Page 181 - Instrumentation Reference Book 3E

Analysis methods that measure terminal velocity 165

Mass on balance pan M - /

t
In, t
Figure 11.9 Sedimentation balance - plotsof Mand
dAUd In t against In t.

and Bostock. They have means for introducing the
homogenous suspension into the column above Figure 11.10 ICIsedimentationcolurnn
the balance pan. In some, the fluid beneath the
pan initialy contains no dust. The pan is counter-
balanced incrementally by small amounts to mini-
mize pan movement, although some is bound to
occur and this causes pumping, ie., some of the
finer particles transfer from above to below the
pan. A11 the columns are contained in thermostatic
jackets, constancy of temperature being important
to the constancy of fluid viscosity and density.

Sedimentation colzinzns A sedimentation column
works on the same principle as a sedimentation
balance but instead of weighing the sediment con-
tinuously the sediment is removed at preset times
and weighed externally enabling a higher quality
balance to be used.
The IC1 sedimentation column, Figure 11.10,
tapers at the bottom to a narrow tube a few
centimeters long. It is fitted with taps top and
bottom. A side branch in the narrow section con-
nects to a clear fluid reservoir. The dust sample is Figure 11.11 BCURA sedimentation column.
introduced into the main column and mixed using
a rising current of compressed air through tap A, With the top tap closed, surface tension in the
with tap B also open and tap C closed. At time capillary supports the column of fluid. At the
t = 0, taps A and B are closed and C is opened. prescribed times, a container of clear fluid is
Particles sediment into the narrow tube below the brought up around the open capillary, breaking
side branch. At present times, tap A is opened the surface tension, and the sediment passes out
allowing clear fluid to wash the sediment into a into the container. The container is then
centrifuge tube or beaker. Negligible sedimenting removed. In principle, no sedimenting fluid is
fluid is lost. The sediment is filtered, dried, and lost, but usually a small initial loss occurs until
weighed. a partial vacuum forms at the top of the column.
The BCURA sedimentation column also uses The above systems have the advantage of
an air supply for mixing, Figure 11.11. In this cheapness, but they are subject to errors because
case, however, the lower tap and narrow tube the tapered walls affect sedimentation and par-
are replaced by a length of 1 mm capillary tubing. ticles adhere to the tapered walls.