The basic rheological properties and flow behavior of materials were discussed in Chapter 2. In this chapter we describe how these properties are measured and the various instruments available for these measurements. Rheometers are used to characterize the rheological properties of various systems in quality control, processing, applications, and research and development. As pointed out in Chapter 2, it is necessary to characterize a system by measuring a range of properties, as the system may “see” different application conditions in very short periods of time. An example is a paint being sprayed on a vertical wall. The paint is subjected to very high shear rates in a matter of seconds after it was at rest, and its leveling and sag resistance properties depend on its ability to recover its viscosity and elasticity after the external stress has been removed. Ideally, a rheometer should detect changes in the rheological properties of a system in situations similar to its behavior in real-life applications. The geometry of the testing device is very important and should be similar to the one described in Chapter 2, Fig. 1: The material should ideally be tested between two parallel plates, one stationary and the other movable. Many devices, ranging from very simple and inaccurate to very sophisticated and accurate, have been designed to measure the rheological properties of 36various systems (1-6). Following are descriptions of most of the instruments used to date to measure rheological properties.