Some fluids and many plastic solids, including lubricating grease, exhibit non-Newtonian behavior. In other words, the viscosity (applied shear stress/ shear rate) is not constant; instead, it depends on shear stress and shear rate at a given temperature. Thus, non-Newtonian fluids are described by their apparent viscosity. Conventional types of viscometers with uncontrolled shear rates are not suitable for measuring the viscosity of Non-Newtonian materials.