Here is the first test of my Android Windmeter.
It works with two small magnets attached to the anemometer axis and rotating above the smartphone's magnetometer.
This produces a very small fluctuation of the magnetic field. When applying a fast fourier transform you can get the rotation speed.
Then the App allows to calibrate the speed using the GPS and display it on a gauge.

Well, see it into action :
https://www.youtube.com/watch?v=jA16slSEuuM

Prusa

i3 MK2S

No

Yes

200µm

100% but could be less