Page 99 - Fluid Power Engineering

W i nd Measurement 77

a met-tower with a life of about 20 years is considered
permanent. Most temporary met-towers are tilt-up with no
foundation, while permanent met-towers have a concrete
foundation. However, some states in the United States, like
Florida and Ohio, require even a temporary met-tower to be
Rev-G compliant, which may require a foundation.
Maximum height of met-tower. Some of the more common

heights of met-towers are 30, 50, 60, 80, and 100 –meters.The
60 m is the most popular met-tower height and the trend is
toward taller met-towers.

Anemometer
Anemometers are used to measure wind speed. There are different
types including:

Cup anemometer—this is the most widely used. Most modern

anemometers contain three cups with a vertical axis of rota-
tion. The rotation speed of the cups is proportional to the wind
speed. The output signal of an anemometer is a low-level AC
sine wave; the frequency of the sine wave is proportional to
the wind speed. Figures 6-1a and 6-2 show cup anemometers.
Propeller anemometer. As the name suggests, a propeller is

used to measure wind speed. The axis of rotation of propeller
anemometer is horizontal. In order to align the axis of rota-
tion with the direction of wind, this type of anemometer also
contains a wind vane. This instrument serves two purposes:
Wind speed and wind direction measurement. A form of pro-
peller anemometer is used to measure the vertical component
of wind speed; in this case, the axis of rotation is fixed to be
vertical. Figure 6-1b shows a propeller anemometer.
Sonic anemometer. Ultrasound waves are used to measure wind

speed and direction. Three-dimensional velocity vectors are
computed by measuring travel time of sonic pulses between

(a) (b) (c)
FIGURE 6-1 Anemometer and wind vane. (a) NRG#40 cup anemometer;
(b) RM Young propeller anemometer; and (c) NRG200P wind vane.
(With permission of NRG Systems.)