Reliability of Chilled-Mirror Hygrometer (WP4C) for Soil Total Suction Measurement

The WP4C chilled-mirror dew-point hygrometer is widely used for measuring soil total suction. This study aimed to evaluate its reliability in determining total soil suction through the vapor equilibrium technique. Results showed offsets between WP4C measurements of soil specimens and its corresponding vapor equilibrium saline solution, particularly for suctions below 10 MPa. This difference can be attributed to the difference in vapor diffusion behavior because of the presence of both liquid and solid evaporation surfaces in the soil case, as opposed to just a liquid surface in the saline solution case. To address this issue, the study modified the WP4C sample cups to present varying solution surface areas to the WP4C, representing varying volumetric water contents of the soil. The results reveal that the total suction measurement increases as the surface area decreases for a specific salt solution. This outcome was used to develop a new WP4C correction equation based on solution surface area. The proposed correction equation was validated through experiments and literature data, showing that it significantly improves the measurement precision of the WP4C.