An optimized radar system for tank level measurement in dispersive bypass pipes

The conventional approach for radar level measurement of liquids in tanks is to perform echo measurements in the free space above the liquid. In this paper, the alternative approach of distance measurement using a bypass, which is a communicating pipe connected with the tank, is discussed. The goal behind the presented work was to optimize these systems in order to allow for high precision level measurements. It is shown with a system theoretical model of the cylindrical bypass pipe that the given dispersion might introduce severe systematical measurement errors. A time-variant inverse filtering technique has been developed to compensate for the dispersion, and the implementation in a Frequency Modulated Continuous Wave (FMCW) radar level meter is discussed. For further optimization of the system's performance, especially in the case of weakly-reflecting liquids, a metallic float is used as an improved radar target. The proposed concepts have been evaluated by means of measurements with a 5.7 to 6.7 GHz FMCW radar system for level measurement using a bypass pipe with an inner diameter of 38 mm. Measurement results show that the distance measurement error is decreased from +/-4.96 mm to +/-1.44mm by means of the proposed inverse filtering approach.