Vaisala WS425 - Manual (Page 19)

Chapter 2 __________________________________________________________ Product Overview
VAISALA _______________________________________________________________________ 19
speed measurement taken over the preceding averaging interval is
summed and the sum is then divided by the number of measurements.
The sensor computes the true running average. If the Data Acquisition
System requests data before the initial averaging interval completes, the
sensor returns the best possible running average.
In SDI-12 submode B, the average is determined over the averaging
interval specified, which ranges from three seconds to one hour.
In RS-232 mode, the average is determined over the averaging interval
specified, which ranges from one to nine seconds.
Wind direction is a circular function with a discontinuity at north, where
360 degrees is equal to zero degrees. For example:
359
° + 5° = + 4°
0
° - 5° = 355°
The microprocessor translates the circular function to a linear function so
that 359
° + 5° is translated to 364°. Also, 0° - 5° translated to 355°. This
way the wind direction average stays representative of the true situation
even if individual samples occur in both sides of the zero direction.
Vector Averaging of Wind Speed and
Direction
The vector average is available in SDI-12 submode B and serial mode.
Each x velocity and y velocity measurement over the averaging interval
is added and then divided by the number of measurements. The resultant
average x velocity and average y velocity are converted to polar direction
and magnitude, returning as the average direction in degrees and speed in
the chosen units. The average is determined over the specified averaging
interval (0 ... 9 s for serial mode,
3 s ... 1 h for SDI-12 submode B). The sensor computes a true running
average. If the DAS requests data before the initial averaging interval
completes, the sensor returns the best possible running average. The
averaging calculation uses the direction coast speed.
Wind Direction Coasting
At very low wind speeds, the measured wind direction is meaningless.
Therefore, you can specify a direction coast speed. When the measured
wind speed drops below the direction coast speed, the calculations use
the last wind direction that occurred while the wind speed was at or
above the direction coast speed to the average wind direction. You can