Guided wave radar instruments, like Flexar®, use a cable as a wave guide or antenna. The microwave energy pulses down the cable antenna and reflects off the material surface. The reflected pulse travels back up the antenna to the transceiver.
Obstructions near (within ~12”) of the cable antenna have a high probability of interfering with the microwave energy measurement pulses. Just think of attempting to make a cell phone call while standing inside a metal doorway. Sometimes it works but most times it doesn’t.
Some obstructions result in crazy reflective echoes. These reflections can exceed the strength of that from the material surface. To cancel such large reflection an irritating and often substantial dead zone has to be created along the length of the wave guide cable antenna. Not good.
Obstructions also cause signal degradation. Like the cell phone example, energy from the wave guide cable can be bleed off onto the obstruction. Because the Flexar guided wave radar sensor is measuring and not cooking the silo contents the emitted energy level is rather limited. If enough of this energy is bleed off there is a possibility that insufficient energy will remain to make a viable reflection.
Vessel View, Exterior
Vessel View, Interior
Pictured above is one of the most common examples when it comes to interfering obstructions.
For unknown reasons the factory provided application and mounting recommendations were disregarded and the Flexar instrument was mounted to the pictured stub nozzle. Under the best of circumstances the height of a stub nozzle should not exceed the inside diameter. In this example that guideline has been completely violated. As shown, the nozzle protruding above and below the roof line well exceed the inside diameter. This resulted in crazy pulse reflections which were so strong that the level signal would not follow the
material level. The short term solution required a six foot dead zone to be created on the top portion of the vessel.
This meant the customer lost the ability to monitor the material in the upper six foot section of the vessel. Not good.
The long term solution is to move the Flexar to the center of the forward plate which sits atop a large nozzle which better conforms to the mounting guideline.
Andy
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