Pitot Tubes

Pitot tubes can be used to indicate fluid flow velocities by measuring the difference between the static and the dynamic pressures in fluids

A pitot tube can be used to measure fluid flow velocity by converting the kinetic energy in a fluid flow to potential energy.

pitot tubes

The principle is based on the Bernoulli Equation where each term of the equation can be interpreted as pressure

p + 1/2 ρ v2 + γ h = constant along a streamline         (1)


p = static pressure (relative to the moving fluid) (Pa)

ρ = density (kg/m3)

v = flow velocity (m/s)

γρ g =  specific weight (N/m3)

g = acceleration of gravity (m/s2)

h = elevation height (m)

Each term of the equation has the dimension force per unit area N/m2 (Pa) - or in imperial units lb/ft2 (psi).

Static Pressure

The first term - p - is the static pressure. It is static relative to the moving fluid and can be measured through a flat opening in parallel to the flow.

Dynamic Pressure

The second term - 1/2 ρ v2 - is called the dynamic pressure.

Hydrostatic Pressure

The third term - γ h - is called the hydrostatic pressure. It represent the pressure due to change in elevation.

Stagnation Pressure

Since the Bernoulli Equation states that the energy along a streamline is constant, (1) can be modified to

p1 + 1/2 ρ v12 + γ h1

    = p2 + 1/2 ρ v22 + γ h2

    = constant along the streamline         (2)


suffix1 is a point in the free flow upstream

suffix2 is the stagnation point where the velocity in the flow is zero

Flow Velocity

In a measuring point we regard the hydrostatic pressure as a constant where h1 = h2 - and this part can be eliminated. Since v2 is zero, (2) can be modified to:

p1 + 1/2 ρ v12 = p2         (3)


v1 = [2 (p2 - p1) / ρ] 1/2         (4)


p2 - p1 = dp (differential pressure)

With (4) it's possible to calculate the flow velocity in point 1 - the free flow upstream - if we know the differential pressure difference dp = p2 - p1 and the density of the fluid.

Pitot Tube

The pitot tube is a simple and convenient instrument to measure the difference between static, total and dynamic pressure (or head).

pitot tubes

The head - h - (or pressure difference - dp) can be measured and calculated with u-tube manometers, electronic pressure transmitters or similar instrumentation.

Air Flow - Velocity and Dynamic Head Chart 

The charts below are based on air density 1.205 kg/m3 and water density 1000 kg/m3

Air Flow - Velocity and Head

Water Flow - Velocity and Dynamic Head Chart

Air Flow - Velocity and Head

Related Topics

  • Fluid Flow Meters - Flow metering basics - Orifice, Venturi, Flow Nozzles, Pitot Tubes, Target, Variable Area, Positive Displacement, Turbine, Vortex, Electromagnetic, Ultrasonic Doppler, Ultrasonic Time-of-travel, Mass Coriolis, Mass Thermal, Weir V-notch, Flume Parshall and Sluice Gate flow meters and more

Related Documents

Tag Search

  • en: pitot tube flow velocity speed volume measurement
  • es: medición del volumen de la velocidad de la velocidad del flujo tubo de Pitot
  • de: Staurohr Strömungsgeschwindigkeit Geschwindigkeit Volumenmessung

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