Brookeld Engineering Labs., Inc. Page 28 Manual No. M02-313E1209
APPENDIX A - Variables in Viscosity Measurements
As with any instrument measurement, there are variables that can affect a Viscometer measure-
ment. These variables may be related to the instrument (Viscometer), or the test uid. Variables
related to the test uid deal with the rheological properties of the uid, 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 measure-
ments. We can then work with these uids to suit our lab or process conditions.
There are two categories of uids:
Newtonian - These uids 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 uids 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 Viscom-
eter 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 nd that the P values decrease with time, the material is
thixotropic.
Brookeld publication, “More Solutions to Sticky Problems” includes a more detailed discus-
sion of rheological properties and non-Newtonian behavior.
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