A procedure to quantitatively analyze torque/rotor speed data in order to extract viscosity/shear-rate curves using non conventional geometries is presented. The approach is based on the Couette analogy. It is first validated using a relatively simple geometry for which the equivalent internal radius used in the analogy can be analytically obtained. The results showed that the equivalent internal radius depends only slightly on the nature of the fluid. The experimental results with complex geometries and complex fluids are found to coincide, within experimental errors, with those obtained using standard geometries over a wide range of shear rates. For certain fluids such as tomato ketchup, only a specific mixing device (the helical ribbon impeller in this case) can be used to obtain reproducible viscosity/shear-rate curves.