How many more technologies will emerge in the world market for measuring continuous level for solids and liquids materials? It seems to be an unending barrage of advertisements with statements of performance, high success, breakthroughs, reliability, and on and on, but are these level technologies really fact or fiction in their claims? With all of the level technologies at the availability of end users, it is becoming really difficult for people to make a wise selection of what technology will provide them with the best results for their application. There are many factors involved in deciding which technology is best suited for a level application, not to mention the thoughts of the technologies cost, the installation time, and the learning curve, if any, on the technology chosen. In the market for level measurement, there are technologies that are being pushed as the end all, beat all solution for liquids and solids level, but let me tell that there is no universal technology. There are some technologies that will have a broader application base for solving level, but never, ever rely on just one for your measurements. Focusing on continuous level will be the direction of this article and a detailed discussion on the mystique of microwave radar as the "chosen" technology.
Is it the panacea for all level applications? The answer is absolutely not, as overshadowed in the background of this highly promoted technology is the technology of acoustic wave. Not to be mistaken with ultrasonic wave, but this acoustic wave technology uses low frequency design harnessed within a short cylindrical, but narrow diameter transducer package. Different than anyone else on the market today, but this unique design drives the low frequency resonating mass to produce a pressure wave at the face of the transducer. With this pulsing to the face of the sensor, there is a self-cleaning effect that eliminates any type of build-up. So, low frequency (5 KHz to 30 KHz) combined with high power provides measurement capability in the toughest of applications. Now, I am not saying that acoustic wave technology is the chosen "ultimate" technique for measuring level. There are technologies and there are application conditions, and the two items have to be considered together and not separately. Many companies make the mistake of trying to shoe horn fit one technology into a plethora of applications, and this does not work well at all. Understand the application, and what the parameters are, as well as the customers' requirements, and then discuss the technology for the measurement.
So, if there isn't a universal technology for measuring level in liquids or solids, why is microwave radar being sold into a large majority of level applications? It is being falsely promoted as the solution to almost every application, and has claims that it can perform under all conditions of duress. Conditions of duress would be heavy dust in the airspace, heavy build-up on the antenna emitter, wet and moist conditions, the ability to read through foam, high temperatures, and just plain every other condition that other technologies fail at. Many, many times over, one can visit a plant site and see microwave radar designs installed in applications where they should not have been used, and yet they were embellished to be the solution. With microwave radar technology, like any other technology in the marketplace, there are advantages and limitations for its applicability and performance success. These things need to be understood by the end users.
Microwave radar is not the ultimate solution for all level applications, and that's what this article will further point out. For this technology, difficult application conditions for liquids such as heavy vapors above the liquid surface, high temperatures beyond 300 F, pressures in excess of 50 psig, and turbulent conditions on the material surface would deem this technology as the most suited. It does not mean though that another technology could not be considered such as guided wave radar, capacitance, or differential pressure, but when presented with conditions, it makes sense to evaluate each technology and perform a process of elimination. With these conditions stated, would traditional ultrasonic be applicable? The answer is no and the reason being is that the vapors from the hydrocarbon will stratify with the changes in temperature in the vessel. The speed of sound depends on the temperature of air and it highly affected by the vapor density in the airspace. Errors in the measurement will occur if the air medium is different than pure air as the sound wave will be somewhat attenuated and travel will change. Additionally, the ultrasonic technology will be affected by the condensation in the vessel caused by the changing temperatures, as traditional ultrasonic is usually transducer frequencies of 40 to 55 KHz. With this higher frequency band, there are no self-cleaning properties because there is not enough power to create a pressure wave on the transducer face. So microwave radar would be an ideal choice for this liquid application.
As seen from above in the liquids market, microwave radar has its place and can work quite well in applications, especially when there are some harsh conditions like heavy vapors, strong turbulence, and temperature/pressure extremes. But again, it is not the broad brush solution to every level application, and that's what has to be understood. Level applications that have build-up as a possibility are a real question mark for any technology. Applying a contact technology in an application with build-up or coating is not the smart choice. In that case, the application of a non-contact technology should be the first thing on the mind and then the thought about the type of build-up. Regardless of the build-up on the sensor of microwave or acoustic wave, there has to be either maintenance schedule of cleaning performed on a periodic basis OR the use of a self-cleaning technique to keep the build-up or coating off of the transducer face. It's not to say that acoustic wave is the solution with any build-up, as it does depend upon the dielectric of the build-up from a microwave radar standpoint, but in general, the high power pressure wave created does eliminate the coating from happening.
Now, when the application involves the measurement of solids materials, like powders, grains, metal ores like copper, iron, coal, and cement materials, then applying the right technology takes additional thought. From a microwave wave radar perspective, the technology was introduced into the solids level market in the 2003 timeframe, and was promoted as being the new technology for measurement in all solids applications. Think about it from a level standpoint, a technology that could be the solution to difficult solids applications with conditions like heavy dust, dealing with angles of repose, long range measurements beyond 200 feet, high temperatures, and more. Although the technology sounds admirable and stellar in thought, it doesn't fit the bill from a universal applicability standpoint. It has hit the level market and portrayed as the technology that could provide reliable and accurate measurements under any conditions. The microwave technology absolutely took the level market by storm, and in many cases, cannibalized other technologies in the process, especially in the solids industry. The use of non-contact ultrasonic and acoustic wave has taken a direct hit from a loss of sales standpoint and it is a result of the over promotion of microwave radar.
Certainly, the technology does warrant merit as it is a very solution oriented design, however, when it comes to solids and the addition of moisture into any application, there are no self-cleaning properties like acoustic wave. With the measurement of solids, it must be emphasized that there are industries that have dry solids, and then applications which have moisture. When one speaks about the mining industry (coal, non-ferrous and ferrous minerals), reducing the heavy dust in the environment through the use of water sprayers is an absolute necessity for the work force. There is so much excessive dust throughout a mining site because raw rock and ore material is transported to machinery that performs crushing, movement via conveyor belts, dropping from one transfer chute or another, and is a dust nightmare. So with that said, most mining operations today will make use of water in a spray form that is located at filter screens, ROM bins, rock crushers, conveyor transport, and many more. With powder and water mist combined, the parts of any containment, conveyor belts, transfer chutes, and such will be coated with heavy scaling or build-up. So for level measurement and the notion to keep things clean so that performance can continue, either there has to be lots of periodic maintenance to keep the sensing elements clear of coating or the technology must have that feature built in somehow. Hereto the technology of acoustic wave which has the built in capability to keep the propagating sensor surface free of coating or build-up regardless of the material being wet or dry.
This technology of acoustic wave is what I would refer to as a diamond in the rough, or a pearl hidden behind the shell simply because it has been considered to be the same as ultrasonic technology. Ok, the technologies are similar; however, there is no comparison when it comes to performance in tough solids applications with heavy, wet build-up, moisture in the environment, or shear brute transmission over long ranges in the nastiest airspace conditions. And the magic behind this acoustic wave technology is not just the frequency, as it is a combination of low frequency, like 20 to 5 KHz, along with high power to a narrow diameter balanced resonating mass, and adaptive modulating gain control. Actually, the secret is in the formula for all three of these variables, which is mechanical transducer design, power & energy distribution, and adaptive software control. In the solids industry and level measurement, it isn't just software that will yield the successful results for these harsh application conditions.
When applying acoustic wave technology, the choice of the transducer frequency is not based upon the range of the measurement, but it is really a function of what's in the containment. And with low frequency, high power acoustic wave, it's not really how dusty the airspace above the surface is, but more about the environment inside the containment. If there is moisture, then the frequency will need to be lowered, and accordingly, the power to move that larger resonating mass is taken into account. Let it be understood though that with lower frequency, the energy to the transducer is NOT increased, but distributed differently so that there is focus out into the airspace and not elsewhere. The acoustic technology is not affected by moisture content from an operational standpoint as would microwave radar. Remember that microwave radar is a function of the dielectric value of the material. With air being 1 and water being 80, the low dielectric materials of 1.5 to 10 are insulators, and will be fairly non-absorbing of microwave signals. If the microwave radar transmitter is measuring dry powders, and there is a dust cover over the antenna, then the application is relatively easy. Adversely though, and this is important to understand, that if there is any moisture entrained or apparent in the solids material that causes coating or build-up, then the microwave non-contact radar is virtually doomed for successful performance. The acoustic wave technology is not affected by the coating or build-up because the low frequency and high power create strong frequent pressure wave activity on the transducer face and this keeps the material off. This pulsing power on the transducer provides the self-cleaning feature that gives the performance under these tough conditions.
In addition to the self-cleaning properties for clean signal propagation on acoustic wave in wet, moist dusty environments, it is of great merit to discuss the other noted features that allow this technology to stand out amongst others and raise this sleeping giant of a technology.
The low frequency combined with the high energy allows this design to be used in solids applications without the typical aiming assembly for dealing with angles of repose. Enough focused energy is produced from the transducer face, and then a focalizer is used to harness the energy so that signal reflection back from the material surface is collected.
Inherent false echoes and excessive ringing will occur with traditional ultrasonic technology as most of the transducers are mounted onto flanges that contact metal containment structures. A simple acoustical coupling abatement flange is designed now to completely isolate the transducer and eliminate the ringing issues.
Acoustic wave design is impervious to any crosstalk directly related to electrical noise (variable frequency drives, pumps, etc.) because the signal coming out of the transducers are digital RS485 in their output and not some small millivolt output. Additionally, with the balanced transducer design at the crystal array, any noise is also not a problem.
Acoustic wave technology deals extremely well with low bulk density materials. The issue with low bulk density materials (lightweight powders that have a soft surface) is that energy from microwave devices will allow the energy to pass through the material, and provide a small amount of reflection back, but mostly propagate through the material and reflect off of the metal containment bottom. Obviously, this is not an accurate measurement. The acoustic wave technology will allow much of the signal to reflect back due to some design features, which makes it ideal for these applications.
The message that should be clear from this article is that every technology for level measurement has its place in applications. Whether it is liquids or solids materials, the selection of the technology must be reviewed carefully and all application conditions of dust, condensation, wet material, turbulence, mounting, surface conditions, temperature & pressure, and many more have to be considered. One technology like microwave radar should not be considered the universal solution to all level measurement applications. As pointed out in this article, there are other technology solutions that will fit more appropriately in an application, and one that has been lying quietly in the background like acoustic wave, especially for nasty solids materials, fits the conditions to provide top notch performance. With almost 30 years of experience in the level industry, it has been seen countless times where a microwave radar design or other technology has been installed in the wrong application, yet it was promoted to the customer as the right technology. The hype for microwave radar needs to be filtered, as it is being sold as the technology for anything, when in fact it is finding more problems now as the application installed base increases. Don't overlook the sleeping giant of acoustic wave as good things can come in small packages.
Applying the right level technology is an engineered process, and should not be a commodity sale. A careful review of the application conditions as well as the customers' requirements should be examined before just offering something that has been promoted as the ultimate solution. Whether the customer is in the liquids or solids industry, the right level solution will provide many years of good successful performance with a low cost of ownership.
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