Ultrasonic Sensors are a reliable and cost effective instrument often used to measure the level of liquid and solid materials. In operation the sensor is mounted over the surface of the liquid or solid. To determine the distance to the material, it transmits a sound pulse to the target that is then reflected from that surface, and measures the time for the echo to return. (1)
Occasionally vapors from the materials being measured can increase the attenuation of the ultrasonic signal, or the characteristics of the surface of the material can reduce the reflectivity of the ultrasonic signal.
The range of detection of the sensor will be reduced in cases like this. Advanced ultrasonic sensors can help overcome the reduction in echo amplitudes.
Structural components such as a small pipe or conduit, can be located in the acoustic path between the ultrasonic sensor and the liquid or solid surface. 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 material.
This article explains how advanced ultrasonic sensors can overcome these problems.
Increasing Reduced Echo Levels and Ignoring False Echoes with an Advanced Sensor
Figure 1 details an example of an ultrasonic waveform. It displays 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 liquid or solid, and the second target represents the echo from the surface of the material.
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 material and provide an incorrect level reading.
The MassaSonic® PulStar® Plus Sensors permits 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 waveform after the detection threshold levels were adjusted. As displayed, the threshold in the region over the false target has been increased significantly above the amplitude of the echo. The sensor will now ignore the echo from the false target and detect only the echo from the surface of the material, and therefore provide correct level information.
The PulStar® Plus also contains a power level control feature. Therefore, the strengths of the reduced amplitude echoes that fall below the detection threshold can be increased and detected. These features enable the PulStar® to operate in many applications that are difficult for other ultrasonic sensors. A MassaSonic ® PulStar® Plus Sensor is shown in Figure 3.
Massa Products Corporation
Increasing Power to Overcome Sound Attenuating Vapors or Poor Targets and Avoiding False Echoes When Using Ultrasonic Sensor
Ultrasonic Waveform from a MassaSonic® PulStar® Plus Sensor Showing a False Target Being Detected Instead of the Echo from the Surface of a Liquid or Solid Material
MassaSonic® PulStar® Plus Ultrasonic Waveform With the Same Targets As In Figure 1, But With the Detection Threshold Modified to Ignore the False Target
MassaSonic® PulStar® Plus Sensor
This information has been sourced, reviewed and adapted from materials provided by Massa Products Corp.
For more information on this source, please visit Massa Products Corp.
 A more complete overview of the fundamentals of ultrasonic sensors is contained in the two part article entitled “Choosing an Ultrasonic Sensor for Proximity or Distance Measurement,” written by the author of this article and published in the February and March 1999 editions of Sensors Magazine. Reprints of this article can be obtained at www.massa.com/about/publications.