As previously discussed, the speed of radar changes in different media. While this has little impact in vacuum and light gas applications, it creates a tremendous difference in high density vapor and interface applications. Because the height, or distance, to the surface of a product is based on time of flight, a change in speed of radar impacts the accuracy. A higher dielectric will produce a level measurement that appears lower than it actually is. Using guided wave radar the change in velocity can be compensated by creating an echo at a fixed distance. The offset between the physical distance and measured distanced is used to compensate for a change in dielectric and speed of radar.
This phenomenon was first observed when determining the lowest dielectric that could be used with guided wave radar. In products with a low dielectric the end of probe measurement appeared longer than the physical length of the probe, due to the product slowing the radar speed. After some experimentation it was determined the level of the product in a tank could be reliably measured using this offset. This is commonly know as Probe End Projection. Since this method was so reliable for measuring level of low dielectric products, it was theorized the same method would work for vapors with a relatively high dielectric. It was determined this was indeed the case.
Dynamic Vapor Compensation works by creating a fixed echo at the top of a guided wave radar instrument. As the dielectric of the vapor changes, the distance of the echo changes. Any offset in this distance is realized as a change in radar speed due to the change in vapor dielectric. This change in speed is than applied to the echo created by the surface of the product. Not only is the measurement repeatable, it is accurate and reliable. This strategy works great for boiler drum level and any other application where the vapor density changes.
A great application for this technology is on boiler drums. Boiler drum level has historically been a difficult level measurement. Using a differential pressure (DP) level solution was the most common technology, but required frequent adjustment. Since this was a differential pressure solution, it required the same static pressure on both sides of the sensor. This meant a water leg needed to be maintained on the low-side (top) of the drum. This leg also protected the sensor from the steam temperature. Any deviation in this leg created an error in the measurement. When radar was first introduced in this application, it did not produce an accurate level due to the change in vapor dielectric. Dynamic Vapor Compensation with guided wave radar addresses this problem.
It is important to keep in mind this is a specific technology solution with guided wave radar. As with the Point End Projection, it cannot be done with non-contact radar. Moreover, the correction cannot be done external to the device (radar technology only reports one level). Having said that, it is a great strategy for measuring level in previously difficult applications.
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