For the first time, the capability of three-dimensional ERT to monitor systems with a significant level change under realistic process conditions and scales has been demonstrated. Level change within a 6001 vessel has been investigated with the addition of a brine tracer. Using the new method of a moving reference, the three-dimensional images can be reconstructed without level artifacting, allowing un-obstructed viewing of the mixing patterns. A moving finite element model has also been used to show the images with the actual level change. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.