1. There is a tendency to pit ultrasonic level sensors and radar level sensors against one another when in reality each sensor has advantages and disadvantages that give each sensor its own place.
2. We will look at the differences in how these two technologies work and will cover the applications each sensor is best suited for.
ULTRASONIC LEVEL SENSORS
1. As the name implies, ultrasonic level sensors operate by emitting a burst of sound waves in very rapid succession. The sound waves emitted from the sensor travel at a known speed (the speed of sound) to the intended target where they bounce off the target and return to the sensor.
2. Using the amount of time it takes for the sound burst to return to the sensor after being sent, the distance between the sensor and the substance being measured or the level of the substance is calculated. With help from other parameters programmed into the sensor or control system, volume, weight, or other similar measurements can also be calculated from the measured distance.
3. In order to accurately determine the level of the target substance, ultrasonic sensors require an unobstructed air column between the sensor and the target. Things like physical obstructions, excessive foam, heavy vapors, thick dust, and light powders can all deflect or absorb the signal, or act as a false surface leading to erroneous readings from the ultrasonic sensor.
COMMON USES OF ULTRASONIC SENSORS
1. Liquid Level Measurement - Ultrasonic sensors are most commonly used for measuring the level in a body of liquid, such as in a tank, well, pit, lake, or another body of liquid. Ultrasonic level sensors can also be used to measure liquid in non-linear containers. This usually requires a strapping chart to adjust the reading to a volume as liquid rises and falls.
2. Solids Level Measurement - Ultrasonic sensors are an excellent choice for solids level measurement, especially in comparison with radar level sensors. The ultrasonic sound waves are much easier to manage and measure than radar’s electromagnetic waves when coming off a substance with an angle of repose. It is important to note that using an ultrasonic sensor for solids level measurement usually reduces the effective range of the sensor by half. Additionally, environments with dusts or powders scatter sound waves, leading to erroneous readings. But an ultrasonic sensor with an appropriate range used in an environment with little to no air-borne particles will provide extremely reliable solids level measurement.
3. Open channel flow - Open channels are a primary means of transporting, filtering, and metering water, often used in water treatment plants, environmental monitoring, and irrigation canals. Often, in this type of application, ultrasonic level sensors are paired with an appropriate controller for flow calculations.
4. Presence Detection and Object Profiling - Last but not least, ultrasonic sensors are a great fit for presence detection and object profiling. If object detection is required in dirty or wet environments where the targets are fairly slow-moving, an ultrasonic sensor can be used. They are able to handle harsh conditions, making them a suitable choice.
RADAR LEVEL SENSORS
1. Radar sensors, by contrast, work not with sound waves, but with electromagnetic waves. This is the key difference between ultrasonic and radar sensors. Like the waves from ultrasonic sensors, the waves emitted from the radar sensor bounce off of objects and travel at a known speed (much faster than ultrasonic waves). Unlike ultrasonic sound waves, radar’s electromagnetic waves react differently to certain materials as they are reflected off the surface.
2. Radar sensors are affected by different variables than other level sensors. Radar level transmitters are affected less by temperature than ultrasonic sensors, improving consistency and accuracy. Radar level sensors are also well suited for specialty applications, such as working in a vacuum, or in higher pressures (as long as the housing can handle it). Radar sensors are also less affected by foam, vapors, powders, and dust that can interfere with signals on ultrasonic sensors and lead to erroneous readings. This can make radar sensors a better choice for these applications.
3. An important factor for radar sensors is the target material’s dielectric constant. A material with a low dielectric will not reflect an electromagnetic wave, so radar tends to pass right through. These materials are typically non-conductive and have low moisture content, such as dry powders and granules.
4. To be fair, radar can sense a lot of these materials, but the energy of the returning waves is so small that precise alignment with the signal is paramount. This introduces techniques such as guided wave radar, or special antenna. Measuring materials with a low dielectric constant is not always impossible, just very challenging.
THE BOTTOM LINE: ULTRASONIC VS. RADAR
1. In the end, if your application is straight-forward, an ultrasonic sensor is a good bet for high-quality performance and excellent longevity. For tanks, wells, channels, or reservoirs, an ultrasonic can be configured to meet your needs.
2. However, as soon as foam, vapors, powder, dust are introduced to the equation, radar sensors become the answer to your measurement needs. Environmental variables just don't affect radar measurements the way they do ultrasonic sensors.
3. A necessary caveat here is that good tech will always trump bad tech. Yes, radar sensors are better than ultrasonic sensors in foamy, dusty applications, but a poorly-designed radar sensor is always going to perform poorly. Similarly, ultrasonic sensors are usually cheaper than radar for easier liquid or clean-air solids level measurement, but a cheap ultrasonic will never be anything better than a cheap ultrasonic. Ultrasonic and radar sensors don't compete with each other, they complement each other. Neither one is a one-size-fits-all level measurement solution.
ULTRASONIC AND RADAR LEVEL SENSORS: COMPLEMENTARY, NOT COMPETITIVE TECHNOLOGIES
1. Ultrasonic and radar level sensing technologies are usually described as competitors, pitted against each other, in a struggle for level-sensing supremacy. “Ultrasonic is better!” “Radar is the best!” The truth is that they are far more complementary than they are competitive, with each excelling in different situations. Anybody pitching either technology as a One-Tech-Fits-All level measurement solution isn’t telling the whole truth about the technologies and their products.
2. A basic non-contact level measurement technology assumption is “Go with ultrasonic, because it’s cheaper, unless the application is complicated. Then use radar.” While that assumption is partially true, it doesn’t tell the whole story. Yes, measure for measure, ultrasonic sensors are cheaper. And yes, radar does handle most not-so-simple applications better, but it doesn’t handle all of them better. Sometimes, the simplicity of a “simple application” is in the return signal of the sensor, not in the eye of the engineer or systems manager deciding which sensor tech to buy.
ULTRASONIC LEVEL SENSORS & PILING SOLIDS
1. Ultrasonic level sensors, in general, are better at dealing with piling solids than radar. The signal from a single-frequency radar will be partially absorbed, partially reflected, and partially deflected by the angled target surface of the piling substance. Ultrasonic waves, however, will reflect enough to get a decent signal, as long as the sensor is capable of reading flat surfaces twice as far away as the angled surface. And while dual-frequency radar sensors are capable of dealing with the complicated reflections off the angle of repose, the signal processing required is much more complicated than what is need for single-frequency radar or ultrasonic.
RADAR LEVEL SENSORS & ATMOSPHERIC INTERFERENCE
1. Radar level sensors provide better results than ultrasonic sensors when there is atmospheric interference. Dust, foam, and vapors are the three most often named culprits, but almost any physical substance or object that could come between a sensor and the target surface is easier to “see through” with a radar sensor than with an ultrasonic sensor.
2. This includes dual-surface interface detection: radar can be set to look past the initial surface to the point of interface between two (usually) liquids. Ultrasonic sensors just can’t do that. And since radar sensors generally have a narrower beam spread, they can often be used in tighter, more cluttered spaces than ultrasonic sensors.
3. Here’s the huge caveat: if you use bad tech, it isn’t going to work well. A poorly designed sensor, whether it uses sound waves or electromagnetic waves, won’t produce a good measurement, no matter how well it should fit the application. For example, full-power radar sensors promise to be the only non-contact level sensor you’ll ever need. But because the premise is faulty, the sensors end up causing as many problems as they solve.
ARE FULL-POWER RADAR SENSORS THE ANSWER?
1. Full-power radar sensor manufactures promise a narrow beam-spread angle, which sounds great. But what they don’t tell you is that the secondary side-beams generated by their full-power pulse create just as many echoes as a wider primary-beam. When you have a full-power primary wave and noisy secondary waves, everything around the sensor and the target will generate echoes. Everything.
2. So, in order to compensate for making all that racket, a full-power radar sensor has to deploy a huge amount of front-end signal processing. Every echo that returns to the sensor has to be analyzed: “Is this the echo I’m looking for?” All that processing means a longer delay between sending the initial pulse and generating an output, and more time means more power being used by the sensor.
3. Those blackouts that full-power radar sensors use to ignore echoes from stationary objects are also a problem in and of themselves. In order to compensate for creating too many echoes, a full-power radar sensor creates blind spots within the target range. When the target level moves through a blackout area, the sensor is literally guessing what the output should be. Sure, it can base the output on the immediately previous rate-of-change for the target level, but if the rate-of-change suddenly changes, you have no way of knowing until the echo from the target surface emerges from the blackout area.
4. Full-power radar sensors also need to “guess” when it comes to dual-surface interface detection. Because the sensor fills the measurement environment with echoes, the return signal from a dual-surface interface looks like one big echo, instead of distinct echoes for each surface. So once again, the differentiating of unwanted echoes from the desired primary surface echo and dual-surface interface echo is left to signal processing. At some point, it’s fair to ask if the sensor manufacturer is as good at designing signal processing software as it is poor at designing radar sensors.
5. Another design feature of some radar level sensors is a convex antenna. The selling point is that a shorter antenna means the radar level sensor can be used in smaller, tighter areas, where a traditional cone antenna might not fit. There are, however, two problems related directly to choosing an antenna of this shape.
6. First, any condensation that forms on the antenna will collect at the apex of the curve. Excess condensation will drip off, but what remains directly interferes with the radar signal. Second, honest radar level sensors with convex antenna will advertise the necessity of a companion air compressor to supply antenna-clearing compressed air. That’s right, you need to buy an air compressor with these radar level sensors. The less honorable manufacturers won’t tell you about the need for cleansing compressed air until you have problems with your already-installed radar level sensor.
7. From the booming extra echoes of full-power radar to the required air compressor accessories for convex-shaped antenna, poorly designed radar level sensors simply create more problems on their way to measuring levels. Choosing to use these kinds of radar sensors simply because “Radar is better than ultrasonic” is choosing to trade one set of problems for another. Yes, there are applications where a radar level sensor is a better choice than an ultrasonic level sensor, but that radar level sensor has to be well designed. Poorly engineered equipment always leads to level measurement problems.
Source:
https://www.apgsensors.com/about-us/blog/radar-and-ultrasonic-sensors
https://www.apgsensors.com/about-us/blog/radar-and-ultrasonic-level-sensors-complementary-not-competitors
