proportionally to the temperature, are used for temperature measurements. The sensor type is indicated by the third letter “P”, for example: FKP or FRP. FK designates the sensor of the heat source (e.g. collector sensor) and FR the reference sensor (e.g. store sensor). these sensor types have the same electrical features. they differ only in the connecting cable.