Set the
PConfig
parameter to Low Level AC and the
POption
parameter to Frequency.
The expression for wind speed (U) is:
U = Mx + B
where
M = multiplier
x = number of pulses per second (Hertz)
B = offset
(p. 10) lists the multipliers to obtain miles/hour or meters/second when the
measurement instruction is configured to output Hz.
Table 7-2: Wind speed multiplier
Model
Miles/Hour
Output
Meters/Second
Output
05103, 05103-45, or 05106
0.2192
0.0980
05108 or 05108-45
0.3726
0.1666
05305
0.2290
0.1024
Set the offset to zero since the helicoid propeller calibration passes through zero (Gill, 1973;
Baynton, 1976).
7.2.2 Wind direction
The wind vane is coupled to a 10 kΩ potentiometer, which has a 5-degree electrical dead band
between 355 and 360 degrees. A 1 MΩ resistor between the signal and ground pulls the signal to
0 mV (0 degrees) when wind direction is in the dead band (between 355 and 360 degrees).
Wind direction is measured by the
BRHalf()
instruction.
Some CRBasic measurement sequences can cause the measurement of the wind direction to
return a negative wind direction (–30º) while in the dead band. To overcome this problem, all
program examples use a delay of 20 ms (20,000 μs) and set any negative wind direction values to
0.0: If WindDir < 0, then WindDir = 0.0.
The excitation voltage, range codes, and multipliers for the different data logger types are listed
in
Wind direction measurement theory
(p. 24) has additional information on the
BRHalf()
measurement instruction.
Wind Monitor Series
10
