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Water Level Sensor and Datalogger Testing and Demonstration
www.itrc.org/reports/pdf/WaterLevelSensor.pdf
ITRC Report No. R 99-002
Chapter 4 – Ultrasonic Sensors
Overview
General. Ultrasonic sensors transmit a series of cone-shaped sound waves through the
air. These sound pulses reflect off the liquid surface and are in turn received by the
sensor, which measures the time interval between the transmitted and received signal.
Electronics then convert this time interval into a distance measurement using the speed of
sound in air. No part of the sensor ever touches the water, a distinct advantage of this
sensor type.
Installation. The ultrasonic transmitter angle, water surface turbidity, and distance to the
water surface can affect ultrasonic sensor accuracy. Ultrasonic sensors have a specified
minimum measuring distance, called the blanking distance (usually 30 – 45 cm). If the
water surface rises to within the blanking distance the sensor will not be able to
distinguish between a sound transmission and its echo, resulting in an unreadable water
level (see later description). However, an ultrasonic sensor should be placed as close as
possible to the water surface for maximum accuracy. It also must be installed absolutely
perpendicular to the water in order for the sound waves to reflect directly back to the
sensor (Figure 32).
Beam
Angle
Correct
Figure 32. Ultrasonic sensor installation.
Incorrect