Wind measurement principle - WXT530

WXT530 Series User Guide

Document code
M211840EN
Revision
F
Language
English (United States)
Product
WXT530
Document type
User guide
WXT536 WXT535 WXT534 WXT533 WXT532 WXT531

The transmitters use Vaisala WINDCAP sensor technology for wind measurement.

The wind sensor has an array of 3 equally spaced ultrasonic transducers on a horizontal plane. The unit determines wind speed and wind directions by measuring the time it takes the ultrasound to travel from one transducer to the other two.

The wind sensor measures the transit time (in both directions) along the 3 paths established by the array of transducers. The transit time depends on the 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 up-wind direction transit time increases and the down-wind transit time decreases.

The unit calculates wind speed from the measured transit times using the following formula:

Vw = 0.5 × L × (1/tf – 1/tr)
Vw Wind speed
L Distance between the two transducers
tf Transit time in forward direction
tr Transit time in reverse direction

Measuring the 6 transit times allows Vw to be computed for each of the 3 ultrasonic paths. The computed wind speeds are independent of altitude, temperature, and humidity, which are cancelled out when the transit times are measured in both directions, although the individual transit times depend on these parameters.

The Vw values of 2 array paths are enough to compute wind speed and wind direction. A signal processing technique ensures that wind speed and wind direction are calculated from the 2 array paths with the best quality.

The wind speed is represented as a scalar speed in selected units (m/s, kt, mph, km/h). The wind direction from which the wind comes is expressed in degrees (°). North is represented as 0°, East as 90°, South as 180°, and West as 270°.

The wind direction is not calculated when the wind speed drops below 0.05 m/s. In this case, the last calculated direction output remains until the wind speed increases to the level of 0.05 m/s.

The average values of wind speed and direction are calculated as a scalar average of all samples over the selected averaging time (1 ... 3600 s) with a selectable updating interval. The sample count depends on the selected sampling rate: 4 Hz (default), 2 Hz, or 1 Hz. The minimum and maximum values of wind speed and direction represent the corresponding extremes during the selected averaging time.

You can select the computation of the wind speed extreme values in one of 2 ways:

  • Traditional minimum/maximum calculation
  • 3-second gust & calm calculation recommended by the World Meteorological Organization (WMO). In this case the highest and lowest 3-second average values (updated once a second) replace the maximum and minimum values in reporting of wind speed, while the wind direction variance is returned in the traditional way.

The transmitter constantly monitors the wind measurement signal quality. If poor quality is detected, the wind values are marked as invalid. If over half of the measurement values are considered invalid, the last valid wind values are returned as missing data. In the SDI-12 protocol, the invalid values are marked as zero.