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1、911VISCOSITYViscosityisapropertyofliquidsthatiscloselyrelatedtotheresistancetoflow.Itisdefinedintermsoftheforcerequiredtomoveoneplanesurfacecontinuouslypastanotherunderspecifiedsteady-stateconditionswhenthespacebetweenisfilledbytheliquidinquestion.Itisdefinedastheshearstressdividedbytherateo
2、fshearstrain.Thebasicunitisthepoise;however,viscositiescommonlyencounteredrepresentfractionsofthepoise,sothatthecentipoise(1poise=100centipoises)provestobethemoreconvenientunit.Thespecifyingoftemperatureisimportantbecauseviscositychangeswithtemperature;ingeneral,viscositydecreasesastemperatu
3、reisraised.Whileontheabsolutescaleviscosityismeasuredinpoisesorcentipoises,forconveniencethekinematicscale,inwhichtheunitsarestokesandcentistokes(1stoke=100centistokes)commonlyisused.Toobtainthekinematicviscosityfromtheabsoluteviscosity,thelatterisdividedbythedensityoftheliquidatthesametempe
4、rature,i.e.,kinematicviscosity=(absoluteviscosity)/(density).Thesizesoftheunitsaresuchthatviscositiesintheordinaryrangesareconvenientlyexpressedincentistokes.Theapproximateviscosityincentistokesatroomtemperatureofetheris0.2;ofwater,1;ofkerosene,2.5;ofmineraloil,20to70;andofhoney,10,000.Absol
5、uteviscositycanbemeasureddirectlyifaccuratedimensionsofthemeasuringinstrumentsareknown,butitismorecommonpracticetocalibratetheinstrumentwithaliquidofknownviscosityandtodeterminetheviscosityoftheunknownfluidbycomparisonwiththatoftheknown.Manysubstances,suchasthegumsemployedinpharmacy,havevari
6、ableviscosity,andmostofthemarelessresistanttoflowathigherflowrates.Insuchcases,agivensetofconditionsisselectedformeasurement,andthemeasurementobtainedisconsideredtobeanapparentviscosity.Sinceachangeintheconditionsofmeasurementwouldyieldadifferentvaluefortheapparentviscosityofsuchsubstances,t
7、heinstrumentdimensionsandconditionsformeasurementmustbecloselyadheredtobytheoperator.MeasurementofViscosity—Theusualmethodformeasurementofviscosityinvolvesthedeterminationofthetimerequiredforagivenvolumeofliquidtoflowthroughacapillary.Manycapillary
