Tensiometric
The tensiometer consists of a water-filled porous ceramic cup attached to a vacuum gauge.
Water is withdrawn from the cup by the matric forces in the soil until the pressure potential inside the tensiometer is equal to matric potential of the soil water. The cup is permeable to salts but not to air and soil colloids, so the osmotic potential of the soil solution has no effect on the reading. The useful range of the tensiometer is about 0 to -85 J/kg.
Very accurate measurements are possible in this range, and this is the range in which most
moisture flow occurs in soil. In theory, the tensiometer could be used to measure matric
potentials much lower than -85 J/kg, since the adhesive and cohesive forces of water are
sufficient to withstand tensions of many hundreds of joules per kilogram (c.f. in the xylem of
plants), but impurities in the water and on the surfaces inside of the tensiometer, and dissolved
gases in the water cause cavitation when the absolute water pressure approaches zero. Once a
gas phase in present in the tensiometer, negative pressures are impossible
UMS tensiometers, which can be monitored continuously Irrometer tensiometer