Method and apparatus for determining rheological parameters of a fluid

A method of determining Herschel-Bulkley parameters of a fluid 4 comprises providing an inner member 6 and an outer member 2 to create a fluid volume, the inner member and the outer member being rotatable relative to each other; providing the fluid in the volume; rotating the inner member and the outer member relative to each other at three different relative rotational velocities; obtaining corresponding pairs of rotational velocity and torque value for each of the different relative rotational velocities; using the Herschel Bulkley model to derive a differential equation in which the change of angular velocity as a function of distance is expressed in terms of the torque value and the Herschel-Bulkley parameters: solving the differential equation by means of integration; and using the corresponding pairs of rotational velocity and torque value to derive the Herschel-Bulkley parameters using the solution. An apparatus for performing the method is also provided.