Refractory material sensor for determining level of molten metal and slag and method of using

An apparatus and method for indexing the slag-metal interface level in a metallurgical container uses an insulating refractory brick having a plurality of embedded conductor wires. Distal ends of the conductor wires are exposed at know, fixed distances from the distal end of the stick and flush with the brick face. The proximal ends of the conductor wires protrude a predetermined distance from the proximal end of the brick. The brick is attached to the inner lining of the container at a known distance from the floor of the container. The proximal wire ends are joined by a suitable connector to a signal-transmission cable containing a matching number of individual conductors, which is connected to a multi-channel voltmeter. Output from the multi-channel voltmeter can be input to a multiple-channel PC-based signal interpretation instrument. During operation the voltage output of each sensor circuit is continuously monitored. A multiple-channel impedance-measuring device is also incorporated into each sensor circuit to continuously monitor individual circuit impedance without disturbing the internally generated DC voltage signal, ensuring that the value of the displayed voltage accurately reflects sensor output and not some unrelated property of the electromagnetic environment. If the circuit impedance falls outside predetermined limits, the system is disabled and cannot be reset, providing an indication of the need to take alternative action.

FIELD OF THE INVENTION
 This invention relates to an apparatus and method for measuring the level
 of molten steel in a steel containment vessel. More particularly, the
 invention relates to the fabrication and use of a non-conductive
 refractory brick attached to the side wall of a steel containment vessel a
 predetermined distance from the floor of the vessel, the brick having a
 plurality of embedded conductors connected at one end to a
 multiple-channel voltmeter for determining the location of the interface
 between molten steel and slag, and slag and air, relative to the top or
 bottom of the vessel.
 BACKGROUND OF THE INVENTION
 In a steelmaking and casting operation, batches of partially refined molten
 steel are tapped from a basic-oxygen or electric-arc furnace into a
 refractory-lined ladle. Final refining to the specified chemical
 composition is performed in the ladle which is then drained into a
 bathtub-type vessel, also refractory lined, called a tundish, that
 simultaneously drains into water-cooled copper molds where the steel
 solidifies into a specific shape such as slab, bloom or billet.
 The initial transfer from furnace to ladle is executed by tilting the
 furnace and draining the liquid contents through an opening in the furnace
 shell known as a taphole. After refining, and with the ladle upright, the
 transfer of liquid steel to the tundish is controlled by a slide-gate
 valve attached to a refractory nozzle in the ladle bottom. Likewise the
 draining rate from the tundish is controlled by one or more
 slide-gate/nozzle combinations in the tundish floor, or by a vertically
 movable refractory plug over the nozzle known as a stopper.
 The cast steel is pulled continuously into a cooling bed by pinch rolls
 underneath the mold. While in motion, the hot slabs, blooms or billets
 issuing from the casting machine are cut to length prior to further
 rolling.
 Typically, a string of ladles of refined steel is drained sequentially into
 the same tundish before changing tundishes by an operation known as a
 tundish "fly."
 An inevitable consequence of the furnace-ladle-tundish-mold transfer
 operations is the presence of a slag layer over the molten steel. In the
 steelmaking furnace, a significant amount of "oxidized" slag is generated
 that is detrimental to final refining of the steel to the targeted
 composition. Thus, in the transfer of steel from furnace to ladle through
 the taphole, it is desirable to prevent significant carryover of furnace
 slag. In ladle refining, the objective is to form a "reducing" slag that
 is prepared by deliberate addition of appropriate fluxing agents. Although
 this reducing slag is not deleterious to the refined steel from the
 standpoint of chemical reactivity, carryover into the tundish in the form
 of entrained droplets, if not completely de-entrained before the steel
 solidifies, compromises the surface and internal quality of the cast
 product. Furthermore, some slag is eventually generated in the tundish
 itself by melting of; (1) "free-opener" sand, and; (2) insulating powder
 added to the tundish to form a protective blanket over the liquid surface.
 If a significant amount of liquid slag inadvertently reaches the mold, the
 rate of heat extraction by the mold is diminished, creating an opportunity
 for the liquid core in the solidified steel shell to break out, a highly
 unwelcome event that disrupts operations, causes equipment damage and
 carries the risk of a life-threatening explosion.
 In the initial transfer of liquid steel from a furnace to a ladle,
 technology has been developed to limit the amount of furnace slag carried
 over into the ladle. For example, in an electric-arc furnace equipped with
 an eccentric-bottom-tapping (EBT) system, virtually slagfree tapping is
 achieved by the geometric configuration of the taphole relative to the
 furnace hearth and by melting surplus steel scrap that is retained in the
 furnace as a liquid reserve, known as a heel, after filling the ladle to
 the desired weight. However, in the event the scrap charge is "short,"
 there is a risk of significant carryover of oxidized slag into the ladle.
 This slag must be removed by skimming, an operation affecting overall
 productivity, yield, and electrical energy consumption. Thus, in order to
 maximize the benefit of the EBT configuration, a slag-detecting system is
 required that shuts off the liquid flow automatically before the amount of
 slag in the ladle becomes significant. Effective slag-detecting devices
 installed near the taphole are available, but no art has been developed
 for measuring the amount of slag retained in the furnace. Knowledge of the
 amount of slag retained in the furnace has significant value, particularly
 in the production of low-phosphorus steel.
 Effective slag-detecting devices have also been developed that limit the
 amount of ladle slag carried over into the tundish. However, such devices
 rarely achieve an operational availability of 100 percent. If, for
 example, the slag alarm fails in only one of ten ladles drained into a
 particular tundish, the amount of slag present before the tundish is
 removed from service is subject to a serious degree of uncertainty,
 undermining the effectiveness of tundish weight measurements (determined
 by load cells) to gauge the depth of the steel bath. To be certain that
 slag does not reach the mold when an unknown amount is present in the
 tundish, the tundish is shut off early, incurring a yield penalty in the
 form of a larger-than-necessary tundish "skull." In addition, the
 lining-wear profiles of individual tundishes vary over time, amplifying
 the lack of precision in the relationship between tundish weight and steel
 bath depth. Furthermore, as mentioned above, casting operations are
 susceptible to adverse events if the steel bath depth happens to stray
 outside safe limits.
 Various systems have been developed for measuring the liquid level in
 remote storage vessels such as water tanks. Some examples are shown in
 U.S. Pat. No. 3,461,722 to Martens and U.S. Pat. No. 4,903,530 to Hull.
 One method is based on a change in the magnitude of an electrical current
 flowing in a circuit when an insulated electrode, placed at a known
 elevation inside the tank, makes or breaks contact with the liquid
 surface. The electrical circuit requires a power source, which typically
 supplies a constant DC voltage, allowing circuit resistance to be measured
 directly. Since the electrical circuit is open when the electrode is not
 in contact with the liquid, the change in resistance between an open and
 closed condition is massive, typically several orders of magnitude.
 In baths of molten metal, particularly molten steel, measurement of liquid
 level is complicated by the presence of a supernatant slag layer of
 unknown thickness. U.S. Pat. No. 4,365,788 to Block; U.S. Pat. No.
 3,395,908 to Woodcock; U.S. Pat. No. 3,505,062 to Woodcock; U.S. Pat. No.
 4,413,810 to Tenberg; and U.S. Pat. No. 3,663,204 to Jungwirth teach that
 a change in the resistance of a sensing circuit can be utilized to detect
 the steel-slag interface level. However, the theoretical basis and
 application of such a resistance measuring device is suspect, for example,
 if the containment vessel for the liquid steel has an internal diameter at
 the slag-steel interface of 3 meters and contains an extraordinarily thick
 layer of molten slag of 0.5 meter, the resistance of such a layer, top to
 bottom, is approximately 0.00007 to 0.002 ohm, far below the threshold
 needed for reliable interpretation. As a practical matter, the minimum
 length of copper-conductor cable required to deliver the sensing circuit
 signal to a signal converter or control pulpit a safe distance away from
 the hot vessel, is approximately 20 to 30 meters. The resistance of a
 single strand of 16 gage copper wire, 20 meters long, is approximately 0.5
 ohm at ambient temperature. Despite molten slag having a specific
 resistivity about 7000 times greater than liquid steel, which in turn has
 a specific resistivity about 70 times higher than ambient copper, such
 differences are overwhelmed by the relatively large volume of, and short
 conducting distances in, the liquid phases such that these methods are not
 easily and economically usable.
 U.S. Pat. No. 4,365,788 to Block also teaches that combination electrodes
 embedded in the wall of a metallurgical vessel can be used to measure
 variables such as lining wear, liquid level, steel-slag interface level
 and temperature, based on changes in resistance of a circuit with an
 applied power source. However, Block does not teach how a change in the
 single parameter of resistance can distinguish between multiple
 phenomenological causes. Furthermore, internally generated DC voltages at
 junctions between dissimilar conductors at high temperature, such as
 caused by Seebeck and double-layer effects, make circuit resistance
 changes difficult, if not impossible, to interpret.
 U.S. Pat. Nos. 4,037,761, 4,150,974, and 4,235,423, all to Kemlo, disclose
 interface level detection based on a change in voltage output from a
 single conducting probe. Three designs are disclosed, one for measurement
 of slag-layer thickness and interface level in a full ladle, and two for
 interface level detection during ladle draining. For the measurement in a
 full ladle, a moveable electrode is disclosed that is suspended above the
 ladle and that can be made to travel vertically through the slag layer by
 means of a winching device. The moveable electrode assembly comprises a
 conducting metal rod or tube connected by a non-conducting black-rubber,
 or ebonite, bushing to a second tube. A conductor wire from a suitable
 instrument passes through an opening in the upper tube through the bushing
 to the lower rod where it terminates.
 However, such a device cannot provide a reliable indication of slag-layer
 thickness because of the extreme hostility of the environment above a
 ladle of molten steel. It is well known that a piece of metal immersed in
 molten slag would immediately become coated with frozen slag that would
 then take some time to melt off. Once the electrode is in contact with
 molten metal the only way of knowing whether the tip barely protrudes
 through the slag layer or is several inches below it, is to withdraw the
 probe. Since a solid conductor in contact with liquid metal would start to
 dissolve and/or melt, capture of the exact elevation of the slag-metal
 interface is problematical. In addition, as a practical matter, the slag
 surface could be solidified into a hard crust that requires a rugged
 "push-pull" mechanism and a second, sacrificial probe to penetrate.
 For the interface level measurement during ladle emptying, one embodiment
 is a conducting probe embedded in a refractory sleeve of a stopper rod
 employed for draining the ladle. With the advent of effective slide-gate
 valves, the use of stopper rods to control steel flow from ladles has
 virtually disappeared. In an alternative embodiment a single electrode of
 steel, graphite or molybdenum is embedded in a lining brick located near
 the centerline of the ladle trunnions to detect when the steel-slag
 interface passes this elevation as the ladle is drained. However,
 regardless of the selection of electrode material, this design is
 susceptible to two performance-impairing phenomena. The first is caused by
 the electrode having a higher thermal conductivity than the refractory
 brick in which it is embedded, known as electrode "fogging," where a skin
 or "skull" of solidified metal forms over the electrode, effectively
 extending it downwards by an unknown distance. The second factor is
 electrode "blinding," that arises from the widespread practice of
 extending ladle lining life by a patching practice known as "gunning," in
 which a jet of refractory slurry is directed at high wear areas of the
 ladle lining. Here the objective of always maintaining contact between the
 electrode and molten steel is at loggerheads with the objective of maximum
 time interval between ladle relines.
 Some additional examples of interface level detection are disclosed in U.S.
 Pat. No. 4,345,746 to Schleimer, U.S. Pat. No. 5,827,474 to Usher et al.
 U.S. Pat. No. 5,375,816 to Ryan; U.S. Pat. Nos. 5,549,280 and 5,650,117 to
 Kings et al disclose methods for detecting the presence of a non-metallic
 liquid phase in a flowing steel stream and interpreting the properties of
 the sensing-circuit signal.
 Thus, there remains a need for an economical and practical device that
 determines the location of the molten metal-slag interface and slag-air
 interface in containment vessels that overcomes the limitations of the
 prior art and provides a reliable signal for halting a liquid transfer
 operation at the optimum moment in time. Furthermore, there is a need for
 a device that can determine the location of the molten metal-slag
 interface in a tundish so that inadvertent discharge of slag or overflow
 of steel can be prevented.
 SUMMARY OF THE INVENTION
 Accordingly, it is a primary object of the present invention to provide an
 apparatus and method for determining the location of the metal-slag
 interface in refractory-lined containment vessels employed in steelmaking
 and casting processes.
 Another object of the present invention is to provide an apparatus and
 method for making a molten metal-slag interface measuring device having a
 plurality of probes embedded in a refractory brick attached to the
 refractory lining of a tundish.
 Another object of the present invention is to provide an apparatus and
 method for measuring the voltage generated between an electrode and a
 molten metal bath in order to determine the location of the interface
 between the molten metal and the supernatant slag layer and for measuring
 the circuit impedance in order to determine the location of the interface
 between liquid metal or slag and air.
 Another object of the present invention is to provide an apparatus and
 method for measuring the depth of molten metal and slag in a vessel using
 a plurality of electrodes embedded in a brick made from refractory
 material and placed at a predetermined distance from the container bottom.
 In accordance with one aspect of the invention, a sensor for determining
 the level of molten metal and slag in a container having a refractory
 lining includes a brick having first and second electrically conductive
 wires embedded therein. The wires are placed at predetermined locations
 within the brick, with each of the wires having an exposed distal end and
 a proximal end that is preferably exposed. A measuring device, for example
 a voltmeter, is electrically connected to the proximal wire ends for
 measuring the voltage generated at the interface between the distal wire
 ends and the molten metal, or between the distal wire ends and the slag.
 Internally generated DC voltages between dissimilar conductors in contact
 with each other at high temperatures are caused by various effects,
 including Seebeck and double layer effects. The Seebeck effect refers to
 an electromotive force (emf) or voltage generated at the point of contact
 between different electronic conductors, the magnitude of which is
 temperature dependent. The Seebeck effect is most commonly utilized in
 thermocouples to measure temperature. The double-layer effect is an emf
 between the bulk of an ionically conducting phase such as slag, and a
 boundary layer of slag in contact with an electronic conductor such as
 metal or graphite.
 In accordance with another aspect of the invention, the brick could be in
 the form of a special shape of refractory commonly known as "furniture".
 In accordance with another aspect of the invention, the brick is fabricated
 from a material similar in composition to the container refractory lining.
 This material can be approximately 40 to 95 percent alumina, or
 approximately 80 to 100 percent magnesia.
 In accordance with another aspect of the invention, the wires embedded in
 the brick are an oxidation resistant metal or oxidation resistant metal
 alloy.
 In accordance with another aspect of the invention, a second conductive
 material covers the exposed distal ends of the embedded wires. The second
 conductive material can be zirconia, thoria, alumina-graphite,
 zirconia-graphite, magnesia-graphite, a closed-one-end zirconia tube or a
 closed-one-end thoria tube. The alumina-graphite, zirconia-graphite and
 magnesia-graphite conductive materials have compositions within the range
 of approximately 40 to 80 percent of alumina, zirconia or magnesia,
 respectively. Additionally, each of the alumina-graphite,
 zirconia-graphite and magnesia-graphite materials contain approximately 10
 to 40 percent carbon, of which approximately 30 to 100 percent is in the
 form of graphite. Thus, the alumina-graphite, zirconia-graphite and
 magnesia-graphite conductive materials contain approximately 3 to 40
 percent graphite.
 In this manner, a brick having at least two embedded wires, or electrodes,
 at known intervals is attached to the wall of a container. The proximal
 ends of the electrodes are connected to a multi-channel voltmeter; each
 electrode forming an individual circuit insulated from other electrode
 circuits. When molten metal is poured into the container, a voltage is
 generated between the exposed distal end of the electrode and the molten
 metal that can be measured by a circuit through ground. The magnitude of
 the voltage is a function of the electrical properties of the liquid phase
 with which it is in contact. Thus, when the electrode is in electrical
 contact with molten metal, an electronic conductor, a first voltage is
 produced. When the electrode is in electrical contact with slag, an ionic
 conductor, a different voltage is produced. As the molten bath rises or
 falls, a change in voltage is observed as each electrode contacts a
 different phase. Since the physical location of each electrode is known,
 the level of each liquid phase can be readily tracked.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 A seen in FIG. 1, a level detector 10 for determining the depth of molten
 steel 12 and slag 14 in a tundish 16 in a continuous casting facility 18
 for molten steel is illustrated. The continuous casting facility 18 will
 be described first in order to allow a clearer understanding of the nature
 of the present invention.
 A ladle container 20 is disposed over a tundish 16. The ladle 20 contains a
 quantity of molten steel 22 covered by a layer of slag 24, that thermally
 insulates the molten steel 22 and isolates it from ambient oxygen. Between
 the upper surface of the molten steel 22 and the bottom surface of the
 slag 24 is a molten metal-slag interface 26. Similarly, an air-slag
 interface 28 is present at the top surface of the slag 24 that is likely
 to be solidified into a "crust." At the bottom of the ladle 20 is an
 outlet nozzle 30 connected to a slide-gate valve (not shown), that is
 connected to a tube or shroud 32, that allows molten steel 22 to flow into
 the tundish 16, without exposure to ambient oxygen.
 When the molten steel 22 is released from the ladle 20 into the tundish 16,
 some slag 24 is also released before the ladle slide gate is closed. Over
 a period of time, a new layer of slag 14 is formed in the tundish 16 over
 the steel bath 12. Similarly, a molten steel-slag interface 34 and
 air-slag interface 36 are also formed in the tundish 16.
 In the bottom of the tundish 16 is a nozzle 38, allowing the molten steel
 12 to flow from the tundish 16 through a tube 40 into a water-cooled
 copper mold 42. The steel flow is controlled by a second slide gate
 attached to the underside of the tundish or, alternatively, by a
 refractory rod suspended in the liquid bath.
 The tundish 16 is typically a trough or wedge-shaped container with sloping
 walls 44, having a capacity of about 25 tons to about 60 tons of molten
 steel. Objects, known as tundish furniture (not shown), may be placed in
 the tundish 16 for flow distribution control. The depth of the
 molten-metal bath during a casting operation may be between about two feet
 to about five feet above the floor 46 of the tundish 16. The tundish
 vessel 16 typically has an outer shell 48 of reinforced steel plate, a
 refractory safety lining 50 and a refractory "working" lining 52.
 For each operational cycle of the tundish 16, that is, prior to being
 prepared for receiving molten steel 12, the residue from the previous
 operational cycle is removed and the working lining 52 coated with a
 contact lining which may be sprayed onto or otherwise applied to the
 working lining.
 As shown in FIGS. 1-4, the level detector 10 in accordance with an
 embodiment of the present invention is a sensing device in the form of a
 refractory brick that can distinguish between contact with molten steel,
 molten slag or air, having embedded electrodes 54 placed at predetermined
 intervals. The level detector 10 is mounted on the working lining 52. The
 level detector 10 is a piece of electrically-insulating refractory brick
 56, with embedded metallic conductor wires, or electrodes 54, exposed at
 fixed distances from the distal end of the brick. In the preferred
 embodiment, when the brick 56 is attached to working lining 52, the brick
 56 projects into the interior of the tundish 16. The brick 56 is
 preferably positioned such that the top of the brick 56 is in close
 proximity to the top of the tundish 16, and the bottom of the brick 56 is
 proximate to the bottom of the tundish 16. For example, the top of the
 brick 56 can be almost flush with the lip 17 of the tundish 16 (FIG. 1).
 The brick 56 is prepared by a conventional casting process in which a
 slurry of refractory particles is introduced into a mold of suitable
 dimensions in which the conductor wires 54 are held. The slurry is
 vibrated to achieve uniform density. The refractory material is an
 alumina-based material containing about 40-96% alumina, and preferably
 between 70-85% alumina. Alternatively, the refractory material can be
 80-100% magnesia. In the preferred embodiment, the dimensions of the brick
 56 are approximately two inches deep by three inches wide by twenty four
 to forty eight inches long. However, the brick can be fabricated into any
 shape and size that meets the needs of the liquid transfer operation.
 After the slurry has set, the "green" brick 56 is removed from the mold
 and excess wire snipped off. The green piece is hardened at a normal
 baking temperature of approximately 500 F to 600 F to remove most of the
 water of hydration. The remaining chemically-combined water is released
 during tundish preheat and during the initial tundish fill with molten
 steel. In the preferred embodiment, the rectangular brick 56 contains
 three embedded wires, or electrodes 54.
 The brick 10 (FIG. 2) is mortared onto the working lining 52 a fixed
 distance from the floor of the tundish 46 so that the distal wire ends 58
 are at known and fixed distances above the tundish floor 46 and from each
 other. A ground circuit measures the voltages generated by each electrode
 54 in contact with a liquid phase in the tundish 16. The wires 54 are
 preferably formed from an oxidation-resistant nickel-based alloy or
 molybdenum. This three-wire configuration has the capability of
 controlling bath level during casting as well as closely controlling the
 steel-bath depth at the time of termination of draindown. Referring to
 FIGS. 3-4, the castable refractory brick 10 contains embedded sensor wires
 54a, 54b and 54c. It will be understood that the distal wire ends 60, 62
 and 64 will melt and co-mingle with molten steel in the tundish 16, but
 the cylindrical volume originally occupied by solid wire in the refractory
 brick 10 remains filled with liquid metal. In the event of exposure of the
 wire ends to the atmosphere from a drop in bath level, any liquid "wire"
 is retained in the brick 10 by gravity. Oxidation of wire ends 58 while
 temporarily exposed to the atmosphere does not impair operability. Upon
 re-contact with molten steel, any iron, nickel or chromium oxide that may
 have formed on the wire ends 58 is reduced by the aluminum dissolved in
 the steel, and metal-to-metal contact is rapidly reestablished.
 Co-mingling of molten steel with melted wire also does not impair
 operability, although the magnitude of the Seebeck effect between molten
 steel and alloy wire may slowly change because of a possible change in the
 length of metal of a composition between the bulk steel and the original
 wire. Also note that the protrusion of the brick from the inside of the
 tundish vessel reduces the risk of skull formation over the wire ends 58.
 Effective electrical contact between the wire ends 60, 62 and 64 and the
 molten bath is maintained by the ferrostatic pressure of the molten bath
 on the refractory wall.
 The proximal wire ends 66, 68 and 70 are connected via a three-conductor
 signal-transmission cable 72 to an enclosure 76 containing a three-channel
 voltmeter, analog-to-digital converter cards, a flat panel industrial
 microcomputer or a microprocessor such as a programmable logic controller
 (PLC) and impedance-measuring cards. The panel monitor 78 displays "live"
 traces of voltage and impedance for each of the three sensor circuits.
 The impedance cards continuously monitor individual circuit impedances
 without disturbing the internally generated DC voltage signals, ensuring
 that the value of the displayed voltages accurately reflect sensor outputs
 and not some unrelated property of , the electromagnetic environment. If a
 circuit impedance falls outside predetermined limits, a disabled signal is
 displayed that cannot be reset, providing a timely warning to take
 alternative action.
 The instrumentation "package" depends only on changes in the internally
 generated circuit voltages and is independent of their polarity. The
 algebraic sum of all Seebeck and "double-layer" effects in each circuit is
 believed to determine the magnitude of the measured DC voltage.
 FIG. 5 shows a voltage and impedance trace from a single electrode 54. The
 voltage trace 80 shows that when the electrode 54 is in contact with
 molten steel 12, the measured voltage is approximately 20 millivolts and
 the circuit impedance 82 is approximately 20% of scale, or about 2.5 ohms,
 i.e. essentially shorted. As the tundish 16 drains and the molten
 steel-slag interface 34 passes the electrode 54, the voltage signal 80
 suddenly deflects to negative 150 millivolts without a measurable change
 in circuit impedance, clearly demarcating the transition between the
 molten steel 12 and slag 14. As the steel-slag interface continues to
 drop, the circuit impedance rises to 100 percent of scale--an open circuit
 condition indicative of an electrode pulling clear of slag and in contact
 with air.
 With electrodes 54 positioned at predetermined distances, and the brick 56
 installed at a predetermined location on the tundish wall 44, the depth of
 the molten-steel bath 12 can be readily tracked and the liquid transfer
 operation stopped at the optimum moment in time.
 Colored lamps 84 (FIG. 1) mounted on enclosure 76 can display the
 operational status of each individual sensor circuit, according to any
 desired convention. For example, white can mean unarmed; green, armed;
 red, level alarm; amber, disabled. The computer 76 can be programmed to
 set off an alarm when one of the electrodes 54 contacts a phase other than
 the predetermined default phase. In the preferred embodiment, during
 casting the default contact phase for the lowest sensor 60 is liquid
 steel, the default contact phase for the top sensor 64 is air, while the
 default contact phase for the middle sensor 62 is steel. If sensor 64
 comes in contact with molten steel or slag during casting, a visual or
 audible alarm is set off, indicating a high bath level. Likewise when
 sensor 62 comes in contact with air or slag, a low level alarm is
 activated. During draindown, sensor 60 is armed and immediately sets off
 an alarm when the steel-bath level falls below it, preparing the tundish
 16 for imminent shut off. With appropriate interfacing between the
 enclosure 76 and the caster control computer, the draindown operation can
 be stopped automatically.
 It will become apparent to one skilled in the art that more than three
 electrodes 54 can be placed in the brick 56. Additionally, the size of the
 brick 56 can be adjusted to fit the size of the tundish 16 and that brick
 56 could be attached to an article of tundish furniture such as a baffle
 for flow-distribution control or could be an integral part of the article
 of tundish furniture. Likewise, the brick 56 can be placed in the ladle 20
 to measure the level of molten steel 22 and slag 24. Brick 56 can also be
 used to measure the height of any suitable molten material compatible with
 the material used for the electrodes 54.
 With appropriate modifications, brick 56 with embedded electrodes 54 can be
 used to measure phase interface levels in foundry ladles, EBT electric arc
 furnaces, basic oxygen converters (BOF), argon-oxygen refining vessels
 (AOD), vacuum-oxygen refining vessels (VOD) and electric furnaces equipped
 with tapping valves.
 Additionally, the device 10 can be used in pressure casting, for example,
 of steel or aluminum wheels. With an effective level indicator 10 in a
 ladle holding vessel, a good decision can be made by the user on when to
 terminate casting, eliminating the risk of a partial cast or leaving
 excess liquid metal in the ladle.
 In another embodiment, shown in FIGS. 6 and 7, the distal wire ends 60, 62
 and 64 are in electrical contact with a second electrically conducting
 material 86 such as zirconia or thoria that can be closed-one-end tubes.
 The second conductors 86 are exposed to the molten steel bath 12 at the
 same predetermined distances as the distal wire ends 60, 62 and 64.
 In yet another embodiment the distal wire ends 60, 62 and 64 are in
 electrical contact with a second electrically conducting material such as
 alumina-graphite, magnesia-graphite or zirconia-graphite. The second
 conductors are exposed to the molten steel bath 12 at the same
 predetermined distances as the distal wire ends 60, 62 and 64.
 In a further embodiment, part of the brick 56 encasing the wires 54 is
 permeable and can be connected to a source of unreactive gas. For example,
 the gas can be one of argon, nitrogen and carbon dioxide. A flow of
 unreactive gas may help to maintain a clean surface on brick 56 and extend
 the number of phase changes detectable by wires 54.
 Although the present invention has been described with reference to
 preferred embodiments thereof, it will be understood that the invention is
 not limited to the details thereof. Various modifications and
 substitutions have been suggested in the foregoing description, and others
 will occur to those of ordinary skill in the art. All such substitutions
 are intended to be embraced within the scope of the invention as defined
 in the appended claims.