This sensor is slick!
A hall effect array sensor and a rotating magnet provides for a completely friction less wind direction sensor. A diametrically polarized magnet is held in the bottom of the vane shaft which runs in a bronce shaft. A ball bearing in the housing make the vane spin completely freely. Built into the sensing chip are 4 separate Hall sensors that measure the direction and strength of the magnetic field from the magnet on the shaft. The built in logic then calculates the absolute position of the magnet in respect to its case and resolves the rotation into a voltage. The result can be read as an analog voltage increasing from 0-360 degree.
The first prototype of this sensor is still in use after more than 7 years of nearly constant movement by the wind.
See the download section for specs.