As we’ve mentioned in previous posts, selecting the appropriate measuring system is essential for making accurate rheometry measurements. Measuring systems, also known as fixtures, tools, and geometries, hold the sample in place during the test and ideally should have no direct influence on the data. Consequences of using an unsuitable measuring system include wonky results that can be caused by sample coming out of the gap, wall slip, and the measuring system not being sensitive enough to quantify very low or very high viscosity samples.
While common measuring systems such as plates, cones, and cylinders are readily available in various dimensions and materials, there are times when a custom measuring system is best. These instances include dual-analyses rheology research projects, along with specific mixing simulations and testing uncommon samples.
Both academic and industrial labs often combine rheometry with other analytical measurements to gather optical, spectroscopic, or other microstructural information simultaneously with the rheology data. Methods such as rheology-coupled particle velocimetry, rheo-SANs, and rheometry with light scattering entail use of measuring systems that allow for lasers and other instrumentation to interact with the sample without significantly disrupting the rheometry measurement. Upcoming advanced techniques often employ customized measuring systems specially adapted to the unique conditions of these measurements.
Other situations that benefit from using custom measuring systems include mixing studies. When characterizing a mixture, colloid, or other dispersion for the ability to mix quickly and effectively, the rheometer should be equipped with a measuring system that is similar to the mixing blades used in processing. Although extreme congruency may not be possible, stirrers with geometries that are like that of the mixer help reproduce mixing conditions far better than standard cylinders or plates.
Once the preferred shape of the measuring system is decided, it is best to have a custom measuring system made by the manufacturer of your rheometer. Although costs may be perceived as much greater through using the professionals, the trade-off in performance and accuracy is vital. First of all, modern rheometers rely on variables known as the Constant Shear Rate value (CSR) and Constant Shear Stress value (CSS). The CSR and CSS are calculated numbers that directly tell the instrument how to produce data collected with that specific measuring system. When a new measuring system is inserted, the rheometer cannot run without being given a CSS and a CSR. A new measuring system is worthless if it does not have a known CSS or CSR value.
Additionally, advanced manufacturing processes ensure that measuring system surfaces and dimensions are as exact as possible. The flatness of a plate surface should be micro-scale smooth, and the concentricity of cylinders in cups is extremely important for accurate data. Even a competent machine shop is not outfitted with the tools necessary for these requirements.
On top of the accuracy concerns, the worst situation that can occur with a home-made measuring system is it damaging the rheometer. For example, home-made measuring systems intended for rheo-tribology, a special technique involving applied normal force, fast rotational speeds, and intentional contact between measuring system and lower plate, can exert forces on the rheometer’s expensive air bearing motor. Such a case can result in a five-digit repair bill on account of saving a couple thousand dollars by making a measuring system independently of the rheometer manufacturer.
Measuring system choice - especially the use of a custom measuring system - is worth taking time to consider for your sample and test measurements.
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