The term viscosity describes the flow characteristics of liquid systems. Two liquid layers, which slide one against the other generate a mechanical resistance against the movement (toughness, inner friction). This is called flow stress. If several layers are moved with different speeds, viscous forces are generated. These forces decelerate the layers with higher speed and accelerate the layers with lower speed. These phenomena follow physical laws and can be defined by equations, with regard to shear rate and shear forces. Viscosities are always dependent on temperature.

Viscosity measurements are frequently performed to optimise the consistency of pasty materials or to optimise its viscosity for food processing lines. Yogurt with and without fruit pieces, sauces, mustard, tomato puree and oils are frequently tested products of the food industry. Medical gels, paint, emulsions, cremes and oils are examples for tested products in other industries.

The back extrusion rig – an alternative rotation viscometers

The back extrusion rig offers an alternative solution to rotation viscometers. The principle of the back extrusion rig is based on the displacement of the test material through an annular gap between piston and back extrusion cell. The cell is filled with the test substance and the piston is lowered into the container, pressing the material up through the annular gap. When the piston is raised, the material flows back through the annular gap into the cell. The force is measured in the downand upcycle at a defined position. The dynamic viscosity is evaluated, from the measured force difference and the flow rate of the liquid in the annular gap.

The system can repeat this cycle as often as desired with freely adjustable shear rates. Thus the test can be optimally adapted to your test material, and with only one single test you can measure the viscosity across the entire shear range.