Rocking and Shocking the PARIOTM: How Sensitive Is ISP-Based Particle-Size Analysis to Mechanical Disturbance?

Among soil physical analyses, determination of the soil particle-size distribution (PSD) is arguably the most fundamental. The standard methodology combines sieve analysis for sand fractions with sedimentation-based techniques for silt and clay. Established sedimentation methods include the pipette and hydrometer techniques. More recently, the Integral Suspension Pressure (ISP) method has become available, which derives PSD by inverse modeling of the temporal evolution of suspension pressure measured at a fixed depth in a sedimentation cylinder. Since ISP is based on the same physical principles as the pipette and hydrometer methods, their results should, in principle, agree.

The ISP methodology has been implemented in the commercial instrument PARIO (METER Group, Munich). While elegant, the method relies on pressure change measurements with a resolution of 0.1 Pa (equivalent to 0.01 mm of water column). Consequently, the PARIO manual strongly advises avoiding any mechanical disturbance such as thumping, bumping, clapping, vibration, or other shock events. This warning is essentially precautionary, because to date no systematic experimental investigation of such disturbances has been reported.

To explore this issue, we prepared a single 30 g soil sample following standard PSD procedures and subjected it to 26 PARIO repeated measurement runs over a period of five months, each run lasting 12 h. Between runs, the suspension was remixed but otherwise not altered. The first ten runs (over ten days) were conducted without intentional disturbance to establish baseline repeatability. This was followed by eight runs with deliberately imposed and timed disturbances that generated single or repeated vibrations (“rocking and shocking”). After approximately two and five months, we conducted additional sets of five and three undisturbed runs, respectively.

We report how these mechanical disturbances, along with temperature variations during measurement and the time elapsed since sample pre-treatment, affected the derived PSD. The results provide a first quantitative assessment of how fragile—or robust—the ISP method and PARIO system really are when reality refuses to sit perfectly still.