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The Facts about Acoustic Wave and Radar

December 3, 2012

Many level applications pose special problems for process level equipment and technologies. Whether the industrial site is a mine, power generation facility, or cement plant, or any other plant wanting to make a solids measurement, these sites all require technologies that will withstand the tough environmental conditions as well as the harsh nature of the solids applications. These include heavy dust in the airspace, steep angles of repose, high temperatures, changing process conditions, corrosive media, abrasive solids materials, moisture in solids and more. In addition, so many different sizes and shapes of containment mean that many installations have to deal with obstructions like mechanical bracing for structural support.
    So let's look at the non-contact technologies, both new and older in the market place today. The technology known as ultrasonic has been around for many years, and it is as the name implies, sub sound technology in the kilohertz frequency band. The designers of ultrasonic technology have made valiant attempts to solve the difficult solids applications with frequencies down to as low as 8 to 12 kHz and various transducer designs in size and shape, but the overall measurement success has been inconsistent at best. Then along comes non-contact microwave technology with the claims that it is the new "sexy" technology to measure the long range, dusty solids measurements. Great claims for something that performs well in dry materials, but induce moisture into the solids materials along with heavy dust, water sprayers for dust abatement, and that's a formula for disaster.
    This new technology is not the panacea for all level applications as many companies tout, and it definitely does not have carte blanche performance in the industries like coal, metal mining, minerals, and other solids industries. With the less than desirable results on solids using "ultrasonic" and the through air radar not capitalizing in the mining industries, what technology is out there to solve these applications?
    Well the overlooked technology, which is a variation on a technology theme of ultrasonic, but designed in a way to offer significant application benefits, is acoustic wave technology. The magic behind this technology is the fact that it utilizes audible frequencies (5 to 30 KHz) in a transducer design that is harnessed as a balanced resonant mass. The combination of low-frequency, high applied power, and variable adaptive gain control, makes this acoustic wave technology a real solids solution that can't be beat and is really underestimated. On the transducer, the low frequency with high applied pulsing power to the face creates a pressure wave that literally offers consistent and proven self-cleaning properties. Effectively, there are no materials that will adhere to this transducer face regardless of their moisture or sticky properties.
    Let’s take a look at microwave radar for solids applications. With microwave radar, the frequency of the device used and the antenna design is important in how well it will perform in these dusty conditions. Non-contact microwave radar designs typically operate in the frequency band from 5.8 to 26 GHz, and some even go higher than that, with use of either pulse or FMCW technique. The technique of pulse wave radar seems to be most often used these days, and a frequency band of 24+ GHz. The correct size and type of antenna is essential when choosing this technology for solids level measurements. The antenna type should be a horn style and the size should be as large as possible, but most manufacturers offer 2- to 6-in.-diam, with some offering 10-in. parabolic dish type versions. Applying a 2- or 3-in. size horn antenna is not appropriate for solids applications, as there is not enough of a collection source at the receive area for the microwave signal. So choosing a horn diameter of 4 in. or larger is best for penetrating the dust in the airspace, as well as allowing for a better collector on the returning signals. The technology works well on measurement ranges up to 125 to 150 ft, but after that, the readings become somewhat unreliable, and usually build-up of dust becomes a major deterrent to the propagation of the microwave energy.
   Jerry Boisvert is a regional business manager with Hawk Measurement. He has been in the process instrumentation market for 30 years and specializes in level controls for the liquids and solids industries.

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