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.
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)
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 this equation has the dimension force per unit area - N/m2 or in imperial units psi, lb/ft2.
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.
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 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)
suffix 1 is a point in the free flow upstream
suffix 2 is the stagnation point where the velocity in the flow is zero
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)
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
Charts based on air density 1.205 kg/m3 and water density 1000 kg/m3.
Water Flow - Velocity and Dynamic Head Chart