Ultrasonic Sensors for Water Level Measurement
Avoiding False Echoes from Fixed Structures
By Donald P. Massa
The water and wastewater industry has many requirements for measuring the level of water in applications such as flumes, weirs, pipes, tanks, wells, basins, and cooling towers. Ultrasonic sensors are reliable, cost-effective instruments for these applications. In operation, the sensor is mounted over the water. To determine the distance to the water, it transmits a sound pulse that reflects from the surface of the water and measures the time it takes for the echo to return.
Sometimes structural components, such as a small pipe, are located in the acoustic path between the ultrasonic sensor and the water. They can reflect a portion of the sound and produce a false echo that interferes with the ability of the sensor to properly detect the echo from the surface of the water. Advanced ultrasonic sensors can be adjusted to ignore these false echoes, which therefore enables them to provide accurate water level measurements.
How It’s Done
Figure 1 contains an example of an ultrasonic waveform. It shows graphs of both the magnitude of the received ultrasonic signal and the magnitude of the detection threshold as a function of distance from the sensor. The black curve shows the magnitude of the received ultrasonic signal, and the red curve is the magnitude of the detection threshold for the normal factory setting. The plot of the ultrasonic signal contains two target echoes. The first target represents a false echo, such as one produced by a pipe located in the acoustic path between the sensor and the water, and the second target represents the echo from the surface of the water. As can be seen, the false target is above the normal factory-set detection threshold. The sensor would therefore interpret it as the surface of the water and provide an incorrect level reading.
Figure 1: Ultrasonic Waveform from a MassaSonic™ PulStar™ Plus Sensor Showing a False Target Being Detected Instead of the Echo from the Surface of the Water.
Figure 2: MassaSonic™ PulStar™ Plus Ultrasonic Waveform With the Same Targets As In Figure 1, But With the Detection Threshold Modified to Ignore the False Target.
The MassaSonic™ PulStar™ Plus Sensor allows the user to display an ultrasonic waveform on a computer screen and to change the levels of the detection threshold at different distances from the sensor. The threshold can therefore be increased around the false target so that it is not detected. Figure 2 shows the ultrasonic wave form after the detection threshold levels were adjusted. The threshold in the region over the false echo target is increased significantly above the amplitude of the echo. The sensor now ignores the echo from the false target and detects only the water echo, and therefore provides correct water level information. The sensor will always accurately detect the echo from the water, even if the water’s surface moves to the same location as the false target. This is because the magnitude of the water echo is significantly greater than the level of the detection threshold that was set over the false target.
A MassaSonic PulStar Plus Sensor.
About the Author: Donald P. Massa is president and chief technical officer of Massa Products Corporation.