Measuring Soil Water Tension with a Stationary Dihedral Tensiometer: Principles and Applications.

Developed at the Brazilian Agricultural Research Corporation (Embrapa) the dihedral sensor (WO 2011/079367) was built as a stationary soil water tension prototype by Tecnicer Tecnologia Cerâmica, São Carlos, SP, Brazil, with a measuring rang of 0 to -160 kPa. In this instrument the soil water tension (T) is read as a function of the distance between the water meniscus and the dihedral vertex, using the expression {T = -2σ/[L tg(α)]}, in which σ is the water surface tension and α is the dihedral angle. The instrument encasement is a robust and impervious alumina tube (450 mm long, 27 mm id, 32 mm od) with a high bubbling pressure porous kernel material (>500kPa). At the base, the tube sphericity promotes soil contact. A 30º oblique plane at the top end enhances visibility for the flat glass plate (2x22x50 mm) glued 40 mm away from kernel neighboring vertex formed the dihedral sensor display window. Liquid glue applied behind a 0.036 mm rod spacer forms a rigid body fixing the spacer, the glass plate and kernel. After water immersion during a day the instrument is installed at half the effective crop rooting depth to measure water tension (0 to 160 kPa). Soil water tension equilibrium, with this instrument is a function of the water tension, however, in most desorption uses under 80 kPa, 90% equilibrium was achieved in less than one hour. Results are presented for its applicability in sandy to heavy clayed soils.