Processing Magazine

The Top 10 mistakes made when applying ultrasonic level sensors

And how to avoid them to obtain the best possible performance

May 1, 2013

APG SensorsBy Jon Cox

Of all our sensing technologies, ultrasonic is one of the most versatile and yet can be one of the trickiest. Ultrasonic makes for a fantastic sensing solution, suitable to a wide array of applications. However, avoiding some simple mistakes can assure the best performance with the least amount of headaches.

An ultrasonic level or distance sensor bounces short bursts of high-frequency sound waves off a target surface and measures how long it takes the reflected sound waves to return to the sensor. This is converted into a distance or a level measurement. The transmitted sound travels out in a conical shape similar to the beam of a flashlight, growing wider and weaker as the distance increases.

Following are the top 10 mistakes for ultrasonic sensors and tips to avoid making them.

#10: Poor quality power source
Because the sensors must detect and amplify what is often a very small electrical signal, a clean source of power is critical to keep electrical noise from overpowering the desired signal and triggering false readings. Low quality power supplies should be avoided.

#9: Improper cable routing
For the same reason that a clean power source is essential, care should be taken to keep instrumentation cable runs away from large electric motors, generators or other devices with strong electromagnetic fields that can induce electrical noise in the cable.

#8: Not using shielded cable
As stated in mistakes #9 and #10, electrical noise is a common problem with high gain (signal amplification) instrumentation like an ultrasonic sensor. Using shielded cable and proper grounding practices is one of the best ways to ensure proper sensor operation.

#7: Choosing a sensor with an adequate sensing range
The required sensing range of a given application is not the only consideration that needs to be taken into account. Factors such as high temperatures, targets with poor sound reflective characteristics and the size of the target (the amount of surface area perpendicular to the sensor) can all be good reasons to select a sensor with a sensing range greater than the actual maximum distance to the target.
Even the width of the sensing path to the target is a critical factor. A short-range sensor forced to run at higher gain (signal amplification) settings will have a wider detection beam than a long-range sensor operating at lower gain settings.

#6: Chemical incompatibility
It is always good practice to validate that the sensor materials are compatible with any chemicals that might be present in the application.

#5: Poor mounting angle
The transducer face of the sensor must be perpendicular to the target in order to receive the maximum sound wave energy reflecting back. Proper alignment becomes even more important as the sensing range increases. 

#4: Inappropriate application
Not all materials and objects are good targets for an ultrasonic sensor, nor are all environments compatible with transmitting or receiving sound-wave signals. Highly vaporous liquids create a constantly shifting atmospheric density, which affects the speed of sound, and can greatly reduce the accuracy of the readings. Soft or sound-absorbent materials, such as powders or foamy surfaces, can drastically reduce sensing ranges. Areas with heavy suspended dust can attenuate the signal. There are many other sensors that can do the job of an ultrasonic when the application is not a fit.

#3: Poor mounting location
The sound wave signals travel in the shape of a cone, similar to the beam of a flashlight (getting wider as distance increases). Mounting too close to potential unwanted targets, such as pipes, fill streams or even tank walls, if they aren’t smooth, can cause false or unstable readings.

#2: Ignoring the sensor’s blanking distance
An ultrasonic transducer cannot generate an ultrasonic signal and simultaneously detect a signal returning from a target. Because of this, there is an inherent dead-zone near the sensor face where signal echoes are returned too quickly to be distinguished from the transmitted signal. This dead-zone is known as the blanking distance, and extends anywhere from a few inches to more than a foot, depending on the frequency of the sensor. The sensor needs to be mounted in a location where the target will not come closer than the published minimum sensing range of the sensor.

#1: Improper sensor adjustments
Ultrasonic sensors are often adjusted incorrectly for the application. For example, some targets move quickly, others very slowly. Settings that work well for one will not be ideal for the other. It is human nature to think that more is better, which often leads to higher gain settings and faster target sampling than is required and overzealous signal filtering settings. It is good practice to take a minimalistic approach to sensor settings, not setting values to levels that are higher than required by the application. Don’t be afraid to contact the manufacturer if you need help determining the proper settings.
Keep these tips in mind as you select, install and use ultrasonic sensors. While they can be tuned to match your application like a glove, doing it wrong can have the opposite effect.

Jon Cox is an application engineer with Automation Products Group, Inc., Logan, UT. Automation Products Group (APG) is an ISO 9001 certified manufacturer, specializing in liquid level sensor and pressure transducer design and manufacture. 888-525-7300,
www.apgsensors.com, sales@apgsensors.com