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.
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)
γ = ρ 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).
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.
The second term - 1/2 ρ v2 - is called the dynamic pressure.
The third term - γ h - is called the hydrostatic pressure. It represent the pressure due to change in elevation.
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
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.
The pitot tube is a simple and convenient instrument to measure the difference between static, total and dynamic pressure (or head).
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.
Water Flow - Velocity and Dynamic Head Chart
- 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