The objective of this paper is to present results of measurements of granular materials density changes during dynamic silo discharge using Electrical Capacitance Tomography. Tomography allows to observe the bahoviour of bulk solid density in horizontal cross-section during silo discharge, especially in the zone near the silo wall. The tests were carried out using 12-electrode sensors with a factor of length to silo diameter equal to 1. The sensors were located outside of the cylindrical silo model made of perspex at 2 different levels. The silo model was supported by a non-deformable steel frame. During the experiments, dry sand was used with various initial densities. The influence of the outlet diameter and wall roughness on the results was also investigated.