The viscosity η is then calculated in the rheometer software according to the viscosity law from the quotients of the shear stress τ and shear rate $\dot$.Īs mentioned, the shear stress correlates with the torque. The user sets the shear rate profile – the different speeds of rotation – and the rheometer determines the required shear stress. When measuring ketchup for instance, its viscosity should be measured with a shear-rate-controlled rotational test. All further parameters, such as viscosity, are not measured but calculated. Rheological parameters are calculated from the measured values torque, deflection angle, and speed using conversion factors (see Table 1). However, the rheometer user is usually more interested in the rheological parameters. In fact, it is always the current – and therefore the torque – which is provided and the parameter of either speed or torque is set via a quick controller. In a measurement, modern rheometers can set either the torque or the speed – the other parameter is then measured. The speed is calculated from the deflection angle and the time. The sample provides resistance or a reset torque to the setting and the resulting deflection angle is then measured very precisely by the rheometer’s encoder. This means that in a viscosity measurement the rheometer presets a certain current that correlates to a defined torque. They measure the torque and the deflection angle of the measuring bob. To measure the rheological properties of a material, rheometers are used.
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