# Velocity Pressure (VP)

## Definition - What does Velocity Pressure (VP) mean?

In a moving fluid, velocity pressure (VP) is the pressure that would induce an equivalent velocity if applied to move the same fluid through an orifice, so that all pressure energy is converted into kinetic energy. It is used in air conditioning, heating and ventilating work to determine air velocity.

Velocity pressure is a function of air density and velocity. VP will only be exerted in the direction of airflow and is always positive.

Velocity pressure, along with static pressure and pressure due to elevation, are used in Bernoulli's principle as an energy balance on a closed system. The three terms are used to define the state of a closed system of an incompressible, constant-density fluid.

## Corrosionpedia explains Velocity Pressure (VP)

Velocity pressure is that pressure required to accelerate air from zero velocity to some velocity (V) and is proportional to the kinetic energy of the air stream.

For example, when a fan is moving air through a duct system, two types of pressure are encountered: velocity pressure and static pressure. The sum of these pressures is referred to as total pressure. Velocity pressure is the pressure caused by air in motion. Units of pressure are expressed in inches of water, designated as W.G. Velocity pressure can be calculated from the difference between the total pressure and static pressure.

Velocity pressure is useful in aerodynamics to determine how the aerodynamic stress varies, and in particular, when it reaches its maximum value. The point of maximum aerodynamic load is a critical parameter in many applications, such as during spacecraft launch.

The measurement of free-stream velocity pressure is fundamental to indication of airspeed. Airspeed indicators are simply pressure gauges, which measure velocity pressure, related to various airspeeds.

Velocity pressure cannot be measured directly. The easiest way to determine flow velocity of air is to measure the velocity pressure in the duct with a pitot tube assembly connected to a differential pressure sensor. The pitot tube assembly includes a static pressure probe and a total pressure probe.

A total pressure probe, aligned into the airflow, senses the duct velocity pressure and the static pressure, which equals the total pressure. A static pressure probe aligned at a right angle to the airflow senses only the static pressure. The difference between the total pressure reading and the static pressure reading is the velocity pressure.