Abstract:
A magnetic flow metering device and method is disclosed for the measurement of corrosive flow streams. The device utilizes a unibody construction wherein the flow conduit is constructed entirely from an insulative, non-conducting material without resorting to a metallic outer housing. The portions of the electrodes in contact with the flow stream are made of a suitable conductive polymer material, resistant to the corrosive media. The electrodes also feature shields that are molded into the electrode assembly to reduce background electrical noise. The invention also utilizes an electrical configuration that actively drives the electrode shield circuit (electrodes as well as cabling) to provide a more accurate measurement of the electromotive force.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the monitoring and measurement of fluid flow in a conduit and particularly to magnetic flow meters for use in the semiconductor industry.  
       BACKGROUND OF THE INVENTION  
       [0002]     The theory of operation of a magnetic flow meter (‘magflow meter’) is based on Faraday&#39;s law of induced voltage, wherein an electromotive force (EMF) is produced that is proportional to the velocity of a conducting medium that flows through a magnetic field. Typically, in the case of a magflow meter, the flowing medium is a conductive medium passed through a section of conduit that is subjected to a transverse magnetic flux. As the conductive fluid passes through the conduit, the resulting EMF is detected by electrodes that are mounted to the conduit walls and in physical contact with the medium. To prevent shorting of the EMF, the conduit walls are constructed of a non-conductive material.  
         [0003]     Magflow meters have found application in the process control industries (chemical, food-and-beverage, pulp and paper, water treatment) because they have low measurement error (0.2% of reading attainable) over a broad range (typically 30:1) and they require no moving parts (unlike turbine meters or paddlewheel technology) or flow restriction (unlike differential pressure meters) to operate. Magflow meters can also be configured to resist the deleterious effects of the harsh chemicals of the flow stream medium through proper selection of the materials for the electrodes and conduit walls.  
         [0004]     Previous designs have utilized a conduit fabricated from a non-conducting material to provide the desired electrical isolation. The electrodes are mounted so that the tips are flush with the interior wall. Other designs have implemented a conduit constructed of a non-conducting liner within a metallic outer housing, with the tips of the electrodes protruding into the flow stream.  
         [0005]     Various prior art designs possess certain disadvantages that prevent the realization of the full benefits of the magflow meter. Most magflow meters utilize a dielectric sleeve within a metal housing. Outfitting a metal housing with a dielectric liner is a costly process. Current magflow meters are complicated assemblies requiring hardware to be welded or otherwise attached to the metal housing for the mounting of the electrode and electromagnetic assemblies. Furthermore, these approaches typically teach the use of metallic electrodes, which are incompatible with applications and processes in other industries.  
       SUMMARY OF THE INVENTION  
       [0006]     Despite the technological advantages of magnetic flow meters, they have not typically been used or applied in the semiconductor industry. This is thought to be due to the ultra pure and highly corrosive nature of the liquids (acids and bases such as HCl and NH 4 OH) and gases commonly used in the industry. Purity must be maintained to sub-parts per billion (“PPB”) levels. To maintain this level of purity, materials in contact with the corrosive fluid must neither corrode nor produce any ionic contamination. Therefore, flow meters completely constructed of non-metallic and non-corroding wetted materials such as PTFE or PFA (or other polymers in the same family) are strongly preferred. This requirement has eliminated magnetic flowmeters from use because the electrode in contact with the liquid must be able to conduct the EMF signal and therefore are usually constructed of metals such as 316 stainless steel, hastelloy or platinum.  
         [0007]     In addition, magflow meters are typically large, bulky devices not conducive to the small size and flow rate requirements of the semiconductor industry. A primary driver of the size is the requirement in the process industry to fiction properly over a wide range of pressures and temperatures, necessitating the formation of the conduit from either an expensive material such as ceramic or a PTFE or PFA lined metallic pipe.  
         [0008]     The invention in the following example embodiments is a magnetic flow meter wherein the flow conduit is constructed entirely from an insulative, non-conducting material without a metallic outer housing. The non-conducting conduit has a flow cross-section that defines a wetted perimeter containing a fluid that flows along an axis normal to the flow cross-section. The conduit is fitted with a pair of magnetic poles that spans the flow cross-section in a diametrically opposed configuration. The magnetic poles define a first lateral axis that substantially intersects the flow axis. A pair of electrodes is also disposed on the sides of the conduit, defining a second lateral axis that intersects both the flow axis and the first lateral axis formed by the magnetic poles. The electrodes are made of a conductive polymer material that is resistant to the corrosive media of the flow stream. The electrodes penetrate the wetted perimeter of the conduit to make contact with the fluid flowing within.  
         [0009]     An advantage of the various embodiments of the invention is that the conduit is fabricated from a dielectric material, without incorporating a metallic outer housing, thus reducing the cost and complexity of lining the conduit. Furthermore, the conduit is fabricated to easily and directly accept the electrode and electromagnetic assemblies, further reducing cost and assembly complexity.  
         [0010]     Another advantage of the various embodiments of the invention is that the conductive polymer (or plastic) electrodes are resistant to chemical attack. As disclosed in U.S. Pat. No. 5,449,017, the conductive polymer (or plastic) electrodes may be constructed from a polymer material suitable for the particular medium under measurement, including but not limited to blends of PTFE or PFA. These electrodes also feature shields that are molded into the electrode assembly to reduce background electrical noise.  
         [0011]     An aspect of the invention addresses a problem of signal attenuation in electromagnetic flow meters. When an electrode contacts an electrolytic fluid (e.g. water or an acid or a base), the electrical connection between the fluid and the electrode is not a simple resistance. Rather, the fluid/electrode interface creates a complex impedance (an impedence having a reactive component) that is also a function of several physical properties or factors, such as the electrical conductivity of the fluid with which it is in contact, as well as the size and material of the electrode. With a standard metal electrode, this complex impedance forms a voltage divider with the capacitance of the electrical connection means that connects the electrode to the electronics. Therefore, an attenuated voltage is presented to the amplifier, the attenuation being a function of the capacitance of the connection means relative to the complex impedance of the fluid/electrode interface. The dominance of the voltage divider effect is especially prevalent in small magnetic flow meters, because the electrodes are compact and have a high complex impedance at the fluid/electrode interface.  
         [0012]     Another advantage of one of the various embodiments of the invention utilizes an electrode construction that electrically drives an electrode shield circuit (electrodes and cabling), thereby reducing the effective cable impedance relative to the complex impedance of the fluid/electrode interface to provide a more accurate measurement of the electromotive force.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a cut away view of a prior art magnetic flow meter;  
         [0015]      FIG. 2  is a cross-sectional view normal to the flow channel, with schematic of appurtenances, of the invention;  
         [0016]      FIG. 3A  is a cross-sectional view of the invention, normal to the flow channel;  
         [0017]      FIG. 3B  is an isometric projection of the invention;  
         [0018]      FIG. 3C  is a cross-sectional view of the invention, normal to the flow channel;  
         [0019]      FIG. 3D  is a cross-sectional view of the invention, normal to the flow channel;  
         [0020]      FIG. 4A  is a cross-sectional view of the invention, through the plane of the flow channel;  
         [0021]      FIG. 4B  is a cross-sectional view of the invention, through the plane of the flow channel;  
         [0022]      FIG. 5  is a schematic of a prior art electrode assembly;  
         [0023]      FIG. 5A  is a schematic of a prior art electrode assembly;  
         [0024]      FIG. 5B  is a schematic of a prior art electrode assembly; and  
         [0025]      FIG. 6  is a schematic of an electrode assembly according to the present invention. 
     
    
       [0026]     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The invention is generally directed to an apparatus and a system for measuring the flow velocity of corrosive chemical fluids in a semiconductor fabrication facility. While the invention is not necessarily limited to such an application, the invention will be better appreciated using a discussion of example embodiments in such a specific context.  
         [0028]     Magnetic flowmeters are used to measure the volumetric flow rate of electrically conductive liquids. They operate on Faraday&#39;s principle of induced voltage, expressed by
 
emf∝B·L·U
 
 where emf is a electromotive force (volts), B is a magnetic flux density (gauss), L is a spanwise length or thickness through the conductive liquid across which emf is generated (e.g., cm), and U is the local velocity of the conductive liquid being metered (e.g., cm/sec). 
 
         [0029]     Referring to  FIG. 1 , there is illustrated a prior art magnetic flow meter  1  that includes a housing  2  defining a conduit  3  having a central flow axis  4 , a wetted perimeter  5 , and containing a fluid flow  6  that flows substantially parallel to flow axis  4 . A pair of magnetic poles  7  is situated on the perimeter of housing  2  generating a magnetic field B there between. A pair of electrodes  8  is disposed on either side of housing  2  and penetrate wetted perimeter  5  so as to be in contact with fluid flow  6 . Electrodes  8  are connected to a read out device  9  for detection of an electromotive flux emf.  
         [0030]     Referring now to  FIG. 2 , an example embodiment of a magnetic flow meter  10  according to the invention is shown in cross-section. Meter  10  includes a unibody housing  20 , which is represented as a hollow cylinder that defines a conduit  30  and a wetted perimeter  37 , configured for containing a fluid flow  40 . Magnetic poles  50  are mounted on the top and bottom of housing  20 . Because housing  20  is constructed of a dielectric material, each magnetic pole  50  is mounted in a bottomed port  52  that is formed within the wall of housing  20 . Bottomed ports  52  are so-defined because they do not penetrate wetted perimeter  37 , but instead terminate within housing  20 , thus defining a bottom portion  57 . Bottomed ports  52  are aligned along a first lateral axis  95  that passes through flow axis  35 .  
         [0031]     The embodiment of  FIG. 2  also illustrates a pair of electrodes  70  as being located on the same plane as and about 90-degrees with respect to magnetic poles  50 . Each electrode  70  is mounted in a through-port  72  that penetrates housing  20  and wetted perimeter  37 , thus creating a fluid communication between each through-port  72  and conduit  30 . Electrodes  70  are aligned along a second lateral axis  105  that intersects both flow axis  35  and first lateral axis  95 . The 90-degree orientation, though preferred, is not necessary for the magnetic flow meter to be operative. The cross section of  FIG. 2  shows the relationship between a magnetic field  60  and an electromotive flux (“EMF”) field  90  that is sensed between electrodes  70 .  
         [0032]     Electrodes  70  are also connected to a read out device  80  that senses a voltage potential caused by EMF  90 . Read out device  80  may be configured to convert the voltage to engineering units (e.g., cm/sec.) before displaying.  
         [0033]     Referring to  FIG. 3A , another embodiment of the invention is shown in cross-section wherein magnetic flow meter  10  is formed from an insulative, non-contaminating, chemically inert material mass or body  110 . As used herein, the term “insulative” refers to a property of the material of mass or body  110  that is both electrically non-conducting and chemically resistant and inert to a corrosive chemical fluid flow  40 , thereby “isolating” fluid flow  40 . Conduit  30  is formed to flow through insulative mass  110 . A pair of electromagnetic coils  130  is housed within mass (or body)  110 . In this configuration, magnetic poles  50  are driven by electromagnetic coils  130 , which are connected by a magnetic return path  100 .  FIG. 3A  also shows electrodes  70  as being terminated with an electrode connector  120 . Electrode connectors  120  are each connected to instrumentation cable  125  that is subsequently routed to a read out device  80  (not shown).  
         [0034]      FIG. 3B  shows an isometric projection of another embodiment of the invention that is shrouded and protected from the environment. A flow passage  30  is formed through insulative mass (or body)  110  with the various components (magnetic poles  50 , electromagnetic coils  130 , shielded electrodes  70  and magnetic return path  100 ) contained within insulative mass  110 . By housing the components within insulative mass  110 , the components are protected from typical operations in the manufacturing environment, such as dust and dirt, maintenance wash downs and chemical spills. The cost and complexity of manufacturing is also significantly reduced.  
         [0035]     Referring to  FIG. 3C , another embodiment of the invention is shown that includes an inner portion  140  of a housing  20 C. Inner portion  140  is capped off with outer portion  150 A and  150 B. This arrangement allows the electromagnetic coils  130  to be mounted in housing  20 C and then capped off so as to be protected from the environment.  
         [0036]     A similar embodiment of the invention is shown in  FIG. 3D . This embodiment shows outer portions  150 A and  150 B configured in a clamshell arrangement. In this embodiment, the outer portions  150 A and  150 B combine to circumscribe inner portion  140 . Electrode connectors  120  are then connected to electrode  70  to form a hermetic seal that protects the interior components from the environment.  
         [0037]     Referring to  FIG. 4A , a depiction of a side view of the  FIG. 3C  embodiment is shown in cross-section. This figure shows flow conduit  30  passing through inner portion  140  along flow axis  35 . An end  75  of electrode  70  can also be seen on the wetted perimeter  37  of flow conduit  30 . Note that end  75  is in fluid contact with fluid flow  40 .  
         [0038]     Another related embodiment of the invention is illustrated in  FIG. 4B , which includes flow conduit  30  is formed into a convergent/divergent flow passage  160 . This geometry of conduit  30  acts to constrict fluid flow  40  as the fluid flow passes through magnetic field  60 , thereby increasing flow velocity U. Because the electromotive flux generated is proportional to U, convergent/divergent flow passage  160  acts to generate a greater electromotive flux  90 , thereby improving the signal-to-noise ratio detected by read out device  80 .  
         [0039]     Referring now to  FIGS. 5A and 5B , a prior art electrode assembly  165  is pictorially and schematically represented. Assembly  165  includes a central conducting member  190  connected to the non-inverting input  253  of an amplifier  250  via an electrical connecting means  215 . The inverting input  257  of amplifier  250  is connected to an electrical ground  230 . Central conducting member  190  passes through housing  2  and a dielectric liner  193  and is electrically isolated from housing  2  by means of an insulative sleeve  191 . Central conductor  190  contacts an electrolytic fluid  195  (e.g. water or an acid or a base) at a fluid/conductor interface  198 . A complex impedance  200  having an active component  201  and a reactive component  202  develops between fluid  195  and central conductor  190 . Complex impedance  200  forms a voltage divider with the impedance of electrical connection means  215 . Therefore, an incorrect or attenuated voltage is presented to amplifier  250 . This “voltage divider effect” is a function of a parasitic capacitance  260  of the connection means  215  (represented in  FIG. 6  by a capacitor in phantom) and the complex impedance  200 . The dominance of the voltage divider effect is especially prevalent in compact magnetic flow meters with metallic probes, because the electrodes are small and therefore complex impedance  200  at interface  198  is high. Also, impedance  200  is a function of several physical properties, including the electrical conductivity of fluid  195 , the size of central conductor  190 , and the material of central conductor  190 . Because the conductivity of fluid is susceptible to change unrelated to the flow rate, the attenuation can be dynamic.  
         [0040]     An embodiment of an electrode assembly  167  according to the invention is shown in  FIG. 6 . Assembly  165  includes an electrically shielded electrode  170 . Electrode  170  has a longitudinal axis  180  along which central conducting member  190  is located. Central conductor  190  is in a concentric arrangement with an annular conducting member  225 . Both the central conducting member and the annular conducting member are connected to a signal amplifier  220  via electrical connecting means  215  ( 215 A,  215 B). Annular conducting member  225  and central conducting member  190  are electrically isolated from each other by way of a first insulative member  210 . A shield member  240  surrounds and is concentric with annular conducting member  225 . Shield member  240  is connected to electrical ground  230 . The schematic of a signal amplifier  220  shows amplifier  250  with non-inverting input  257  connected to central conductor  190  and inverting input  253  connected to annular conducting member  225 . A jumper connection  270  connects the inverting input  253  to the op amp output  280 . The  FIG. 6  embodiment operates to drive inherent capacitance  260  to a low impedance, thereby reducing the dynamic error caused by the interaction between the complex impedance  200 .  
         [0041]     Conducting members  190  and  225  may be fabricated from a conductive plastic, as disclosed in U.S. Pat. Nos. 5,316,035 and 5,449,017, both of which are hereby incorporated by reference. Herein, the term “plastic” refers generally to polymers, fluoropolymers or other dielectric materials particularly suited to resist the deteriorating effects of a corrosive atmosphere environment both within and outside the magnetic flow meter. Examples of a “plastic” include, but are not limited to, polyvinylidine fluoride (PVDF), polyetheretherketone (PEEK), perfluoroalkoxy (PFA), polytetrafluoroethylene (PTFE) or other materials known to persons of skill in the art to be of suitable chemical resistance. Wherein this application reference is made to a “conductive plastic,” the plastic is filled with particles or fibers of a conductive material that are added integrally and distributed throughout the plastic. The conductive material thus impregnated may include, but is not limited to, carbon or iron or both. Such plastics may be used in the central and annular conductive plastic sensing elements  190  and  225 , and shield member  240 .  
         [0042]     While the particular magnetic flow meter embodiments presented and discussed in detail above are fully capable of obtaining the objects and providing the advantages stated, it is to be understood that they are merely illustrative of the present invention. Various other modifications and changes with which the invention can be practiced and which are within the scope of the description provided herein will be readily apparent to those of ordinary skill in the art.