User manual VAISALA WS425

[. . . ] USER'S GUIDE Vaisala WINDCAP® Ultrasonic Wind Sensor WS425 M210361EN-D PUBLISHED BY Vaisala Oyj P. O. Box 26 FI-00421 Helsinki Finland Phone (int. ): +358 9 8949 1 Fax: +358 9 8949 2227 Visit our Internet pages at http://www. vaisala. com/ © Vaisala 2009 No part of this manual may be reproduced in any form or by any means, electronic or mechanical (including photocopying), nor may its contents be communicated to a third party without prior written permission of the copyright holder. The contents are subject to change without prior notice. Please observe that this manual does not create any legally binding obligations for Vaisala towards the customer or end user. [. . . ] For the WS425 sensor, the time between each consecutive wind direction measurement is one second. Each wind direction measurement taken over the preceding averaging interval is summed and the sum is divided by the number of measurements. The same averaging interval is used for both the average wind speed and average wind direction. The sensor computes a true running average. If the data acquisition system requests data before the initial averaging interval completes, the sensor returns the best possible running average. Averaging Method These settings affect the calculation of wind speed and direction. The available options are as follows: a. Scalar averaging Vector averaging Scalar Averaging When the scalar averaging is selected, the wind direction is a circular function with a discontinuity at due north, where 360° is equal to 0°. For example, 359° + 5° = + 4° and 0° - 5° = 355°. VAISALA _______________________________________________________________________ 47 USER'S GUIDE____________________________________________________________________ The microprocessor translates this circular function to a linear function, that is, 359° + 5° is translated to 364° and 0° - 5° translates to - 5°. To calculate the scalar average wind direction, each translated wind direction measurement taken over the preceding averaging interval is summed and the sum is divided by the number of measurements. Vector Averaging Each x velocity and y velocity measurement over the averaging interval is added and then divided by the number of measurements. The resulting average x velocity and average y velocity are converted to polar direction and magnitude, returning as average direction in degrees and speed in the chosen units. Output Interval The output interval can be selected in full seconds between 1 and 9 seconds. These settings are independent of the averaging interval (configuration parameter 3, see section Average Interval on page 47). Regardless of the length of the output interval, the last measurement sample before transmission is always the last sample of the averaging window. Therefore, the transmitted data is always based on the latest measurements. If the output interval is set to zero (0), polling is used in data acquisition. NOTE When using the NMEA Standard as the operating mode (configuration parameter 1), there must be a non-zero setting for output interval since polling is not supported in the NMEA Standard mode. Sensor ID Character The sensor ID character must be a single capital letter from A to Z. Numbers or small letters are not accepted. After an ID is defined for a sensor, the configuration menu can be opened by typing open <id>. This is useful if several sensors are sharing the same communication line. When running the NMEA Extended or WAT11 mode, the sensor ID is part of the polling string. 48 __________________________________________________________________ M210361EN-D Chapter 4 ________________________________________________________________ Operation Wind Direction Coast Threshold At very low wind speeds, the measured wind direction is meaningless. Therefore, you can specify a direction coast speed. When the measured wind speed drops below the direction coast speed, the calculations use the last wind direction that occurred while the wind speed WS at or above the direction coast speed to average wind direction. You can specify a direction coast speed between 0. 0 and 9. 9 units of measurement. [. . . ] After this, the programmed communication parameters will become effective. You have 5 seconds time to type: open<Enter> Make sure that the sensor is already on when you type the first letter. If you do not succeed, try several times and check your connections. After a successful OPEN command, the sensor should display a configuration menu. [. . . ]