How The Weather Station Instrument Works - Furuno WS-200 Operator's Manual

4. HOW THE WEATHER STATION
INSTRUMENT WORKS
About the Ultrasonic Wind Sensor
The ultrasonic wind sensor (an ultrasonic anemometer) measures apparent wind
speed and direction. The Weather Station instrument contains four ultrasonic
transducers, visible through the four holes in the top of the sensor's wind channel
(see the figure below). These transducers operate in pairs—one transducer injects
a pulse into the air, and the other (directly opposite to it) listens for the arrival of that
pulse. Each pulse bounces off the metal plate at the bottom of the wind channel
and is carried by the wind to arrive at the opposing 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, 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 to cover all eventualities of wind direction.
A microprocessor within the Weather Station instrument then combines the mea-
surements from all four transducers to calculate the resultant wind speed and di-
rection. Throughout this process, the sensor monitors the air temperature, to
compensate for the fact that the speed of sound in air changes with temperature.
Note: Do not use wind data in the WIND mode of an autopilot, because the data is
not sufficiently accurate.
True and Apparent Wind
The Weather Station instrument has the unique ability to display both true and ap-
parent 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.
Weather Station ultrasonic wind sensor
Waterproof film
Wind channel
Metal plate
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