In HVAC/R applications it is helpful to understand the methods used to determine air velocity. Air velocity (distance traveled per unit of time) is most often expressed in feet per minute (FPM). Multiplying air velocity by the area of a duct allows you to determine the air volume flowing past a point in the duct per unit of time. Volume flow is generally measured in cubic feet per minute (CFM).
Air velocity is measured by sensing the pressure that is produced through the movement of the air. Velocity is also related to air density with assumed constants of 70° F and 29.92 in Hg. The two most common technologies to measure velocity are capacitive based pressure sensors and hot-wire anemometers.
There are two types of pressure that need to be known to measure velocity; total pressure and static pressure. Both can be measured using a Pitot or averaging tube. Velocity pressure is calculated by taking the difference between the total pressure and static pressure.
To measure the velocity pressure, connect a Pitot or averaging tube to a velocity sensor and place the tube into the air flow of the duct. The actual velocity requires either a mathematical calculation or a calibrated sensor that directly shows velocity.
V = 4005 x square root (delta P)
Determining air flow is a matter of multiplying the cross-sectional area of a duct by the air velocity. If the dimensions of the duct are known, then the cross-sectional area can be easily determined and the volumetric flow calculated.
The one thing to keep in mind is that the air velocity is not uniform at all points of the duct. This is true because the velocity is lowest at the sides where the air is slowed down by friction. To account for this, using an averaging Pitot tube with multiple sensing points will more accurately reflect the average velocity.
Importance of measuring Velocity:
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