Page 353 - Fundamentals of Air Pollution 3E

308 19. Transport and Dispersion of Air Pollutants

Fig. 19-8. Propeller vane wind system. Source: Photo courtesy of R. M. Young Co.

The instrument should be placed away from other instrumentation and
the propeller axis carefully aligned to be vertical. The specifications of this
sensor are the same as those of the wind sensor. Because this instrument
will frequently be operating near its lower threshold and because the eleva-
tion angle of the wind vector is small, such that the propeller will be
operating at yaw angles where it has least accuracy, this method of measur-
ing vertical velocity is not likely to be as accurate as the measurement of
horizontal fluctuation.
Rather than using separate systems for horizontal and vertical wind
measurements, a u-v-w anemometer system (Fig. 19-9) sensing wind along
three orthogonal axes, with proper processing to give average wind direc-
tion and <r a from the combination of the u and v components and w and
cr w from the w component may be used.