Radar level transmitters are often referred to as “non-contact” radar because the antenna or “horn” does not come into contact with the product or process being measured. This is in contrast to Guide Wave Radar (GWR) which requires contact with the product being measured.
The measurement principle of radar is; Time-of-Flight (ToF) using microwave energy. Extremely short microwave pulses at a given wave length are transmitted by the antenna system to the measured product. The pulses are reflected by the product surface and received back by the antenna system. The time from transmission to reception of the signals is proportional to the distance or product level in the vessel (ToF). A special time stretching procedure ensures reliable and precise measurement of the extremely short transmission periods and the conversion into a level measurement.
The typical radar sensor operates with low emitted power in the C, K ,or W band frequency ranges, each of which offer high reliability and tight accuracy in their respective applications:
Low frequency C band sensors (~6GHz) are typically used for continuous level measurement of liquids under difficult process conditions. They are suitable for applications in storage tanks, process vessels or standpipe. C band radars perform well in process vessels where build-up, foaming, or strong agitation is present. The C band frequency can have difficulties when measuring short ranges or when nozzle space limited due to the larger antenna systems they require.
Mid frequency K band sensors (~26GHz) are suitable for continuous level measurement of almost all liquids. They are likely the most common used radar frequency and are suitable for storage containers, reactors and process vessels, even under difficult process conditions. With the a large variety of antenna systems and materials, K band are a common solution for almost all typical applications and processes. The exception being where heavy vapors or dense foams are to be expected.
High frequency W band sensors (~80GHz) are suitable for continuous level measurement of liquids and bulk solids, in addition they can have particular advantages where K & C band radars have historically struggled. Small process fittings available with the W band sensors offer flexibility in small vessels or tight mounting spaces. The increased signal focus can increase the reliability in narrow silo or vessels with stirrers, agitators, baffles, heating spirals, etc. and avoid the common false signals from obstruction. The W band is also successful in measuring very low dielectric liquids such as liquified natural gases.
Over the past 3-4 decades, radar has emerged as one of the preferred level technologies in the process control industry. While every level application has specific details that require thought and consideration; the development and optimization of these 3 frequency ranges has made an ever-increasing number of applications solvable (and more importantly reliable) with non-contacting technology.