be compatible. Both eBay and Amazon sell the older
model 7911.
The cups in the Davis unit rotate a magnet past a
magnetic sensor once every revolution. The calibration is
extremely simple. You count the number of pulses
occurring in 2.250 seconds: 10 pulses = 10 MPH; 20
pulses = 20 MPH; and so on. The unit also contains a 20K
ohm continuous-rotation potentiometer which can be
used to track the direction of the wind. However, I didn’t
use the pot in this project.
The output of the anemometer is filtered to
eliminate any noise from the sensor or the 75 feet of
cable running through the attic. I didn’t employ any
lightning arrestors, although whenever a
thunderstorm comes along I can’t help but think
about whether the next lightning strike is going to
electrify my desk and me with it!
The pulses from the cups are counted by an
Arduino (also used in this project) for exactly 2.250
seconds and stored in a small array. This counting
process repeats every 2.250 seconds until a full
minute has elapsed, so there are approximately 25
readings/minute. Next, the peak wind speed from
the 25 readings is determined and stored as the next
data point to be plotted, i.e., each minute the peak
during the past minute is plotted. Of course, you
could plot the average value for the past minute
instead of the peak.
30 February 2016
■ FIGURE 5. The Davis anemometer
measures the number of rotations in 2. 25
seconds; 10 rotations/2.25 sec = 10 MPH.
■ FIGURE 3. The control box contains three processors: a
Parallax SX28, Arduino Mega 2560, and Raspberry Pi 2. ■ FIGURE 4. Here, 8-10 GPS satellites are
typically in use (as shown on the LCD), even with
the GPS receiver mounted indoors.
