A rheometer is a laboratory device used to measure the way in which a liquid, suspension or slurry flows in response to applied forces. It is used for those fluids which cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer. It measures the rheology of the fluid. There are two distinctively different types of rheometers. Rheometers that control the applied shear stress or shear strain are called rotational or shear rheometers, whereas rheometers that apply extensional stress or extensional strain are extensional rheometers. Rotational or shear type rheometers are usually designed as either a native strain-controlled instrument (control and apply a user-defined shear strain which can then measure the resulting shear stress) or a native stress-controlled instrument (control and apply a user-defined shear stress and measure the resulting shear strain).
How to Use a Rheometer?
A rheometer measures the way in which a fluid responds to applied forces. This is similar to a viscometer, which provides the single quantity of viscosity to define a fluid. However, a rheometer provides a set of parameters to describe fluids with more complex behavior than simple viscosity. There are different types of rheometers, depending on the specific aspect of fluid behavior that you're measuring. For example, a shear rheometer measures the shear stress of a fluid.
Measure shear stress with a capillary rheometer. This type of rheometer consists of a tube with a constant cross section and known dimensions. Force liquid into the tube under conditions of laminar (non-turbulent) flow. This test may be performed with a constant flow rate so that the pressure drop may be calculated or vice versa. You can then calculate the shear stress from these values.
Test the shear stress with a rotational cylinder rheometer. This type of rheometer has a container that consists of a one cylinder inside of another. Place the sample in the container between the two cylinders and rotate one of the cylinders at a known speed. Measure the force with which the sample drags the moving cylinder and convert this to a shear stress. The speed of a rotational cylinder rheometer can vary from 3 to 600 revolutions per minute (rpm), depending on the fluid and specific type of modeling required.
Use a cone and plate rheometer. Place the test fluid onto a horizontal plate and then place a shallow cone that is supported by a torsion bar onto the plate. Rotate the plate and use the torsion bar to measure the force that the test fluid applies to the cone. Use the degree of twist on the torsion bar to calculate the shear stress. You can then calculate the shear rate from the dimensions of the cone and rotational speed.
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