[AllyCat]

I was really thinking about how one might make their own using the Sensirion
SDP3xx to produce a signal that Weatherduino could accept. I assume the Darrera one uses similar, (if not the same) SDP3xx sensors.

Hi,

I'm rather confused that you seem to be mixing pressure sensors with an "Ultrasonic Time Of Flight" windspeed sensor.

The latter uses two pairs of Ultrasonic "Loudspeakers" and "Miicrophones" (at right angles) to measure how long it takes for sound to travel between one and the other.  The time delay reduces if the sound travels "with" the wind and increases if "against" the wind.  It's quite a difficult measurement as the speed of sound is around 300 metres/second.   The microcontroller then uses fairly simple trigonometry to convert the two windspeed vectors to a combined (linear) speed and direction.

It's quite an "advanced" technique, so the price is not overly surprising (for the claimed accuracy).  I'm more surprised that the Netatmo appears to do it for less than £100.  But that it uses 4 x non-rechargeable AA cells is probably a clue.  

Cheers,  Alan.