Non-invasive systems of measurement are basically known as the through-air radar systems. They usually employ a horn antenna or a rod antenna for sending microwave beams onto the surface of the liquid being measured. These antennas mounted at the top of the tank then receive the reflected microwave signal back from the fluid surface. A timing circuit is incorporated in the systems which measures the time of flight and hence the distance between the antenna and the fluid level is determined.
These systems can pose measurement problems if the dielectric constant of the fluid being measured is very low. “The reason is that the amount of reflected energy at microwave (radar) frequencies is dependent on the dielectric constant i.e. εr of the fluid, and if εr is low, most of the radar’s energy enters or passes through. Water ( εr = 80) produces an excellent reflection at the change or discontinuity in εr.”
Besides, this radar level measurement technique faces the same beam divergence issues which affects ultrasonic level transmitters. Moreover, issues like internal piping, antenna deposits, and manifold wave reflections from tank coatings and obstructions may lead to inaccurate results. In order to get rid of these errors, advanced algorithms employing fuzzy logic should be integrated with these radar transmitters. However, these arrangements would make the transmitter setup very complicated.