Abstract:
Liquid leaks from a vessel cause shorts between at least one elongate sensing wire and another conductor when the fluid absorbs into the porous sheath of the sensing wire. The other conductor may comprise a second elongate sensing wire having similar porous sheath or a conductive tray or other conductive collection means. The sensing wire is placed in proximity to the vessel, such as beneath or immediately adjacent. Shorts are detected from the electrical characteristics of a circuit including the sensing wire and location is determined therefrom.

Description:
TECHNICAL FIELD 
   The present invention is generally related to liquid leak detection. More particularly, the invention relates to robust detection and localization of liquid leaks. 
   BACKGROUND OF THE INVENTION 
   Many processes and applications rely heavily on the use of liquids. Liquids are typically stored in tanks, such as reserving tanks, and are transported from such reserving tanks to process stations where they are required by way of piping lines. 
   It is desirable in any storage, transportation or process use of fluids to be aware of any tank or plumbing fluid leaks or breaches. Even minimal fluid leaks can be detrimental, not only from a material loss standpoint, but also from the standpoint of environmental and safety considerations if such fluids happen to be hazardous materials. 
   A variety of fluid leak detection schemes are known. For example, so-called differential pressure techniques may be used to detect the existence of leaks in a tank or piping. However, such techniques generally fail to identify localization of a leak and may have difficulty if not total inability to detect the existence of very small leaks. Furthermore, such techniques may experience significant time lag between when a leak first occurs and when the leak is detected rendering such detection techniques undesirable in time critical leak detection applications. 
   Piping and tank leaks may also be detected by way of discrete, uninsulated electrode pair placements wherein a fluid leak that causes the fluid to bridge the electrodes is detected as a short across the electrodes or as a significant change in the resistance between the electrodes. The fact that such a technique uses exposed or uninsulated electrodes may be problematic in applications using flammable fluids as they represent a potential ignition source. 
   Certain other techniques have been proposed which utilize a coaxial conductor cable wherein the dielectric layer that is intermediate the solid central and braded exterior conductors is porous. Infiltration of the fluid from a leak to be detected into the porous intermediate layer causes a substantial change in the permitivity of the layer at the infiltrated location. Pulse reflection distortion techniques are then utilized to detect the presence and location of the leak. This proves to be a solution requiring expensive and sophisticated electronics for generating and interpreting signals. Furthermore, it is recognized that such techniques may be slow to detect leaks due to wicking effects of the braided outer layer which slows the infiltration of the leaked fluid through the porous intermediate layer and may distribute the liquid over a exceptionally long length of the cable. 
   Yet another option in fluid leak detection is disclosed in commonly assigned and co-pending U.S. patent application Ser. No. 09/881,389, Attorney Docket No. 67,200-438. In that application, a thermal sensing fluid leak detection scheme is described. Temperature changes in the detector due to fluid contact are detected in such a scheme. While the invention described therein is regarded as an improvement over the prior art, it may not meet all requirements of certain fluid leak detection applications. For example, a plurality of such individual thermal leak detection apparatus may be required to adequately canvas an area of interest for the purpose of leak detection. Even then, the granularity of localizing the source of such leaks may be greater than that desired in a particular application. 
   SUMMARY OF THE INVENTION 
   Therefore, it is one object of the present invention to detect the existence of fluid leaks. 
   It is a further object of the present invention to localize such detected fluid leaks. 
   It is a further object of the present invention to accomplish the detection and localization of fluid leaks quickly after such a fluid leak ensues without delay. 
   It is yet a further object of the present invention to provide localization of such fluid leaks in accordance with a continuous sensor element and effectively only limited by the resolution afforded by the sensing electronics. 
   It is yet a further object of the present invention to provide for such a fluid leak detection system that has an additional visual indication of the existence and localization of a fluid leak. 
   In accordance with these and other objects and advantages, the present invention comprises a pair of electrical conductors located in relative proximity to a fluid vessel, the pair of electrical conductors comprising at least one insulated conductor characterized by an electrically insulative, porous sheath. Circuitry coupled to said electrical conductors is effective to measure resistance of the combination of the pair of electrical conductors and an electrical short there between caused by a liquid leak, whereby the resistance indicates the existence of a leak and the relative location of the leak along said at least one insulated electrical conductor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1A  illustrates a pair of single element conductors arranged in parallel adjacency suitable for implementation of the present invention; 
       FIG. 1B  is a sectional view through a pair of electrically shorted single element conductors arranged in parallel adjacency suitable for implementation of the present invention; 
       FIG. 2  illustrates a dual element conductor suitable for implementation of the present invention; 
       FIG. 3A  is an exemplary utilization of a leak detection system for a liquid tank or conduit in accordance with the present invention; 
       FIG. 3B  is a sectional view taken as indicated through the leak detection system for a liquid tank or conduit of  FIG. 3A  in accordance with the present invention; 
       FIG. 4A  is an alternate exemplary utilization of a leak detection system for a liquid tank or conduit in accordance with the present invention; 
       FIG. 4B  is a sectional view taken as indicated through the leak detection system for a liquid tank or conduit of  FIG. 4A  in accordance with the present invention; 
       FIG. 5  is an alternate exemplary utilization of a leak detection system for a liquid vessel in accordance with the present invention; 
       FIG. 6  is a schematic exemplary utilization of a leak detection system for multiple liquid vessels in accordance with the present invention; 
       FIG. 7  is a schematic exemplary utilization of a leak detection system for a liquid injection manifold in accordance with the present invention; 
       FIG. 8A  is a schematic plan view of an alternate leak detection system in accordance with the present invention; 
       FIG. 8B  is a sectional view taken as indicated through the leak detection system of  FIG. 8A  in accordance with the present invention; and 
       FIG. 9  is a schematic diagram of a liquid leak detection system in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   When used herein, the term conduit shall be understood to mean any piping, plumbing, manifolding or the like generally used for the transportation, mixing or movement of liquid from one location to another. When used herein, the term tank shall be understood to mean any tank, container or the like generally used for long term or short term, open or closed, storage or containment of liquid. When used herein, the term vessel shall be understood to mean either a tank or conduit. 
   The present invention has particularly beneficial utility in application to leak detection in so called wet processing in the semiconductor industry. However, the invention is not so limited in application and while references may be made to such wet bench applications to illustrate the technology and benefits of such a system, the invention is more generally applicable to leak detection in a variety of industrial and product applications. 
   Advanced logic integrated circuits are fabricated using in excess of 300 fabrication steps. About 50 of those steps may involve some type of wet processing. Wet processing steps may generally be categorized into one of three areas; critical cleaning, critical etching and photoresist stripping. Conventional wet bench equipment may include fourteen or more storage tanks, a plurality of working tanks (e.g. reaction chambers, such as baths) wherein electronic component precursors (i.e. in process wafers) are exposed to various liquids, and a variety of plumbing conduits including valves, filters, recirculators and injection manifolds for the intermixing of a liquid carrier stream and various process liquids. Additionally, wet bench equipment may employ processors such as personal computers, programmable logic controllers (PLCs), or embedded processors. The processing system may also include one or more controllers. Suitable controllers for use in the present invention include for example the processors previously described. 
   Many of the process liquids are hazardous. Integrity of the fluid system is therefore a major consideration for the integrated circuit manufacturer. When leaks do occur, it is imperative that it be detected and that the source of the leak be located without delay. Process liquids are generally stored in tanks in a concentrated form and diluted to a usable concentration in an injection manifold. These process liquids include, without limitation, aqueous solutions of hydrochloric acid and buffers comprising the same, ammonium hydroxide and buffers comprising the same, hydrogen peroxide, sulfuric acid and buffers comprising the same, mixtures of sulfuric acid and ozone, hydrofluoric acid and buffers comprising the same, chromic acid and buffers comprising the same, phosphoric acid and buffers comprising the same, acetic acid and buffers comprising the same, nitric acid and buffers comprising the same, ammonium fluoride buffered hydrofluoric acid, solutions of sulfuric acid with ozone, sulfuric acid and ozone and/or hydrogen peroxide, inorganic acids such as sulfuric acid, nitric acid, chromic acid, and phosphoric acid, and hydrogen peroxide. Various drying fluids including alcohols such as methanol, ethanol, 1-propanol, isopropanol, n-butanol, secbutanol, tertbutanol, or tert-amyl alcohol, acetone, acetonitrile, hexafluoroacetone, nitromethane, acetic acid, propionic acid, ethylene glycol mono-methyl ether, difluoroethane, ethyl acetate, isopropyl acetate, 1,1,2-trichloro-1,2,2-trifluoroethane, 1,2-dichloroethane, trichloroethane, perfluoro-2-butyltetrahydrofuran, perfluoro-1,4-dimethylcyclohexane or combinations thereof may also be employed in a wet bench. No distinction is made herein between process liquids and drying liquids. 
   According to the present invention, detection of a liquid leak from a tank is exemplified in FIG.  5 . Tank  50  is illustrated having a substantially cylindrical shape though any regularly or irregularly shaped tank may be employed. Tank  50  contains a liquid which may be transported via conduit  51 . Beneath tank  50  in relative proximity thereto is drip tray  53 , the general purpose of which is to contain any leak from tank  50  above. Disposed along the bottom of drip tray  53  is sensing wire  55 . Sensing wire  55  may comprise a single conductor or double conductors. Double conductors may take the form of a pair of individually insulated conductors that are arranged adjacent one another, preferably in parallel adjacency, as illustrated in FIG.  1 A. The adjacency may be immediate or spaced. Double conductors may also take the form of integrally insulated parallel conductors as illustrated in FIG.  2 . Embodiments of the invention utilizing double conductors will be taken up immediately while discussion of single conductor embodiments will be taken up later in conjunction with  FIGS. 8A and 8B . 
   A section through a double conductor sensing wire  55  comprising a pair of individually insulated conductors is illustrated in FIG.  1 B and corresponds to the section line indicated in FIG.  5 .  FIG. 1B  is also useful in further understanding important aspects of the present invention. Features of the sensing wire  55  are exaggerated for clarity. Each conductor comprises an internal conductor wire  11  and an outer insulation or sheath  13 . Sheath  13  comprises an electrically insulative but porous material. For example, Teflon® is a preferred material. Though the section of  FIG. 1B  shows a relatively thick sheath, thinner sheaths have the characteristic advantage of quicker penetration of fluid. 
   As arranged in the embodiment of  FIG. 5 , the conductors lie against the bottom of drip tray  53 . As shown in  FIG. 5 , the sensing wire  55  is laid out in a sinuous or zigzag pattern for the purpose of substantially evenly distributing the double conductor sensing wire  55  over the majority of the bottom of drip tray  53 . The tighter the pattern the less granular the resolution of the leak location aspect of the invention. If a leak occurs, the fluid will eventually bridge a section of the sensing wire  55  between two adjacent individually insulated conductors. Such fluid bridging is labeled  17  in FIG.  1 B. The portions of the sheaths  13  appearing mottled or dotted represents absorbed fluid  17 . The bridged fluid  17  effectively provides an electrical current path at a point in the run of sensing wire  55 . The ionic nature of the fluid makes this current path an effective short between the conductors. The presence and location of such a fluid short along the sensing wire may be inferred by monitoring electrical parameters of the sensing wire as described later. 
   Additionally, the sensing wire  55  may be treated chemically to provide a visual indication of its contact with a fluid. For example, the sheath may be coated with copper sulfate (CuSO 4 ) or copper sulfate may be intermixed with the sheath material prior to overmolding on the conductor  11 . A mixture of 5% to 40% of copper sulfate to Teflon® has proven to be an acceptable formulation. Copper sulfate in the presence of water will crystallize and turn blue. Since most of the fluids in a wet bench are water diluted, contact of such fluids with the sensing wire  55  so treated will provide a visual indicator of the location of the leak. An additional benefit of such treatment for visually indicating a leak is that commonly used deionized water has significantly higher resistivity than do the other process fluids, and a leak of the deionized water may be difficult to detect electrically. 
     FIG. 5  also illustrates drip tray  53  having a slight pitch  57  from horizontal. Such a pitch may reduce time to detection of smaller or slower leaks by allowing the fluid to trickle toward a section of the sensing wire as opposed to pooling and accumulating for a period of time sufficient to passively reach the sensing wire. 
   The integrally insulated parallel conductors as illustrated in  FIG. 2  may be substituted for the pair of individually insulated conductors previously described. Fluid absorption into the sheath thereof will operate in the same manner to bridge the conductors therein. 
   Semiconductor manufacturing typically employs a dozen or more tanks having a variety of process fluids per piece of wet bench equipment. The schematic of  FIG. 6  illustrates the applicability of the invention to leak detection of a plurality of tanks  65 . In that Figure, tanks  65  are above a single drip tray  67  configured similar to the description of drip tray  53  below a single tank in FIG.  5 . 
   Similarly, a trayed system has applicability to leak detection at an injection manifold  71  as exemplified schematically in FIG.  7 . Injection manifold  71  comprises a deionized water inlet  72  and a process or working fluid outlet  74 . There between is located a bank of mixing valves  76 - 79  which receive the deionized water from inlet  72  and process fluids from a corresponding plurality of process fluid inlets  73 . Drip tray  75  is equipped as generally described previously with a sensing wire for the detection of the presence and location of a fluid leak from the manifold  71 . 
     FIGS. 3A and 3B  illustrate another application of the present invention in detecting leaks that is particularly well suited to conduit runs but which may be applicable to a tank also. Conduit  30  has contained therein and flowing therethrough a fluid. The underside of the conduit, substantially at the lowest surface thereof, is sensing wire  31 . Preferably, sensing wire  31  comprises integrally insulated parallel conductors as illustrated in FIG.  2 . Sensing wire  31  is in surface contact with conduit  30  and may be held in place by any of a variety of means including adhesives, clips, or wire ties or wraps.  FIG. 3B  is a sectional view taken along the section line illustrated in FIG.  3 A. Surface tension of the fluid will, particularly in the event of a slow leak from conduit  30 , result in the fluid trickling along the profile of the conduit to the underside thereof whereat sensing wire  31  is strategically place to sense the leak as previously described. Underside application of sensing wire  31  to a tank is, as alluded to above, an additional application of this sensing technology. 
     FIGS. 4A and 4B  illustrate yet another application of the present invention in detecting leaks that is particularly well suited to conduit runs. In this instance, conduit  40  carrying a liquid is located above a trough  45  similar in function to a fluid containing drip tray as described earlier herein. Placed on the upper side of trough  45  is sensing wire  41 . Leaks from conduit  40  above trough  45  are likely to drip off of conduit  40  into trough  45  below and be sensed as previously described by sensing wire  41 .  FIGS. 4A and 4B  illustrate a single conduit  40 ; however, a plurality of adjacent or bundled conduits may similarly be placed above a trough in similar fashion. 
   A single conductor embodiment of the present invention is illustrated in  FIGS. 8A and 8B . In  FIG. 8A , a plan view of a drip tray  83  is shown. Sensing wire  85  is, similar to the embodiment described corresponding to  FIG. 5 , laid out in a sinuous or zigzag pattern for the purpose of substantially evenly distributing the single conductor sensing wire  85  over the majority of the bottom of drip tray  83 .  FIG. 8B  is a sectional illustration taken through the section line as shown in FIG.  8 A. Single conductor sensing wire  85  has sheath  86  comprising an electrically insulative but porous material. Drip tray  83  is electrically conductive and is preferably characterized by a resistivity that is low as compared to the resistivity of the sensing wire. Drip tray  83  is also shown to be electrically grounded. In this embodiment, drip tray  83  provides a portion of the electrical circuit needed to determine the presence and location of a fluid leak by the sensing wire  85 . A fluid leak bridging the sensing wire  85  and the drip tray  83  is labeled  87  in FIG.  8 B. The portion of the sheath  86  appearing mottled or dotted represents absorbed fluid  87 . The bridged fluid  87  effectively provides an electrical current path between the sensing wire  85  and drip tray  83  at a point in the run of sensing wire  85 . The ionic nature of the fluid makes this current path an effective short between the sensing wire  85  and the drip tray  83 . The presence and location of such a fluid short along the sensing wire may be inferred by monitoring electrical parameters of the sensing wire as described below. 
     FIG. 9  illustrates schematically the basic electrical aspects of the various embodiments of the invention as described above and as further described to follow. Sensing wire is labeled  93  in FIG.  9  and comprises first and second electrical conductors  93 A and  93 B. In the double conductor embodiments described, conductors  93 A and  93 B comprise a double conductor sensing wire. Conductors  93 A and  93 B in a double conductor embodiment are substantially equivalent lengths. Node  95  at the distal or remote end of the sensing wire  93  is schematically illustrated as a simple twisted connection but may be any electrical termination that short circuit couples the two conductors  93 A and  93 B such as soldered leads, terminal blocks, insulation displacement splice, etc. The proximal or local end of the sensing wire is coupled such as by a connector  91  to control and sense circuitry  90 . 
   In the case of a single conductor embodiment described, conductor  93 A comprises a single conductor sensing wire. Conductor  93 B comprises a conductive drip tray or other conductor which, in the event of a liquid leak, provides a low resistance ground terminal coupled to the distal end of the conductor  93 A such as by a grounding terminal or by otherwise sharing a common ground with the control and sense circuitry  90 . 
   Control and sense circuitry may comprise an ohmmeter which measures simple resistance. In a double conductor embodiment of the invention wherein the sensing wire has a length of substantially L, the resistance reading R would be substantially in accordance with the following formula.
 
 R=r*l/A   (1)
 
   This is the general formula for resistance in a conductor where r is the electrical resistivity of the sensing wire conductor material, A is the area of the cross section of the conductor and l is the length of the conductor. Receptivity r may vary significantly with the choice of conductor material, for example copper has a resistivity at 20 C of substantially 1.7×10 −8  ohm*m whereas nichrome has a resistivity at 20 C. of substantially 100×10 −8  ohm*m. The maximum resistance R expected in a double conductor sensing wire embodiment is equal to
 
r*2L/A   (2)
 
where no leaks bridge the conductors and, in accordance with the same relationship, the minimum resistance R expected in a double conductor sensing wire embodiment is substantially equal to zero where a leak bridges the most extreme proximal end of the sensing wire. The value of the sensing wire resistance R varies proportionally between zero and the maximum in accordance with the linear distance of a leak bridging the sensing wire as measured from the proximal end thereof. Of course a normal condition where the distal end of the sensing wire is short circuited as described will be a resistance R in accordance with the maximum expected as described above. A leak bridging the sensing wire at a distance of substantially one-half of the length L will result in a resistance reading of substantially one half of the maximum resistance. A leak bridging the sensing wire so described will similarly provide a resistance reading that is in direct proportion to the leak&#39;s location along the sensing wire.
 
   Alternatively, the node  95  may be eliminated and the pair of conductors of the sensing wire allowed to float. Liquid bridging the conductors at the distal end will result in a maximum resistance reading while liquid bridging the conductors at the proximal end will result in a minimum resistance reading of substantially zero. A leak bridging the sensing wire will similarly provide a resistance reading that is in direct proportion to the location along the sensing wire. 
   In a single conductor embodiment, the results are similarly obtained. The fact that the drip tray may be of dissimilar metal from that of the sensing wire can be made an insignificant distinction where the sensing wire is characterized by a substantially higher resistance than the drip tray contributes thereby rendering the drip tray resistance contribution negligible. Hence, the resistance measured will vary from a maximum to a minimum of substantially zero in direct proportion to the location of the bridging leak along the sensing wire. 
   It may be desirable to have a sensing wire to provide, for a given length of sensing wire, a relatively substantial resistance. Leak location resolution may be improved thereby and less sensitive circuitry may be employed. The profile or gauge of wire selected can provide some flexibility in this regard with smaller diameter wires providing a greater per unit resistance. Alternatively or in conjunction, the material selection plays an important role. As between the two examples of conductors given above, copper and nichrome, all else being equal nichrome will provide a resistance per unit length that is substantially 100/1.7 times greater than copper. 
   Additionally, a relatively high resistance wire in a single conductor embodiment wherein the grounded drip tray has significantly lower resistance provides another alternative. Similarly, a double conductor sensing wire may comprise dissimilar conductor material choices—one of a significantly higher resistivity than the other. 
   Finally, while the circuitry for reading the resistance has been described as an ohmmeter  90  which returns a reading that is related to the linear location of a detected leak along the sensing wire, alternative circuitry is also envisioned. For example, the ohmmeter readout may be directly translated to a location or region readout (not shown) for convenience of the operator or servicing technician. Personal computers  100 , programmable logic controllers (PLCs)  102 , or embedded processors  104  as commonly employed in wet bench apparatus may be employed to provide the function of an ohmmeter or equivalent. For example, a predetermined current provided to the sensing wire and a voltage sensing circuit provide voltage and current quantities in the sensing wire that may be used to calculate the resistance and hence the position of a liquid leak or more directly to determine the existence and location of a fluid leak directly from the sensed voltage and predetermined current. Alternatively, a predetermined voltage provided to the sensing wire and a current sensing circuit will also provide voltage and current quantities in the sensing wire that may be used to calculate the resistance and hence the position of a liquid leak or more directly to determine the existence and location of a fluid leak directly from the sensed current and predetermined voltage. 
   The invention has been described with respect to certain preferred embodiments intended to be taken by way of example and not by way of limitation. Certain alternative implementations and modifications may be apparent to one exercising ordinary skill in the art. Therefore, the scope of invention as disclosed herein is to be limited only with respect to the appended claims. 
   The invention in which an exclusive property or privilege is claimed are defined as follows.