Vaisala WMT700 - Manual (Page 37)

Chapter 3 _______________________________________________________ Functional Description
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the functionality of WMT700.
Operating Principle
WMT700 uses the Vaisala WINDCAP
®
ultrasonic sensor
technology in wind measurement. The sensor has an onboard
microcontroller that captures and processes data and
communicates over serial interfaces.
The wind sensor has an array of three equally spaced ultrasonic
transducers on a horizontal plane. Wind speed (WS) and wind
direction (WD) 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 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 upwind direction
transit time increases and the downwind transit time decreases.
Figure 11 on page 36 shows how the time shift of the ultrasonic
signals is measured and how tail wind and forward wind affect the
measurement.
VAISALA ________________________________________________________________________ 35