Abstract:
A leak detector for detecting volumetric changes in a liquid volume includes a biased piston disposed in a housing and a liquid passage extending from the liquid volume to an expansible chamber defined by the housing and piston. The leak detector further includes a magnetostrictive sensor including a magnetostrictive waveguide, a magnet operably coupled to the piston and moveable therewith and pulsing and detection devices for detecting the position of the magnet along the magnetostrictive waveguide. A method of using the detector includes exposing the expansible chamber to liquid from the liquid volume and sensing the changes in the liquid volume magnetostrictively by causing relative movement between the waveguide and the magnet to obtain data representative of piston movement which is responsive to volumetric changes in the liquid volume.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional patent application Ser. No. 60/760,116 filed on Jan. 19, 2006, the disclosure of which is expressly incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to leak detection and has particular application to the detection of leakage from pressurized fuel delivery lines in dispensing operations such as gas stations. 
       BACKGROUND OF THE INVENTION 
       [0003]    Leakage into the environment of petroleum products, including gasoline, is damaging to surrounding soils and water. Once a leak is detected, clean up or remediation is costly and time consuming. 
         [0004]    In a dispensing operation such as a gas station, fuel is typically stored in underground storage tanks (“UST”) from where it is pumped through pipes to an above ground pump or dispensing unit for dispensing into vehicles. Leaks of fuel from the tank or from the interconnecting pipe lines to the dispensers can cause significant environmental damage. The United States Environmental Protection Agency has set certain standards for the detection and prevention of environmental leaks. It accordingly has been a goal of manufacturers of this equipment to detect leaks and to meet EPA standards in this regard. 
         [0005]    For example, as this application is filed, the EPA requires detection methods sufficient to detect volumetric leak rates of 0.1 gallons per hour. In addition, the EPA requires a detection rate of no less than at least 95% accuracy with false leakage alarms no more than 5%. 
         [0006]    A variety of devices operating on a variety of physical principles have been proposed to meet these standards and thereby warn of leaks and provide means for stopping the leaks as quickly as possible to reduce the impact on the surrounding environment. One such device and system is described in U.S. Pat. No. 5,091,716, which patent is expressly incorporated herein by reference as if fully expressly reproduced herein. 
         [0007]    In that device, a detector is spliced into a fuel delivery line. The detector has a port allowing fuel in the fuel delivery line to engage a piston and displace it against the bias of a constant rate spring. In more particular detail, the patent discloses a line leak detector having a liquid reservoir which is adapted to be mechanically coupled to the liquid fuel line. The liquid reservoir is preferably in the form of a cylinder. A piston is also included in the liquid reservoir with the piston being moveable in response to the volume of liquid in the reservoir. A spring is also connected to the piston to provide a restoring force to movement of the piston. The piston includes a core mechanically coupled thereto for movement in response to the movement of the piston. A coil surrounds the core so that the core is moveable in the coil in response to the liquid in the reservoir. Movement of the core changes the electrical inductance of the coil. Accordingly, the exact position of the piston in the reservoir may be monitored by measuring the inductance changes in the coil. According to the patent, measurement of the coil inductance provides a highly accurate measurement of the volume of liquid in the reservoir so that an accurate measurement of a line leak may thereby be provided. 
         [0008]    While such a device is accurate to a certain degree, certain features of the device limit the overall performance and the degree of accuracy which can be attained. For example, the device is spliced into a fuel line which is typically of a relatively larger cross-sectional flow area than the more restrictive cross-sectional flow area of the device. Since all the fuel must flow through the sensing device, the device thus constitutes a choke point in the fuel line, restricting the delivery of fuel through the line beyond the capacity of the larger line itself. 
         [0009]    Moreover, it will be appreciated that the displacement sensing operation of the device works on the principle of electronic induction, i.e. a coil surrounding a movable core which changes the inductance of the coil as the core moves with the piston. Such devices are capable of measuring changes in the linear position of the core over certain ranges of movement, but the data provided is limited in that such devices can only register certain finite changes of position over the design stroke of the piston. These measurable changes are relatively large, as compared to the following description of the invention. As such, this known device has certain accuracy limitations in terms of the preciseness of its ability to detect minute positional changes in piston displacement smaller than the capacity of the inductive system to measure. 
         [0010]    In addition, the collection of the limited data which is sensed in such devices requires certain minimum time periods. In other words, it takes a significant time period to collect sufficient data for analysis. The combination of a limited range of positional changes or data points with minimum times necessary for accumulation and data analysis restrict the sensitivity and timeliness of such prior devices, thus limiting the accuracy of the devices and their responsiveness in detecting leaks and providing leaking control. These limitations, together with others, result in a system which is better than having no detection, but which can still permit undetectable leakage or leakage over small periods of time which can accumulate to adversely impact the environment. 
         [0011]    It is also recognized that even though current EPA standards are restrictive, small leaks which are undetectable under current EPA detection performance standards can accumulate over time to constitute massive long term environmental damage. Finally, it is recognized that the frequency of catastrophic leaks has increased recently, even with the leak detection systems which comply with current EPA standards. 
         [0012]    Accordingly, it has been one objective of the invention to provide improved leak detection that exceeds current EPA standards. 
         [0013]    A further objective of the invention has been to provide improved leak detection apparatus and methods capable of measuring smaller leaks over shorter time periods than heretofore known, while at the same time avoiding flow rate restrictions in fuel delivery lines. 
       SUMMARY OF THE INVENTION 
       [0014]    To these ends, the invention contemplates leak detection apparatus and methods wherein volumetric fuel changes are measured across a large range of multiple positional related data points not heretofore available in fuel detection, and over a shorter time than heretofore available. A biased piston moves in a bore ported to a fuel delivery line without restriction of the cross-sectional flow area of that line. The piston is connected to a magnet moving over a magnetostrictive waveguide and piston movement is detected by a pulsed magnetostrictive apparatus providing thousands of “positional measuring points” over the entire range of piston movement, such as, for example, 10,000 detectable position points along two inches of piston travel. Pulsed clock time along the magnetostrictive waveguide is preferably on the order of 64 MHz or greater, and with a counter running asynchronously to the waveguide, a combination of measurements of displacement are produced, providing a large number of data points in a small time period and a sensing result more accurate by orders of magnitude than the inductive device of U.S. Pat. No. 5,091,716. 
         [0015]    In a preferred embodiment of the invention, at least 100 data points of piston movement can be quickly obtained to determine whether changes in fuel line volume indicate a leak or some other change not indicative of a leak. 
         [0016]    Also, the invention in various embodiments contemplate use of a biasing member to bias the piston against the anticipated ranges of volumetric changes for the fuel volumes to be tested. For example, the biasing member may be configured as a constant or variable rate spring, a weight, an electromagnet, a permanent magnet, a regulated fluid supply, or a sealed gas pocket. This combination of elements may eliminate artifact sensed volumetric changes resulting from the progressive compression and expansion of air within the liquid volume sensed as well as provide other advantages. 
         [0017]    Accordingly, the invention provides numerous and significant advantages over prior leak detection systems such as those of U.S. Pat. No. 5,091,716. More data points are gathered in significantly less time. Significantly smaller changes in volume are detected over significantly reduced time periods, thus minimizing absolute leak volumes. The impact of thermal variation caused changes is substantially reduced since thermal changes require time and the test window provided by the invention is significantly shorter than in prior systems, resulting in less artifact impact from thermal changes. Existence of air pockets in the fuel volume may be eliminated as a volume change factor resulting from use of a constant force or pressure biasing member. And significantly, fuel line flow rates are not adversely impacted upon installation of the invention since it is simply ported to the line and not inserted or spliced into the line as a choke point as was required in prior devices. 
         [0018]    As a result of these and other features of the invention, a larger number of data points gathered over a shorter period of time, as compared to past systems, can be analyzed by appropriate algorithms (not part of this invention) to provide a more realtime indication of smaller leaks which can be acted on to prevent costly adverse environmental impact, and without restricting delivery system flow. EPA standards or minimums are exceeded on the favorable side and the leak detection capabilities of the invention provide enhanced integrity of fuel delivery systems. 
         [0019]    While the invention finds primary application in petroleum product delivery and transport systems, these and other applications will be readily apparent to those of ordinary skill in the art from the following detailed description of preferred embodiments of the invention and from the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention. 
           [0021]      FIG. 1  is a diagrammatic illustration of an exemplary fuel dispensing system; 
           [0022]      FIG. 2  is a perspective disassembled view of an embodiment of a line leak detector in accordance with the invention; 
           [0023]      FIG. 3A  is a cross-sectional view of the line leak detector shown in  FIG. 2  after assembly; 
           [0024]      FIG. 3B  is another cross-sectional view of the line leak detector shown in  FIG. 2  after assembly; 
           [0025]      FIG. 4  is a cross-sectional view of an alternate embodiment of the line leak detector in accordance with the invention; 
           [0026]      FIG. 5  is a cross-sectional view of an alternate embodiment of the line leak detector in accordance with the invention; 
           [0027]      FIG. 6  is a cross-sectional view of an alternate embodiment of the line leak detector in accordance with the invention; 
           [0028]      FIG. 7  is a cross-sectional view of an alternate embodiment of the line leak detector in accordance with the invention; 
           [0029]      FIG. 8  is a cross-sectional view of an alternate embodiment of the line leak detector in accordance with the invention; and 
           [0030]      FIGS. 9A-9C  are a flowchart illustration of an exemplary leak detection system using embodiments of the leak detector of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    An exemplary fuel dispensing system of the invention is shown in  FIG. 1  and generally includes an underground storage tank (“UST”)  10  for storing a fuel, a submersible pump  12  located in the tank  10 , and a fluid conduit line  14  that transports the fuel under pressure to one or more dispensing units  15 , shown schematically in  FIG. 1 . Typically, the fluid conduit line  14  is coupled to the submersible pump  12  via a pump manifold  16  that is typically located external to tank  10 , such as in a covered manway. Pump manifold  16  includes a check valve  18  for preventing fuel from flowing back into tank  10 . Because check valve  18  prevents any fuel from flowing back into tank  10 , when the dispensing unit  15  is off, thus preventing fuel from flowing from conduit line  14 , the fluid conduit line  14  defines a closed system containing an amount or volume of fuel that depends on several factors including length of conduit line  14 , size of conduit line  14 , and other factors. As mentioned above, to meet EPA regulations, the integrity of the fluid conduit line  14  must be regularly tested and the amount of any fuel leakage therefrom monitored. 
         [0032]    To this end, the fuel dispensing system includes a volumetric leak detector, generally shown at  20 , for determining the volume of fuel leakage, if any, from conduit line  14 .  FIGS. 2 ,  3 A and  3 B illustrate an exemplary embodiment of the leak detector  20 . Leak detector  20  includes a generally cylindrical body or housing  30  having a proximal end portion  32 , a distal end portion  34  and a generally cylindrical interior bore  36 . The distal end portion  34  is adapted to be coupled to a port  22  in manifold  16  and may, for example, include a set of external threads that cooperate with a corresponding set of internal threads in port  22  to threadably couple leak detector  20  with pump manifold  16 . The invention is not limited to the threaded connection described herein, however, as those of ordinary skill in the art will recognize other ways to couple leak detector  20  with pump manifold  16 . Those of ordinary skill in the art will further recognize that the leak detector  20  is not limited to being coupled to pump manifold  16 , but may be positioned at any point along the fluid conduit line  14  between the check valve  18  and the dispensing unit  15 . The distal end portion  34  of leak detector  20  also includes at least one (two shown) fluid channel  38  having one end open at an end face  40  of distal end portion  34  and the other end open to the interior bore  36 . The fluid channel  38  provides fluid communication between the interior bore  36  and fluid conduit line  14  so that when leak detector  20  is coupled to pump manifold  16 , fluid may flow into interior bore  36  via fluid channel  38 . 
         [0033]    In one aspect of the invention, leak detector  20  is not spliced or inserted into conduit line  14  in a manner that restricts flow through conduit line  14 , as with prior leak detection systems, but may be ported to a pump manifold  16  without occluding, choking or otherwise creating a flow restriction within conduit line  14 . To this end, the distal end portion  34  of the leak detector  20  does not extend beyond port  22  and into the conduit line  14 , but instead ends prior to, or at most ends with, the end of port  22  in pump manifold  16 , as shown schematically in  FIG. 1 . Thus, the cross-sectional flow area of conduit line  14  is not restricted by the presence of leak detector  20  and provides a near zero resistance to flow within fluid conduit line  14 . In this way, the fuel dispensing system has the capacity to test for a leak in fluid conduit line  14  without also creating a flow restriction that limits the rate at which fuel may be dispensed at the dispensing units  15 . 
         [0034]    An expansible chamber  41  ( FIG. 3B ) is defined by a piston  42  positioned in the cylindrical interior bore  36 . Piston  42  is adapted to be movable within bore  36  between high and low positions adjacent proximal and distal end portions  32 ,  34 , respectively. Piston  42  includes a first seal  44 , such as one or more O-rings, along the periphery of piston  42  so as to create a seal between the piston  42  and the wall of interior bore  36  and therefore prevent or otherwise minimize any fluid leakage around piston  42 . Piston  42  further includes a central passageway  48  adapted to receive a hollow shaft  50  that extends along a central axis  52  of interior bore  36  from the proximal end portion  32  to distal end portion  34 . Distal end portion  34  includes a blind bore  54  adapted to receive hollow shaft  50  and rigidly affix shaft  50  to cylindrical body  30 . For example, hollow shaft  50  may include a set of external threads that cooperate with a corresponding set of internal threads in blind bore  54  to secure hollow shaft  50  to body  30 . The invention, however, is not so limited as those of ordinary skill in the art will recognize other ways to couple the hollow shaft  50  to cylindrical body  30 . As bore  54  is a blind bore, no fluid may escape leak detector  20  through blind bore  54 . Alternately, the end of hollow shaft  50  inserted into bore  54  may include a seal such as a suitably sized O-ring. Additionally, piston  42  further includes a second seal  56 , such as one or more O-rings, along central passageway  48  to create a seal between the piston  42  and the hollow shaft  50 . In this way, and as shown in  FIG. 3B , pressurized fluid that enters the expansible chamber  41  via conduit line  14  and flow channel  38  contacts a lower face of piston  42  and moves piston  42  along central axis  52  but is bounded by piston  42  without a loss of fluid around piston  42 . 
         [0035]    The proximal end portion  32  of leak detector  20  includes a cap  58  that closes off interior bore  36  and includes a central aperture  60  adapted to receive an adaptor  61  for securing hollow shaft  50  at the proximal end portion  32 . Cap  58  may include a set of threads for threadably engaging cap  58  with cylindrical body  30  and a seal for sealing with body  30 . Leak detector  20  may further include a vent port  62  adjacent proximal end portion  32 . Vent port  62  is in fluid communication with interior bore  36  at one end and in fluid communication with the ullage space in tank  10  ( FIG. 1 ) at the other end via a vent line  63 . The vent line  63  prevents any build up of air pressure behind piston  42  as the piston  42  is moved toward proximal end portion  32 , such as when bringing the conduit line  14  up to full line pressure. To this end, as the piston  42  is moved toward proximal end portion  32 , air is forced through vent port  62  and vent line  63  and therefore maintains a relatively constant pressure behind piston  42 . 
         [0036]    Leak detector  20  further includes a biasing member that biases the piston  42  toward the distal end portion  34  of detector  20  so that fluid entering expansible chamber  41  must have sufficient pressure in order to move the piston  42  toward the proximal end portion  32  and against the force of the biasing member. For example, and as shown in the figures, in one embodiment the biasing member may be a spring  64 . The spring  64  may be a constant rate spring wherein the force imposed by the spring, which then determines the fluid pressure in conduit line  14 , varies as a function of displacement of the piston  42  within interior bore  36 . For a constant rate spring, leak detection then occurs under variable pressure conditions within the conduit line  14 . Detection of leaks under variable pressure conditions is relatively more complicated and may give rise to effects undesirable to highly accurate leak detection, including the compression and expansion of air pockets within conduit line  14 . 
         [0037]    While a constant rate spring is contemplated for use in embodiments of the invention, in another embodiment of the invention, spring  64  may be a variable rate spring configured such that the biasing force imposed by the spring, and thus the fluid pressure in conduit line  14 , remains constant over substantially the entire displacement of the piston  42  within interior bore  36 . In this way, any leaks that occur in conduit line  14  occur under constant pressure conditions, which are relatively easier to analyze and identify. Additionally, any volumetric effects from air pockets in conduit line  14  are minimized because their size, which depends on fluid pressure, remains constant during a testing period. 
         [0038]    Leak detector  20  further includes a displacement sensor for measuring the linear displacement of piston  42  within interior bore  36 . While a great number of displacement sensors exist, the invention contemplates the use of magnetostrictive technology to determine the displacement of piston  42 . While magnetostrictive technology is generally known in the art, such sensors are not known to have been used heretofore in line leak detection systems. There may be several reasons for this. One reason may include the expense of more accurate systems than that of less accurate prior art systems, which met less stringent EPA standards or even current standards, even though history shows that more accurate systems have been needed. While there has been a need to provide a more accurate leak detection, it is apparent the industry has not appreciated or recognized the potential use of magnetostrictive technologies and the advantages of the combination of that technology in leak detectors as described herein. 
         [0039]    Accordingly, in one embodiment, the displacement sensor may be configured as a magnetostrictive sensor  66  including a magnetostrictive waveguide  68  positioned within hollow shaft  50  and extending the length of interior bore  36  between the proximal and distal end portions  32 ,  34 . As recognized by one of ordinary skill in the art, magnetostrictive waveguide  68  may be formed from a suitable ferromagnetic material, such as iron, nickel or cobalt. Magnetostrictive sensor  66  also includes a permanent magnet  70  coupled to piston  42 . For instance, magnet  70  may have an annular configuration having an opening through which magnetostrictive waveguide  68  may be positioned. In this way, as the piston  42  moves due to volumetric changes in conduit line  14 , the magnet  70  moves relative to magnetostrictive waveguide  68 . The displacement of magnet  70  relative to magnetostrictive waveguide  68  can be sensed by magnetostrictive sensor  66  and may be used to determine if a leak exists in fluid conduit line  14 , as explained in more detail below. 
         [0040]    Magnetostrictive sensor  66  further includes a sensor control unit  72  coupled to cap  58  of leak detector  20 . Control unit  72  houses the necessary electrical components and systems for operation of magnetostrictive sensor  66 . In operation, control unit  72  includes an electrical pulse signal generator that generates and sends a current pulse along magnetostrictive waveguide  68 . The pulse is transmitted down the magnetostrictive waveguide  68  creating an electromagnetic field along the length of the magnetostrictive waveguide  68 . The permanent magnet  70  also generates a magnetic field that interacts with the magnetic field from the current pulse that causes a mechanical twisting of the waveguide  68  (Wiedemann effect) at the location of the permanent magnet  70 . The mechanical twisting of magnetostrictive waveguide  68  causes a return pulse in the form of an ultrasonic wave along waveguide  68 . Control unit  72  includes a pickup capable of detecting the return ultrasonic pulse. The control unit  72  may be electrically coupled to a central control  74  ( FIG. 1 ), such as by a suitable cable  75 , for collecting and analyzing the data signals from magnetostrictive sensor  66 . Those of ordinary skill in the art will recognize that some, if not all, of the electrical components in control unit  72  may alternately be located in central control  74 . 
         [0041]    The location of the piston  42  within the interior bore  36  may be detected by first applying a current pulse to the magnetostrictive waveguide  68 . At the same time, a high-speed counter located in control unit  72  is started. When the pulse reaches the permanent magnet  70  an ultrasonic wave is generated and travels back up magnetostrictive waveguide  68  and is detected by the pickup. The counter is then stopped. Since the speed of the ultrasonic wave in waveguide  68  is known, i.e., speed of sound in the waveguide material, the elapsed time between the current pulse and the returned pulse provides an indication of the position or location of the piston  42  along waveguide  68 . The control unit  72  repeatedly sends pulsed signals along magnetostrictive waveguide  68  and by comparing the location of the piston  42  for the multiple signals, a displacement of the piston  42  may readily be determined. Because the geometry of interior bore  36  is known, i.e., cross-sectional area of interior bore  36 , the displacement of piston  42  may be directly correlated to volumetric changes in conduit line  14   
         [0042]    The use of magnetostrictive sensor  66  to determine location (displacement) of the piston  42  has several advantages for line leak detection systems. A primary advantage is the increased sensitivity of the magnetostrictive sensor  66  to displacements of the piston  42  relative to previous sensors currently being used in line leak detection systems. By way of example, magnetostrictive sensors can sense movements on the order of 0.0005 inch while many current displacement sensors used in line leak detection systems require movements one to two orders of magnitude larger. As a result, the sensitivity of the magnetostrictive sensor  66  to relatively small displacements permit a large number of data points to be sampled and analyzed in order to determine if a leak exists in conduit line  14 . For example, for a two inch stroke of piston  42  in interior bore  36 , approximately 10,000 data points corresponding to detectable positions of piston  42  may be sampled and analyzed. The increased number of data points in turn permits a more accurate and reliable determination of the presence of a line leak. 
         [0043]    The sensitivity of the magnetostrictive sensor  66  also permits a relatively large data set to be sampled in a shorter period of time than heretofore available in current leak detection systems. Thus, for example, with a magnetostrictive sensor, a data set that may be reliably used to determine a line leak may take orders of magnitude less time than the devices currently being used. For instance, in one embodiment of the invention, thousands of valid data points may be used to determine if a leak exists in conduit line  14 . Due to the sensitivity of magnetostrictive sensor  66 , these thousands of data points may be taken in a very short period of time relative to the amount of time of current sensors to generate an equal amount of data points. 
         [0044]    The ability to obtain a reliable data set in a relatively short period of time provides additional advantages for line leak detection systems. For instance, the negative effects of thermal variations in the conduit line may be minimized by minimizing the sampling or testing time. As recognized by those of ordinary skill in the art, thermal variations in the conduit line (e.g. temperature differences between the fuel and piping system and ground) may be characterized by a thermal time constant that generally indicates the amount of time for the system to react to the thermal variations. For fuel dispensing systems, such as at gas stations, this time constant may be on the order of several minutes. Accordingly, the ability of the magnetostrictive sensor to gather a reliable data set in a relatively short period of time and in a period of time shorter than the thermal time constant of the dispensing system, eliminates, or at least significantly reduces, the impact of thermal variations on the determination of a leak in the conduit line. Eliminating thermal effects removes this as a potential source of volume change and thus increases the accuracy and reliability of line leak detection systems. 
         [0045]    Yet another advantage of using magnetostrictive sensor  66  to determine the location (displacement) of piston  42  is that much smaller volumetric changes in the conduit line  14  may be accurately detected. Small leaks that would not otherwise be detected by many of the current line leak detectors would be detectable using magnetostrictive sensor  66 . Thus, prophylactic measures may be taken at a much earlier stage than currently possible to prevent a possible catastrophic event and to prevent or reduce the amount of environmental damage resulting from the leak. 
         [0046]    It is anticipated that in the future, more restrictive EPA regulations will be implemented that require gas stations and other fuel dispensing systems to detect much smaller leaks than are currently required. In that case, many current line leak detectors will be inadequate and unable to meet the more stringent regulations. The leak detector of the invention, however, is capable of such small leak detection, and detection may be accomplished in a relatively short period of time and with the required accuracy and reliability required by such leak detection systems. 
         [0047]    While the biasing member in  FIGS. 2-3B  is shown and described as spring  64 , the biasing member is not limited to a spring as there are other ways to apply a biasing force against movement of the piston  42  in accordance with alternate embodiments of the invention. For example, and as illustrated in  FIG. 4  in which like reference numerals refer to like features in  FIGS. 2-3B , instead of a spring applying the biasing force to the piston  42 , the biasing member may take the form of a weight  75  coupled to piston  42 . The weight  75  may be integrally formed with the piston  42 , or alternately the weight  75  may be coupled to the piston  42  through a separate assembly process. In any event, the weight  75  operates in a similar manner as the variable rate spring in that the biasing force imposed by weight  75 , and thus the fluid pressure in conduit line  14 , remains constant over substantially the entire displacement of the piston  42  within interior bore  36 . Thus, any leaks that occur in conduit line  14  occur under constant pressure conditions, which as noted above are relatively easier to analyze and identify. 
         [0048]      FIGS. 5 and 6 , in which like reference numerals refer to like features in  FIGS. 2-3B , show additional embodiments in accordance with the invention. In these embodiments, the biasing member is configured as a magnet for imposing the biasing force on the piston  42 . In  FIG. 5 , the magnet is configured as an electromagnet  76 , such as a solenoid. In particular, the electromagnet  76  includes a coil operatively coupled to a power source (not shown) for creating a controllable magnetic field when a current flows through the coil. The electromagnet  76  may be configured such that the magnetic filed then imposes a relatively uniform force, i.e., the biasing force, for resisting movement of the piston  42  toward the proximal end portion  32  of leak detector  20 . Again, similar to the variable rate spring, the biasing force imposed by electromagnet  76 , and thus the fluid pressure in conduit line  14 , remains constant over substantially the entire displacement of the piston  42  within interior bore  36 . Thus, any leaks that occur in conduit line  14  occur under constant pressure conditions. 
         [0049]    The embodiment shown in  FIG. 6  also uses a magnet as the biasing member, but in this embodiment, the magnet is a permanent magnet  77 . This may result in a simpler design, eliminating the need to couple the biasing member to a power source. The permanent magnet  77  may be positioned adjacent the proximal end portion  32  of the leak detector  20 . In this case, the magnet  77  may have a polarity the same as that of magnet  70  on piston  42 . In this way, magnet  77  creates a biasing force that opposes motion of the piston  42  toward the magnet  77  (i.e., biases the piston  42  toward the distal end portion  34 ) due to similar pole magnets repelling each other. In an alternate embodiment similar to that shown in  FIG. 6 , the permanent magnet  77  may be positioned adjacent the distal end portion  34  (shown in phantom in  FIG. 6 ). In this case, the magnet  77  may have the opposite polarity as that of magnet  70  on piston  42 . In this way, magnet  77  creates a biasing force that opposes motion of the piston  42  away from magnet  77  (i.e., biases the piston  42  toward the distal end portion  34 ) due to opposite pole magnets attracting each other. For the embodiments shown in  FIG. 6 , the permanent magnet  77  operates similar to a constant rate spring in that the force imposed by the magnet  77 , which then determines the fluid pressure in conduit line  14 , varies as a function of separation of the magnet  77  relative to magnet  70  on piston  42 . For a permanent magnet, leak detection then occurs under variable pressure conditions within the conduit line  14 . The attracting/repelling force of magnets as a function of separation distance, are readily known by those of ordinary skill in the art and may be accounted for in determining whether a line leak has occurred. Although magnet  77  operates in cooperation with magnet  70  on piston  42 , a separate magnet may be coupled to piston  42  for use with magnet  77 . 
         [0050]    In still another embodiment, and as shown in  FIG. 7 , the biasing member may be configured as a pressurized fluid supply, shown schematically at  78 , for imposing the biasing force on the piston  42 . For example, the fluid supply  78  may be a gas supply for pneumatically pressurizing the interior bore  36  above the piston  42 . Alternately, the fluid supply  78  may be a liquid supply for hydraulically pressurizing the interior bore  36  above the piston  42 . In either embodiment, the fluid supply  78  may be regulated so that the biasing force imposed by the pressurized fluid above the piston  42 , and thus the fluid pressure in conduit line  14 , remains constant over substantially the entire displacement of the piston  42  within interior bore  36 . Thus, any leaks that occur in conduit line  14  occur under constant pressure conditions. In this embodiment, the vent port  62  and vent line  63  may be eliminated, or alternately include a valve for selectively opening and closing fluid communication between the interior bore  36  above the piston  42  and the tank  10 , as previously described. 
         [0051]      FIG. 8  shows yet another embodiment of the invention, wherein the biasing member is configured as a compressible gas pocket  79 . In this embodiment, the proximal end portion  32  of the leak detector  20  is totally sealed off, i.e., the vent port  62  and vent line  63  would be eliminated or again include a valve for selectively opening and closing fluid communication between the interior bore  36  above the piston  42  and the tank  10 . When the pocket  79  is sealed off, the gas contained therein imposes a biasing force on the piston  42 , due to the pressure of the gas. The gas pocket  79  operates similar to a constant rate spring in that the biasing force on the piston  42  varies as a function of displacement of the piston  42  within interior bore  36 . For example, as the piston  42  moves toward the proximal end portion  32 , the pressure of the gas pocket  79  and thus the biasing force on the piston  42  would increase. Moreover, as the piston  42  moves toward the distal end portion  34 , the pressure of the gas pocket  79  and thus the biasing force on the piston  42  would decrease. Accordingly, leak detection then occurs under variable pressure conditions within the conduit line  14 . The compressibility of gases, such as air or other suitable gases, are readily known by those of ordinary skill in the art and may be accounted for in determining whether a line leak has occurred. 
         [0052]    While leak detection occurs under variable pressure conditions within conduit line  14 , the embodiment shown in  FIG. 8  provides a number of advantages. In one aspect, the piston  42  may be configured as a simple buoyant float positioned on top of the liquid within leak detector  20 . Because the gas pocket  79  created between the top of the liquid and the proximal end portion  32  of the leak detector  20  provides the biasing force, there is no longer a need to maintain a fluid tight seal between the piston  42  and the wall of interior bore  36 , such as that performed by seals  44 . There is also no need to maintain a fluid tight seal between the central passageway  48  of piston  42  and the hollow shaft  50 , such as that performed by seals  56 . Thus, the piston  42  may have an outer dimension less than the dimension of the interior bore  36 . Similarly, the central passageway  48  may have a dimension greater than the dimension of the hollow tube  50 . The piston  42  in this embodiment therefore has a much simpler design, incorporating fewer parts (e.g., no seals). Moreover, friction effects between the movable piston  42  and the stationary portions of leak detector  20  are no longer problematic in this design, as close contact between the piston  42  and the interior bore  36  and hollow shaft  50  is avoided. This in turn may provide more accurate data. This design also avoids problems of wear on the piston seals and leaks past the piston, which may give a false indication of a leak. 
         [0053]      FIGS. 9A-9C  are flow charts illustrating operation of leak detector  20  in an exemplary leak detection system  80 . With the fuel dispensing system at normal operations, a leak test timer is started at  82 . The leak detection system  80  then operates in a loop configured to run a test after a sufficiently long period of inactivity. For example, if a dispenser unit  15  has not operated in several hours, e.g. the dispensing unit  15  is inactive during overnight hours; a test may be initiated after such a period of inactivity. To this end, leak detection system  80  continually checks if the test timer has expired at  84  and whether a dispensing unit  15  is requesting the submersible pump  12  at  86 . If the test timer has expired due to a sufficiently long period of inactivity, the leak detection system  80  checks if the submersible pump  12  has been disabled at  88 . For instance, a pump  12  may be disabled if a prior test indicated that a leak exists in the conduit line  14 . Of course, the leak detection system  80  will not permit any fuel dispensing from a dispensing unit  15  coupled to conduit line  14  for which a leak has been detected. If the submersible pump  12  is disabled, the inability to test is recorded at  90 . If the submersible pump  12  is not disabled, then the leak detection system  80  proceeds to a first stage test at  92 . 
         [0054]    If prior to the expiration of the test timer, a dispensing unit  15  requests pump  12 , the leak detection system  80  again checks if the submersible pump  12  has been disabled at  94 . If the pump  12  has been disabled (e.g. a prior leak test indicated a leak in the conduit line  14 ), the dispenser request is denied and recorded at  96 . If, on the other hand, pump  12  has not been disabled then the pump  12  is started at  98  in order to dispense fuel from dispensing unit  15 . The leak detection system  80  then operates in a second loop configured to run a leak test at the end of the dispensing process, as the conduit line  14  is at full line pressure. To this end, the leak detection system  80  checks if the dispensing unit  15  is still being requested at  99 . If the dispensing unit  15  is no longer being requested, i.e., the dispensing process has ended, then the leak detection system  80  proceeds to the first stage test at  100 . 
         [0055]    The first stage test is configured to determine whether a catastrophic leak exists in conduit line  14 . To this end, the submersible pump  12  is powered up at  102  in order to bring conduit line  14  up to normal, full line pressure. A timer is started at  104  to determine the amount of time it takes for the piston  42  to reach its high or top most position within interior bore  36 . The high position is reached when the pressure in conduit line  14  is at the full line pressure. In operation, the leak detection system  80  checks whether a dispensing unit  15  is requesting the submersible pump  12  at  106  and also checks the proper operation of the magnetostrictive sensor  66  at  108 . If a dispensing unit  15  is requesting the pump  12 , the test is abandoned at  110  and the leak detection system  80  returned to normal operations at  112 . If the magnetostrictive sensor  66  is not working properly, the malfunction is reported to an operator at  114  and the test is abandoned at  116  and the leak detection system  80  returned to normal operations at  118 . If the piston  42  reaches its high position before the expiration of the timer, there is no catastrophic leak in the conduit line  14  and the system  80  proceeds to a second stage test at  120  as discussed below. If, on the other hand, the piston  42  does not reach its high position before the expiration of the timer, then a catastrophic leak exists in the conduit line  14  and the submersible pump  12  is disabled at  122 . The catastrophic leak is then reported to an operator at  124  and the leak detection system  80  otherwise returned to normal operations at  126 . 
         [0056]    If the dispensing system passes the catastrophic test, as described above, then the leak detection system  80  proceeds onto a second stage test where a more precise leak test is conducted. In this stage, a timer is started at  128 . Again, the leak detection system  80  checks whether a dispensing unit  15  is requesting the submersible pump  12  at  130  and also checks the proper operation of the magnetostrictive sensor  66  at  132 . In addition, the leak detection system  80  checks to ensure that the piston  42  has not reached its low or bottom most position at  134 . If a dispensing unit  15  is requesting the pump  12 , the test is abandoned at  136  and the leak detection system returned to normal operations at  138 . If the magnetostrictive sensor  66  is not working properly, the malfunction is reported to an operator at  140  and the test is abandoned at  142 . The leak detection system  80  is then returned to normal operations at  144 . If the piston  42  has reached its low position, then an inconclusive test is recorded at  146  and the leak detection system  80  returned to normal operations at  148 . 
         [0057]    If the submersible pump  12  is not being requested, the magnetostrictive sensor  66  is working properly, and the low position of the piston  42  has not been reached, then the leak detection system  80  collects and analyzes position data of the piston  42  at  150  to determine volumetric changes in the conduit line  14 . To this end, the data may be sent to a central control  74  where the data is analyzed according to a pre-programmed algorithm configured to identify a leak in conduit line  14  based on the data. The algorithm is not a part of the present invention, but is used to draw a conclusion about a leak in conduit line  14 . One outcome of the algorithm is that the data indicates the presence of a leak in conduit line  14 . Such an outcome is indicated at  152 . In this case, the submersible pump  12  is disabled at  154  and the leak is reported to an operator at  156 . The leak detection system  80  is then otherwise returned to normal operations at  158 . Another outcome of the algorithm is that the conduit line  14  is tight and no leak exists, as indicated at  160 . In this case, the tight line is reported to an operator at  162  and the leak detection system  80  is returned to normal operations at  164 . Yet another outcome of the algorithm is that the data is inconclusive. In this case, the leak detection system  80  returns to  166  to continue the testing process. If a conclusion of the test is not reached prior to the timer expiring, an inconclusive test is recorded at  168  and the leak detection system  80  returned to normal operations at  170   
         [0058]    While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in numerous combinations depending on the needs and preferences of the user.