The Stolf impact penetrometer is a dynamic cone penetrometer to measure soil resistance, in use since its release in 1982. In the original design, a centimeter scale to measure penetration depth was engraved on the proper penetration rod. The idea was to make the equipment easier to understand for farmers and to allow direct readings on the penetration rod while also avoiding additional parts. The purpose of this study was to incorporate a millimeter ruler to enable the operator to read the depth at eye level, to avoid squatting and to increase the reading accuracy. The equipment use and reset procedure of the depth reading (zero) were described and illustrated. The two reading types were compared in terms of sampling time, using a randomized block design. A significant difference (t test, 1 %) was observed, resulting 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, which was finally incorporated into the commercial model. A retrospective of the technical development of the penetrometer is presented; this study represents the first modification of the project, which had remained unaltered for 30 years.
dynamic soil penetrometer; millimeter ruler; operation; accuracy and cost