Air Movement & Flow

Manometers are devices that are used to measure pressure. Water or some other fluid is placed in a U shaped tube separating a sealed container of gas and the gas in the atmosphere. A difference in pressure between the two gasses can be measured by observing the change in height of the liquid.

Pressure (P) is a measure of force (F) applied to a surface area (A). Force is measured in Newtons (N). Area is usually measured in square meters (m2) so pressure can be measured in N/m2 which is known as a Pascal (Pa).  Other common pressure units of measure include pounds per square inch (psi), millibars, atmospheres (atm), millimeters of mercury (mm Hg), and inches of water (in H2O).

To use a manometer to measure the pressure (P) the difference in the pressure in the manometer (P1) and the pressure outside the manometer (P2) can be computed by using the density of the liquid in the manometer (ρ), the gravitational constant (g) and the height (h = h2-h1). 

A Pitot tube is a type of manometer that can be inserted into an air duct with the tip pointing towards the air flow and will measure the velocity pressure of the air.

To convert the velocity pressure into the actual velocity of the air, the above equation is needed. Multiply 4005 by the square root of the change in pressure (ΔP measured in inches of water column - in WC; essentially the height of liquid change) to get the velocity (V in feet per minute - ft/min).