The pattern of solids flow out of a silo has a strong influence on the wall pressures, and consequently on the silo's structural integrity. However, measuring the pattern of flow is very difficult because the material is opaque and movement cannot be easily detected from outside the structure. Most flow pattern studies have used laboratory models, but extrapolation of these observations to full scale industrial silos is most uncertain. The flow patterns described occurred in a recently constructed 250 tonne steel silo of circular planform storing ground gypsum mineral powder. Radio frequency tags were placed in a systematic pattern in the solid using a specially designed deployable seeding device. Many tags were accurately placed in the solid at different levels, giving up to 300 measures of the movement of the solids during discharge. Three methods (contours of residence time, contours of mean velocities and computer visualisation) were used to infer the flow pattern from the radio tag marker data. The tests show that a commercial discharge aid can have a major effect on the flow pattern, leading to a flow pattern which is different from any documented in the literature.