Electrical resistance tomography (ERT) allows to measure hydrocyclone internal flow dynamics in situ. A dual-plane high-speed ERT system is proposed to map two-phase distributions with varying operational parameters. Experimental results are presented in terms of air-core diameter and its size variation for various feed flow rates and design variables. The total variation algorithm provides the best results compared to Tikhonov and Gauss-Newton algorithms. The influence of finite element method mesh size on air-core resolution is evaluated. High-speed video imaging and a volume-of-fluid-based two-phase computational fluid dynamics (CFD) model are adopted to determine the air-core size. Cross validation of air-core size and shape is performed by ERT measurements with CFD simulations and high-speed video imaging. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.