An instrument used to obtain the rheological properties of fluids. It involves a small quantity of the fluid being entrapped between two geometric surfaces with one surface static and the other in motion at constant speed. This defines the shear rate. The torque to resist the motion defines the characteristic shear rate. A limitation with the rotational speed is the need to avoid secondary flow patterns that give false readings. Parallel-disc, cone and plate rheometer, and Couette are all forms of rheometer.