Unformatted text preview: OTHER UNITS Other units of viscosity have come about because of the way viscosity is measured. For
example REDWOOD SECONDS comes from the name of the Redwood viscometer.
Other units are Engler Degrees, SAE numbers and so on. Conversion charts and
formulae are available to convert them into useable engineering or SI units.
1.2.4 VISCOMETERS The measurement of viscosity is a large and complicated subject. The principles rely on
the resistance to flow or the resistance to motion through a fluid. Many of these are
covered in British Standards 188. The following is a brief description of some types. © D.J.DUNN 5 U TUBE VISCOMETER
The fluid is drawn up into a reservoir and allowed to
run through a capillary tube to another reservoir in the
other limb of the U tube.
The time taken for the level to fall between the marks is
converted into cSt by multiplying the time by the
ν = ct
The constant c should be accurately obtained by
calibrating the viscometer against a master viscometer
from a standards laboratory.
This works on the principle of allowing the
fluid to run through an orifice of very accurate
size in an agate block.
50 ml of fluid are allowed to fall from the level
indicator into a measuring flask. The time
taken is the viscosity in Redwood seconds.
There are two sizes giving Redwood No.1 or
No.2 seconds. These units are converted into
engineering units with tables. Fig.1.4 © D.J.DUNN 6 FALLING SPHERE VISCOMETER
This viscometer is covered in BS188 and is based on measuring
the time for a small sphere to fall in a viscous fluid from one
level to another. The buoyant weight of the sphere is balanced
by the fluid resistance and the sphere falls with a constant
velocity. The theory is based on Stokes’ Law and is only valid
for very slow velocities. The theory is covered later in the
section on laminar flow where it is shown that the terminal
velocity (u) of the sphere is related to the dynamic viscosity (µ)
and the density of the fluid and sphere (ρf and ρs) by the
Fig.1.5 µ = F gd2(ρs -ρf)/18u F is a correction factor called the Faxen correction factor, which takes into account a
reduction in the velocity due to the effect of the fluid being constrained to flow between
the wall of the tube and the sphere.
There are many types of viscometers, which use the principle that it requires a torque to
rotate or oscillate a disc or cylinder in a fluid. The torque is related to the viscosity.
Modern instruments consist of a small electric motor, which spins a disc or cylinder in
the fluid. The torsion of the connecting shaft is measured and processed into a digital
readout of the viscosity in engineering units.
You should now find out more details about viscometers by reading BS188, suitable
textbooks or literature from oil companies. ASSIGNMENT No. 1 1. Describe the principle of operation of the following types of viscometers. a. Redwood Viscometers. b. British Standard 188 glass U tube...
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