ADS Environmental Services IETG FlowHawk QR 775012 A4 - Manual (Page 57)

System Overview 2-21
Ultrasonic Depth Sensor
This sensor first transmits an ultrasonic signal downward from one
of the crystals on the sensor face that bounces off the flow surface
and returns to the other crystal. The monitor measures the travel
time elapsed between transmission and reception of the reflected
sound signal. Based on the elapsed time and the speed of sound, the
monitor determines the range. The range represents the distance
between the sensor face and flow surface. The monitor calculates
the depth of the flow by subtracting the range from the pipe
diameter. It compensates for the speed of sound in the air using the
temperature recorded by a temperature sensor housed within the
ultrasonic depth sensor.
The ultrasonic depth sensor is located in the front, horizontal
portion of the sensor and has two downward-facing ultrasonic
crystals covered with a clear, moisture-shedding Teflon
® coating.
Surface Velocity Sensor
This sensor measures the velocity of the flow also by bouncing
signals off the flow surface using a method similar to the ultrasonic
depth sensor, but the velocity signals are angled toward the
oncoming flow surface.
The surface velocity sensor crystals reside in the two descended
portions toward the rear end of the sensor.
IETG does not recommend using this sensor for measuring velocity
at locations with smooth flow surfaces or velocities of less than 0.3
meters per second (1 ft/sec).