Rheometers characterize the viscous properties of fluids, as well as the reading their elastic responses. Their name is derived from rheology, which is the study of the behavior of fluids.  Rheometers quantify the properties of fluids through creep testing including dynamic tests (oscillation method), normal force measurements, shear, stress jumps, or a combination of the above. 

Rheometers use a number of different technologies to measure the properties of fluids.  Common among these technologies are rotational rheometry, capillary rheometry, falling ball testing.  In rotational rheometry, torque is required to rotate a spindle at constant speed while immersed into the sample fluid. The torque is proportional to the viscous drag on the immersed spindle, and thus to the viscosity of the fluid.

Capillary rheometers measure the flow rate of a fixed volume of fluid through a small orifice at a controlled temperature. The rate of shear can be varied from near 0 to 106 s-1 by changing capillary diameter and applied pressure. A specific volume of fluid passes through the orifice and the time it takes for the volume of fluid to pass through the orifice is proportional to the fluid viscosity. However, it also depends on the density of the fluid since the denser the fluid, the faster it will flow through the orifice. The property being measured in this example is then the kinematic viscosity and not the dynamic viscosity.

In falling ball tests, the viscosity is proportional to the time required for a ball to fall through the test liquid contained in a precise and temperature controlled glass tube.

Other less common types of rheometers include gravity testers, positive displacement models, and needle testing rheometers.

Video credit: Anton Paar / CC BY-SA 4.0

Rheometers may be used to measure both Newtonian and non-Newtonian fluids.  A Newtonian fluid is one in which the viscosity does not depend on shear rate. Regardless of the level of shear that is applied, the viscosity stays the same. In many applications, however, this is not the case and, as the fluid is sheared at greater rates, the viscosity will change. These types of liquids are known as non-Newtonian and there are many classifications. Many common solvents, mineral base oils, synthetic base fluids and fully formulated single-grade oils obey Newton's Viscosity Law.  Non-Newtonian fluids are a mixture of pigments, suspenders, polymers, solvents, etc. Each of these items by itself may or may not have Newtonian characteristics, but when combined they shear in unique, non-linear ways.