Emulsions are often found in formulated products. They contain small liquid droplets dispersed into an immiscible liquid that remains as a continuous phase. Emulsions are not thermodynamically stable systems; therefore their properties depend strongly on the preparation method and material history. 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 sulphate) in water using vessels of different diameter (D = 0.9 and D = 0.15 m) with high shear disk agitators (D/T = 1/3). To monitor the dissolution times we use a single probe conductivity measurement and volume averaged conductivity obtained by 3D electric resistance tomography. We compare the validity of the methods and highlight the difference between dissolution times for highly viscous fluids and mixing times for tracers with the same viscosity as water.