MEASUREMENT OF VISCOSITY IN FLUIDS BASIC AND TUTORIALS


Using Flow Through Orifices
In viscosity determination using Saybolt or Redwood viscometers, the time for the flow through a standard orifice, of a fixed quantity of the liquid kept in a cup of specified dimensions is measured in seconds and the viscosity is expressed as Saybolt seconds or Redwood seconds.

The time is converted to poise by empirical equations. These are the popular instruments for industrial use. The procedure is simple and a quick assessment is possible. However for design purposes viscosity should be expressed in the standard units of Ns/m2.


Rotating Cylinder Method
The fluid is filled in the interspace between two cylinders. The outer cylinder is rotated keeping the inner cylinder stationary and the reaction torque on the inner cylinder is measured using a torsion spring.

Capillary Tube Method
The time for the flow of a given quantity under a constant head (pressure) through a tube of known diameter d, and length L is measured or the pressure causing flow is maintained constant and the flow rate is measured.

ΔP = (32 μ VL)/d2 (1.8.5)

This equation is known as Hagen-Poiseuille equation. The viscosity can be calculated using the flow rate and the diameter. Volume flow per second, Q = ( π d2/4) V. Q is experimentally measured using the apparatus. The head causing flow is known. Hence μ can be calculated.

Falling Sphere Method
A small polished steel ball is allowed to fall freely through the liquid column. The ball will reach a uniform velocity after some distance. At this condition, gravity force will equal the viscous drag.

The velocity is measured by timing a constant distance of fall.

μ = 2r2g (ρ1 – ρ2)/9V (1.8.6)

(μ will be in poise. 1 poise = 0.1 Ns/m2)

where r is the radius of the ball, V is the terminal velocity (constant velocity), ρ1 and ρ2 are the densities of the ball and the liquid. This equation is known as Stokes equation.

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