Xylem FP111, FP211, & FP311 GLOBAL WATER FLOW PROBE Users Manual - Manual (Page 6)

Global Water
800-876-1172 • globalw.com
6
V. Average Velocity
The Flow Probe is used to measure the average water velocity.
Stream flow velocity varies for several reasons:
a. The velocities vary throughout the flow’s cross-section due
to the friction between the water and the channel. 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).
c. The water is not always moving in the same direction.
Even the smoothest water flow contains turbulence that
causes the water to move in different directions. These
velocity vectors not parallel to the flow will be seen as
moving slower because only a part of the vector parallel
to flow. Unlike other types of water velocity measuring
devices, the Global Water’s Flow Probe propeller’s
protective housing will channel the water parallel to flow
and help to eliminate errors caused by improperly
measuring velocity vectors not parallel to flow.
d. Large obstructions in the flow such as rocks and trees will
cause the water to flow around them, which may cause
the flow to change direction for a short distance from these
obstructions. To properly measure the true water velocity,
the flow probe needs to be directly inline with the flow.