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Theory of Operation DYACON UA-1 Manual
6.0 Theory of Operation
6.1 Wind Speed and Direction Sensing
Ultrasonic anemometers measure wind speed and direction by timing sound propagation through
air.
The ultrasonic transducers in the sensor individually emit a “click” and each of the other
transducers detect the sound. The time delay for the sound to reach the other transducers is
affected by air movement. A shorter delay indicates that the air is moving from the emitter
toward the detector.
Ultrasonic anemometers must correct for air temperature. Colder air is more dense than warmer
air, resulting in an increase in the speed of sound. Timing algorithms in the sensor adjust for this
change.
6.2 Practical Considerations
Each instrument and technology will have some advantages and disadvantages. Users should not
deploy any weather station instrument and expect it to operate flawlessly without the necessary
service.
Users should periodically inspect the wind sensor for fouling from insects and birds. Any
material that is placed in the sensing gap will affect the wind measurements.
Rain, snow, and ice can similarly disrupt an ultrasonic sensor.
The active sensing principle requires ultrasonic sensors to emit pulses even when there is no
significant air movement. In some cases, this can lead to excess current draw. Users should
evaluate their current budget. Adjusting the polling rate (frequency Modbus requests) will also
significantly affect the current usage.
6.2.1 Wind Speed Sensing
The anemometer uses two magnets, 180° opposite at the perimeter of the anemometer hub. A
reed switch is used to detect rotation.
A 4-sample running average of the switch time is used for wind speed. Measurement response is
a function of wind speed. At 2.5 m/s, the wind speed response is about 1 s.
The wind speed value is updated with each switch closure or half turn of the anemometer.
A deceleration algorithm is used to ensure the running average is accurate as the wind speed
approaches zero.
6.3 Boot Up
When first powered, UA requires 10 s to begin the measurement processes. An additional 10 s to
30 s may be required before valid measurements are available in the Modbus registers.
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