Instruments for Measuring Viscosity

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Instruments forMeasuring Viscosity

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• All viscometers apply a stress to the fluid and
  measure the resistance to flow
• Most viscometers are in 2 major categories
• Rotational type viscometer
• Tube type viscometer

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Tube Viscometers

DPpr2tprL
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• With this as a starting point we gan dreive the
  Hagen-Poiseuille Equation

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Capillary Viscometer

• In the glass capillary viscometer, the tube
  diameter is small (0.1-4 mm). The pressure drop
  is provided by hydrostatic head of fluid
• DP rgh

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• If we switch from Q (ml/s) to the average
  velocity ltugt -gt Q pR2
• We could then show that the Hagen-Poiseuille
  equation is

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• We can define the kinematic viscosity
• The average time for a fixed volume of fluid to
  flow through the tube is

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• So that the kinematic viscosity is given by

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Capillary Viscometer Pros/Cons

• ADVANTAGES
• Simplest rheometer, yet most accurate for steady
  viscosity
• High shear rates possible
• Sealed system limits evaporation
• May simulate process
• POSSIBLE ERRORS
• Wall slip with concentrated dispersions (fluid
  layer forms at wall with
• different concentration than bulk)
• Melt fracture
• Reservoir pressure drop
• Entrance effects
• Viscous heating

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DRAG FLOW/ROTATIONAL VISCOMETERS

• Shear is generated between a moving and a fixed
  surface

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Common Geometries

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Concentric Cylinders

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• The inner cylinder rotates at a speed W rpm.
  This is related to the shear rate.
• The instrument sensor measures a torque T. The
  torque is opposed by the shear stress on the bob
  emanating from the viscous forces in the fluid

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• It can be shown

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• The geometric parameters are usually accounted
  for in the instrument. The Brookfield has a
  conversion chart other viscometers calculate the
  constants by software.

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