Static Pressure vs Dynamic Pressure vs Total Pressure

The Three Pressures of Fluid Flow

When measuring pressure in a moving fluid, what exactly are you measuring? The answer depends on how your gauge is oriented relative to the flow. According to Bernoulli's principle, the total energy of a flowing fluid at a given elevation is the sum of its internal pressure and its kinetic energy.

This leads to three distinct definitions of pressure: Static, Dynamic, and Total (Stagnation) pressure.

1. Static Pressure ($P_{static}$)

Static pressure is the actual thermodynamic pressure of the fluid. It is the pressure that pushes outward uniformly in all directions, regardless of whether the fluid is moving or standing still.

• How it acts: It pushes against the walls of the pipe, attempting to burst it.
• How to measure it: A pressure tap drilled perfectly flush into the wall of the pipe (perpendicular to the flow) measures pure static pressure. The moving fluid simply glides past the tap without converting its kinetic energy into pressure.

2. Dynamic Pressure ($P_{dynamic}$)

Dynamic pressure (often called velocity pressure) is the kinetic energy of the fluid expressed in units of pressure. It represents the extra pressure that would be created if you forced the moving fluid to suddenly come to a complete halt.

The formula for dynamic pressure is:

Where:

• $\rho$ = Fluid density ($kg/m^3$)
• $v$ = Fluid velocity (m/s)

Dynamic pressure cannot be measured directly on its own. It must be derived.

3. Total Pressure / Stagnation Pressure ($P_{total}$)

Total pressure is the sum of the static and dynamic pressures.

• How to measure it: You insert a tube into the pipe with its open opening facing directly into the oncoming flow. The fluid rams into the tube and is brought to a dead stop (stagnates). The kinetic energy is fully converted into pressure. The gauge on this tube will read the Total Pressure.

Application: The Pitot Tube

The relationship between these three pressures is the exact operating principle of the Pitot-Static Tube (invented by Henri Pitot), widely used to measure airplane speed and fluid flow in large ducts.

A Pitot-Static tube has two ports:

1. A front-facing hole that measures Total Pressure ($P_{total}$).
2. Side-facing holes that measure Static Pressure ($P_{static}$).

A differential pressure transmitter measures the difference between them:

By knowing the density and rearranging the equation, the instrument instantly calculates the fluid velocity:

Try the Calculator

Use the Pipe Hydraulics Calculator on this page to input your fluid density and pipe velocity. The calculator will instantly output the Dynamic Velocity Head ($P_{dynamic}$) in kPa!