Protótipo de sensor de umidade do solo

Abstract

Water content is one of the most important dynamic attributes of soil, especially for agriculture, and it is extremely necessary for irrigation management. The determination of soil moisture in the laboratory has low applicability on a large scale. There are high-precision sensors on the market, but they are generally expensive. The objective of this work was to calibrate and prototype low-cost soil moisture sensors. Two types of sensors were used: resistive (models hl-69 and HD-38) and capacitive (model V1.2). The price of these sensors on the market varies from BRL 10.00 to 70.00. The TDR (Time Domain Reflectometry) sensor, model HH2 with ML3 ThetaProbe rod (Delta-T), with nominal accuracy above 90%, was used as a reference for fitting calibration equations for the low-cost sensors. The calibrations were carried out on two soil samples, with clay content of 398 and 136 g/kg soil, collected in the Ipameri, Goiás, Brazil. Calibration tests were carried out in 5L pots, saturated with water to field capacity and left to dry in open air for an indefinite period of time. Daily readings were made between February and May 2023. The coefficient of determination was used to evaluate the goodness-of-fit of the calibration models, linear and exponential. The equation for the hl-69 resistive sensor, which is the lowest cost, showed R2 > 0.85 for both samples. The HD-38 sensor presented the best result for the soil with clay content of 136 g/kg, reaching R2 = 0.90. The capacitive V1.2 sensor showed better results for the sample with clay content of 398 g/kg, R2 = 0.89, and R2 = 0.75 for another sample. The portable prototype was built using PVC pipe and a microcontroller module with WiFi technology to transfer reading data. A 9V battery powers the system. The prototype allows the adoption of both calibrated moisture sensors. The final cost of the prototype using the hl-69 resistive sensor was BRL 110.00.