Basic Documentation

Room Pressurization Control Application Guide
Differential Pressure
A pressure measurement is always a measurement of the pressure difference between two
points or locations. Every pressure measurement can be termed a differential pressure
measurement. In most instances, a plainly visible barrier separates the two “locations.” For
example, when measuring automobile tire pressure, the measurement is the difference
between the pressure inside the tire and atmospheric pressure outside the tire (with the tire
wall being the “barrier”). Similarly, boiler steam pressure is a measurement of the difference
between the pressure inside the boiler and atmospheric pressure outside of the boiler. In
these and most other common pressure measurements, it is generally understood that the
pressure value is the difference between the measured pressure and atmospheric pressure.
Also, in most common pressure measurements it is usually apparent which location is at the
higher pressure. For example, the pressure inside a steam pipe or inside an inflated tire is
higher than outside of the pipe or tire.
When referring to ventilation system pressures (typically room static pressure), it is usually
necessary to indicate the two locations that comprise the pressure measurement since this is
not always apparent. It is also necessary to use a positive or negative prefix to indicate which
location is at the higher or lower pressure. Thus, when referring to the static pressures
associated with ventilation systems, it is usually necessary to add a prefix. The prefix
“positive” indicates that the point or location is at the higher pressure, while the prefix
“negative” indicates that the area or location is at the lower pressure.
Summary of Pressure Components
The following is a summary of important factors that relate to ventilation system pressure
components.
• Total Pressure—the force per unit area exerted by air in motion. It is measured by
sensing the force exerted by a moving air stream in the direction of airflow.
• Static Pressure—the force per unit area exerted by air in motion or at rest. When air
is in motion (such as in a duct), it is measured by sensing the force exerted by a
moving air stream perpendicular (at right angles) to the direction of airflow. When air
is at rest, the static pressure can be measured by sensing the force exerted in any
direction.
• Velocity Pressure—the difference between total pressure and static pressure.
Velocity pressure is mathematically related to the velocity of a moving air stream.
Knowing the velocity pressure of a moving air stream enables one to determine its
airflow velocity using the following expressions:
Airflow Velocity
Ft/sec.
= 4005 (dP
inches w. c.
)
1/2
OR
Airflow Velocity
M/sec.
= 1.29 (dP
Pa
)
1/2
48 Siemens Building Technologies, Inc.