Radar technology is a vastly popular means of process instrumentation. From grain silos to oil refineries to chemical facilities, radar is trusted to keep processes running at maximum efficiency. As products and processes have grown more complex over time, radar technology has adapted to keep pace with market demands. One of the ways these instruments have advanced is by increasing the dynamic range. This paper explains dynamic range and its importance in solid and liquid applications.
Dynamic range vital to level instrumentation versatility
Large dynamic range allows radar technology to accurately measure liquids and solids in a variety of applications. Measured in decibels (dB), dynamic range (also called dynamics) is an indicator of sensitivity; it refers to the range of usable signals a device can detect. The larger the dynamic range of radar instrument, the smaller the signals it can measure in operation, and the more useful it is to multiple industries.
To gain a conceptual understanding of dynamic range, consider two common types of scales found at home. A standard digital bathroom scale measures weights up to 330 pounds (150 kg) and does so in 0,2 lbs (0,1 kg) increments. Considering 0.2 lbs. is a small value and 330 lbs. is quite heavy, it is safe to say that a bathroom scale has good range. Moving to the kitchen, we would find that the produce scale has good range as well. Granted, the typical produce scale only measures up to 44 pounds (20 kg), but it does so in two ounce (55 g) intervals. Each scale is limited to a set range of weights it can measure, and we can compare the size of those ranges.
Just as with radar instruments, the value of a scale’s range differs depending on application. A dieter depends on the range of a produce scale to weigh in ounces (grams) the blueberries he eats for breakfast. He also depends on the range of the bathroom scale to measure in tens of hundreds of pounds (kilograms) the effects of his healthy diet. Substituting one scale for the other will not give the dieter accurate data and would likely result in a bathroom full of blueberries and the busted remains of the produce scale scattered across the kitchen floor. No one wants that.
Now, imagine a radar level device with the dynamic range of 96 dB is a bathroom scale. After weighing in, you could step off the scale, remove one hair and step back on, and the scale would detect the difference. That is amazing, but 96 dB is hardly the top of the mountain when it comes to the dynamics of modern radar instruments. Many level devices offer a dynamic range of 120 dBs. If one of these units were a scale, you park a truck on it and remove a hair, and the scale would furnish a lighter weight. The market’s most technologically-advanced radar devices have a dynamic range of 128 dB. Applying those dynamics to a scale, you could do the hair trick while sitting in a train, and the scale would show the difference. Installing these radar devices in a plant is the equivalent of owning a home scale that reaches the light end of the produce scale’s range, the heavy end of the bathroom scale’s range, and far, far beyond.
For example, let us compare the first and last devices in our analogy. The 24 dB difference in dynamic range corresponds to our factor of 200. This means signals appear 200 times larger in the 120 dB instruments than they do in the 96 dB instrument. The 8 dB difference between the second and third radar device also represents a large chasm in performance than one might assume, as 8 dB corresponds to a factor of five. The incredible sensitivity is an important feature to plant operations in various industries.
Dynamic range in demand for measuring difficult media
The need for radar sensors with large dynamic range, changes based on the media being measured. Power plant operators rely on radar sensors to detect the height of coal stockpiles and to control the position of conveyor belts that feed product to the pile. In this application, a sensor does not need large dynamics because coal is highly reflective of radar signals, so a device with small dynamic range will work just fine. This is not the case for media with poor reflective properties.
The chemical industry, for example, is thick with solids and liquids that do not easily reflect radar signals due to low dielectric constant (dK). These low-dK materials demand a radar instrument that performs more like a produce scale than a bathroom scale to detect their miniscule signals. In other words, they require a radar instrument with large dynamics. Radar sensors with dynamic range of 120 dB can measure any liquid: In fact, it is safe to say the days of looking up dK values are over. However, chemical composition is not the only reason to choose a sensor with large dynamics to measure process liquids.
Dynamic range helps a radar sensor perform in difficult conditions. For instance, foaming runs rampant in chemical vessels that use an agitator to mix or dissolve liquid product. Operators hesitate to use radar instruments in these applications because foam attenuates radar signals. In extreme cases, foam can completely absorb radar signals and prevent them from returning to the sensors. However, radar devices equipped with large dynamic range compensate for foam with greater signal strength than their predecessors. By emitting a stronger signal to the liquid, modern sensors detect the small signals that make it back through the foam. The Vegapuls 64, VEGA’s new liquid level measurement device, is such a radar sensor.
Conclusion
Dynamic range of a radar device refers to its sensitivity to – and ability to measure – small signals. Process instrumentation manufactures are responding to the growing demand for large dynamic range by bringing to market radar devices that accurately measure increasingly difficult media. The result is greater application versatility. Radar is now a viable technology in industries that previously ruled it out, giving plant operators a new, reliable option for process measurement.
For more information contact Chantal Groom, VEGA Controls SA, +27 (0)11 795 3249, [email protected], www.vega.com
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