User guide

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APPENDIX C - Variables in Viscosity Measurements
As with any instrument, there are variables that can affect a viscosity measurement. These variables may
be related to the instrument (Rheometer) or the test fluid. Variables related to the test fluid deal with the
rheological properties of the fluid, while instrument variables would include the Rheometer design and
the spindle geometry system utilized.
Rheological Properties
Fluids have different rheological characteristics that can be described by Rheometer measurements. We
can then work with these fluids to suit our lab or process conditions.
There are two categories of fluids:
Newtonian - These fluids have the same viscosity at different Shear Rates (different
RPM’s) and are called Newtonian over the Shear Rate range they are
measured.
Non-Newtonian - These fluids have different viscosities at different shear rates (different
RPM’s). They fall into two groups:
1) Time Independent non-Newtonian
2) Time Dependent non-Newtonian
The time dependency is the time they are held at a given Shear Rate (RPM). They are non-
Newtonian, and when you change the Rheometer spindle speed, you get a
different viscosity.
Time Independent
Pseudoplastic - A pseudoplastic material displays a decrease in viscosity with an increase
in shear rate, and is also known as “shear thinning”. If you take Rheometer
readings from a low to a high RPM and then back to the low RPM, and the
readings fall upon themselves, the material is time independent pseudoplastic
and shear thinning.
Time Dependent
Thixotropic - A thixotropic material has decreasing viscosity under constant shear rate. If
you set a Rheometer at a constant speed, recording viscosity (cP) values over
time and find that the viscosity (cP) values decrease with time, the material
is thixotropic.
Brookfield publication, “More Solutions to Sticky Problems” includes a more detailed discussion of
rheological properties and non-Newtonian behavior.
Rheometer Related Variables
Most fluid viscosities are found to be non-Newtonian. They are Shear Rate dependent on the
measurement conditions. The specifications of the Rheometer spindle and chamber geometry will affect
the viscosity readings. If one reading is taken at 25 rpm, and a second at 50 rpm, the two viscosity (cP)
values produced will be different because the readings were made at different shear rates. The faster the
spindle speed, the higher the shear rate.
The shear rate of a given measurement is determined by: the rotational speed of the spindle, the size and
shape of the spindle, the size and shape of the container used, and therefore, the distance between the
container wall and the spindle surface.