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 velocities by converting kinetic energy in the 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 this 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)
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 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