# Design of Capacitive Paddy Moisture Sensor Based on Electrical Impedance Spectroscopy Analysis
## 2 Materials and Methods  
Wu Yu Jing 3 (_Oryza sativa_ L.) was selected as the research paddy variety. The paddy samples were obtained from the field of thousand Li village, Gu Yang Town, Dantu District, Zhenjiang City, Jiangsu province, China at harvest time. The initial moisture content of Wu Yu Jing 3 was approximately 29.46% (w.b.). The paddy moisture content in the following were in wet base. The paddy samples were sieved to remove foreign matter, and immature and broken seeds. The paddy samples were sealed in polyethylene bags and stored in a refrigerator at approximately 4 \({}^{\circ}\)C before the experiments.  
This study adopted the method of sampling while drying. Paddy samples with different moisture content were obtained by sampling directly from the paddy samples during the drying. This made sure that the paddy samples with different moisture contents were in accordance with the actual paddy grain conditions during actual drying. Because of the possible natural drying of the samples during the paddy impedance electrical measurements, the paddy moisture content was determined immediately after the electrical measurements.  
The paddy dielectric properties were determined by the impedance measurement system which was mainly composed of Solartron impedance analyzer 1260A + 1294, computer testing software SMaRT, a constant temperature and humidity chamber, and a coaxial cylindrical capacitor, as shown in Figure 1.  
Before conducting the experiments, the Solartron impedance analyzer needed to be preheated for 1 h to ensure detection stability and measurement accuracy. The impedance analyzer was programmed to measure the paddy dielectric properties at 61 frequencies ranging from 100 Hz to 1 MHz in the logarithmic scanning mode. The measurement results contained impedance magnitude, impedance phase degree, admittance magnitude, and capacitance magnitude. In a constant temperature, the paddy sample was dried, and its dielectric properties were detected until the moisture content of the paddy sample was 14% or below. The temperature ranged from 40 \({}^{\circ}\)C to 80 \({}^{\circ}\)C with 5 \({}^{\circ}\)C intervals. The ranges of research parameters are shown in Table 1.  
Taking a temperature of 50 \({}^{\circ}\)C as an example, the experimental steps were as follows. Firstly, a 200 g (error \(\leq\) 0.1 g) paddy sample was selected evenly and distributed in a stainless steel disc uniformly with a layer thickness of 10 mm. The paddy sample was dried in a drying chamber (with accuracy of \(\pm\)0.5 \({}^{\circ}\)C) with drying temperature of 50 \({}^{\circ}\)C. Secondly, the paddy sample was taken out of the drying chamber every 30-60 min, the coaxial cylindrical capacitor was filled with the paddy sample, and the paddy sample was settled with increasing intensities. Then, manually pressing into the coaxial cylindrical capacitor was performed in to obtain higher densities and the uniform volume. Thirdly, the coaxial cylindrical capacitor which was filled with paddy sample was placed into constant temperature and humidity chamber with a temperature of 50 \({}^{\circ}\)C and a humidity of 70%. After the temperature and humidity became stable, the test software SMaRT was used to measure and collect the impedance spectrum of the paddy sample. Fourthly, after the measurement of paddy impedance spectrum, the quality of all paddy samples was measured, the current moisture content of the paddy sample was calculated and recorded. Fifth, the second, third and fourth steps were repeated until the paddy moisture content reached below its safe moisture value of 14%.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Frequency** & **Paddy Moisture** & **Temperature** \\ \hline Range & 100 Hz–1 MHz & 14–29\% & 40–80 \({}^{\circ}\)C \\ \hline \end{tabular}
\end{table}
Table 1: Ranges of research parameters.  
Figure 1: Impedance measurement system (length unit: mm).