Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 191
Mean Wind Vector
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Section 7. Installation often included to zero the measurement when it equals the offset so that WindVector() can reject measurements when wind speed is zero. Standard deviation can be processed one of two ways: 1) using every sample taken during the data storage interval (enter 0 for the Subinterval parameter), or 2) by averaging standard deviations processed from shorter sub-intervals of the datastorage interval. Averaging sub-interval standard deviations minimizes the effects of meander under light wind conditions, and it provides more complete information for periods of transition (see EPA publication "On-site Meteorological Program Guidance for Regulatory Modeling Applications"). Standard deviation of horizontal wind fluctuations from sub-intervals is calculated as follows: where: is the standard deviation over the data-storage interval, and are sub-interval standard deviations. A sub-interval is specified as a number of scans. The number of scans for a sub-interval is given by: Desired sub‐interval (secs) / scan rate (secs) For example, if the scan rate is 1 second and the data interval is 60 minutes, the standard deviation is calculated from all 3600 scans when the sub-interval is 0. With a sub-interval of 900 scans (15 minutes) the standard deviation is the average of the four sub-interval standard deviations. The last sub-interval is weighted if it does not contain the specified number of scans. The EPA recommends hourly standard deviation of horizontal wind direction (sigma theta) be computed from four fifteen-minute sub-intervals. 7.8.5.2.1 Measured Raw Data Si: horizontal wind speed Θi: horizontal wind direction Uei: east-west component of wind Uni: north-south component of wind N: number of samples 7.8.5.2.2 Calculations Mean Wind Vector Resultant mean horizontal wind speed, Ū: 191
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