Cone-and-plate viscometers are suitable and popular devices for measuring non-Newtonian fluid flow characteristics because the shear rate of fluids in the gap between the cone and the plate is constant and they are easy to operate. Since the theoretical equations derived are complicated, especially for a Herschel-Bulkley model fluid, the first and second approximate expressions of the equations are derived. It is found that the first approximate expression is unsuitable for the calculation in the case that the included angle between the cone and the plate is over 2° to keep the relative errors under 1%. It is confirmed that the second approximate expression is applicable to the calculation in the case that the included angle is up to 4° under the condition in which the relative error is less than 1%.
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