A model for predicting the mixing cavern dimensions for Bingham plastic fluids based on the assumption of equal torque is developed. Experimental data has been collected for the purpose of verifying this model, using a novel technique, for both axial and radial flow impellers in a tank of Heinz ketchup. The mixing caverns were found to be the shape of an elliptical torus. The ellipse aspect ratios were determined for both impeller types and are assumed to be constant. The model was able to predict the cavern diameter and cavern height within the experimental uncertainty. (c) 2005 Elsevier Ltd. All rights reserved.