In modern rheometry many different measuring systems are available. It is possible to rheologically characterize almost all substances. To select the best measuring system for your sample, the following needs to be considered:
The following figure gives an overview and rough classification of different substances, from low-viscosity liquids to solids. Each column lists the most common test types and a recommended measuring system. The lower the viscosity of a sample, the larger the surface of the measuring bob should be.
Low-viscosity samples are often measured in cylinder measuring systems. At high shear rates the centrifugal forces lead to turbulence in the measuring gap (Taylor vortices). This leads to an apparent increase in the viscosity. Therefore, a critical shear rate limit should not be exceeded.
Recommendation: The more viscous the substance, the more likely it is that a cone-plate or parallel-plate measuring system should be used.
Rheology Road (DG: double-gap measuring system, CC: cylinder measuring system, CP: cone-plate measuring system, PP: parallel-plate measuring system, SRF: Solid Rectangular Fixture, UXF and SER: Extensional Rheometer Systems)
Sometimes the choice of measuring system is limited because only a small volume of sample is available and therefore cylinder measuring systems cannot be used.
In this case it is recommended to use a cone-plate or parallel-plate system. With a CP40-0.3 (diameter: 40 mm, angle: 0.3°) cone you only need, for example, 0.09 mL of sample.
If the measuring system is difficult to clean after measurements, a parallel-plate or cone-plate system should be used instead. Cylinder measuring systems are relatively difficult to clean.
There are disposable systems available for all measuring system types. These can be disposed of along with the sample or cleaned separately. This is required for samples which undergo a curing process.
If the sample contains solvent and parallel-plate, cone-plate or double-gap systems are used, the sample may dry out around the edge. The resulting measured viscosity is too high.
If possible, a cylinder measuring system should be used. Even if the sample surface dries out, this does not affect the results when using cylinder measuring systems.
When using cone-plate and parallel-plate measuring systems there are several ways of stopping the sample drying out:
If the sample tends to sediment, the viscosity values may be too low as only the liquid phase is measured. For such samples using cylinder measuring systems is recommended.
If the sample slides or slips, sandblasted or profiled measuring systems should be used. For very aggressive samples measuring systems made of special materials can be requested. If the sample contains particles which are bigger than 1 mm, using the Ball Measuring System or a vane geometry is recommended.
When using special systems it is important to measure the sample in the same way every time in order to obtain comparable results.
Learn more about rheological measuring systems, applications, and industry solutions in a comprehensive knowledge base: Basics of rheology