Apparatus for measuring surface tension

An apparatus for the measurement of the surface tension of liquids by means of a capillary tube for supplying gas, the tube having a connector and a nozzle, may be used in a measuring device. In order to create an apparatus that is easy to manufacture and handle, and that can be used reliably in a variety of liquids, the capillary tube is arranged, at least partially, in a crucible for receiving the liquid and is mounted on the wall of the crucible, whereby the connector is arranged outside the interior of the crucible. The connector is connected to the gas line of the measuring device and to a pressure sensor and/or a flow meter.

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 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/C1 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 leads 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 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 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 than 1 mm, and 
particularly not greater than 0.5 mm, and/or that the product of the 
thermal conductivity (W/K.multidot.m) and the wall thickness of the 
capillary tube at the outer end of the nozzle be smaller than 
5.5.times.10.sup.-3 W/K (Watt/Kelvin) at 1400.degree. 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 reproducibility 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.

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. 
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 reproducibility 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 
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. 3a, 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. 3b and d, or as a conical expansion in the direction of the 
outer end of the nozzle, as shown in FIGS. 3c and f. A combination of the 
two latter nozzle shapes is shown in FIG. 3e; 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 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 in 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. 
FIG. 8 illustrates another alternative embodiment of the capillary tube 2 
having the nozzle 7 located on the side of the capillary tube 2. The 
nozzle 7 has a diameter which increases at the nozzle end. 
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.