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Using Short Soil Moisture Probes with High-Bandwidth Time Domain Reflectometry Instruments

Department of Bioresource Engineering, Gilmore Hall, Oregon State Univ., Corvallis, OR 97331

* Corresponding author ( kellys{at}ucs.orst.edu).

ABSTRACT

Time domain reflectometry (TDR) is used to measure moisture content and salinity of soils. Most TDR systems have a bandwidth of 2.5 GHz or less, limiting the precision of measurements using short probes. The primary objective of this research was to develop and test short probes for use with a high-bandwidth (20-GHz) TDR instrument. The secondary objective was to determine moisture content in highly conductive media by insulating the probes with Teflon heat-shrinkable tubing. Laboratory-packed test cells with probe lengths of 0.025, 0.05, and 0.075 m were used for the primary research objective, and 0.075-m probes for the secondary objective. Linear relationships between the apparent refractive index and volumetric moisture content were developed for each probe length and type. The accuracy of the probes was ±0.0125, ±0.05, ±0.025, and ±0.035 m³/m³ for the probes of lengths 0.075, 0.075 (insulated), 0.05, and 0.025 m, respectively. Moisture measurements in highly conductive media were possible using insulated probes.

Received for publication March 11, 1994.