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PharmaceuticalRheology.
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Bubble viscometer This is based on a similar principle. A series of sealed standard tubes have calibrated oils covering a range of viscosities. Each has a small air bubble of exact geometry. The unknown sample is placed in an empty tube and stoppered so that it is identical in bubble content to the standards. The unknown and standard tubes are inverted, and the bubble rise times are compared to determine the standard most resembling the unknown.
Example
3.5 (Viscosity)
A ball of density 3.0 takes 100 s to fall the fixed distance of an inclined tube viscometer when a calibrating liquid of density 1.0 and viscosity 8.0 poise is used. (A) Calculate the instrumental constant. (B) What would be the viscosity of a sample oil of density 0.8 if the fall time under similar conditions was 125 s?
Solution
K
=
1'/
(Ps-A)t
=
8.0
poise
(3.0-1.0)(100)(g/mL)(s)
=
0 04 pOlseg . -I mL s-I
.
B. 11= 0.04 (PB-PoH)t =
(0.04)(3.0 - 0.8)(125)
=
11 poise
Rotational Viscometers Principle: These instruments are based on the fact that a solid rotating body immersed in a liquid is subjected to a retarding force due to the viscous drag, which is directly proportional .to the viscosity of the liquid.
Cup and bob viscometers As represented by Figure 3.19, the cup and bob viscometer comprise two members, a central bob or cylinder and a coaxial or concentric cup. One or both are free to rotate in relation to each other. Between these is the test substance, in the annulus. Three basic configurations have been utilized. 1. Couette type: Rotating outer cup with strain measurement on the central bob. For example, the MacMichael viscometer. 2. Searle type: Rotating central bob with strain measurement on the cup. For example, the Stormer viscometer and the Brookfield viscometer. 3. Fixed cup with both rotation and strain measured on the bob. For example, the Contraves viscometer, Epprecht viscometer and Rotovisko.