Basic Documentation

Glossary
Standard Air Density (Standard Atmosphere)
This has been set at 0.075 lb. per cubic foot (I/P units) and is approximately the weight of one
cubic foot of air at 70°F and 29.921 inches of mercury (Hg), where 29.921 inches of mercury
approximately equals 14.696 psi.
Static Pressure
Pressure exerted by air at rest or the force of air that is exerted perpendicular to the direction
of airflow when air movement is present. Static pressure is not a resultant of air movement.
The static pressure of an air stream will approach maximum value when the airflow velocity is
reduced to zero by an obstruction such as a closed damper in a duct.
Total Pressure
Maximum pressure of a moving air stream that is exerted in the direction of airflow. Total
pressure is measured by sampling the pressure directly in line with the normal direction of
airflow. The total pressure of a moving air stream is always higher than the static pressure,
since total pressure also adds the force produced by the moving air stream. As airflow
velocity falls, total pressure also falls until at zero velocity the total pressure equals the static
pressure.
Velocity Pressure
Mathematical difference between the total pressure and static pressure of a moving air
stream. It is the additional pressure component that the moving air stream exerts in the
direction it is moving due to its mass.
Siemens Building Technologies Glossary-3