Integration of a Ruler to Measure Depth in the Design of the Stolf Impact Penetrometer: A Retrospective Study

Abstract

The Stolf impact penetrometer is a dynamic cone penetrometer to measure soil resistance that has been used since its release in 1982. A centimeter-scale to measure penetration depth was engraved on the penetration steel rod in the original design. The idea was to make the equipment easier to understand for farmers and allow direct readings on the penetration rod and avoid additional parts. This study aimed to incorporate a millimeter-scale ruler to enable the operator to read the depth at eye level, avoiding crouching and increasing the reading accuracy. The equipment use and reset procedure of the depth reading (zero) were described in detail and illustrated. The original equipment with reading on the rod and the proposed equipment, with reading on the ruler, were compared in terms of the resistance values and sampling time in a soil profile. A significant difference (t-test, 1%) was found, resulting, on average, in 21% less time spent for readings on the ruler. Besides increasing the reading accuracy and reducing the sampling time, the users preferred the new design due to its higher comfort, which was finally incorporated into the commercial model. A retrospective of the technical development of the penetrometer is presented, and this study represents the first design modification, which has remained unaltered for 30 years.