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Velocity-Only Sensor
Section 4
4.12.3.4 Compensating for Velocity Direction
When programming the velocity sensors you may select one of three Velocity
Direction options and the velocity cutoff/velocity default:
4.12.3.5 Placing the Sensor and Mounting Band into the Pipe
Point the front of the sensor (the side opposite the cable entrance) into
the flow. See Figure 42.
Slide the mounting band as far into the pipe as possible to eliminate
drawdown effects near the end of the pipe. Locate the sensor at the bottom-
most point, in the channel. If excessive silt is present on the bottom of the
pipe, rotate the band in the pipe until the sensor is out of the silt.
Direction Description
Upstream
Use this option at sites with fairly consistent velocities, and low to medium turbulence. The flow stream
over the sensor should be relatively straight, with no drops or turns near the measurement point.
Mount the sensor in the pipe, beveled edge facing into the flow, where the flow stream enters the
measurement area. (See Figure 42.)
Downstream
Use this option when the sensor is installed downstream of the measurement point (where the flow stream
exits the site). This option is useful when more than one flow stream enters a site and the combined flow of
all streams at a single exit point is measured.
Mount the sensor in the downstream direction rather than the typical, upstream direction. Mounting the
sensor 'backwards' in this manner (see Figure 42) causes the velocity direction readings to be the
opposite of actual stream flow. By selecting the Downstream choice when programming, the logger
reverses the measured signal to show actual flow direction (beveled edge downstream).
Note: The maximum velocity obtained in this type of installation is 5 fps.
Always Positive
Extremely turbulent conditions can make it difficult to detect the flow direction. Particles in the flow stream
(particularly near the surface of the stream) may travel in several different directions, even though most of
the flow is moving in one general direction. Although the velocity magnitude is generally consistent in
these cases, reflections from the particles moving in a positive direction (same direction as the flow
stream), are so mixed up with those from particles moving in a negative direction (opposite direction as
flow stream) that determining actual direction is difficult.
The Always Positive option logs all readings as positive, regardless of the measured signal direction. Do
not select this option at sites where negative flows normally occur, such as tidal effects on ocean outfalls.
Velocity Cutoff/
Velocity Default
The Velocity Cutoff option compensates for problems encountered at sites with very low velocities and
extremely clean water. Flow is difficult to measure in these conditions because clean water contains very
few reflecting particles, and extremely low velocities lack the turbulence to add air bubbles (which also
make good reflecting targets) to the flow stream.
The Velocity Cutoff allows you to enter a default velocity value that is used when the Velocity Cutoff
set-point is reached, rather than report erratic velocities.