Before we get started with how to measure viscous materials, let’s have a quick look at how a rotational viscometer works. A typical rotational viscometer used for quality control measures the viscosity according to the “Searle principle”.
A cup is matched with a so-called spindle that is placed in the sample. The speed on the viscometer is preset and the spindle starts to rotate. The sample in the cup follows this movement and in further consequence the torque (force) required for turning the spindle against the fluid’s viscous forces is measured. A spring connects the motor of the viscometer and the spindle. The rotation (torque) of the spindle deflects the spring. Optical sensors detect the deflection and, as a result, we get the viscosity of the sample. Different spring-types of rotational viscometers allow measuring substances from low to high viscosity. In case of low-viscous substances, the spring needs to be sufficiently sensitive, whereas for samples in the high viscosity range, a more robust spring is required. In the next chapters, you will learn how to measure the viscosity of different chemical samples with such a spring-type viscometer.