Vaisala WS425FG - Manual (Page 61)

Chapter 5 _________________________________________________________ Operating Principle
VAISALA_______________________________________________________________________ 59
CHAPTER 5
OPERATING PRINCIPLE
This chapter provides you with a brief explanation of the operating
principles of Vaisala Ultrasonic Wind Sensor WS425 F/G and its power
supply.
Wind Measurement Principle
The Ultrasonic Wind sensor uses ultrasonic technology. It has an on-
board micro controller that captures and processes data and performs
serial communications.
The wind sensor has an array of three equally spaced ultrasonic
transducers in a horizontal plane. The sensor measures transit time, the
time that it takes the ultrasound to travel from one transducer to another.
The transit time is measured in both directions.
Transit time depends on the wind velocity along the ultrasonic path. For
zero wind velocity, both the forward and reverse transit times are the
same. With wind along the sound path, the up-wind transit time
increases and the down-wind transit time decreases.
The microprocessor of the micro controller computes the wind velocity
in a direction parallel to each possible transducer pair from the transit
times using the formula: