Free Energy of Water-Suction-in Filter Papers

In this paper, a critical evaluation of the filter paper method is presented. The paper evaluates wetting and drying filter paper suction calibration and total and matric soil suction measurement techniques using the filter paper method. Calibration of the method was investigated by evaluating the processes of wetting and drying the filter papers through vapor transfer and of wetting and drying the filter papers through fluid transfer. Recent research shows that different calibration curves result from two different experimental measurement programs (vapor transfer, no contact method and fluid transfer, contact method) for total and matric components of suction. In this study, a filter paper wetting curve was constructed using osmotic suction potentials of sodium chloride (NaCl), ammonium chloride (NH4Cl), calcium chloride (CaCl2), and sodium sulfate (Na2SO4) salt solutions and Schleicher & Schuell No. 589-WH filter papers. Equilibrium time and temperature were two weeks and 25°C with ±0.1°C accuracy, respectively. The adopted laboratory testing protocol shows that suction measurements as low as 50 kPa can be made reliably using the wetting calibration curve obtained in this research study. The calibration curves obtained from the processes of drying the filter papers through vapor transfer (from salt solutions) and drying and wetting the filter papers through fluid transfer (pressure plate type devices) were evaluated based on recently published literature. Although there is hysteresis between the wetting curve with salt solutions and the drying curve with pressure plates, the marked differences between the curves at very low suction levels may not be explained with hysteresis.