A rheometer does not simply measure flow--a viscometer does that. A rheometer measures the effects of flow at different pressures--especially key characteristics of the flow like shear (how difficult it is to change the flow) or how the flow of a substance tends to drag surrounding materials. The output of a rheometer is typically a set of curves that depict these forces as the pressure on the flowing material changes.
Rheometers are almost as varied as the substances and forces that they measure. The simplest type of rheometer forces the test material through pipes of known dimension and measures shear at increasing pressures. The shear forces are recorded as the pressure varies. Rheometers that measure the ability of a material to drag surrounding materials tend to have two concentric cylinders. The outer cylinder is fixed and the inner cylinder turns. Still other speciality rheometers use shallow cones or force material between two electrically charged plates.
Most rheometers are in use because of the function they play in highway construction. It turns out that the rheometer is the best predictor of how long new asphalt or concrete will last. New batches can still be changed if they are found to be substandard before they are poured. Rheometer examination can prevent the pouring of substandard concrete and asphalt which in turn prevents the loss of lives and the waste of repairing substandard highways.
Rheometers also have important functions in the food and medical industries. In both cases rheometers function to detect "improper batches." In the food industry, rheometers are the best predictors of shelf life and other important qualities. Rheometers can also detect how a new substance will be absorbed by the body and thus redirect the formulations of new pharmaceuticals before they are tested on living subjects. Another function or rheometers is the testing of new formulations of paints and inks. Rheometer tests are the most accurate predictors of how easily these substances apply and how they will appear when dry.