Development of a heat pulse sensor for measuring matric suction on soilless substrates

Abstract Crops cultivated in greenhouse normally use soilless substrates as a growing medium. In this production process, strict control of the application of water and fertilizers is required. The availability of sensors that measure accurately and at low cost the matric suction in the culture medium enables the automation of irrigation. Currently available sensors have deficiencies such as reduced sensitivity at low suctions, interference from ambient temperature and being affected by the salinity of the substrate. The cost of commercially available sensors is also a challenge. In this context, the objective of this study was to develop a heat dissipation sensor for measuring matric suction in the range of 0 to 5 kPa, as required by most agricultural substrates. The sensor consists of two parts, the first being an active core comprised of a heat source and a temperature sensor. The second part is a specially designed porous body that comes to hydraulic equilibrium with the medium in which the sensor is inserted. The porous body was produced using sintered glass microspheres. The data acquisition system uses the Arduino Uno platform. Sensors were calibrated and tested using a coconut fiber substrate submitted to water tensions between 0 and 10 kPa, using Haines’ funnels. Good correlations were observed between temperature variations in the sensor and water tension variations in the substrate between 0 and 5 kPa. The low cost and the adequate sensitivity of the sensor indicate that it can be used in the automatic control of irrigation systems in cultivation of plants in substrates.