ADS Environmental Services FlowShark Pulse QR 775004 A1 - Manual (Page 159)

Parameter Settings 8-69
Determining the zero point for the pressure measurement cell often
involves measuring the current depth level using a yardstick, a ruler,
or another similar device without removing the sensor from the flow.
This method, however, can produce errors. Inserting the ruler (or
yardstick) into the flow can create a surge leading to erroneous
measurements depending on the current flow velocity.
CAL Menu: Velocity
Based on the continuous profiling technology employed, velocity typically does not
require calibration. However, under unusual site conditions where velocity requires
constant adjustment, contact ADS for assistance in performing calibrations.
When flow falls below the minimum depth required for proper operation of the
velocity sensor, (i.e., only 2.2 inches of flow exists between the water ultrasonic
sensor face and the flow surface or a total depth of 2.6 inches of flow exists when the
bottom plate of the sensor is at the bottom of the channel), use an automatic
estimation (auto calculation) of velocity. Use this function for computing very low
flow velocities under low depth conditions, such as unusually low minimum nightly
flows. Do not implement this function at locations that experience backwater
conditions.
Make sure that obtaining reliable depth measurements down to 0 is possible through
an independent, external depth measurement device (e.g., an air-ultrasonic sensor).
With falling depth levels, at some point it may no longer be possible to measure flow
velocity. When depths reach this designated minimum level, h_crit (critical level, the
lowest level at which flow velocity can be measured reliably), the FlowShark Pulse
will create an internal table of Q & V vs h values using the last valid flow velocity
reading before dropping below this critical depth. This table of values enables flow
calculations to continue, even when velocity measurements are not available, by
assigning an estimated velocity to a corresponding depth level (based on the
Manning-Strickler relationship). The unit automatically will compensate for the
shape of the pipe in the curve.
The erroneous measurements that develop from using this method in
the context of unstable hydraulic conditions may be significantly
greater than the errors that arise from measuring the actual flow
velocity and depth levels.
This method is suitable only for very low flow volumes that are
backwater-free.