3d Printed Anemometer Under 5$

I am building a small stand alone meteorological station. As always, I am trying not to buy each instrument but building them as far as I can because it is :
much more fun and instructivemuch cheaper
So I needed to find a way to measure the wind speed. An anemometer is basically a rotor with hemispherical cups mounted on it so it was a good pretext to get my 3D printer working. I also needed a way to measure the rotation speed of the rotor. I thought that a magnet and a hall sensor was a good solution.
Each time the south pole pass in front of the hall sensor, a service interrupt routine is called and a counter is incremented in the microcontroller. Each minute, the counter value is written on an SD card and reset to zero. For the moment, the result is only in rpm but if you want to have it in your favorite unit (m/h or km/h) you need to calibrate it. I will explain how I will do it. The anemometer is already on site so I will do it when returned.”