The design procedure of mixers to agitate non-Newtonian flow is commonly based on several correlation methods that associate the power consumption of agitating newtonian flow with the power of agitating non-Newtonian flow. Therefore, the knowledge of the viscosity function of the non-Newtonian fluid is necessary. For the agitation of fluids with solid fractions, the viscosity is usually unknown and the methods are not applicable. Here, a method to determine the viscosity of pseudoplastic fluids with a solid fraction is presented.