Radar level measurement is considered the “best practice” technology within a process plant. However, there are still a number of challenges with it. Foam, agitation and condensing liquids are not the easiest to measure. As a result of improved radar technology, configuration and device settings allow them to be used in these applications. However, low dielectric products remain difficult as not enough radar energy can be reflected from the product surface. Guided wave radar (GWR) is generally used in low dielectric applications as it has a lower limit than non-contact radar. What is interesting is that guided wave radar will even work be used when a surface reflection cannot be measured.
When radar level measurement first came to the process market, it was hailed as being “media independent”. Unlike ultrasonic devices, the vapor space between the radar instrument and the fluid did not impact the level measurement. For most level applications this is basically true is it has negligible effect on the measurement accuracy. However, the speed of the radar energy is impacted by the dielectric of a fluid. The higher this constant, the slower the speed. Using guided wave radar, this can work to you advantage.
As guided wave radar level technology was being developed, it was noted the measured end of probe was offset from the physical end of probe with low dielectric products. Because manufacturers were trying to determine the physical limits of the technology, little attention was paid to this. However, after some experimentation, it was determined this offset was measurable and repeatable based on the dielectric. With low dialectic fluids, the level can be calculated by measuring this offset.
Radar level technology is based on time of flight of the radar energy. The longer it takes for the instrument to transmit and receive the energy, the greater the distance to the surface. The measurement assumes a constant energy velocity. However, the speed of the radar energy changes as it propagates through a low dielectric product. The energy will take longer to return to the device and appear as a lower surface level. With guided wave radar the length of the probe is known. A surface level that appears below this end or probe is actually due to the change of speed rather than a lower level. The greater the offset, the higher the level.
Naturally, the dielectric of the product will need to be known. Any error in the value will lead to inaccuracy in the measurement. It is important to realize the measurement will be repeatable, but not necessarily accurate. However, if the instrument, at some point, does receive enough energy to realize a product surface, it can calculate the actual dielectric based on the end of probe offset. This can be extremely beneficial for applications with changing dielectric products. It should be noted this technology cannot be done with non-contact radar, as there is not any physical benchmarks for the calculation.
Previous limitations with guided wave radar technology no longer need to be considered. This reinforces that radar level is indeed the “best practice” technology for process level measurement. Now if something could be done for vapor compensation in high pressure steam…
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