space a sensor element of the sensor device detects the heat transfer over time between the sensor element and the fluid the sensor element which has a temperature dependent electrical conductivity and to which a voltage has been applied is brought to a temperature which lies outside the range of the fluid temperatures in the passive phase the fluid is brought to a constant passive temperature by a heat transfer between the sensor element and the surroundings the sensor device provides a constant passive output voltage a heat transfer between the sensor element and its surroundings takes place in the active phase by changes in the fluid filled space the sensor device provides an active output voltage which is different from the passive output voltage when a difference between the output voltages is exceeded the signal is generated the device can be used for triggering the flushing action in sanitary installations and for keeping a level constant for example in aquarium installations application_number US 29899299 A description fig1 shows a sensor device 10 with a sensor element 12 with lines 14 a 14 b to which voltage has been applied as well as a heating element in the form of a heating resistor 16 which is connected via lines 18 a 18 b to a current or voltage source not represented.