Submitted to: IEEE Sensors Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 10, 2012
Publication Date: May 21, 2012
Citation: Casanova, J.J., Evett, S.R., Schwartz, R.C. 2012. Design of access-tube TDR sensor for soil water content: Testing. IEEE Sensors Journal. 12(6):2064-2070. Interpretive Summary: Measuring the amount of water in soil is important in managing crop irrigation. Current methods of soil water estimation are limited by accuracy, precision, ease of installation and cost. This paper presents a new sensor design and investigates the performance of the sensor using tests on prototypes in a variety of media. These laboratory tests confirm physical theory established in a previous paper.
Technical Abstract: Soil water measurement is important in water management for irrigation and hydrologic sciences. The purpose of this paper is to develop and test the design of a cylindrical access-tube mounted waveguide for use in time-domain reflectometry (TDR) for in-situ soil water content sensing. Several prototypes, with varying geometrical parameters, were constructed. The sensors were compared by evaluating characteristics of reflected waveforms from a (200 ps) step pulse in different media, including air, triethylene glycol, deionized water, and over a range of water contents in sand and a clay loam soil. Sensors with greater separation between electrodes, by means of tube diameter or separation angle, tend to have a greater field penetration in both sand and clay. In addition, the sensors with the shortest electrode separation show greater sensitivity to soil electrical conductivity. Together, these trends demonstrate that the propagating electromagnetic fields above 0 Hz do not take the transverse electromagnetic form commonly assumed in analysis of TDR probes.