Average Velocity - Global Water FP101 User Manual

The Flow Probe is used to measure the average water velocity. Streamflow velocity varies for two reasons:
1) 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.
2) 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.
1) 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.)
The Flow Probe uses true velocity averaging. When the average and maximum velocities are zeroed by
pushing both buttons, a running average is started. As long as the probe remains in the flow, the averag-
ing continues. One reading is taken per second, and a continuous average is displayed. For example,
after 10 seconds, 10 readings are totaled and then divided by 10 and this average is displayed. Once the
average reading becomes steady, the true average velocity of the stream is obtained. When you pull the
probe from the water, this average value is frozen on the display until it is reset.
2) For larger streams and rivers where the Flow Probe can't easily be moved throughout the flow, divide the
stream into subsections 2-3 feet wide. We recommend dividing subsections on your graph paper diagram
of the flow profile. Run a measuring tape across the stream for reference. Obtain a vertical flow profile at
the center of each subsection: zero the averaging function and move the Flow Probe vertically from the
surface to the bottom, up and down, slowly and smoothly for 20-40 seconds to obtain a good average.
The average velocity (obtained with the Flow Probe) times the area of the subsection (use your graph
paper diagram) equals the flow for the subsection (Q=VxA). Once the flow of each subsection is obtained,
add all of the subsection flows to obtain the Total Streamflow.
3) For the USGS "6 tens method", the Flow Probe is placed at the center of the subsection at a depth from
the surface of 0.6 of the total depth. The Flow Probe is held in place and the average velocity is obtained
over a period of 40 seconds. The 0.6 depth is assumed to be the average velocity point for the vertical
profile. Therefore, this average is similar to that obtained in technique 2 (above), however, we feel that
technique 2 is more accurate.
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A A v v e e r r a a g g e e V V e e l l o o c c i i t t y y
FP101/FP201 - 2
Global Water Instrumentation, Inc.