Comparison of Shear Methods

Chapters S and 6 have described and evaluated the important shear rheometers and indexers. If measurements of torques, forces, and velocities or pressures and flow rates are made and interpreted properly, all these rheometers should measure the same shear material functions. To conclude these two chapters, we make a few comparisons between rheometers to emphasize this point. Table 6.5.1 highlights some of the strengths and weaknesses of each. 

It is very valuable to compare different rheometers in your own laboratory. Of course comparisons are helpful in checking instrument calibration or finding such errors as a mislabled cone angle. Juxtaposition of rheometers can often identify more insidious errors, such as secondary flows, slip, or evaporation. Comparisons are also very helpful in selecting which rheometer is most useful for a particular material, deformation, or temperature range. Comparisons give confidence to operators that they are really seeing material behavior and not some instrument artifact. 

Sliding plates (S.2) Simple design Homogeneous Linear motion high C(t, y) t 10 s Edges limit y 10 Gap control Loading 

Falling ball (5.2) Very simple Ne e better Sealed rheometer High T, p Not very useful for viscoelastic fluids Nonhomogeneous Tran Muent fluid Need p 

Concentric cylinders (5.3) (Couette flow) Low q, high y Homogeneous if Ri/Ro 0.95 Good for suspension settling End correction High 1 fluids are difficult to clean Ni impractical 

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