METHOD FOR DETERMINATION OF CO- EFFICIENT OF VISCOSITY

MECHANICAL TIPS AND STUDY MATERIAL

various experimental methods are used to determine coefficient of viscosity of a liquid. These are as under.

1) Rotating cylinder method

2) Orifice type viscometer
3) Capillary tube method
4) Resistance by falling sphere method

Rotating cylinder method

 DETERMINATION OF CO- EFFICIENT OF VISCOSITY

Look at the figure, you will find two concentric cylinders with inner cylinder having radii R1 and outer cylinder with radii R2. The space between the two cylinders is filled with liquid whoseviscosity is to be determined. Keeping the inner cylinder stationary, outer cylinder is rotated at constant angular speed (w).  The inner cylinder is held stationary by the help of torsional spring. The torque acting on inner cylinder is equal and opposite in nature to that acting in outer cylinder. The torsional spring with the help of dial and pointer measures the magnitude of torque acting in inner cylinder.

How is torque transmitted from inner to outer cylinder?


Torque acts on the outer cylinder the layer of liquid which is in contact with the inner cylinder's outer surface experiences torque. This torque is transmitted to subsequent layers of liquid and finally to outer cylinder.


Let w = angular speed of outer cylinder


Coefficient of viscosity is given by


                   2(R2 - R1)CT

            ---------------------------------------------
         Π R1^2[4HCR2 + R1^2(R2 - R 1)

WhereR1 = inner radii

R2 = outer radii
C = Clarence at bottom
H = height of liquid


Orifice viscometer


In your engineering course you will come across instruments like Saybolt and redwood viscometer, these are orifice type viscometers.



The liquid whose viscosity it to be calculated is allowed to flow through a capillary tube. The time taken by certain quantity of liquid to flow the tube is noted down and it is compared with coefficient of viscosity of a known liquid.
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