Comparison of height‐averaged and point‐measured capillary pressure–saturation relations for sands using a modified Tempe cell

Tempe cells and similar devices are often used for measurements of capillary pressure versus saturation relationships (retention curves) of soils. Average water saturation within the sample is often used as the representative saturation. For cases where the saturation distribution along the cell height is nonuniform, use of the height‐averaged water saturation artificially smoothes the retention curve when the capillary pressure head reaches the displacement pressure head. This leads to an inaccurate representation of the retention curve in flow modeling. For nine test sands with a wide range of displacement pressure head values (6–78 cm of water), height‐averaged and point‐measured retention curves were simultaneously obtained to investigate the effect of the height‐averaging procedure. A Tempe cell was used, which was modified by installing a small time domain reflectometry probe and a tensiometer at the midpoint of the sample height to measure water saturation and pressure, respectively. When the sample height is large relative to the displacement pressure head, the bulk drainage (initial drainage at the top of the sample) leads to inaccuracies in the determination of retention curves. For such cases, point‐measured saturation should be used, otherwise height correction is necessary.