92168.pdf

246 Rheology

dv/dr

Figure 9.1 An Ostwald viscometer

simple Ostwald viscometer (Figure 9.1). Sufficient liquid is introduced

into the viscometer for the levels to be at B and C. Liquid is then

drawn up into the left-hand limb until the liquid levels are above

A and at the bottom of the right-hand bulb. The liquid is then

released and the time for the left-hand meniscus to pass between the

marks A and B is measured.

Since the pressure at any instant driving the liquid through the

capillary is proportional to its density,

17 = kpt (9.2)

where k is the viscometer constant, p is the density of the liquid and t
is the flow time. Therefore, for two different liquids,
*h Pi'i
(9.3)

Accurate thermostatting is necessary, owing to the marked depend-
ence of viscosities on temperature. Dust and fibrous materials, which