Brookfield DV3T - Appendix C: Variables in Viscosity Measurement

Brookeld 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uid.Variablesrelatedto
thetestuiddealwiththerheologicalpropertiesoftheuid,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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
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nd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.
Brookeldpublication,“More Solutions to Sticky Problems”,includesamoredetaileddiscussionof
rheologicalpropertiesandnon-Newtonianbehavior.