A number of methods are used to measure the viscosity of fluids.
These are typically based on one of three phenomena—a moving
surface in contact with a fluid, an object moving through a
fluid, and fluid flowing through a resistive component. These
phenomena utilize three major viscometers in the industry, i.e.,
a rotating viscometer, a falling-ball viscometer, and a
capillary viscometer. The falling ball viscometer typically
measures the viscosity of Newtonian liquids and gases. The
method applies Newton’s law of motion under force balance on a
falling sphere ball when it reaches a terminal velocity. In
Newton’s law of motion for a falling ball, there exist buoyancy
force, weight force, and drag force, and these three forces
reach a net force of zero. The drag force can be obtained from
Stokes’ law, which is valid in Reynolds numbers less than 1.1–3
The falling ball viscometer is well-suited for measuring the
viscosity of a fluid, and the method has been stated in
international standards.4,5 In the internatio