Influence of Electrical Double Layer in the Electrical Model for Capacitance- based Soil Moisture Sensor on Measuring Soil Moisture

Abstract

The reasonable electrical model for Capacitance-Based Soil Moisture Sensor (CBSMS), namely relationship between soil moisture and sensor output voltage, is the basic requirement of soil moisture measurements. In the present study, two CBSMS (PA-1 for the self-developed CBSMS and EC-5 for decagon company’s CBSMS) were employed to assess influence of electrical double layer in the electrical model for CBSMS on measuring soil moisture based on 15 soil samples with moisture ranging from 2.6~30%. Results showed that significant deviations of experimental results and estimated values were determined mainly in dependence on the electrical models (Model-1: with electrical double layer, Model-2: without electrical double layer), soil moisture range and sensor itself. Significant deviations were observed in the moisture range of 2.6~6%, 22~30% for EC-5 and of 2.6~10%, 22~30% for PA-1 with Model-2, whereas the deviations did not exist for both CBSMS in the moisture range of 2.6~10% with Model-1. For both CBSMS, in the moisture range of 22~30%, the deviation between estimated values from Model-1 with experimental results was obviously less than that from Model-2. These results suggest that the electrical double layer should be considered in the electrical model for CBSMS, especially for measuring low soil moisture.