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Instruments for Measuring Viscosity

Instruments for Measuring Viscosity. 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. Tube Viscometers. D P p r 2 = tp rL.

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Instruments for Measuring Viscosity

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  1. Instruments forMeasuring Viscosity

  2. 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

  3. Tube Viscometers DPpr2=tprL

  4. With this as a starting point we can derive the Hagen-Poiseuille Equation:

  5. 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

  6. If we switch from Q (ml/s) to the average velocity <u> -> Q = pR2 • We could then show that the Hagen-Poiseuille equation is

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

  8. So that the kinematic viscosity is given by

  9. 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

  10. DRAG FLOW/ROTATIONAL VISCOMETERS • Shear is generated between a moving and a fixed surface

  11. Common Geometries

  12. Concentric Cylinders

  13. 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

  14. It can be shown viscosity shear stress shear rate

  15. 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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