Positron Emission Particle Tracking observations of charge and slurry flow have shown that the pressure drop is effectively described by an Euler equation modified by a Darcy-like term. Here we consider the inverse problem: constructing and testing approximate solutions to tht new Dynamic Ergun Equation. These solutions allow the prediction of transport in tumbling mills and other systems given information on the pressure drop and charge motion. (C) 2016 Elsevier Ltd. All rights reserved.