In this work, the effect of scale on the macroinstability is examined using data from a 1.22 m diameter tank. In a previous study coherent macroinstabilities (MI) in a stirred tank agitated with a pitched blade turbine were captured in a small tank of diameter T = 0.24 m. Exactly the same dimensionless frequency (f(MI) = 0.186N) or Strouhal number (St = f(MI)/N = 0.186) is observed at both scales, with greater coherence over a wider range of off-bottom clearances and Reynolds numbers at the large scale. Large eddy simulations (LES) successfully predict the same frequency. The mechanism driving the macroinstabilities is compared to the feedback mechanism driving coherent frequencies for a range of other confined jet geometries. In plant scale vessels, coherent frequencies such as this force cyclic loading on solid surfaces, which can result in mechanical failure of baffles, coupling bolts and even impeller shafts. They can also cause vibrations of equipment so severe that units must be shut down. (C) 2003 Elsevier Science Ltd. All rights reserved.