Evaluation of a Direct-Coupled TDR for Determination of Soil Water Content and Bulk Electrical Conductivity
Introduction
Signal degradation in coaxial cables and interconnects is a long-standing problem in the practical deployment and calibration of time domain reflectometry (TDR) for soil water content monitoring. Acclima, Inc.1 has recently commercialized a TDR sensor (TDR-315) with all electronics required for waveform acquisition embedded in the probe head thereby avoiding signal degradation. Our objectives were to (i) carry out apparent permittivity (Ka) and bulk electrical conductivity (aa) calibrations for the TDR-315 using conventional TDR methods, (ii) complete a water content calibration for a fine-textured soil, and (iii) utilize a saturated column displacement experiment to examine the dependency of measured Ka on oa while avoiding the confounding effects of soil water content changes. In all of these evaluations, TDR-315 responses were compared with conventional TDR.
Sensor Description
- Planar three-conductor transmission line 150 mm in length, rod separation distance of 19 mm, and rod diameters of 3.2 mm (Fig. 1).
- Potted sensor head consisting of a step function generator, precision time base generator, 5 ps resolution waveform digitizer, thermistor, and SDI-12 communications circuits.
- Function generator launches a step pulse with a 10-90% rise time of 100 ps (approx. 3.5 GHz spectral bandwidth .
- Firmware in on-board memory acquires pertinent waveform features, measures temperature, calculates travel time, Ka, and aa, and transmits the information to compliant data loggers using SDI-12.
- Full waveforms can be acquired using a specialized interface. Timing circuit limitations restricts amplitude sampling to 20.4 ns. Four sensors (SN 684 to 731) had firmware to sample amplitudes at 3 ps based on microprocessor cycles.
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