Global Water FP101 User Manual page 6

800-876-1172 • globalw.com
The Flow Probe uses true velocity averaging. When the average and
maximum velocities are zeroed by pushing the top button, a running
average is started. As long as the probe remains in the flow, the
averaging 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.
b. 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.
c. 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.
Global Water
6