METER ATMOS 41 - Barometric Pressure Sensor; Tilt Sensor; Theory; Wind Speed and Direction

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ATMOS 41
3.8 BAROMETRIC PRESSURE SENSOR
The barometric pressure sensor is located behind the Teflon screen next to the relative
humidity sensor. It measures the atmospheric pressure of the environment in which the
ATMOS41 is deployed. With a range from 50 to 110 kPa, it is suitable for measurement
across a wide range of elevations, but keep in mind that the magnitude of sensor output will
depend chiefly on the installation altitude with subtle changes caused by weather.
When powered on, the ATMOS41 measures the barometric pressure once every 60 s and
records the instantaneous values. When queried, the ATMOS41 outputs the average of the
instantaneous measurements since the last query.
3.9 TILT SENSOR
The ATMOS41 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41 remains level at all times.
Regularly check X and Y tilt data to ensure the ATMOS41 is level; if it has tilted, return to the
site and level again. Even a slightly off-level ATMOS 41 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. For reference, a positive X tilt indicates that the
ATMOS41 is leaning north, and a positive Y tilt indicates that the ATMOS 41 is leaning west.
When powered on, the ATMOS41 measures the tilt in the X and Y orientation once every 60 s
and records the instantaneous values. When queried, the ATMOS41 outputs the average of
the instantaneous measurements since the last query.
3.10 THEORY
The following sections explain the theory of wind speed, wind direction, and air
temperature measurements.
3.10.1 WIND SPEED AND DIRECTION
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 3.
Equation 3
c = 20.067 T(1+
0.32e
p
For a given sound path length, d (m), the number of wavelengths, n, in still air is
determined with Equation 4.
Equation 4
n =
vd
c