Pitot Tubes
Pitot tubes can be used to indicate fluid flow velocity by measuring the difference between the static and dynamic pressures in fluids
Sponsored Links
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)
where
p = static pressure (relative to the moving fluid) (Pa)
ρ = density (kg/m3)
γ = specific weight (kN/m3)
v = flow velocity (m/s)
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 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, dynamic and total 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
Sponsored Links
Related Topics
- Fluid Flowmeters - 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
Sponsored Links
Related Documents
- Density, Specific Weight and Specific Gravity - An introduction and definition of density, specific weight and specific gravity - formulas with examples
- Dynamic Pressure - Dynamic pressure is the component of fluid pressure representing the fluid kinetic energy
- Energy and Hydraulic Grade Line - The hydraulic grade and the energy line are graphical presentations of the Bernoulli equation
- 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
- Velocity Head - Dynamic pressure and velocity head
