Comparison of the integral suspension pressure (ISP) and the hydrometer methods for soil particle size analysis

A recently developed Integral Suspension Pressure (ISP) sedimentation-based particle size analysis technique was tested as an alternative to the hydrometer method. The ISP method determines the particle size distribution using an electronic device, namely the PARIO meter. The PARIO meter provides an alternative to collecting manual readings with the hydrometer, by automatically recording differences in suspension pressure during a settling period of up to 24 h at a depth of 18 cm below the surface of the test suspension. The particle size distribution of the samples is then determined by inverse modeling the recorded pressure data. The objective of this study was to validate the accuracy of the ISP method compared to the hydrometer method using a number of soil mixtures manufactured from materials with known particle size composition. Nine soil mixtures with known reference values were analysed by the hydrometer and ISP methods. Statistical comparison of the results revealed that both measurement methods overestimated clay content and underestimated silt content, the hydrometer method providing more accurate results compared to the ISP method. Thirty-eight mixtures of soil were tested using the ISP method to further validate the accuracy. The results indicated that the accuracy of the ISP method depends on the mass of clay-sized particles and the species composition of the clay minerals in suspension. The ISP method obtained more accurate results for soil mixtures containing expandable clay minerals than to those containing non-expandable clays.