(2)viscosity measurement -2015

7/23/2019 (2)Viscosity Measurement -2015

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VISCOSITYMEASUREMENT OF

GLASS


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1 Pa-s = 10 P


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1 Pa-s = 10 P


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EXPERIMENTAL ISSUES

1. elevated temperatures

the materials used in the construction of the apparatus must becapable of withstanding the required temperatures withoutundue deformation under the mechanical or thermal stressesinvolved, and without being corroded signicantly by the

atmosphere or by the melt.

our choice!platinum-rhodium alloys, or else pure ceramics such asrecrystalli"ed alumina.

2. glass melts is highly sensitive to temperature

# provide a furnace chamber with the minimum possibletemperaturevariation throughout the volume occupied by the sample# $onstancy to within 1%$ should be the aim.


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3. the measurement of temperature

a. thermocouples so placed as to give the best estimate of the

samples average temperature.

b. Thermocouple wires should enter the furnace cavity long a route

designed to minimize differential thermal gradients and electrical noise

pickup

c. the wires should be sufficiently long to allow proper connection to the

measuring device.

d. thermocouples suffer aging in service to some extent, it isnecessary to calibrate them at intervals against a subsidiary standard

thermocouple reserved for that purpose.

4. Sample preparation and handling

i. avoid clearly visible inhomogeneities or contamination

ii. Excessive heating can cause change of composition due to

volatilization or reaction with the surrounding atmosphere

iii. &rinding a sample to powder while e'posed to the usuallaboratory atmosphere could encourage water absorption with

consequent composition change, and is best avoided.


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ROTATO!A" #S$O%&T&RS

range 10 to 10(poise

either the crucible or

the plunger is rotated

and the relationshipbetween the measured

torque and the

rotational speed gives a

measure of the viscous

drag exerted by the

liquid.


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'(&R &"O!)ATO! #S$O%&T&RS

range 10)to 101*

measuring the rate of extension of a glass fiber suspended vertically in asuitable furnace


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%iber extension determines the dynamic viscosity of the glass by measuring the

elongation of the sample under uniaxial stress, implicitly assuming that the flow

behavior is &ewtonian.

. %ibers are drawn with diameters between '.( and '.)mm to yield test

samples between *' and ''mm in length

+. Each end of the sample is fused to a bead about +mm in diameter to

allow the sample to be suspended in the furnace and coupled to a loading

device.

. "ny surface layer resulting from the drawing process is removed by

etching, and samples with visible imperfections scratches, inclusions, non$

constant diameter must be discarded

-. The sample is mounted as indicated linked by a wire or ceramic coupling

to a thin vitreous silica rod which extends outside the furnace to accept the

weights used for loading


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The method is absolute in that the viscosity is calculated directly from

knowledge of the sample geometry, the loading and the measured extension

rate. #rovided that the dead weight of the fiber and the effects of surface

tension of the glass may be neglected

the rate of extension of the loaded fiber

where is the dynamic viscosity, m is the mass of the system that loads the fiber,

g is the acceleration due to gravity, lis the length of the fiber at the start of a

measurement, l+is the length of the fiber at the end of the measurement, /dt0 is

the time interval over which the measurement is taken, and " is the surface areaof the fiber specimen 1excluding the balls at the ends2.

dl = l1+

l


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urface tension / opposes the applied stress barrier againstincreasingthe surface area of the glass when the ber elongates. o, the

downwardforce is a balance between gravitational load and surface tension! = 234g - 5/r

annealing point! elongation rate = .('10-6 l7d8 strain point ! rate = 0.0*16 ' rate at the annealing point


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Fiber temperature is monitored by several thermocouples distributed

along the length of that portion of the furnace chamber occupied by

the sample

3easurements may be made with the furnace temperature rising steadily or at

constant temperature

To limit the danger of the fibers necking and producing anomalous results, the

total extension of one particular sample should not exceed '4 over the series

of readings

5ittleton softening point, which is a variation on the fiber extension method. 6ere,

a glass fiber of specified dimensions is sub7ected to a defined constantly rising

temperature environment and allowed to extend under its own weight. The

measurement depends on establishing the temperature at which

the fiber tip moves downward at a rate of mm8minute


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(&!*!) (&A% #S$O%&T&RS

range 109to 101(poise

specimen in the shape of a rod or beam supported near its ends and

loaded midway between the supports.

beams are about mm in diameter and the distance between the

supports is about -'mm.


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:hen a load is applied the initial response of the sample is a rapidelastic de;ection, followed by a slower de;ection as ;ow occurs.Provided the sample is behaving in a


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diameter of the beam is not less than about '4 of the span between the lower

supports and when the total mid$point deflection has not exceeded about *4 of

that span.

(2)viscosity measurement -2015

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