The control and stability of the air core and factors affecting the flow split to the underflow or ‘heavy’ product are two of the least understood aspects in the operation of hydrocyclone and dense medium separators. Existing methods for assessing these factors using conventional and lomographic measurement instrumentation are described. New results obtained using electrical resistance tomography (ERT) are described for a pilot-scale large diameter coal dense medium separator (LARCODEMS). Such data have important implications for the development of: (a) empirical correlations to describe air core characteristics as a function of operational variables (feed rate, feed slurry viscosity, feed density, etc.) and their role on separation efficiency, and (b) computational fluid dynamics models of liquid and slurry flow in centrifugal separators.