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.

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