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TABLE OF CONTENTS Chapter 3 - Physical Properties of Fluids: Fluid Viscosity
Viscosity is the internal resistance of a fluid to flow. Water has low viscosity,
while syrup has high viscosity. When syrup is cold it is very much more viscous
than when it is hot. Viscosity is highly temperature dependent.
Absolute Viscosity and Kinematic Viscosity
There are two ways to measure the viscosity of a fluid. One is to rotate a cup or
disk in the fluid and measure the rotational speed along with the torque required to
keep it rotating. The density of the fluid doesn't play any part in the measurement.
Viscosity is the torque divided by the speed and divided by the contacting surface
area. When measured this way, the viscosity is called "absolute viscosity." Its
units are centipoise, pascal-seconds, or lb/ft-sec.
Another way to measure viscosity is to allow a certain volume of fluid to drain
by gravity out of a container through a capillary tube or some other restriction.
The time to drain is directly related to viscosity and is often reported in "seconds."
Since the flow is by gravity, the force causing the flow depends on the density of
the fluid. Viscosity measured this way is called "kinematic viscosity." Its units are
centistokes, m2/sec, or ft2/sec. As suspected, centipoise (cP) equals centistokes
(cSt) multiplied by density. Some equivalents are:
cP = (cSt)(density in kg/l) lb/ft-sec = 0.000672(cP) Pa-sec = (m2/sec)(density in kg/l) |
Figure 3-9 shows the relationship between many of the kinematic viscosity
scales.
Gas viscosities are treated exactly as are liquid viscosities, but in general the
absolute viscosity of a gas is much lower than that of a liquid.
Temperature has a very great effect on the viscosity of a fluid. For liquids,
increasing temperature will lower viscosity, but, for gases, increasing temperature
increases the viscosity. Figure 3-10 presents graphs that allow estimation of viscosity
of a liquid at one temperature if viscosity is known for that liquid at some
other temperature. Table 3-4 and Table 3-5 give equations and constants for the
calculation of viscosity values for a number of specific liquids and gases.
Many flowmetering phenomena actually depend on kinematic viscosity, but it
is common to show absolute viscosity in the formulas and include density in the
formula.
