ADS Environmental Services FlowShark Pulse QR 775004 A1 - Manual (Page 33)

System Overview and Operation 4-3
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.
Pressure Depth Measurement
The combi 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 combi 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 combi sensor has a piezo crystal 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. The
size and shape of a particle determines the result of the reflected signal. Together, the
returned signals produce a reflection pattern (Figure 4-2) received by the piezo
crystal. This pattern is converted into electric signals that are loaded into a digital
signal processor (DSP) housed in the sensor.
Fig. 4-2: Conditions at first signal detection
After a certain period, the sensor sends a second ultrasonic pulse into the flow. The
DSP also saves the new reflected signal.
Flow velocities differ at varying depth levels, creating a velocity profile. Therefore,
the distances (or movement) between reflective particles’ initial and subsequent
positions also vary at different depth levels, resulting in a distorted reflection pattern
(Figure 4-3). Several other factors also influence particle reflection and velocity
profile measurement. Rotating particles may exhibit multiple shapes for reflection;