Airmar PB150 Technical Information download pdf (Page 17)

Appendix A—How the WeatherStation instrument Works

About the Ultrasonic Wind Sensor

The ultrasonic wind sensor (an ultrasonic anemometer) measures apparent wind

speed and direction. The WeatherStation instrument contains four ultrasonic

transducers, visible through the four holes in the top of the sensor’s wind channel

(see Figure 9). These transducers operate in pairs—one transducer injects a

pulse into the air. The pulse bounces off the metal plate at the bottom of the wind

channel and is carried by the wind to arrive at the listening transducer a short time

later.

When there is no wind, the pulse travels at the speed of sound from the sender to

the receiver. Whenever the wind is blowing in that direction, the pulse will arrive

sooner than if the air is still. Similarly, whenever the wind is blowing in the

opposite direction, the pulse will arrive later than if the air is still. The four

transducers take turns in sending and receiving pulses.

A microprocessor within the WeatherStation instrument then combines the

measurements from all four transducers to calculate the resultant wind speed and

direction. Throughout this process, the sensor monitors the air temperature, to

compensate for the fact that the speed of sound in air changes with temperature.

Understanding True and Apparent Wind

The WeatherStation instrument has the unique ability to display both true and

apparent wind. True wind is the actual motion of the air relative to the earth.

Apparent wind is the wind which an observer experiences while moving or on

board a boat. It is the result of two motions—the actual motion of the air (the true

wind) and the motion of the boat. If the vessel is not moving, then the true and

apparent wind will be the same.

There are two components to any wind measurement: speed and direction. By

convention, the wind direction is an angle representing the direction from which

the wind is blowing. Sometimes this angle is referenced to true or magnetic north,

and sometimes it is referenced to the bow of the vessel. Both true and apparent

wind use these same references.

Figure 9. WeatherStation ultrasonic wind sensor

4 transducer

metal plate

behind waterproof film

wind channel

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Airmar PB150 Technical Information Page 17