g = acceleration of gravity
Δh = h2 - h1 = height difference (fluid column)
Note! - in the basic equation the head unit is with reference to the density of the flowing fluid. For other units and reference liquids - like mm Water Column - check Velocity Pressure Head.
The pitot tube is a simple and convenient instrument to measure the difference between static, total and dynamic pressure (or head).
The head - Δh - (or pressure difference - Δp) can be measured and calculated with the help of u-tube manometers, electronic pressure transmitters or similar instrumentation.
The charts below are based on air density 1.205 kg/m3 and water density 1000 kg/m3.
Note that as indicated in the diagram above - pitot tubes are not suited for low velocity flow. Due to low dynamic pressure (head) the readings will be inaccurate.
The point velocities in a duct, channel or pipe can be measured by traversing the cross-sectional area of the conduit. The point velocities can be used to calculate the average velocity that can be used to estimate the flow.
The average velocity can be calculated as
va = Σ vn / n
= (2 g Σ hn / n )1/2 (6)
va = average velocity (m/s, ft/s)
vn = point velocity (m/s, ft/s)
hn = point height difference pitot meter (m, ft)
n = number of point velocities
The volume flow in the conduit can be calculated as
q = va A (7)
q = flow (m3/s, ft3/s)
A = cross-sectional area (m2, ft2)
For round ducts larger than 10 inches the average velocity can be calculated as the arithmetic middle of a 10 point traverse from inward side wall with distance
For smaller ducts the the average velocity can be estimated to 81% of centerline velocity.
Flow metering principles - 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.
Conservation of energy in a non-viscous, incompressible fluid at steady flow.
Calculate the discharge length from the open end of a partially filled horizontal pipe.
A limited comparison of flowmeter principles - regarding service, rangeability, pressure loss, typical accuracy, upstream pipe diameters, viscosity and relative costs.
An introduction to density, specific weight and specific gravity.
Dynamic pressure is the kinetic energy per unit volume of a fluid in movement.
The hydraulic grade line and the energy line are graphical presentations of the Bernoulli equation.
Introduction to accuracy in flow measurement devices.
Energy possessed by an object's motion is kinetic energy.
The orifice, nozzle and venturi flow rate meters makes the use of the Bernoulli Equation to calculate fluid flow rate using pressure difference through obstructions in the flow.
Inclined and vertical u-tube manometers used to measure differential pressure in flow meters like pitot tubes, orifices and nozzles.
Dynamic pressure or velocity head.
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