Beta Testing Implementation of the Purdue Time Domain Reflectometry (TDR) Method for Soil Water Content and Density Measurement

Abstract

The Purdue TDR method is a new technology for simultaneously measuring soil water content and dry density insitu. An ASTM standard for using TDR to measure soil water content and dry density based on Purdue TDR method was approved during the time span of this project and is designated ASTM D6780. The primary objective of this study was to take the Purdue TDR Method to the point where it is widely field tested by users on a broad spectrum of soils around the country. This goal was achieved by involving researchers at other universities and practitioners in federal and state agencies and in private practice firms. The results and feedback was obtained from Beta Partners by Purdue University for evaluation and further analysis. Major achievements in this research include: 1) Involvement of Beta Partners - Six Beta partners including two universities, two private firms and two state DOTs were involved in this project and instructions were provided to each Beta Partners. Involving these Beta partners provided a large span of field applications and research feedback. Results from testing performed were compared with existing technologies and provided the basis for the precision and bias statements needed for ASTM D6780. Based on feedback from extensive field tests, testing procedures were improved. Meanwhile, the testing equipment was refined and integrated, which made the testing system both more robust and easier to handle. The overall cost of the testing system has also been significantly reduced, which made it more economically competitive for mass production. 2) Testing automation - A new generation of electronics was identified and incorporated into the Purdue TDR test, the new TDR100 by Campbell Scientific, Inc. Corresponding software for automation was designed and systematically improved. This provided a user friendly interface and facilitated performance of TDR testing process. Preliminary feedback from using the computer software is satisfactory. The efforts and achievements on testing automation also built up the basis for a developing a more compact package in the future. 3) One step method for TDR testing - A simplified procedure to that described by ASTM D6780 to measure soil water content and dry density was discovered, which is an important product of this project. The simplified procedure is called the one-step method since it only requires one field TDR reading. The one-step method achieved this simplification by incorporating information of bulk electrical conductivity from TDR signal in addition to the apparent