Vaisala WXT520 - Manual (Page 27)

Chapter 3 ______________________________________________________ Functional Description
VAISALA_______________________________________________________________________ 25
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the measurement principles and heating function
of Weather Transmitter WXT520.
Wind Measurement Principle
WXT520 uses Vaisala WINDCAP
®
sensor technology in wind
measurement.
The wind sensor has an array of three equally spaced ultrasonic
transducers on a horizontal plane. Wind speed and wind directions are
determined by measuring the time it takes the ultrasound to travel from
each transducer to the other two.
The wind sensor measures the transit time (in both directions) along the
three paths established by the array of transducers. This transit time
depends on the wind speed along the ultrasonic path. For zero wind
speed, both the forward and reverse transit times are the same. With
wind along the sound path, the up-wind direction transit time increases
and the down-wind transit time decreases.