During the preparation of emulsions, it is often desirable to dissolve highly Viscous fluids, such as surfactants or other additives, into an aqueous phase. In this work we measure dissolution times for a non-Newtonian highly viscous surfactant (sodium laureth sulfate) in water using vessels of different diameter (D = 0.9 and D = 0.15 m) with high shear disk agitators (D/T = 1/3 and 0.62). To monitor the dissolution times, we use a volume averaged conductivity obtained by 3D electrical resistance tomography. We compare the validity of the method and highlight the difference between dissolution times for highly viscous fluids and blending times for tracers with the same viscosity as water.