Brookfield CAP2000 Viscometer - Manual (Page 29)

Brookfield Engineering Labs., Inc. Page 29 Manual No. M02-313-G0812
APPENDIX A - Variables in Viscosity Measurements
As with any instrument measurement, there are variables that can affect a Viscometer
measurement. These variables may be related to the instrument (Viscometer), or the test fluid.
Variables related to the test fluid deal with the rheological properties of the fluid, while instrument
variables would include the Viscometer design and the spindle geometry system utilized.
Rheological Properties
Fluids have different rheological characteristics that can be described by Viscometer
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
The time dependency is the time they are held at a given Shear Rate (RPM). They are non-
Newtonian, and when you change the Viscometer 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 Viscometer
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 Viscometer at a constant speed, recording P values over time,
and find that the P values decrease with time, the material is thixotropic.
The Brookfield publication “More Solutions to Sticky Problems” includes a more detailed
discussion of rheological properties and non-Newtonian behavior.