The principle of the method is to measure the force (torque) acting on a rotor when it rotates at a constant angular velocity or rotational speed in a liquid. Rotational rheometers/viscometers are used for measuring the viscosity of fluids, both Newtonian and non-Newtonian. The following procedures are used to determine the viscosity of Newtonian fluids or the apparent viscosity of non-Newtonian fluids. The calculated viscosity of Newtonian fluids should be the same (within experimental error), regardless of the rate of shear (or rotational speed). Given the dependence of viscosity on temperature, the temperature of the substance being measured should be controlled to within ±0.1°, unless otherwise specified in the individual monograph.