System Overview 2-21
Long Range Depth Sensor
This sensor has an ultrasonic transducer with a glass reinforced
epoxy acoustic window and is located inside the integral parabolic
reflector. It first transmits an ultrasonic signal horizontally from
the transducer to the optimized reflector, focusing a very narrow
beam downwards. The signal bounces off the flow and returns to
the reflector where it is concentrated and aimed at the same
transducer. 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
air 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 manhole depth.
It computes the speed of sound in the air using the temperature
recorded by a temperature sensor housed within the ultrasonic
depth sensor.
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