The optimal design of close clearance impellers requires the knowledge of the power demand of the mixing equipment. In non-Newtonian mixing, this can be readily obtained using the Metzner and Otto concept [1]. In this work, this concept and the determination of the K-s value for an atypical helical agitator (PARAVISC system from Ekato firm) have been revised in the case of shear-thinning fluids and a shear-thickening fluid. For poor shear-thinning fluids, it has been shown that for our mixing system the K-s value does not vary strongly with the flow behavior index, and may be regarded as a constant for the mixing purpose design. By contrast, for the shear-thickening fluid, power consumption measurements indicate that the relationship between the K-s values and the flow behavior index is much more complex due to a partial solidification of the product around the impeller.