KR101500920B1 - Method and Apparatus for Characterizing a Glass Melt by Ultrasonic Illumination - Google Patents

Abstract

The present invention relates to a system characterized by molten glass, wherein said waveguide (20a) is acoustically coupled to an outer surface (34) of a vessel (10) containing a large amount of molten glass. The sound waves are separated into glass melted by the first transducer 24a through the first waveguide 20a, wherein part of the sound wave is reflected in the melted glass, is received through the second waveguide 20b, The final signal is generated by the second transducer 24b and analyzed to characterize the molten glass.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a method and apparatus for characterizing a glass melt by ultrasonic irradiation,

The present invention relates to characterizing fluids, and more particularly relates to characterizing a glass melt by irradiating the glass melt with ultrasonic waves.

High quality glass is made by controlled cooling of the glass melt. However, the glass melt may comprise a heterogeneous material such as solids and gas inclusions, and a small volume of deviating densities and chemical compositions are often referred to as cords. In particular, the formation of the cords has local regions with different refractive indices. Due to the localized regions of different refractive index, the final glass is not suitable for a number of precise applications.

In general, the properties of the glass melt are determined by sampling the glass during various stages of processing, or by installing in-line sensors such as thermocouples. At high temperature, which is an industrial glass making process, molten glass is almost impossible to sample because it is contained in and / or flows through a closed conduit insulated with respect to the outer perimeter of the conduit.

On the other hand, the in-line sensor can hardly sustain the action at a high temperature used for glass processing. While thermocouples are used to measure the temperature in the glass melt of some processes, the use of thermocouples is undesirable because thermal cells deteriorate at an unacceptable rate. Moreover, the insertion of the sensor into the glass melt can result in a clogging of the sensor, an interruption of the flow, or a loss of quality of the product by creating an unacceptable heat loss in the system.

The present invention is to provide a method and apparatus for characterizing a glass melt by ultrasonic irradiation.

The present invention characterizes a glass melt non-invasively, wherein a modification process or modification to the glass melt processing parameters can be applied to the sensed properties to produce a final glass having nominal properties. That is, the present invention increases knowledge of glass melt properties, which enhances the quality of the final product by allowing the processing parameters to be applied accordingly.

More particularly, the system can provide a method for characterizing a glass melt by coupling sound waves to the glass melt through the outer surface of the vessel.

In another configuration, a system for characterizing a glass melt comprises a vessel comprising a glass melt.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The figures are not required to be understood, and the sizes of the various elements may be inaccurate for clarity. The drawings illustrate one or more embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

Sound is vibration that moves or propagates through a medium such as liquid or air. The source of this vibration is the repetitive fluctuations of the medium. For example, when you hit a bell, it vibrates. The side of the bell first moves toward the outside with respect to the air surrounding it, creating a high pressure in the air, and then the side moves to create a low pressure while moving inside. The high- and low-pressure regions are known as compression and rarefaction regions, respectively, and propagate through the medium as waves by affecting adjacent molecules of air: molecules in the air are subjected to high pressure and low pressure crossings Moves back and forth, and acts on alternating molecules, which subsequently act on the adjacent molecules. The high and low pressure regions therefore propagate through the medium as waves with limited speed and wavelength.

In accordance with the present invention, a method of characterizing a glass melt employing a basic pulse-echo technique is disclosed. Electrical pulses are generated in a high frequency pulse generator, ) Wavelength, for example, a piezoelectric transducer is acoustically coupled to the glass melt by a primary waveguide. Ultrasonic waves propagate through the waveguide and are transferred to the molten glass through a vessel containing the molten material. The sound waves in the melt are canceled out, if any, dispersed by the heterogeneous material and reflected from the boundary. The reflected wavelength can be detected by a secondary transducer acoustically coupled to the molten glass through a secondary waveguide, and the acoustic wave can be converted back to an electronic signal. The signal can be expanded and processed by an appropriate data acquisition system. For example, the signal may be processed by a computer capable of measuring parameters such as, for example, transit time, amplitude and frequency of a sound wave. These parameters convey information about the physical geometrical properties of the glass melt medium, such as ultrasonic offset, the presence of heterogeneous (bubbles), the flow of the melt, the temperature of the melt, and the like.

The term "glass" includes random (liquid crystalline) amorphous molecular structures. The manufacturing process of the glass requires that the raw material be heated to a temperature sufficient to produce a relatively low viscosity melt. It is not crystallized during cooling but becomes hard. The glass melt may be one of a variety of compositions including soda lime glass, lead glass, borosilicate glass, aluminosilicate glass, 96% silica glass, fused silica glass and alumino-borosilicate glass. The term " glass melt "or" molten glass "includes one of various glass compositions above the respective softening point. Generally, the glass melt is within the range of 1200 ° C to 1700 ° C. The term "acoustic wave" refers to a mechanical vibration transmitted through a medium. In one embodiment, the sound waves are within the ultrasonic range, i.e., about 100 kHz to 300 kHz.

According to one embodiment of the present invention, an apparatus 8 of an embodiment for characterizing a glass melt is shown in Figure 1 and comprises a vessel 10, a pair of acoustic waveguide assemblies 12,14 and a controller 16). The vessel (10) is surrounded by a thermally insulated refractory lid (18).

The container 10 can be one of a variety of configurations including a large amount of glass melt. The container 10 has an open or closed lid and may be self-contained with a large amount of glass melt. In one configuration, the vessel 10 is associated with a waveguide assembly 12, 14 that allows sensing within a given percentage of the cross-sectional area of the flow path to refer to the selected flow path. Thus, the vessel 10 can maintain or receive the flow of the glass melt from the upstream position, allowing the glass melt to flow to the downstream location. The vessel 10 may be, for example, a tube through which the glass melt flows, and Fig. 1 shows a cross-sectional view of the tube.

The container 10 may be composed of a material capable of withstanding the intended operating temperature of the glass melt, generally within the range of about 800 ° C to about 1700 ° C. The vessel preferably comprises a metal selected from metals of the platinum group, including platinum, rhodium, iridium, ruthenium, palladium, osmium or alloys thereof. However, other high temperature materials can also be used. For example, molybdenum may function as an effective container material on its own or in combination with other materials.

The waveguide assemblies 12,14 are acoustically coupled to the vessel 10 and include an acoustic path extending from the primary waveguide assembly 12 through a portion of the vessel wall to the glass melt 15, Is an acoustic path extending through the opposite side of the wall to the remaining waveguide assembly 14.

Figure 1 discloses a pair of acoustically coupled waveguide assemblies 12,14 that provide a straight acoustic path between the waveguide and the assemblies by being positioned in opposite directions radially across the vessel on the same straight line. This arrangement is particularly suitable for detecting foreign matter having considerable dimensions compared to acoustic wavelengths such as large glass bubbles whose diameter across the acoustic path is a few millimeters. Such a pathway can cause amplitude reduction of the ultrasonic signal detected by the receiving transducer. In an alternative arrangement suitable for the detection of small foreign matter such as small bubbles of sub-millimeter size, the waveguides can be arranged in a V-shape.

In another configuration, a plurality of pairs of waveguide assemblies 12,14 can be acoustically coupled to the vessel. The pair of waveguides are preferably collinear and diametrically opposed to the container, but may be in a plurality of V-shaped directions depending on the application, if desired.

Because the waveguide assemblies 12,14 are essentially the same, the following description relates to waveguide assembly 12, and the description of the individual components also applies to the waveguide assembly 14, unless otherwise stated. The components of the waveguide assemblies 12 and 14 are differentiated into suffixes "a" and "b" at the end of each component in the figure. The waveguide assembly 12 includes a waveguide 20a including a core rod 21a and a cladding tube 22a and a transducer 24a and one end of the waveguide 20a is acoustically coupled to the transducer, The other end of the waveguide 20a is acoustically coupled to the vessel 10. Preferably, the waveguide 20a is not only acoustically coupled, but is physically coupled or attached to the vessel 10 as well. In some embodiments, waveguide 20a is also physically coupled to transducer 24a. The method of physically coupling waveguide 20a may include brazing / welding, thread fitting, or other methods that are practically applicable.

The transducer 24a is acoustically couple between the transducer 24a and the refractory vessel 10 to leave the transducer out of the waveguide 20a because it can not reliably function at the elevated temperature associated with the glass melt and the refractory vessel. . This spacing creates a temperature gradient along the length of the waveguide 20a, which allows the transducer to operate at a lower temperature than the refractory vessel and the glass melt.

The transducer 24a is a transducer suitable for generating ultrasound signals. For example, the converter 24a is a Langevin type or Tonpilz type converter. The converter 24a converts an electrical signal generated by the signal generator 23 and amplified by the amplifier 25 into an acoustic signal or a wavelength. The signal generator 23 and the amplifier 25 may be operably coupled to the converter 24a via control lines 29 and 31 in a conventional manner. The controller 16 is operatively coupled to the signal generator 23 via a control line 27. Controller 16 may be, for example, a dedicated processor or a computer.

Acoustically coupling the transducer 24a with the waveguide 20a to transfer the acoustic signal or wavelength to the glass melt may include biasing the waveguide 20a relative to the transducer 24a . Biasing can be accompanied by loading of a separate biasing member, such as a waveguide (or transducer) or a spring. However, more rigid couplings, such as soldering / welding or threaded fittings, can enhance signal transmission from the transducer to the waveguide.

The waveguide 20a is a long member capable of transmitting an ultrasonic signal. It has been found that an elongate member performs as an appropriate waveguide, although the waveguide 20a may be one of a variety of structures such as a stepped or tapered conical or folded structure. Since the waveguide 20a will be coupled to the vessel 10 at one end, the waveguide 20a must be able to withstand the high temperatures experienced in the vessel, while at the same time serving as an effective waveguide. Thus, the waveguide 20a is preferably comprised of a refractory metal core rod 21a (or "metal core"), such as a platinum alloy such as platinum or a platinum rhodium alloy. In one configuration, the outer diameter of the core rod 20a is about 3 mm. The waveguide 20a preferably also comprises a cladding tube 22a. The cladding tube 22a (or "ceramic cladding") is preferably a non-metallic ceramic material such as mullite (3Al ₂ O ₃ 2SiO ₂ ) providing a higher velocity path for sound waves than the core rod. The cladding tube 22a preferably extends along substantially the entire length of the core rod 20a. The outer diameter of the core rod 21a should be selected to fit comfortably within the inner diameter of the cladding die 22a. In the selected configuration, an acoustic coupling reagent may be placed between the core rod 21a and the cladding tube 22a to ensure proper acoustic coupling between the core rod and the cladding. Alternatively, a waveguide cladding 22a may be formed around the core rod 21a.

The cladding tube 22a may be shortened to expose a length of approximately 8-10 mm of the core rod 21a at one end adjacent to the transducer 24a in order to maintain an appropriate operating temperature in the transducer 24a. A cooling air flow may be passed through the exposed portion of the core rod 21a through the passageway 30a so as to be maintained at approximately 50 DEG C or less at the interface of the transducer and the waveguide.

Referring to FIG. 1, the outer tube 32a may be centered relative to each waveguide. The ring must be separated from the waveguide 20a so as to be formed between the waveguide 20a and the outer tube 32a. The ring may comprise, for example, air.

One end of the outer tube 32a is joined to the outer surface 34 of the vessel 10. The engagement of the outer tube 32a with the vessel wall enhances the confinement of the region at the vessel wall to emit and receive acoustic energy (i.e., sound waves). Since the vessel 10 is generally surrounded by a refractory insulating material 18, the outer tube 32 functions to isolate the insulating material from the vicinity of the contact point between the waveguide and the vessel. The outer tube 32a may comprise a ceramic, for example Al ₂ O ₃ . Coupling or physically coupling the outer tube 32a to the container 10 may use, for example, a refractory adhesive.

In one embodiment, waveguide 20a may be coupled to vessel 10 by primary brazing / welding or thread fitting, such as receptacle 35, as shown in FIG. A complementary thread is formed at the end of the waveguide 20 (e.g., at the end of the core rod 21) and the waveguide is coupled to the vessel by threading the waveguide to the receptacle. 2, the receptacle 35 is in the form of internal threading and a collar, while the waveguide 20a is connected to the receptacle 35 and consequently to the vessel 10, . Preferably, the receptacle 35 comprises platinum or a platinum-alloy. Alternatively, the coupling between the waveguide 20a and the vessel 10, as shown in Figure 3, may be accomplished by attaching the screw stub 40 to the vessel 10 (e.g., by soldering or welding) By forming a recess in the waveguide 20a (i.e., the core rod 21a). Accordingly, the waveguide 20a is fitted into the stub 40, so that the waveguide 20a and the vessel 10 can be coupled through the stub 40. [ In a similar approach, the core rod 21a may be soldered / welded directly to the vessel 10.

It has been found very useful to physically ensure good planar contact between the coupling means when waveguide 20a is coupled to fitting 35 or stub 40. [ In other words, the end face of the core rod 21a should preferably traverse the longitudinal axis of the waveguide, and the fittings 35 (not shown) are not threaded so that the interface contact surface 36 between the rod end and the fitting depends on the actual acoustic coupling. ) Should be in complete contact with the base of the internal thread diameter. This principle also applies to the interface contact surface 38 between the core rod 21a and the stub 40 if used.

In operation, the acoustic signal (sound waves) is generated by the transducer 24a and is acoustically coupled to the waveguide 20a. The acoustic signal is transmitted through the waveguide 20a to the vessel 10 and is thereby introduced into the glass melt 15 by the vessel. The transmitted sound waves are reflected by the foreign matter in the molten glass. The reflected sound waves are transmitted through the vessel wall and are received by a secondary waveguide assembly 14 comprising a core rod 21b (or "core") and a cladding 22b (or "cladding tube"). The acoustic signal travels through the waveguide 20b to the acceptance transducer 24b where the corresponding electrical signal is generated by the transducer and is transmitted through the line 37 before reaching the controller 16 via line 40. [ To the preamplifier 33, and to the amplifier 26 via line 39. The electrical signal is then sampled and recorded. A digital oscilloscope is also used to digitize the received signal, where each digitized sample contains a "record ".

The frequency of the sound waves generated by the transducer 24 is selected to detect the presence of small bubbles, cording, or other significant defects. However, the signal frequency should be low enough to allow other losses in the acoustic path. Preferably, the frequency of the sound waves is about 100 kHz to 300 kHz.

The measured transit time of the sound waves in the glass melt corresponds to the temperature of the glass melt. The relationship between travel time and temperature within a certain temperature range was measured to be substantially proportional. Thus, the travel time of the acoustic pulse between the waveguide assemblies can be measured and used to calculate the temperature of the melt. That is, the travel time for the temperature range can be measured. For example, Figure 4 shows the relationship between the temperature for the experimental arrangement with the path through the glass melt of the container portion of the container and the receiving portion of the container is only 55 mm and the travel time of the vertical axis. The figure shows a substantially linear relationship between temperature and time at about 1400 ° C to 1550 ° C. The change in the orientation of the graph between about 1550 ° C and 1575 ° C is believed to be due to the softening of platinum at these high temperatures. By addressing these issues, such as alternative materials, the temperature range can be extended to about 1600 ° C, and with a longer path, it is possible to achieve an accuracy within two degrees.

To detect the presence of bubbles, digitized data for transmission or movement time as a function of pulse time and rating (voltage) of the received signal was examined from the actual time in the storage device or oscilloscope.

Thus, it is possible to determine the acoustic signal transfer time through the glass melt to determine the temperature of the melt as well as the bubble detection in the glass melt. Preferably, the apparatus and method of the present invention can be used in a glass manufacturing system, such as a glass sheet forming system.

Referring to FIG. 5, there is shown a schematic diagram of an exemplary glass manufacturing system 42 in accordance with an embodiment of the present invention. For example, a fusion process is described in U.S. Patent No. 3,338,696 (Dockerty). The glass manufacturing system 42 includes a melting furnace 44 (melting vessel 44), which is melted to form the molten glass 48 after the feedstock feed material is injected, as indicated by arrow 46. The glass manufacturing system 42 also further comprises components made from platinum-containing metals, such as platinum or platinum-rhodium, platinum-iridium, and combinations thereof. However, it may also contain refractory metals such as molybdenum, palladium, rhenium, tantalum, titanium, tungsten or their alloys. The platinum-containing component may be introduced into the reaction vessel through a purification vessel 50 (e.g., a microtubule 50), a fusing unit to a microcoupling tube 52, a mixing vessel 54 (e.g., a stirring chamber 54) (E.g., a bowl 58), a stirring chamber to bowl coupling tube 60, and a downcommer 62. As shown in FIG. The molten glass is supplied to the injection port 64 which is coupled with the molding container 66 (e.g., fusion pipe 66). The molten glass supplied through the injection port 64 to the molding container 66 is divided into two separate glass flows that converge on the outer surface of the molding container 66 and flow down the molding container 66. The two separate molten glass streams recombine at a line that is a convergent molding surface crossing to form a single glass sheet 68. Generally, the molding container 66 is made from a ceramic or glass-ceramic refractory material.

Because the outer surface of the separated glass stream flowing down the converging forming surface of the molding vessel 66 does not contact the forming surface, the combined glass sheet with its inherently outer surface is well suited for the manufacture of liquid crystal displays.

In accordance with embodiments of the present invention, the apparatus 8 can be coupled with one or more of the following components to detect the presence of a heterogeneous material in the molten glass: a molten glass-to-micro tube 52, 50), a pinar to agitation coupling tube (56) or a stirring chamber (54). If foreign matter is detected, remedial measures may be taken as known in the art to reduce such foreign matter. For example, bubbles can be reduced by varying the outside air pressure of the finer (e.g., by increasing the amount of hydrogen contained in the air). The cord can be relaxed by increasing the stirring speed in the stirring chamber. Of course, the method and apparatus of the present invention, as described herein, are not limited to those used in a fused glass manufacturing system, but may be applied to any glass forming operation using metal containers to process molten glass.

The present invention increases knowledge of glass melt properties and this knowledge enhances the quality of the final product by allowing the processing parameters to be applied accordingly.

1 is a schematic cross-sectional view of an apparatus for sensing and / or characterizing a glass melt in accordance with an embodiment of the present invention.
Fig. 2 is a cross-sectional enlarged view of the device of Fig. 1 coupling between walls of a vessel for containing a waveguide and a glass melt. Fig.
3 is a cross-sectional enlarged view of the apparatus of FIG. 1 of another method for coupling a wall of a vessel comprising a waveguide and a glass melt.
Figure 4 is a graph of the relationship between the temperature of the acoustic path and the travel time for acoustic signals at an experimental set-up.
Figure 5 is a schematic illustration of an exemplary glass manufacturing system using the sensing / characterizing device of Figure 1;
6 is a graph showing the relationship between the sample (record) over time and the detection of the converter output voltage.

Several waveguides, each containing a platinum-rhodium core rod of approximately 3 mm diameter each, were dried-in into a mullite cladding tube having an outer diameter of about 9.5 mm. The core rod was welded to the outer surface of platinum-rhodium, a cylindrical crucible having a diameter of approximately 55 mm, in a co-linear relationship with the longitudinal axes of the respective waveguides coinciding. The mullite cladding tube was shortened at each platinum alloy rod to allow the exposed end of the rod to pass vertically through the holes of the stainless steel tube. A commercially available 1 MHz 0.25 inch ultrasonic transducer (e.g., a transducer, a transducer) was lightly compressed against each platinum rod of each waveguide with an ultrasonic couplant located between the transducer and the rod. The stainless steel tube carries a cooled air stream at a temperature of about 50 ° C at the interface of the waveguide and the transducer.

Second, Al ₂ O ₃ The outer tubes are arranged centrally with respect to each waveguide and are attached to a crucible that limits the emission and receiving areas of the crucible. The final assembly was introduced into a tubular oven supplied either through an oven or into a waveguide extending over the oven.

The aluminoborosilicate glass was pre-melted in a separate vessel to ensure the removal of the bubbles already present in the glass. The glass was then transferred to the crucible. In one configuration, the transducer was operated at a maximum pulse width and was operated by a Metrotek MP 217 pulser which brakes the resistance and amplitude. The receiver signal detected by the acceptance transducer via the second waveguide is shipped via Bruel & Kaer 2637 with a 0.1-1.4 MHz filter in combination with a Bruel & Kaer 2638 conditioning amplifier fixed at a 0.05-2 MHz bandwidth 20 dB amplified sound. . The signals were digitized to 8 bit resolution by a LeCroy 9450 Digital Oscilloscope for a total of 2,500 samples recorded at a 10 MHz sample rate starting after an 80 microsecond trigger delay.

In the first part of the experiment with a clean acoustic path in the glass melt, the time domain response for about 250 records was recorded and averaged. A thin ceramic tube having an outer diameter of about 10 mm and an inner diameter of 6 mm was introduced into the melt in the crucible through the ceiling of the furnace. Compressed nitrogen passed slowly through the tube to produce a bubble near the bottom of the crucible that rises to the surface of the melt and slowly squeezed out into the glass melt. A bubble flask in a compressed nitrogen line was created that allows for a rough indication of each bubble momentarily. The temperature of the furnace was about 1570 ° C.

Without the bubble tube, the ultrasonic path was clean and the time domain response indicated by the oscilloscope was very stable. The bubble tube was subsequently introduced into the melt and at about record 250 the first bubble was produced at a very constant rate of one bubble per about 1-2 seconds. During the experiment, an oscilloscope screen was used to indicate the time response fluctuation.

As in the previous example, all records were filtered through a 200 kHz fourth order Butterworth filter, the envelope was calculated and the average envelope over the period before the initial agitation (in this case, the first 150 records) was subtracted. Between records 250 and 550, thirteen bubble paths were observed in the corresponding graph.

Figure 6 shows a simple output representing the sense converter output voltage as a function of the sample (record) over time for the previous experiment. The presence of the detected bubble can be clearly indicated as a consecutive 13 voltage spike starting from a weak record (sample) 250 corresponding to thirteen discharged bubbles. Strangely, a voltage reduction was predicted for the bubble display. It is contemplated that the voltage increase is the result of an incomplete arrangement of the transmit and receive transducer assemblies or a focusing effect of bubbles.

While the invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

10: container, 15: glass melt,
20a, 20b: waveguide, 21a, 21b: metal core,
22a, 22b: cladding, 32a, 32b: outer tube.

Claims (5)

A sound wave whose frequency is between 100 kHz and 300 kHz is passed through a first waveguide 20a including a metal core 21a and a nonmetal cladding 22a coupled to the outer surface 34 of the vessel 10, Lt; / RTI >
Cooling the exposed portion of the metal core (21a) to maintain a temperature below 50 占 폚 at the interface of the transducer (24a) and the first waveguide (20a);
Detecting reflected sound waves from the molten glass; And
And measuring the presence of heterogeneity in the glass melt in accordance with the detected reflected sound waves. ≪ Desc / Clms Page number 20 > The method of claim 1, wherein measuring the presence of a foreign substance in the glass melt measures at least one of transit time, amplitude, and frequency of reflected sound waves. The method of claim 1, wherein coupling the sound wave comprises coupling a first ultrasonic transducer (24a) to the outer surface (34) of the vessel (10) through the first waveguide (20a) ≪ / RTI > The method of claim 3, wherein the reflected sound wave is transmitted through a second waveguide (20b) including a metal core (21b) coupled to an outer surface (34) of the vessel (10) and a non- Converter (24b). ≪ Desc / Clms Page number 13 > A container (10) for containing a glass melt (15);
A metal core 21a coupled to the outer surface 34 of the vessel 10 and a first transducer 24b coupled to the first transducer 24a for acoustically coupling sound waves from the first transducer 24a to the glass melt 15. [ A first waveguide (20a) comprising a nonmetallic cladding (22a) shorter than the metal core (21a) to provide an exposed portion of the metal core (21a) adjacent to the metal core (21a);
A metal core coupled to the outer surface 34 of the vessel 10 to acoustically couple reflected sound waves corresponding to the heterogeneous material in the glass melt from the glass melt 15 to the second transducer 24b, A second waveguide (20b) comprising a nonmetallic cladding (22b) shorter than the metal core (21b) to provide an exposed portion of the metal core (21b) adjacent the second transducer (24b);
A first outer tube 32a disposed in the first waveguide 20a; And
And a second outer tube (32b) disposed in the second waveguide (20b)
A ring is formed between the first waveguide 20a and the first outer tube 32a and the end of the first outer tube 32a is coupled to the outer surface 34 of the vessel 10,
A ring is formed between the second waveguide 20b and the second outer tube 32b and the end of the second outer tube 32b is coupled to the outer surface 34 of the vessel 10 Wherein the glass melt comprises a glass melt.

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