41
ATMOS 41W
Where the overbar indicates an average of the values sampled every 10 s, wind direction is
computed with
Equation 11
.
Equation 11
θ =
tan
−
1
( v / u )
The wind measurement through more frequent SDI-12 commands requires 42 ms to
complete. An additional 60 ms are required for the computations to determine phase
differences. The anemometer samples every 10 s (or more often if requested). The gust speed
reported is the highest instantaneous wind speed measured during the selected averaging
interval (must be >20 s or gusts will equal speed).
The ATMOS 41W uses a wind speed spike rejection scheme to protect against the inclusion
of spurious wind speed spikes in the averaged wind speed data. The ATMOS 41W measures
wind speed every 10 s and keeps a running average of the last 10 measurements. If an
instantaneous measurement is more than eight times the running average, then the
instantaneous measurement is rejected and 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 investigating 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.
NOTE: Cup anemometers average over a much longer interval than 42 ms, so the gusts measured with a sonic
anemometer will have a larger peak-to-mean ratio than one would see with a cup anemometer.
3.2.8 TILT SENSOR
The ATMOS 41W is also equipped with a tilt sensor similar to those found in smartphones.
The primary use of the tilt sensor data is to ensure the ATMOS 41W remains level at all times.
Regularly check tilt data to ensure the ATMOS 41W is level; if it has tilted, return to the site
and level again. Even a slightly off-level ATMOS 41W will cause errors in the solar radiation
measurement, and a 2° tilt can cause large errors in the rainfall measurement. Although this
sensor may be used to level the instrument, it is much easier to use the small bubble level
on the bottom of the anemometer plate. The ATMOS 41W comes with an adjustable leveling
bracket, requiring only a 5/16-in (8-mm) wrench to adjust.
When powered on, the ATMOS 41W measures the tilt once every 60 s and records the
instantaneous values. At the end of the measurement interval, the ATMOS 41W outputs the
average of the instantaneous values measured during that measurement interval.
3.2.9 SOLAR PANELS
The ATMOS 41W comes with two integrated solar panels on the west and east orientation.
These allow the rechargeable NiMH AA batteries to recharge, and allow long-term
deployment with minimal interaction. While the west to east solar panel orientation is
suboptimal for solar recharge, the ATMOS 41W is designed to have a low power draw such
