Basic Documentation

Forces Exerted by Air
STATIC
PRESSURE
LAB0190R1
Figure 17. Static Pressure is Force Per Unit Area Exerted Equally in All Directions and Not
Caused by Air Movement.
Now, consider what happens when air in a confined area (such as a ventilation system duct)
is put in motion. In a confined area, air in motion has both a total pressure and a static
pressure component, and each of these components can be individually measured. However,
to measure the static pressure of a moving air stream, care must be used to ensure that the
pressure measurement is made perpendicular to the direction of air movement to exclude the
effect of the total pressure component.
If airflow in a duct is totally stopped by an obstruction (such as a fully closed damper) there
can no longer be any force component due to air movement. However, even though the air is
no longer moving, it still exerts static pressure in all directions (even in its previous forward
direction). If you tried to measure the total pressure component (in the direction of normal
airflow), you would receive a value that equals the static pressure. Therefore, total pressure
equals static pressure when air is at rest. The static pressure that still exists does not result
from air movement, but rather from the force that is being imposed upon the air. In ventilation
systems the ventilation system’s fans typically create this force.
Velocity Pressure
As previously stated, air can have two distinct measurable pressure components:
• Static pressure
• Total pressure
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