A Porous Cup Sensor to Measure Soil Water Tension By Light Reflection.

The porous sensor named IG sensor (BR1020130097721) is made of a porous cup with a bed of non-sintered particles to measure soil water tension by pneumatic and optical properties. To work in the light reflection mode it comprises:  a porous element for mechanical support and hydraulic connection, a cap to hold a LED light source and one phototransistor, and internally a nucleus partially filled with non-sintered particles. Impregnated by moisture the particle bed bears a dynamic water reflection surface, whose pattern is a function of the soil water tension.  Consequently, the measurement of reflected light on the particle surface is related with the soil water tension. The IG sensor prototype used in this study was made of a porous cup with an outer diameter of 55mm, inner diameter 40 mm and height of 45 mm having internally a bed of transparent glass beads (40 to 100 microns). The obtained responses were sigmoid curves of decreasing voltage ranging from 3.4 to 2.2 V while the water tension ranged from zero to 20 kPa. Simplicity and absence of interferences such as salinity, temperature and soil density suggests that the IG sensor can be a suitable irrigation management tool.