Simple microwave sensor for leak detection and monitoring in underground water pipelines

In 2012, a new method for leak detection in underground water pipes [1] has been introduced (also by one of the Authors). The method consists in posing a bi-wire along the pipe when it is buried and installed. When water spills out, due to a hole or a break in the pipe, the dielectric permittivity of the soil is locally increased. As a consequence, a recognizable feature is produced in the reflectogram obtainable with the well-known time-domain reflectometry technique (TDR). The feature allows the detection and the localization of the leak. A limitation of the TDR method is that it is not feasible for continuous monitoring of a large number of pipes, due to the costs of the measurement apparatus. A second limitation is that the dielectric permittivity of the soil is a function of the position, and varies over time; since the bi-wire senses the permittivity variations for all its length, the reflectogram may have a quite erratic trend, and be difficult to interpret. The authors have developed a sensor [2] for use with the leakage detection setup described in [1], and to overcome the illustrated limitations. The idea is to include a “local” sensing element in a coaxial cable, so that (i) the system is sensitive only in some points (sufficient for the leak detection), and (ii) the presence of leaks can be detected by a cheap transmitter-receiver pair at microwave frequency.