Comparative evaluation of penetrometers with conical and cylindrical tips

Abstract

Soil penetration resistance is a key indicator used to evaluate soil physical conditions, root growth environments, and the suitability of tillage practices. However, measurements obtained with penetrometers equipped with conical and cylindrical tips are not directly comparable because the mechanisms of soil deformation generated by these deformers differ substantially. This study aimed to develop and validate a methodology for harmonizing penetration-resistance measurements obtained using cylindrical and standardized conical penetrometers. A theoretical framework describing soil deformation during cylindrical and conical penetration was developed using soil bearing capacity, volumetric compression, and soil–steel friction relationships. Analytical equations were derived to determine the permissible penetration depth of a cylindrical deformer and to establish an equivalence relationship between the radii of cylindrical and conical penetrometer tips. The proposed methodology was validated through field experiments using a standardized LAN-M penetrometer fitted with interchangeable conical and cylindrical tips. The results demonstrated that reliable penetration-resistance measurements obtained with a cylindrical tip require limiting penetration depth to 0.15 m. An experimental comparison of deformers with identical diameters showed that measurements remain statistically comparable only within the upper 0.05 m of soil depth. At greater depths, significant discrepancies arise because of different soil-penetration mechanisms. A new analytical relationship between cylindrical and conical tip radii was developed and experimentally verified. The proposed equation enables the selection of geometrically equivalent deformers, yielding statistically comparable penetration resistance values across the investigated 0–0.15 m soil layer. The findings support the standardization and cross-comparison of soil-strength datasets obtained using different penetrometer designs. Future research should validate the proposed methodology across wider ranges of soil textures, moisture conditions, bulk densities, and penetration rates, and integrate the developed relationships into digital soil-monitoring and precision-agriculture systems.