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Experimental study on T-TDR sensor soil heat transfer analysis and structural optimization

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  131620130,  2013 Kansas City, Missouri, July 21 - July 24, 2013. (doi: http://dx.doi.org/10.13031/aim.20131620130) @2013
Authors:   Wenting Han, Jun Qiao, Jinghui Xu
Keywords:   T-TDR; Thermal field distribution; Numerical simulation; Soil water content; RMSE; Optimized design.

Abstract. To improve the measurement accuracy of T-TDR (Thermo-Time Domain Reflectometry) sensors and optimize their structural parameters, we studied the heat conduction process of T-TDR sensors using the two-dimensional soil heat transfer equation. By analyzing the initial boundary value problem of T-TDR probe heat transfer, we simulated the dynamic process of heat transfer and the spatial distribution of the thermal field of the sensor in soils with different ambient temperature, thermal conductivities and volumetric heat capacities, and we determined that the thermocouple junction’s ideal location should be 2 mm below the probe midpoint. Three types of T-TDR sensors were manufactured with designed thermocouple junctions at the probe mid-point or 2 mm below the probe mid-point and a reduced junction volume by using spot welding. Experiments were performed to measure the thermal property curves in sand, sandy loam and clayey loam with five different water contents with these sensors and to calculate the water contents. The measurement accuracy was verified using a drying method. The results indicate that the experimental data were consistent with the numerical simulation results; the water content measurement results of the optimized T-TDR sensors were well correlated with the measurements obtained with the drying method (R2 = 0.981, RMSE = 0.0152), indicating improvement in the sensor measurement accuracy compared with sensors using the conventional structural parameters. The research method and results of this paper provided guidance for the optimization of the T-TDR sensor parameters.

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