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A Novel Low-Cost Instrumentation System for Measuring the Water Content and Apparent Electrical Conductivity of Soils

ABSTRACT

The scarcity of drinking water affects various regions of the planet. Although climate change is responsible for the water availability, humanity plays an important role in preserving this precious natural resource. In case of negligence, the likely trend is to increase the demand and the depletion of water resources due to the increasing world population.

This paper addresses the development, design and construction of a low cost system for measuring soil volumetric water content (θ), electrical conductivity (σ) and temperature (T), in order to optimize the use of water, energy and fertilizer in food production. Different from the existing measurement instruments commonly deployed in these applications, the proposed system uses an auto-balancing bridge circuit as measurement method. The proposed models to estimate θ and σ and correct them in function of T are compared to the ones reported in literature.

The final prototype corresponds to a simple circuit connected to a pair of electrode probes, and presents high accuracy, high signal to noise ratio, fast response, and immunity to stray capacitance. The instrument calibration is based on salt solutions with known dielectric constant and electrical conductivity as reference. Experiments measuring clay and sandy soils demonstrate the satisfactory performance of the instrument.

EXPERIMENTAL SETUP

Figure 1. (a) Probe for measuring water content and apparent electrical conductivity of soil ; (b) Probes for measuring temperature, electrical conductivity and relative dielectric constant

Figure 1. (a) Probe for measuring water content and apparent electrical conductivity of soil ; (b) Probes for measuring temperature, electrical conductivity and relative dielectric constant

In this work, low-cost probes to measure θ and σ were developed based on measurement of the electrical impedance of the soil material located between two parallel stainless steel rods. In order to construct the probes, stainless steel rods (length = 130 mm, diameter = 3 mm), liquid polyester resin, a semiconductor temperature sensor (LM35), five-way cable and covers for electrical outlets plugs were used, according to the scheme shown in Figure 1 a. The cover for electrical outlets plugs served as a housing and to fix the rods and the temperature sensor using polyester resin. Figure 1b illustrates the probes implementation.

Figure 3. (a) Signal-conditioning unit circuit diagram; (b) Signal-conditioning unit implemented circuit

Figure 3. (a) Signal-conditioning unit circuit diagram; (b) Signal-conditioning unit implemented circuit

The present work describes the development of an embedded system in order to perform all tasks proposed in Silva, and to measure the soil parameters ε and σ for agriculture purposes. Furthermore, the system measures temperature to evaluate its effect on the measurements of ε and σ. Figure 3 shows the circuit diagram and the signal-conditioning unit. In the circuit diagram shown in Figure 3a, the microcontroller provides Pulse Width Modulation (PWM) signals at different frequencies (100 kHz and 5 MHz) with 50% working cycle. These signals pass through bandpass filters to make them closer to sine functions. Therefore, they can be used as the excitation source for self-balanced bridge circuit.

RESULTS AND DISCUSSION

Figure 6. (a) σ-σ0 relationship; (b) Correlation between temperature and slope of the fitted models for each temperature

Figure 6. (a) σ-σ0 relationship; (b) Correlation between temperature and slope of the fitted models for each temperature

The effect of temperature on the measurement results of σ were evaluated as a function of the slopes of correlation equations between the values of electrical conductivity measured by the proposed system and electrical conductivity values measured by a conductivimeter (σ0) at different temperatures, as shown in Figure 6a. Thus, a model of the average temperature of each solution and the slope of each model was obtained, as shown in Figure 6b.

Figure 10. Relationship between predicted and observed θ for sandy (a) and clay soil (b)

Figure 10. Relationship between predicted and observed θ for sandy (a) and clay soil (b)

Equations (14) and (15) could be considered a recalibration of Topp’s equation, estimating water content based on ε through a third-degree polynomial, whose coefficient values change from soil to soil. Table 2 presents the results from the linear regression and cross-validation related to the models proposed in this work. Figure 10 illustrates the relation between θ predicted by each model and observed using the calibration system.

CONCLUSIONS

This paper presents the development, design and construction of a low-cost instrumentation system for measuring water content, apparent electrical conductivity and temperature of the soil. The measurement method is based on an auto-balancing bridge circuit. Experimental results obtained in the laboratory demonstrate the system accuracy, considered satisfactory for irrigation control. A comparison with other results in the literature also indicates the proposed system as a promising instrumentation device.

The proposed device corresponds to an efficient alternative to automatize irrigation systems, especially due to the satisfactory accuracy and low cost associated. The instrument was designed considering the final cost of the system to the farmer, which is a crucial concern while developing automation solutions for agriculture. Future work includes testing the prototype with slightly different and/or in undisturbed soils, and also manufacturing new devices for testing and field validation.

Source: Federal University
Authors: Alan Kardek Rego Segundo | Jose Helvecio Martins | Paulo Marcos de Barros Monteiro | Rubens Alves de Oliveira | Gustavo Medeiros Freitas

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