Design of a multilevel TDR probe for measuring soil water content at different depths.

摘要

The time domain reflectometry (TDR) method measures soil water content by measuring the dielectric permittivity of the soil using the travel time of an electromagnetic step pulse through a waveguide embedded in the soil. When installing TDR probes at deeper depths, retrieval and re-use is not easy. This study investigated the performance of a multilevel TDR probe created by embedding the waveguide on the outside wall of a PVC pipe and running the coaxial cable through the interior of the pipe as a way to protect the cable. The length of the stainless steel rod at the center of the probe was 0.06 m, and the stainless steel rod forming the outer loop was 0.063 m long. The rods were embedded in a groove made on one end of the wall of a 0.2 m section of PVC pipe. The ends of the stainless steel rods were directed through the wall to protrude inside the pipe and soldered to the coaxial cable. Fifteen sets of probes were built and tested. Several such sections of PVC pipe can be connected in series to create a multilevel probe, which will protect the cables from damage during installation and removal. The reusable multilevel TDR probes were tested in the laboratory and compared with the weighing method. The fitted straight line from the regression analysis between TDR theta _v and weighing method theta _v resulted in good correlation, with R2 of 0.89 and 0.97, respectively, for the first and second laboratory experiments. The field experiment gave an R2 of 0.51 where it was difficult to maintain good soil contact. This multilevel probe is cost-effective, reusable, and can measure soil water content at many different depths.