Level Detection of Materials in Nonmetallic Tank by the DC Component of a Microwave Doppler Module

This paper describes a novel level sensor that is noninvasive to nonmetallic tanks and noncontact to materials stored in them. It detects the materials from the outside of a tank by the use of microwave Doppler modules. The output AC component of Doppler modules has been used as moving object detectors. Here, we focus on the output DC component, which is proportional to the intensity of the reflected microwave signals from the stored material. We propose a sensor to detect the level of very low dielectric materials. We build a theoretical model based on multiple reflections of microwaves from the surfaces between different dielectric materials and demonstrate the sensor's theoretical potential. Experiments were carried out using the sensor to verify its feasibility. The sensor detected levels of Styrene foam with dielectric constant 1.02 from the outside of a 3.0 mm thick acrylic resin tank under optimal conditions. Furthermore, the S/N ratio of the Styrene foam measurement was over 19 dB for the drift noise caused by the change in environmental temperature from 20 °C to 80 °C. The S/N ratio of the change in tap-water's temperature ranging from 0.5 °C to 77 °C was 18 dB at the reference of 23 °C, and for the change in conductivity ranging from 0.35 to 26 mS/cm was 33 dB.