Brookfield Engineering Labs., Inc.
Page 98
Manual No. M13-167-A0415
Appendix C - Variables in Viscosity Measurements
As with any instrument measurement, there are variables that can affect a Rheometer 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 RPMs)
and are called Newtonian over the Shear Rate range they are measured.
Non-Newtonian
- These fluids have different viscosities at different shear rates (different
RPMs). They fall into two groups:
1) Time Independent non-Newtonian
2) Time Dependent non-Newtonian
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
(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 cP values over time
and find that the cP values decrease with time, the material is thixotropic.
- If you take rheometer readings from a low RPM to a high RPM and then
back to the low RPM, and the readings are lower for the descending step,
the material is time dependant, thixotropic.
Brookfield publication, “
More Solutions to Sticky Problems
”, includes a more detailed discussion of
rheological properties and non-Newtonian behavior.
