39
ATMOS 41W
Anemometer
convex surface
Figure 22 Anemometer
When powered on, the ATMOS 41W measures the wind speed and direction once every 3 s
and records the instantaneous wind vector components. At the end of the measurement
interval, the ATMOS 41W outputs the average of the instantaneous values for wind speed and
direction and the maximum instantaneous wind speed value for wind gusts.
The ATMOS 41W measures wind speed every 3 s and keeps a running average of the last
10 measurements. If an instantaneous measurement is more than eight times the running
average, the instantaneous measurement is rejected. It is not reported as the maximum gust
or included in the data that are averaged over the output interval. For normal ATMOS 41W
use cases, this is an effective method for eliminating inaccuracy resulting from spurious
spikes in wind speed (e.g., bumblebee checking out the ultrasonic path). In special use cases
where data are output frequently and large step changes in wind speed are present (e.g.,
turning on a wind tunnel), this spike rejection algorithm may result in an error code being
output temporarily.
WIND SPEED AND DIRECTION THEORY
The theory behind the anemometer comes from Campbell and Unsworth (1979). The speed
c
(m/s) of sound in still air depends on air temperature
T
(K), vapor pressure
e
(kPa), and
atmospheric pressure
p
(kPa), as shown in
Equation 4
.
Equation 4
c
=
20
.
067
T
1
+
0
.
32
e
p
⎛
⎝
⎜⎜
⎜⎜
⎞
⎠
⎟⎟⎟
⎟
