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 FIGURE 2-24
Hot Wire Anemometer
2.15 VELOCITY PRESSURE AND STATIC PRESSURE. There are additional
instruments to be discussed; however, we cannot proceed further without a
discussion of velocity pressure and static pressure.
When you hold your hand out the window of an automobile moving at 55
mph, the force on your hand is due to the pressure of the moving air. This
is called velocity pressure. The instruments described up to this point are
responsive to only velocity pressure. Velocity pressure can be measured
with a device called a manometer as shown in Figure 2-25.
When moving air impacts the end of the sensing tube, it creates a
velocity pressure. This pressure, in turn, causes the water level in the
left leg of the manometer to be lower than the level in the right leg. The
greater the velocity, the greater the difference in liquid levels. Velocity
pressure is measured in inches of water column (or w.c.). A velocity of
just over 4,000 fpm will cause the difference in levels to be exactly
1.0 inch w.c.
2-23
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