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3 Structure / Mode of operation
Wind speed / wind direction:
Wind measurement is based on the hot wire principle. The underside of the housing is equipped
with a heated cylindrical sensor. A PID controller adjusts the temperature of the cylinder to a
temperature that is constantly increased in relation to the environment. The supplied heat energy is
a measure of wind speed.
The metal cylinder contains four temperature-measuring resistors. These resistors are thermally
coupled with the cylinder and positioned according to the 4 points of the compass. When an
incident flow affects the cylinder as a function of the wind direction, this is accompanied by a
temperature gradient which is registered by the measuring resistors. The relationships between the
4 temperature values are used to calculate the wind direction.
In case the wind direction cannot be determined because the wind velocity is 0m/s, the value is set
to 0. Wind from the north is displayed with 360°.
Brightness:
The brightness measurement is carried out via 4 Silicium photo sensors, which are aligned to the
4 cardinal directions in the mean elevation angle (40°).
Twilight:
Twilight means the light diffusion in the atmosphere, which arises with the smooth transition
between day and night before the beginning or after the end of day.
i.e., the solar disc is not visible.
The twilight is direction-independent.
It is calculated from the sum of the 4 measuring values of the direction-independent brightness
sensors.
A change to the mean value from the 4 brightness values is possible by command.
Global irradiance:
A silicon PIN photodiode is used to measure global irradiance. The sensor is positioned horizontally
and registers the diurnal values of the solar irradiation intensity.
Precipitation:
The detection of precipitation is based on capacitance measurement, i.e. the capacity of the sensor
surface varies when wet. The sensor is installed in the housing cover. An integrated heating system
adjusts the sensor area to an overtemperature in relation to the ambient temperature. This
overtemperature (approx. 2K) prevents bedewing of the sensor surface. The thermal output is
increased with precipitation. This accelerates drying of the sensor, allowing the time at which
precipitation ended to be identified more accurately.
