To measure rheological properties, the numerical values of constants that are
included in rheological equations of state of various materials have to be found. Depending
on the choice of equation, experiments are carried out in order to establish the influence
of these constants on results. However, irrelevant of the choice of a model of
rheological behavior, the measurement of two fundamental characteristic functions of
material always plays a central role − viscosity as a function of a shear rate, and viscoelastic
properties, such as dynamic moduli, as a function of frequency.