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Transactions in Measurement and Control A Technical Reference by OMEGA

Our interest in the measurement of air and water flow is
timeless. Knowledge of the direction and velocity of air flow was essential
information for all ancient navigators, and the ability to measure water flow
was necessary for the fair distribution of water through the aqueducts of such
early commuhydrodynamics, pneumatics, aerodynamics) is based on the works of
the ancient Greek scientists Aristotle and Archimedes. In the Aristotelian
view, motion involves a medium that rushes in behind a body to prevent a
vacuum. In the sixth century A.D., John Philoponos suggested that a body in
motion acquired a property called impetus, and that the body came to that
Newton’s third law of motion applies not only to stationary bodies, but also to
objects in motion.

The Flow Pioneers

A major milestone in the understanding of flow was reached
in 1783 when the Swiss physicist Daniel Bernoulli published his Hydrodynamica.
In it, he introduced the concept of the conservation of energy for fluid flows.
Bernoulli determined that an increase in the velocity of a flowing fluid
increases its kinetic energy while decreasing its static energy. It is for this
reason that a flow restriction causes an increase in the flowing velocity and
also causes a drop in the static pressure of the flowing fluid. The permanent
pressure loss through a flowmeter is expressed either as a percentage of the
total pressure drop or in units of velocity heads, calculated as V2/2g, where V
is the flowing velocity and g is the gravitational acceleration (32.2 feet/
second2 or 9.8 meters/second2 at 60° latitude). For example, if the velocity of
a flowing fluid is 10 ft/s, the velocity head is 100/64.4 = 1.55 ft. If the
fluid is water, the velocity head corresponds to 1.55 ft of water (or 0.67
psi). If the fluid is air, then the velocity head corresponds to the weight of
a 1.55-ft column of air.