Evaluation of a Portable Dihedral Tensiometer Designed to Measure Water Tension In Substrates.

A new dihedral tensiometer system to measure soil water tension was developed at the Brazilian Agricultural Research Corporation, Embrapa (patent n. WO 2011/079367) and portable prototypes were produced by Tecnicer Tecnologia Cerâmica in São Carlos, SP, Brazil with the following characteristics:  a 200 mm long, 25 mm wide and 4 mm thick transparent flat glass body; a 50 mm long, 20 mm wide and 4 mm thick flat high bubbling pressure porous element (>100kPa) with a central 2 mm orifice; a transversal 0.16 mm cylindrical rod spacer (a); and a fixing epoxy resin. The dihedral vertex is the front contact line between the flat glass and the flat porous element plates and with the transversal spacer placed 40 mm distant from the vertex, define the dihedral angle. Immersion in water for 10 min followed by contact to dry towel to approximate the meniscus to the 10 mm position set the instrument read to do water tension measurements in the range of 0 to 20 kPa in less than 1min. In this instrument soil water tension (T) is a function of the distance between the meniscus and the vertex (L) according to the dihedral angle and the water surface tension (σ), according to the equation  , where tg(α)=a/L. The dihedral sensors showed linear responses to the substrate water tensions adjusted with a Richards chamber. Sensors were tested in different substrates and response time, reproducibility and precision are presented and discussed.