Viscometry Lab.pdf

II. FALLING BALL VISCOMETERS
FDST 8080 Lab 2011
One very simple type of instrument is the falling ball viscometer. Here, a glass or metal
ball is allowed to fall through the sample. The more viscous the sample is, the longer it
takes the ball to reach the bottom. In the simplest case, this may be just a graduated
cylinder with a steel ball. Once the ball drops, it will soon reach a terminal velocity once
the force of gravity is countered by the frictional forces due to the fluid. By Stokes law:
! = 2(" s # " 2
%
l )gR
&
'
9$
where η is the viscosity, ρ s is the density of the solid ball, ρ l is the density of the fluid, R is
the ball radius, g the gravitational constant, and υ the terminal velocity. The time it takes
for the ball to fall a given distance is determined with a stopwatch, and determines the
velocity (υ=∆x/∆t).
F = mg
g
F = K!"
f
(
)
*