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In the last ICIASF conference a novel particle image velocimetry (PIV) system using two different time intervals by means of three colour flashes was presented. The system was jointly developed for the aerospace, hydraulics and electronic departments of "Politecnico di Milano", for the study of water flow and for the water tunnel simulation of aerodynamic phenomena. Here an improved three colour PIV system version is tested on a fundamental physical model. In this test we study the flow field around an obstacle normal to the free-stream direction. The phenomenon modelled is a water stream flowing over a sill, were we have both upstream and downstream recirculation and large velocity gradient. By using two different time intervals three velocity fields were obtained by applying the cross-correlation algorithm on the same image. This information was used to solve the whole velocity field with a more uniform distribution of the uncertainty of velocity measurement. The latter is mainly due to the displacement of the particles, therefore it was possible to choose the best one, having three different possibilities. Moreover different and close time delay allowed the evaluation of local rate of change of velocity which was computed using a first-order differencing scheme.