BYK CAP 1000+ - APPENDIX C - Variables in Viscosity Measurement

29
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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
measurements.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 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ndthattheP values decrease with time,
the material is thixotropic.