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Using Vane Fixtures in a Rheometer

Introduction

Rheometers are sophisticated instruments that apply precise deformations to materials and measure the torque of the mechanical response. Or, the applied stress can be preset and the resulting sample deformation measured. The same instrument can test thin liquids or thick solids as long as the correct fixture is in place. One class of fixtures, relative measuring systems, does not apply well-defined deformations to materials. It can be difficult to define the flow behavior inside a relative fixture, and the measured torque cannot be expressed in absolute units. The most common relative measuring systems are vanes and the ball/cup system. 

The primary use for vanes is to test materials that may show slip. Vanes are also useful for testing samples with large particles that do not flow homogeneously, and they can be used to test dispersions with weak thixotropic structures. An additional advantage of vanes is sample loading. When a vane is inserted into a material, it creates fewer disturbances than using a standard concentric cylinder bob[1],[2]

However, there are disadvantages to using vanes. As mentioned above, they measure only relative viscosity values. Furthermore, thin fluids may generate secondary flows in-side the vane. 

[1] Mezger, T.G. The Rheology-Handbook; Vincentz Verlag, 2002.

[2] Steffe, J.F. Rheological Methods in Food Process Engineering; Freeman Press:, 1994. {This book can be downloaded from www.egr.msu.edu/~steffe/freebook/offer.html}

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