# Demonstrating the Potential of a Low-Cost Soil Moisture Sensor Network
## 3 Results and Discussion  
### Sensor Performance Evaluation with Known Dielectric Standards and Intersensor Variability  
Although sensors with the same manufacturer part number were ordered, two slightly different probes were received. The first type of probe (nodes 3 to 10) consists of a 2.77 cm-diameter cylindrical head with three, 0.3 diameter tines protruding for 7 cm, while the second type (nodes 1 to 3) consists of a 2.7 cm-diameter cylindrical head with three, 0.4 diameter tines protruding for 5.5 cm. The main difference between the two types is the length of the tines and the response variability between probes in the same sample. In terms of the operation principle, the sensor operates similarly to the WET sensor described previously [12; 20]. Briefly, the sensor returns a voltage at a fixed frequency (70 MHz). Capacitance of the material between the tines is measured, from which dielectric properties of the medium are inferred using a sensor calibration file. In the final step, measured \(\varepsilon\) is used to calculate \(\theta\) according to [PERSON]'s equation [21], where \(\varepsilon\) is the apparent dielectric permittivity of the medium and \(\theta\) is the volumetric water content. \[\theta=4.3\times 10^{-6}\varepsilon^{3}-5.5\times 10^{-4}\varepsilon^{2}+2.92 \times 10^{-2}\varepsilon-5.3\times 10^{-2} \tag{1}\]  
To address the various soil property effects in EM-based measurements for \(\theta\), soil-specific calibrations are often recommended, although in general, suppliers provide factory-determined calibration equations. The performance of such factory calibrations has been reported in detail previously for the most common EM sensors for different soil textural classes [12]. Although multiple equations exist as linear and nonlinear, to estimate \(\theta\) from \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Distance from Gateway (m)** & **40** & **100** & **300** & **460** \\ \hline Node ID & 1, 2 & 3, 4 & 5, 6 & 7, 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Distance from the gateway for each group of sensor nodes. sensor response [12] it is critical to minimise intersensor variability (i.e., the degree of variation in response among different sensor units) to provide reliable data from spatially distributed sensors. Permittivity is the physical property that drives the \(\theta\) determination, and it is easier to provide a known permittivity using dielectric liquids than to provide a known \(\theta\) in soil (i.e., due to soil heterogeneity or hydrostatic water distribution) [22]. The use of liquids with known dielectric permittivity values reduces the variability associated with solid media and provides a reproducible approach to sensor screening. Liquids are \"ideal\" dielectric media because of their well-defined properties which overcome complications associated with the use of soil such as air gaps near conductors and density variations [22]. For this purpose, a range of solvent types and mixtures were selected to cover the \(\varepsilon\) range (Table 2). The SenseCap sensors used only output the temperature and \(\theta\) (%) data, while the \(\varepsilon\) data are only available through the serial connection. Equation (1) (used by the manufacturer) was utilised to solve for \(\varepsilon\) from \(\theta\). The average \(\varepsilon\) presented in Table 2 shows a good correlation between the \(\varepsilon_{\mathrm{s}}\) (standard dielectric permittivity) and \(\varepsilon_{\mathrm{a}}\) (measured dielectric permittivity) for the low range (1-24.5) for all the nodes. A significant shift is noticed starting with methanol, where all the sensors overestimate the \(\varepsilon_{\mathrm{s}}\). A similar effect has been reported before for the 10 HS sensor with slight overestimation in the 0-37 \(\varepsilon\) range [23]. In this case, the error is larger, which suggests that the calibration file used is not ideal. It is possible that calibration was achieved using a two-point calibration (i.e., in air and water).