Rheometers have a limited measurement range. Fortunately, for polymers, the time-temperature superposition principle applies. This means that measurements performed at a higher temperatures correspond with measurements performed at lower velocities and vice versa.
For determining the relaxation time spectrum, a wide range of data is necessary. This can be accomplished by collecting data at different temperatures and shifting it to one reference temperature, thus constructing a master curve. When constructing this master curve, shift factors are determined which are also useful to shift the master curve to different reference temperatures.