ADS Environmental Services Portable FlowShark Pulse QR 775006 A2 - Manual (Page 36)

System Overview and Operation 2-15
degree Centigrade). To ensure accurate depth measurements, all calculations must
compensate for the flow temperature.
Adding the sensor offset to the measured depth value (h
1
) determines the depth total
(h). The sensor offset represents the distance from the bottom of the pipe to the
sensor crystal position. The measured depth value represents the distance from the
crystal to the flow surface.
Hydrostatic Fill Level (Pressure)
Based on the type of sensor selected, the combination sensor may have an integrated
hydrostatic depth measurement device. This piezoresistive pressure sensor operates
according to the relative pressure principle, where the pressure of the standing water
column above the sensor is directly proportional to the depth level. This device can
measure depth levels even when the combination sensor is installed away from the
bottom center of the pipe.
During installation, this sensor must be calibrated based on a manual depth
measurement and the position of the sensor in the pipe.
Flow Velocity Measurement
The water-ultrasonic velocity/combination sensor has a piezo crystal sloped toward
the flow for measuring flow velocity. This crystal sends an ultrasonic pulse at a
defined angle into the oncoming flow. All air and/or dirt particles in the path of the
pulse reflect a small amount of the ultrasonic signal back to the sensor. The size and
shape of a particle determines the result of the reflected signal. Together, the returned
signals produce a reflection pattern received by the piezo crystal. This pattern is
converted into electric signals that are saved to a digital signal processor (DSP).
Conditions at first signal detection
After a certain period of time, the sensor sends a second ultrasonic pulse into the
flow. The DSP also saves this reflected signal.
Flow velocities differ at varying depth levels, creating a velocity profile.
Consequently, the distances (or movement) between reflective particles’ initial and
subsequent positions also vary at different depth levels, resulting in a distorted
Pipe bottom
Wedge Sensor