Average Velocity - Global Water FP101 User Manual

f. The average velocity (calculated with the Flow Probe in feet/second)
times the cross-sectional area (square feet) equals flow in cubic feet
per second (cfs), or Q = V x A.
g. If the propeller gets fouled while measuring flow, clean it until the prop
turns freely and start over.
IV. Average Velocity
The Flow Probe is used to measure the average water velocity.
Streamflow velocity varies for two reasons:
a. The velocities vary throughout the flow's cross-section. In general, the
velocities are greater in the center of the flow and less near the bottom
and sides of the channel.
b. The water surges in velocity with time. In a smooth running stream,
the velocity at a specific point can easily vary 1-2 feet per second over
the period of a minute. This pulsating or surging of flow should be
averaged to obtain an accurate average flow reading (leave the probe
in the flow through a series of flow surges).
The Flow Probe can be used in three ways to determine average velocity
in a stream.
a. For small streams and pipes, the probe can be moved slowly and
smoothly throughout the flow during average velocity measurement.
Move the probe smoothly and evenly back and forth from top to bottom
of the flow so that the probe stays at each point in the flow for
approximately the same amount of time. Keep moving the probe for
20-40 seconds to obtain an accurate average value that accounts for
surging. (Move the probe as if you were spray painting and attempting
to get an even coat of paint over the entire surface.)
Global Water
800-876-1172 • globalw.com
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