Campbell Scientific CR10X Measurement and Control System - Manual (Page 175)

11-1
SECTION 11. OUTPUT PROCESSING INSTRUCTIONS
*** 69 WIND VECTOR ***
FUNCTION
Instruction 69 processes the primary variables
of wind speed and direction from either polar
(wind speed and direction) or orthogonal (fixed
East and North propellers) sensors. It uses the
raw data to generate the mean wind speed, the
mean wind vector magnitude, and the mean
wind vector direction over an output interval.
Two different calculations of wind vector
direction (and standard deviation of wind vector
direction) are available, one of which is
weighted for wind speed.
When used with polar sensors, the instruction
does a modulo divide by 360 on wind direction,
which allows the wind direction (in degrees) to
be 0 to 360, 0 to 540, less than 0, or greater
than 540. The ability to handle a negative
reading is useful in an example where a difficult
to reach wind vane is improperly oriented and
outputs 0 degrees at a true reading of 340
degrees. The simplest solution is to enter an
offset of -20 in the instruction measuring the
wind vane, which results in 0 to 360 degrees
following the modulo divide.
When a wind speed sample is 0, the instruction
uses 0 to process scalar or resultant vector
wind speed and standard deviation, but the
sample is not used in the computation of wind
direction. The user may not want a sample less
than the sensor threshold used in the standard
deviation. If this is the case instruction 89 can
be used to check wind speed, and if less than
the threshold, Instruction 30 can set the input
location equal to 0.
Standard deviation can be processed one of
two ways: 1) using every sample taken during
the output period (enter 0 for parameter 2), or,
2) by averaging standard deviations processed
from shorter sub-intervals of the output period.
Averaging sub-interval standard deviations
minimizes the effects of meander under light
wind conditions, and it provides more complete
information for periods of transition
1
.
1
EPA On-site Meteorological Program
Guidance for Regulatory Modeling Applications.
Standard deviation of horizontal wind
fluctuations from sub-intervals is calculated as
follows:
σ(Θ)=[((σΘ
1
) 2 +(
σΘ
2
) 2 ...+(
σΘ
M
) 2 )/M] 1/2
where
σ(Θ) is the standard deviation over the
output interval, and
σΘ
1
...
σΘ
M
are sub-interval
standard deviations.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 2 Repetitions
02: 4 Samples per sub-
interval (number of
scans, enter 0 for no
sub-interval)
03: 2 Sensor/Output 2 digits:
AB
A
Sensor type:
0 = Speed and
Direction
1 = East and North
B Output option:
0 S,
Θ1, σ(Θ1)
1 S,
Θ1
2 S, U,
Θu, σ(Θu)
04: 4 First wind speed input
location no. (East wind
speed)
05: 4 First wind direction
input location no.
(North wind speed)
Outputs Generated: 2-4 (depending on output
option) per repetition
A sub-interval is specified as a number of
scans. The number of scans for a sub-interval
is given by:
Desired sub-interval (secs) / scan rate (secs)
The EPA recommends that hourly standard
deviation of horizontal wind direction (sigma
theta) be computed from four fifteen-minute
sub-intervals.
In an example where the scan rate is 1 second
and the Output Flag is set every 60 minutes, the
standard deviation is calculated from all 3600