The development and application of an industrial deoiling hydrocyclone equipped with electrical resistance tomographic instrumentation is described. The use of electrical sensors for continuous monitoring of separator operation is. demonstrated. Results show that electrical resistance tomography (ERT) is an effective tool for optimising start-up and operation of the separator. Subtleties of flow within the hydrocyclone can also be sensed, highlighting its potential for use as a validation tool for computational fluid dynamics and design models. (C) 2004 Elsevier Ltd. All rights reserved.