Water retention experiments are performed following a standard protocol (ISO 11274:1998). This protocol involves soil samples being saturated by standing on a piece of saturated sponge in a water bath. After wetting to a constant weight, samples are successively equilibrated at low tensions on sand suction tables. They are then equilibrated at high tensions in pressure membrane cells. While performing the analysis the laboratory temperature needs to be maintained at a constant temperature to prevent changes in the temperature and viscosity of the soil water; if the temperature changes the water release characteristics are changed.
Samples are weighed frequently and are considered to have reached equilibrium when the weight of the core remains constant at each tension on the sand suction tables and when water ceases to leave in the pressure membrane system. When the water content has equilibrated, the tension is increased. At the end of the experiment, equilibrated samples are from the pressure cells, weighed, oven dried and weighed again to determine moisture content.
The tension is converted to diameter using the Laplace equation, mentioned previously, where the contact angle (θ) is taken to be either 0 or 5 degrees and the interfacial tension (γ) is 0.075 Nm-1.