Note: Descriptions are shown in the official language in which they were submitted.
.i
213286
- 1 - PATENT
Attorney Docket
No. 925-241
APPARATUS FOR MEASURING SURFACE TENSION
Field of the Invention
The invention pertains to an apparatus for
measurement of the surface tension of liquids by means of a
capillary tube for supplying gas, which has a connector and a
nozzle, wherein the capillary tube is at least partially
arranged in a crucible for receiving the liquid and wherein
the connector is arranged outside the interior of the
crucible. The invention also pertains to a measuring device
with an apparatus of this type.
Background Of The Invention
An apparatus of this type is known from DE 22 31
598/A1, in which an apparatus and process are disclosed for
determining the surface tension at the interface between
liquids and gases. The apparatus uses a capillary tube for
supplying gas with a connector and a nozzle. The capillary
tube extends from above down into the container which receives
the liquid. For measurement in molten metals the capillary
tube is exposed to the heat which rises from the liquid to be
measured. The capillary tube is mounted outside of the
crucible. Such an arrangement is rather expensive.
A further apparatus of this type is known from DE 29
15 956/A1, in which is described an apparatus for measuring
the surface tension of electrically conductive liquids. This
apparatus has a capillary tube with a connecting sleeve and a
nozzle. The end of the capillary tube that carries the nozzle
is bent in a U-shape. The capillary tube is immersed from
above in a liquid so that the nozzle is pointing in an upward
direction. During operation of the apparatus, gas bubbles
exit the nozzle and rise vertically inside a measuring tube.
Two electrodes supplied with a voltage and connected to a
time-keeping device are arranged on the measuring tube. When
the gas bubbles pass between the electrodes, an interruption
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in the current flowing between the electrodes is brought
about; the frequency of the interruptions that the gas bubbles
cause in the electrical circuit is measured. The capillary
tube is supplied with a flow of gas at a constant pressure, so
that by making use of the frequency of the gas bubbles, the
surface tension of the liquid can be determined.
Apparatus of this type are relatively complicated,
since the capillary tube immersed in the liquid from above
must be additionally mounted, like the measuring tube that has
the electrodes. In conjunction with that, these elements must
at the same time be protected from the increasing heat from
molten metals, for example. Even the necessity for generating
a flow of current within the liquid requires a relatively high
expenditure for operation and safety. The danger of possible
leaking currents also has to be viewed as a problem, if the
gas bubbles do not perfectly insulate the electrodes from one
another, since the results of the measurement can be distorted
as a result of such current leaks.
Further, an apparatus is known from DE 42 28 942/Cl
for measurement of the surface tension in liquids, wherein a
capillary tube is partially arranged in a crucible for
receiving the liquid, and wherein the capillary tube extends
through the wall of the crucible. The gas to be conducted
into the liquid flows through the capillary tube by way of a
gas distribution unit. The gas flows out over the entire
surface of the gas distribution unit, more or less
irregularly, and thereby reaches the surface of the liquid
under constantly changing conditions, where a sampling device
catches a portion of the gas bubbles (as a rule the largest)
and Ieads these as a measuring impulse to an analysis. Due to
the different outlet openings, the different paths of the gas
bubbles to the liquid, and due to the different size gas
bubbles emitted by the distributing unit, as well as due to
the inexactness of the receiving of the sampling device
arranged over the liquid, an exact measurement of the surface
tension with the described apparatus is not possible, since
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such an apparatus as a rule will not completely and correctly
catch the gas bubbles which escape from the liquid at
different places and in different sizes.
An additional apparatus for measurement of surface
tensions is described in EP 0 149 500. Also, the
determination of the frequency of gas bubbles, here in liquid
pig iron, is described in G.A. Irons and R.I.C. Guthrie
"Bubble Formation at Nozzles in Pig Iron," Metallurgical
Transactions B, Volume 9B, pages 101 - 110, March 1978. Shown
here is an apparatus in which the gas bubbles are detected by
means of a microphone.
By making use of the surface tension, apparatus of
this type are used, by way of example, to determine the
properties of molten metals. Knowledge of the surface tension
of molten cast iron makes it possible, among other things, to
draw conclusions concerning the graphite morphology of the
carbon contained in the cast iron, since the surface tension
and the interfacial energy between various phases influence
the microstructure of an alloy. This effect is described in
the article by E. Selcuk and D. H. Kirkwood, "Surface Energies
of Liquid Cast Irons Containing Magnesium and Cerium", Journal
of the Iron and Steel Institute, pages 134 - 140, February
1973. Admixtures of cerium and magnesium with cast iron
accelerate the formation of spheroidal graphite, that is, with
increasing content of cerium or magnesium, the form of the
graphite crystals changes from the lamellar type of graphite
at the beginning to the spheroidal type (spheroidal graphite),
which is sought in the practice of casting, because a graphite
morphology of this type generates optimal strength properties
in the cast iron.
Summary of the Invention
Building on the present state of the art described
above, it is an object of the present invention to create an
apparatus that is easy to manufacture and handle, and that can
be used reliably in a variety of liquids. For an apparatus of
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the above type, this object is achieved by means of the
capillary tube being arranged, at least partially, in a
crucible for receiving and holding the liquid and being
mounted on a wall of the crucible, whereby the connector is
arranged outside the interior of the crucible. An apparatus
of this type is relatively simple to manufacture and ensures a
secure placement of the capillary tube within the liquid with
which the crucible is to be filled, without the danger that
heat rising from a liquid that might be very hot, such as
molten metals, could damage the mount or the measurement
device, since no delicate parts need be placed above the
liquid. An apparatus of this type is suitable for measuring
the surface tensions of a variety of liquids, even for
measurements in liquid cast iron in order to determine, among
other things, the graphite morphology; for the determination
of the sulphur content of pig iron; or in order to assess the
modification treatment of aluminum-silicon alloys.
It is beneficial that the capillary tube be run
through the wall of the crucible, particularly through the
bottom of the crucible, in order to ensure a secure mounting.
When it is arranged in the bottom, the capillary tube can be
aligned vertically so that the gas bubbles can exit the
capillary tube unrestrictedly and in accordance with their
buoyancy. Additionally, it is beneficial if the connector is
mounted on or in the bottom of the crucible and is configured
as a crucible mounting, since the crucible can then be placed
with the connector directly on the gas connecting sleeve of a
gas supply line, and does not need to be fastened by
additional means.
For the uniform formation of bubbles it is
advantageous that the inside diameter of the capillary tube
increases at the nozzle and, in particular, is increased in a
circular manner or, that the nozzle is expanded in a slit-like
shape. In conjunction with this, it is beneficial that the
difference between the inside and the outside diameters of the
capillary tube at the outer end of the nozzle not be greater
2I~3286
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than 1 mm, and particularly not greater than 0.5 mm, and/or
that the product of the thermal conductivity (W/K-m) and the
wall thickness of the capillary tube at the outer end of the
nozzle be smaller than 5.5 x 10-3 W/K (Watt/Kelvin) at 1400°C.
An arrangement of this type ensures the regular formation of
bubbles that always exhibit a practically uniform diameter.
It is also possible for the nozzle to be placed laterally on
the capillary tube, by means of a lateral bore or a slit that
is made (by means of sawing or milling, for example) in the
capillary tube, for example.
It is advantageous, particularly for measurements in
aggressive or very hot media, that the capillary tube be made
of a gas-tight material, such as aluminum oxide, quartz, or
zirconia, since these materials exhibit high temperature
stability and are chemically resistant to many media, such as
cast iron melts. For the accuracy and reproducability of
measurements it is beneficial that the connector be joined to
the crucible in a gas-tight manner.
For a measurement device, the object is achieved by
virtue of the fact that the connector is connected in a gas-
tight manner to a gas connecting sleeve of a gas line and to a
pressure sensor and/or flow meter, since the frequency of the
gas bubbles that appear can be measured by using these
devices.
Brief Description of the Drav~ings
The foregoing summary, as well as the following
detailed description of preferred embodiments of the
invention, will be better understood when read in conjunction
with the appended drawings which show further features and
advantages of the invention. For the purpose of illustrating
the invention, there are shown in the drawings embodiments
which are presently preferred. It should be understood,
however, that the invention is not limited to the precise
arrangements and instrumentalities shown. In the drawings:
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Fig. 1 is a sectional side view of a schematic
representation of an apparatus in accordance with the
invention, with arrangement of the capillary tube in the
bottom of the crucible;
Fig. 2 is a sectional side view of a schematic
representation of the apparatus with arrangement of the
capillary tube in a side wall of the crucible;
Figs. 3a-h are longitudinal section views of several
forms of nozzles for the capillary tube;
Fig. 4 is a schematic representation, partially in
section, of a measuring device incorporating the apparatus of
the invention;
Fig. 5 is a pressure measurement curve for
determination of the bubble frequency;
Fig. 6 is a sectional side view of a schematic
representation of an apparatus similar to Fig. 1, but with the
nozzle formed as a slit-like opening laterally on the
capillary tube; and
Fig. 7 is a sectional side view of a schematic
representation of an apparatus similar to Fig. 2, but with the
nozzle formed as a slit-like opening laterally on the
capillary tube.
Detailed Description of Preferred Embodiment
The crucible 1 shown in Fig. 1 is used for
measurement of the surface tension of cast iron melts.
However, it is also suitable for measurements of aluminum-
silicon alloys or pig iron, for example. The crucible is made
of a heat-resistant material, in the present case of resin-
bonded sand or ceramic fibers. The bottom of the crucible 1
is made stronger (thicker) than the side walls in order to
ensure the necessary stability of the crucible. The capillary
tube 2 is run all the way through the bottom of the crucible
and is fastened there by means of a refractory cement 3 in
such a way as to prevent the melt from running out through the
bottom of the crucible.
213286
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The capillary tube 2 is bonded to a connector 4, on
which a gas connecting sleeve 5 is arranged. The connector 4
and the gas connecting sleeve 5 are used to supply gas to the
capillary tube 2. The gas-tightness necessary for
reproducability of measurements is ensured by means of two O-
rings 6, which are arranged between the connector 4 and the
gas connecting sleeve 5. The crucible 1 is connected to the
gas supply line and the measuring device by means of the gas
connecting sleeve 5. The connector 4 and gas connecting
sleeve 5 are made of metal, and the capillary tube 2 is made
of a refractory material, such as aluminum oxide, zirconia, or
quartz, for example. The capillary tube has an inside
diameter of about 0.7 - 1.5 mm and projects to a height of
about 5 - 25 mm into the interior hollow area of the crucible
1 that holds the melt.
The hollow area of the crucible 1 holds a melt
volume of about 100 ml; smaller volumes can lead to a cooling
of the melt, starting at the crucible wall, that progresses
too quickly to allow an accurate measurement of the surface
tension of the melt, after the melt is poured into the
crucible 1, since a volume that is too small has a
correspondingly low heat capacity and, therefore, cools off
correspondingly quickly. To minimize the influence of this
cooling effect on the measurement, the capillary tube 2 is
arranged approximately on the axis of the rotationally
symmetrical crucible 1, and the nozzle 7 is located about in
the middle of the hollow area of the crucible 1.
The crucible 1 shown in Fig. 2 is designed in a
similar manner. The essential difference, with respect to the
arrangement described above, resides in the arrangement of the
capillary tube 2 in a side wall of the crucible 1.
Fig. 3 shows various nozzle forms for the capillary
tube 2, as they can be used in the apparatus described at the
beginning. What these nozzle forms have in common is that
they are dimensioned in the apparatus in such a way that the
difference between the outside and inside diameters of the
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capillary tube at the outer end of the nozzle amounts to 0.5
mm at most, so that the formation of gas bubbles of uniform
size is assured. In this regard, it is conceivable to keep
the inside diameter of the capillary tube constant over the
entire length of the capillary tube, and to dimension the
outside diameter at the outer end of the nozzle in an
appropriate manner, as is shown in Figs. 3 a, g, and h.
However, it is also conceivable to expand the inside diameter
of the capillary tube at the nozzle in an appropriate manner.
In this regard, the expansion can be carried out in the form
of a cylindrical enlargement of the diameter, as shown in
Figs. 3 b and d, or as a conical expansion in the direction of
the outer end of the nozzle, as shown in Figs. 3 c and f. A
combination of the two latter nozzle shapes is shown in Fig. 3
e; here, the inside diameter of the capillary tube is expanded
in a cylindrical fashion, and an additional, conical expansion
is joined only at the outer end of the nozzle.
A measuring device for the determination of surface
tension is schematically shown in Fig. 4. The crucible 1
containing a cast iron melt 8 is connected to a gas line 9
through which the measurement gas is fed to the cast iron melt
8. The necessary gas flow is controlled by means of a gas
flow regulating device 10 in the gas line 9. A gas that is
inert with respect to the melt, such as argon or nitrogen, is
used as the measurement gas that is blown into the cast iron
melt 8 at about 2 - 15 ml per minute. The gas bubbles 11 are
formed in the cast iron melt at the nozzle 7. In conjunction
with this, a pressure that decreases abruptly following the
release of the gas bubbles 11 from the nozzle 7 is built up in
the gas line 9, and increases during the formation of a new
gas bubble 11 until this bubble 11 releases.
This pressure sequence, which is shown as a function
of time in Fig. 5, is detected and recorded by a pressure
sensor 12. The frequency of these pressure fluctuations
brought about by the formation of the gas bubbles 11 is used
for the calculation of the surface tension of the cast iron
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_ g _
melt 8, or of a physical quantity that stands in a direct
relationship with the surface tension, so that, as described
at the beginning, the graphite morphology in the cast iron
melt 8 can be determined. In this regard, the measurement
time that is available for recording the pressure-time
function is limited as a result of the solidification of the
cast iron in the crucible 1. The available measurement time
is determined by the difference between the pouring
temperature of the cast iron melt 8 into the crucible 1 and
the liquidus temperature of the cast iron, among other things.
In order to monitor the temperature, a temperature sensor such
as a thermocouple, for example, can be placed in the hollow
area of the crucible 1.
Figs. 6 and 7 illustrate further preferred
embodiments, similar to Figs. 1 and 2, where the capillary
tube 2 extends through the bottom or side wall, respectively,
of the crucible. However, unlike Figs. 1 and 2, where the
capillary tube opening or nozzle is formed in the end of the
tube, for example-in one of the forms shown in Fig. 3, the
nozzle 7 in Figs. 6 and 7 is in the form of a circumferential
slit or slit-like opening the lateral wall of the capillary
tube, preferably near its end inside the crucible and remote
from the crucible bottom or side wall. This slit may suitably
be formed by sawing or milling the tube in a radial or near
radial direction, preferably perpendicular to the longitudinal
axis of the tube. In the case of side wall mounting, the slit
preferably faces upward to ensure free release and uniform
bubble formation.
It will be appreciated by those skilled in the art
that changes could be made to the embodiments described above
without departing from the broad inventive concept thereof.
It is understood, therefore, that this invention is not
limited to the particular embodiments disclosed, but it is
intended to cover modifications within the spirit and scope of
the present invention as defined by the appended claims.
