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Appendix C
APPENDIX C - 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.