# Prediction of the Moisture Content in Corn Straw Compost Based on Their Dielectric Properties
## 4 Discussion  
The current testing methods using the compost moisture content include conventional weighing, far-infrared drying, frequency-domain reflection, and time-domain reflection. The conventional weighing method is time-consuming and laborious. By contrast, far-infrared drying reduces the drying time considerably. [PERSON] et al. [37] used the far-infrared drying method to determine the moisture content samples of compost materials with a measurement time of 33 min, and the results were accurate to 0.1%. However, it suffers from the inability to achieve real-time measurements and the equipment is more expensive than the regular drying ovens. The time domain reflection method and frequency domain reflection method are indeed very accurate, with test error control within 3% [38], but have the disadvantage of being expensive. Time-domain reflection and frequency-domain reflection both require expensive equipment; furthermore, frequency-domain reflection is more susceptible to electrical conductivity, which is not easy to calibrate and optimize. Since the physical structure of the compost material is similar to that of soil, soil moisture content detectors can be used directly for the detection of the compost material moisture content. This study proposes a method of real-time compost moisture content detection based on dielectric properties, which can detect within 10 s compared with other moisture content detection methods. Further, the scientific rationality of the method was demonstrated experimentally. The results may contribute to the application of nondestructive testing of dielectric properties and the development of potential new moisture meters for compost moisture content detection based on dielectric properties. In this study, first, the relationships between test frequency, moisture content, temperature, bulk density, the dielectric constant \(\epsilon^{\prime}\), and the loss factor \(\epsilon^{\prime\prime}\) of compost samples were analyzed. In the low-frequency band of less than 100 kHz, ionic conductivity is the primary cause of dielectric loss [39], which results in a gradual decrease in the loss factor \(\epsilon^{\prime\prime}\) with increasing frequency. The higher the water content, the larger the corresponding \(\epsilon^{\prime}\) and \(\epsilon^{\prime\prime}\), because the dielectric properties are more strongly affected by the moisture content than other factors. The variation law holds true in the study of the dielectric properties of nuts [40], pecans [41], and fruit juices [42]. Both \(\epsilon^{\prime}\) and \(\epsilon^{\prime\prime}\) of the compost samples increased with the increase in the moisture content at a constant temperature. The equivalent dielectric constant model revealed that the more significant the proportion of water in the volume of the samples, the larger the dielectric constant of the samples will be. A similar pattern has been reported in other studies on peanut kernel [43], corn [44], and barley [45]. At a constant moisture content, \(\epsilon^{\prime}\) and \(\epsilon^{\prime\prime}\) of the samples increased with the increase in the temperature. The increase in  
Figure 9: Prediction error of the moisture content of the PCA-SVR model. dielectric properties with the increase in the temperature has been observed in corn [46], wheat [47], beans [48], and chestnuts [49]. When the moisture content was greater than 25%, both \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) of the samples increased considerably with the increase in the bulk density. This phenomenon could be attributed to the increase in the bulk density of the samples, increasing the number of dielectrics in the capacitor. Furthermore, under the action of the applied electric field, more dielectrics are polarized, and the ability of the samples to store charge increases, and thus, more electric field energy can be stored, which increases the dielectric constant \(\varepsilon^{\prime}\).