Pitot Tubes

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

A Pitot-static tube can measure the fluid flow velocity by converting the kinetic energy in the fluid flow into potential energy.

pitot tubes

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

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

where

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 this equation has the dimension force per unit area - N/m2 or in imperial units psi, lb/ft2.

Static Pressure

The first term - p - is the static pressure. It is static relative to the moving fluid and can be measured through an 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 the 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)

where

suffix 1 is a point in the free flow upstream

suffix 2 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, h1 = h2 and this part can be eliminated. Since v2 is zero, (2) can be modified to:

p1 + 1/2 ρ v12 = p2         (3)

or

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

where

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 

Charts 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

  • Velocity Head - Dynamic pressure and velocity head
  • U-Tube Manometer - The inclined and vertical u-tube manometers are inexpensive and common in differential pressure measurements of flow meters like pitot tubes, orifices and nozzles
  • Dynamic Pressure - Dynamic pressure is the component of fluid pressure representing the fluid kinetic energy
  • Density, Specific Weight and Specific Gravity - An introduction and definition of density, specific weight and specific gravity - formulas with examples
  • Energy and Hydraulic Grade Line - The hydraulic grade and the energy line are graphical presentations of the Bernoulli equation

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