The present invention concerns a sensor means for determining an interior humidity of an interior volume delimited by a wall, e.g. the passenger compartment of a vehicle, and a fogging up tendency on an inner surface of the wall. The sensor means comprises a dew point sensor with a humidity sensor element and a temperature sensor, wherein the humidity sensor element and the temperature sensor are thermally coupled, and a surface temperature sensor. The sensor arrangement is suitable, in particular, for detecting fogging up on the inner side of a vehicle window and the humidity inside the vehicle.
Several solutions have been suggested to prevent fogging up or to measure the interior humidity. EP 1380481 A2 and EP 1306242 A1 propose mounting of a humidity sensor element directly on the window or in its direct vicinity. This sensor element measures the relative air humidity. The obtained data gives information about the tendency of the window to fog up. EP 1306242 A1 moreover proposes to also consider the temperature in the passenger compartment of a vehicle to prevent fogging up.
WO 02/33395 A1 discloses a dew point sensor. If heat is withdrawn from a gaseous medium, e.g. water vapour, which may happen on the inner surface of a vehicle window, it condenses into liquid, e.g. water, at a certain temperature. This temperature with a certain relative air humidity is called the dew point and can be calculated from a measured relative air humidity (interior humidity) and a measured air temperature. If, at a certain relative air humidity, a surface temperature is close to the dew point, the surface tends to fog up (fogging up tendency), i.e. the liquid condenses on the surface. The dew point sensor has a mounting base on which an air humidity sensor and an air temperature sensor are disposed. The mounting base has a thermally conducting coating which is in thermal contact with the air humidity sensor and the air temperature sensor. Since the air humidity sensor and the air temperature sensor detect data at identical temperatures through thermal contact, the dew point at the location of the mounting base of the dew point sensor can be determined from the detected sensor data.
A disadvantage of the conventional solutions consists in that there is a temperature difference between the humidity sensor and the window surface with the result that the measured relative humidity at the sensor does not correspond to the relative humidity on the window. Exact determination of the fogging up tendency is therefore not possible. Another disadvantage consists in that this arrangement does not provide exact information about the relative air humidity in the passenger compartment, which is a decisive feature of comfort.
It is the underlying purpose of the invention to provide a sensor means and a fastening device for mounting thereof, which eliminate the disadvantages of prior art, wherein, in particular, the fogging up tendency of a support surface, i.e. an inner surface of a wall of an interior volume, in particular, a vehicle window and also the interior humidity of the interior volume, in particular, of a vehicle passenger compartment, are reliably detected.