To calculate the rate of air flow use:
V = 4005 √ VP(d)
Where V = rate of flow in fpm.
VP = velocity pressure; measured with Pitot tube.
d = air density factor; at standard temperature and pressure = 1.0.
c. Some of the anemometers that have previously been discussed, for example,
those with relatively narrow probes and fittings, can also be used to measure air
velocities within the system. No matter which device is used, however, the accuracy in
determining duct velocities or flow rates will be dependent upon proper locations and
number of measurements taken in traversing the duct.
Figure 4-6. Relationships between pressures in an exhausting system.