In a typical rotational test, the shear rate is preset. Torque × conversion factor = shear stress Usually, this is automatically done by the instrument. The viscometer’s speed is converted into shear rate using a conversion factor (the factor depends on the measuring geometry used) and the torque is also converted into shear stress using a conversion factor. The physical properties speed and torque can be translated into the rheological properties shear rate and shear stress if the measurement is performed using a standard measuring geometry according to ISO 3219. When we measure the viscosity on a rotational viscometer we apply to the sample a certain shear stress or a certain shear rate, respectively. Newton's Law defines the dynamic viscosity η as the shear stress divided by the shear rate. This torque has to overcome the viscous forces of the tested substance and is therefore a measure for its viscosity.Ī rotational viscometer measures the dynamic viscosity of a sample. The rotational speed of the bob is preset and produces the motor torque that is needed to rotate the measuring bob.
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Most rotational viscometers/rheometers work according to the Searle principle: A motor drives a bob inside a fixed cup.