4/27/2011 1 Capillary viscometers Instruments used to measure the viscosity of liquids can be broadly classified into seven categories: Orifice viscometers High temperature high shear rate viscometers Rotational viscometers Falling ball viscometers Vib ration al viscometers rason c v scome ers
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Capillary viscometers
Instruments used to measure the viscosity of
liquids can be broadly classified into seven
categories:
Orifice viscometers
High temperature high shear rate viscometers
Rotational viscometers Falling ball viscometers
Vibrational viscometers
rason c v scome ers
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A number of viscometers are also available that combinefeatures of two or three types of viscometers noted above,
such as:
Friction tube
Norcross
Brookfield
Viscosity sensitive rotameter
Continuous consistency viscometers
num er o ns rumen s are a so au oma e or con nuousmeasurement of viscosity and for process control.
Common rheological instruments
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CAPILLARY VISCOMETERS
Capillary viscometers are most widely used for measuring viscosity
of Newtonian liquids.
They are simple in operation; require a small volume of sample
liquid, temperature control is simple, and inexpensive.
Capillary viscometers are capable of providing direct calculation of
viscosity from the rate of flow, pressure and various dimensions of
the instruments.
Most of the capillary viscometers must be first calibrated with one or
more liquids of known viscosity to obtain “constants” for that
particular viscometer.
The essential components of a capillary viscometer
are:
1. A liquid reservoir
2. A capillary of known dimension,
3. A provision for measuring and controlling theapplied pressure
4. A means of measuring the flow rate
. .
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Classification of commercially available capillary
viscometers based on their design:
1. Modified Ostwald viscometers
2. Suspended-level viscometers
3. Reverse-flow viscometers
Glass capillary viscometers
a) UBBELOHDE
b) OSTWALD
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CANNON-FENSKE
Reverse-Flow Viscometer
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Common rheological instruments
Operates on the principle of measuring the rate of rotation of a solid
shape in a viscous medium upon application of a known force or
torque required to rotate the solid shape at a definite angular velocity.
Rotational viscometer
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study the flow properties of non Newtonian materials.
Some of the advantages are:1. Measurements under steady state conditions
2. Multiple measurements with the same sample at different shear
3. Continuous measurement on materials whose properties may be
function of temperature
4. Small or no variation in the rate of shear within the sample
during a measurement.
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Partial section of a concentric cylinder viscometer
The concentric cylinder geometry is most suited for fluids of low
viscosity (<10 Pa s).
At very high values, loading problems appear and entrapment of air
Rotational viscometer
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The determination of the shear stress and shear rate within theshearing gap is valid only for very narrow gaps wherein k , the ratio of
inner to outer cylinder radii, is > 0.99.
concentric cylinder viscometer
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to the shear flow at the bottom of the concentric cylinder geometry.
These include the recessed bottom system which usually entails
trapping a bubble of air (or a low viscosity liquid such as mercury)
beneath the inner cylinder of the geometry.
Alternatively the ‘ Mooney–Ewart ’ design, which features a conical
bottom may, with suitable choice of cone angle, cause the shear rate in
the bottom to match that in the narrow gap between the sides of the
cylinders.
The Mooney–Ewart
eometr
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For k = 0.99, the shear rate may be calculated from:
(1)
where R 2 and R 1 are the outer and inner cylinder radii
respectively, and Ω is the angular velocity.
The shear rate for non-Newtonian fluids depends upon theviscosity model itself.
For k > 0.5 and if the value of (d lnT /d lnΩ) is constant over
For the commonly used power-law fluid model, the shear
rate is a function of the power-law index.
e range o n eres τR1 o τR2 , one can use e o ow ng
expressions for evaluating the shear rates at r =R 1 and r =R 2