METER ATMOS 41 - Temperature

21
ATMOS 41
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41 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41 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41 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.
3.10.2 TEMPERATURE
The ATMOS41 uses an energy balance correction to adjust measured temperature to actual
air temperature according to Equation 11.
Equation 11
T
corr
= T
uncorr
−
α
S
S
t
c
p
k u / d
⎛
⎝
⎜
⎜
⎜
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
⎟
⎟
⎟
where:
α
s
=
the absorptivity of the surface to solar radiation
S
t
=
the total solar radiation measured
c
p
=
29.3 J mol
-1
C
-1
, k is a constant
u =
the wind speed
d =
the characteristic dimension
Although these values can be assumed, some ( α
s
and k) were optimized using a Levenberg
Marquardt Least Squares analysis. Optimal air temperature was obtained using an Apogee
TS-100 Aspirated Radiation Shield. Data were collected over several weeks and final values
are shown in Table 2.
NOTE: A maximum value optimization for S
t
was added because radiation values higher than that cause the corrected
temperature to deviate from actual values more than when a maximum
S
t
was used.