TA Instruments AR 2000, Operator'S Manual, Page: 60 / 144

AR 2000 Operator's Manual
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Choosing the Best Geometry
When selecting the correct measuring geometry to use, it is important that you understand the following:
Exactly what type of experiment do you wish to carry out?
What is the sample behavior like—does the sample contain particles?
And probably most importantly, what is the real-life situation are you trying to recreate?
Sometimes the answers to all of the above questions are not known, but there are some basic guidelines that will
help you. However, it is also important to remember that you are measuring the bulk properties of the material
itself, and this should be independent of the type of geometry used (within reason!).
Cone and Plate/Parallel
Plate Systems
The cone and plate and parallel plate systems both need small sample volumes, are easy to clean, have low
inertia, and can potentially achieve high shear rates. The additional advantage to using a cone and plate is that
the shear rate is uniform throughout the sample, and the parallel plate can accommodate large particles.
Generally, the cone and plate or parallel plate systems can be used for almost any sample. They are easy to set
up and use, making one of these systems the best choice
for optimum results. They are both available in different
sizes, therefore, it is important to understand how to
choose the system with the correct dimensions.
Angles
Cones are supplied by TA Instruments in any angle
from 0° to 4°, usually in 0.5° increments. The 4° cone is
the largest available, as the sample velocity profile
becomes unpredictable at higher angles and the math-
ematical expression of
α
~ tan
α
is no longer valid.
The 4° cone is ideal for creep measurements, because a
longer displacement is required per unit strain.
The smaller the angle (or gap in a parallel plate system),
the higher the maximum shear rate obtainable.
Diameters
The smaller the diameter of a cone or parallel plate
system, the larger the shear stress factor. This means
that a small ( e.g ., 20 mm) diameter geometry should be
used with stiffer materials or medium to high viscosi-
ties. A 40 mm geometry is more versatile and it usually
allows the majority of medium viscosity materials to be
measured.
Figure 6.6
Choosing Geometry Angle and Diameter