The objective of this work is to study quantitatively the errors introduced by the standard Newtonian and power-law assumptions used in the determination of the material properties of viscoplastic fluids from circular Couette experiments. The steady-state circular Couette flow of a Herschel-Bulkley fluid is solved assuming that the inner cylinder is rotating at constant speed while the outer one is fixed. Analytical solutions are presented for certain values of the power-law exponent. It is

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... n that the error in the computed wall shear rate, which is insignificant when the diameter ratio is closed to unity, may grow large depending on the diameter ratio and the material parameters.