Abstract:
A modular TDR probe assembly is presented. The probe assembly includes a first section with a central conductor and an outer conductor having a first impedance, and a second section with a central conductor and an outer conductor having a second impedance. An impedance matching elbow joins the first section and the second section. The elbow contains a solid dielectric insert with a bore passing through it housing a terminal pin that conveys electrical signals between the center conductor of the first section and the center conductor of the second section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of, and claims priority to, copending U.S. nonprovisional patent application entitled “System and Method for Accurately Measuring Fluid Level in a Tank,” having Ser. No. 12/243,511, filed Oct. 1, 2008, which claims priority to U.S. provisional application having Ser. No. 60/976,615, filed Oct. 1, 2007, each of which are hereby incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally related fluid level detection, and more particularly is related to a time domain reflectometric probe. 
       BACKGROUND OF THE INVENTION 
       [0003]    A common problem in many different fields is needing to know the properties of a material within an enclosed vessel. Such properties may include, for example, the height of a liquid in a tank. For example, where an air space is formed above the surface of liquid fuel present in the fuel tank of an automobile or airplane, knowledge of the shape of the tank and the height of the air-liquid interface from the tank bottom will allow one to calculate the amount of remaining fuel. 
         [0004]    Where a plurality of stratifying liquids are present within a tank, it may furthermore be desired to know the height of each stratified liquid layer. For example, where water is mixed with hydrocarbon fuel intentionally, such as when seawater is used as ballast in oil tankers; or unintentionally, such as when water is present in a vehicle fuel tank or such as when groundwater seeps into tanks for fuel pumps at filling stations, it may be desired to know the height of fuel layer(s) as distinct from nonfuel layer(s) for accurate determination of remaining fuel. 
         [0005]    Time domain reflectometry (TDR) applies radar techniques to transmission line theory to detect the location of impedance transitions or discontinuities at interfaces between different layers of materials. In TDR, an interrogation pulse transmitted from a transmitter is reflected from such an impedance discontinuity, and the reflected pulse is received by a receiver. It is possible to calculate the distance (range) to the impedance discontinuity that caused the reflection from the observed round-trip propagation time of the interrogation pulse. 
         [0006]      FIG. 1  is a schematic diagram illustrating a probe  100  as disclosed in copending U.S. nonprovisional patent application entitled “System and Method for Accurately Measuring Fluid Level in a Tank,” having Ser. No. 12/243,511, filed Oct. 1, 2008. As is shown by  FIG. 1 , the probe  100  contains an elongated portion  110 , a shaped arm  120 , and a sensor  130 . The elongated portion  110  is a coaxial tube having a hollow center. The elongated portion  110  is shaped and lengthened to allow for positioning within a fuel tank, wherein a distal end  112  of the elongated portion  110  extends toward a bottom of the fuel tank, in which the elongated portion  110  may be positioned. Having a hollow elongated portion  110  allows fluid to enter the elongated portion  110 , via the distal end  112 , into the hollow portion to enable fluid level determination. 
         [0007]    A proximate end  114  of the elongated portion  110  joins a distal end  122  of the shaped arm  120 . The connection between the elongated portion  110  and the shaped arm  120  is provided in a manner so as to allow the combination of the shaped arm  120  and the elongated portion  110  to create a waveguide for an electromagnetic pulse provided by the sensor  130 . In addition, the combination of the elongated portion  110  and the shaped arm  120  provide a coaxial waveguide. 
         [0008]    The shaped arm  120  may be filled with a dielectric such as Teflon®. The Teflon® fill is a solid dielectric. Use of a Teflon® fill serves at least two purposes. First, the Teflon® fill provides impedance matching, second, the Teflon® provides a means to prevent fluid ingression to a non-gauging portion of the probe  100 , thereby eliminating unwanted reflections due to multiple fluid levels inside of the probe  100 . 
         [0009]    In accordance with probe  100 , an interrogation signal is sent by the sensor  130  into a transmission line, wherein the transmission line includes the combination of the shaped arm  120 , the elongated portion  110 , and beyond the distal end  112  of the elongated portion  110 . The transmission line has three sections. A first section of the transmission line is from an excitation source, such as the sensor  130 , to a top of the probe  100 , also referred to as the distal end  122  of the shaped arm  120  (also referred to as the beginning of the gauge-able area). A second section of the transmission line is from the top of the probe  100  (the distal end  122  of the shaped arm  120 ) to a bottom of the probe  100 , also referred to as the distal end  112  of the elongated portion  110 . The second section of the transmission line is also referred to as the gauge-able area. A third section of the transmission line is from the bottom of the probe  100  to the end of a transmission line that runs past the end, or distal portion  112 , of the gauge-able area. 
         [0010]    As discussed above, a TDR system may have a sensor unit (transmitter/receiver), a probe, and one or more connecting elements, such as an arm. The connecting elements may be, for example, a coaxial cable, or a coaxial waveguide. Reflections may occur unless the impedance of the arm is matched with the sensor and the probe. For example, the sensor may have an impedance of 50 ohms, and the probe may have an impedance of 86 ohms. Such impedance matching has been accomplished by filling the waveguide with a dielectric material with a known dielectric constant, for example, Teflon®. However, a dielectric filled waveguide connector may be relatively expensive compared with, for example, a coaxial cable. Therefore, it is desirable to provide a modular TDR system with low cost interchangeable connecting components. 
         [0011]    One technical challenge involved with a modular TDR system is connecting modular components in such a way as to minimize signal loss while transmitting the interrogation pulse between the sensor and the probe. For example, if there is a signal discontinuity or impedance mismatch at the connection between successive modular components, the connection may generate unwanted reflections, and similarly, absorb or divert energy from the interrogation pulse before it reaches the target medium. Such an energy loss may be problematic, both for decreasing the signal to noise ratio of the interrogation pulse as reflected off the target medium, and for obscuring reflections from the gauge-able area with reflections outside the gauge-able area. 
         [0012]    Signal discontinuities or reflection points along a transmission line may also occur where the transmission line is crimped or bent at a sharp angle, for example, an angle greater than 45 degrees. Even so, there are applications where it is advantageous for the TDR signal to traverse a path that takes sharp turns. In aerospace applications of TDR, where components may by necessity be constrained to fit within the aerodynamic boundaries of the vehicle, it may be advantageous to have the probe section attach to a connector section at a relatively high angle of incidence, for example, in the range of 45 degrees to 135 degrees. Such an angled connector is referred to herein as an elbow. 
         [0013]    In aerospace applications, it may be particularly important for a TDR fuel probe to span the full height of a fuel tank. Therefore, the gauge-able region of the probe should begin at the very top of the tank and extend nearly to the bottom. Since in some application, the fuel tank extends to the very edge of the vehicle, extending the probe to the maximum height requires a probe connection at the end of a gauge-able area to be at a high angle of incidence. Therefore, it would be advantageous to connect the probe to the pulse signal chain at the very top of the tank, while minimizing unwanted reflections usually associated with such a high angle of incidence. 
         [0014]    TDR may be used in applications where the material being monitored is inflammable, for example in fuel tanks. Inflammable material may be ignited by electromagnetic energy arcing across the gap (“spark gap”) between two conductors, such as the center and outer conductor of a coaxial wave guide, creating a spark. Therefore, care must be taken to ensure that a high energy electromagnetic signal traversing the TDR interrogation pulse signal path does not ignite the inflammable material. For example, energy from lightning striking a vehicle may be conducted along the TDR signal path into the fuel tank and arc across the gap between the conductors, potentially igniting vapor in the fuel tank. 
         [0015]    Non impedance matching elbow connectors or joints are known in coaxial cabling systems. However, while such prior art elbows allow the signal to be diverted at a high angle of incidence, they do not provide for impedance matching or adequate spark gaps for TDR purposes. In addition, positioning such a connector at the top of a probe would effectively shorten the measuring range of the probe, as the gauge-able area of the probe would have to end before the elbow connector. If an elbow connector is not used, the TDR signal path would have to be more gradually curved inside the vessel being monitored, decreasing the effective gauge-able area of the probe by using a portion of the span of the vessel for gradually bending the transmission line to prevent signal loss and minimize unwanted reflections. Therefore, there is a heretofore-unmet need for an impedance matching elbow connector that is intrinsically safe in a fuel environment. 
       SUMMARY OF THE INVENTION 
       [0016]    Embodiments of the present invention provide an elbow connector for a TDR vessel probe. Briefly described, the present invention is directed to a modular TDR probe assembly including a first elongated portion and a second elongated portion. The first elongated portion has a first impedance and includes a first central conductor and a first outer conductor, and the second elongated portion has a second impedance and includes a second central conductor and a second outer conductor. 
         [0017]    The modular TDR probe assembly further includes a dielectric insert formed from a solid dielectric material having a first end, a second end, and a bore passing between the first end and the second end. The assembly includes a terminal pin formed of a conductive material having a first end and a second end. The terminal pin is configured to be disposed within the dielectric insert bore with the terminal pin first end extending from the dielectric insert first end and the terminal pin second end extending from the dielectric insert second end. 
         [0018]    The modular TDR probe assembly also includes an elbow having a first end and a second end. The elbow is configured to substantially enclose the dielectric insert and terminal pin disposed within the elbow, wherein the elbow first end is configured to connect to the first elongated portion and thereby place the first center conductor in electrical communication with the terminal pin first end, and the elbow second end is configured to connect to the second elongated portion and thereby place the second central conductor in electrical communication with the terminal pin second end. 
         [0019]    Briefly describe, in architecture, a second aspect of the present invention is directed to a TDR probe connector configured to connect a first coaxial probe with a second coaxial probe section. The first coaxial probe section has a first impedance and a first center conductor, and the second coaxial probe section has a second impedance and a second center conductor. The first impedance is different from the second impedance. The connector includes a substantially cylindrical outer conductor having a conductive material with a first end and a second end. The outer conductor first end is configured to connect to the first coaxial probe section, and the outer conductor second end is configured to connect to the second coaxial probe section. 
         [0020]    The TDR probe connector further includes a dielectric insert formed of a solid dielectric material, having a first end, a second end, and a center bore passing through the dielectric insert from the dielectric insert first end to the dielectric insert second end. The dielectric insert is disposed within the outer conductor first end so that the dielectric insert substantially fills the outer conductor first end. 
         [0021]    The TDR probe connector includes a terminal pin comprising a conductive material having a terminal pin first end and a terminal pin second end at least partially disposed within dielectric insert center bore such that the terminal pin substantially fills the dielectric insert center bore. The terminal pin first end extends past the dielectric insert first end and the terminal pin second end extends past the dielectric insert second end. The TDR probe connector also includes a spark gap, the spark gap being the shortest distance between the outer conductor and the terminal pin. 
         [0022]    Other systems, methods and features of the present invention will be or become apparent to one having ordinary skill in the art upon examining the following drawings and detailed description. It is intended that all such additional systems, methods, and features be included in this description, be within the scope of the present invention and protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principals of the invention. 
           [0024]      FIG. 1  is a schematic diagram of a TDR probe having a dielectric filled arm connecting the sensor and the elongated portion. 
           [0025]      FIG. 2  is a schematic diagram of a first embodiment of a TDR probe having a hollow arm connecting section. 
           [0026]      FIG. 3  is a schematic diagram of a cutaway detail of the elbow portion of the first embodiment of the probe. 
           [0027]      FIG. 4  is a schematic diagram of a detail of the terminal pin. 
           [0028]      FIG. 5  is a schematic diagram of a detail of the dielectric insert. 
           [0029]      FIG. 6  is a schematic diagram of the terminal pin as inserted into the dielectric insert. 
           [0030]      FIG. 7  is a schematic diagram of a second embodiment of a TDR probe with an elbow. 
           [0031]      FIG. 8  is a schematic diagram of a third embodiment of a TDR probe with two elbows. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0033]    Among other embodiments, an elbow for a TDR probe is disclosed. The elbow provides impedance matching between a first connecting section having a first impedance to a second connecting section having a second impedance. The first section transports the interrogation signal via a coaxial cable, and the second section is a coaxial waveguide. The elbow both matches impedances and minimizes reflections from the angle of connection using the geometric properties of the signal conducting elements and the impedance matching properties of a dielectric insert. 
         [0034]    A modular TDR probe may have multiple sections. It is generally desirable for interconnecting sections to minimize reflections caused by impedance mismatches across the component connection boundary. In the case of an elbow connector, the angle of incidence between two adjacent components provides an additional source for unwanted reflections, in addition to any impedance mismatch. 
         [0035]      FIG. 2  is a partial cutaway diagram of a first embodiment of a TDR probe  200  having a hollow arm  220 . The hollow arm  220 , shown in a cutaway view, may connect the sensor unit  130  to an elbow  250 . The elbow  250  connects the hollow arm  220  to the elongated portion  110 . The elongated portion  110 , shown in a partial cutaway view, has a distal end  112 . The TDR probe  200  has a gauge-able area that extends from the elbow  250  to the elongated portion  110  distal end  112 . The hollow arm  220  may alternatively connect the elbow  250  to a second elbow (not shown), or to another connecting element other than the sensor unit  130 . In a second embodiment ( FIG. 7 ), described below, the elbow  250  may connect the elongated portion  110  directly to the sensor unit  130 , with no intervening connecting sections. 
         [0036]    The hollow arm  220  may contain a coaxial cable  210  to conduct the interrogation signal from the sensor  130  to the elbow  250 . The coaxial cable  210  may be less expensive than, for example, a dielectric filled coaxial waveguide. The hollow arm in  FIG. 2  has two spacers  212  that center the coaxial cable  210  within the hollow arm  220 . The spacers  212  may be made of a solid material having a known dielectric constant, for example, Teflon®. It should be noted that more or fewer spacers  212  may be used, for example, zero, one, two or more spacers  212 . 
         [0037]      FIG. 3  is a cutaway detail of the elbow  250  of the first embodiment of the TDR probe  200  and the connection between the elbow  250  and the elongated portion  110 . The elbow  250  may be a single hollowed out component, or may be formed from several individual elements. The elbow  250  is generally hollow, allowing the signal path components to enter from a sensor facing portion  305 , and also acting to transition the signal to a wave guide beginning within the elbow and extending through the elongated portion  110  at a probe facing portion  395 . The sensor facing portion  305  may have a first impedance, for example, 50 Ohms, and the probe facing portion may have a second impedance, for example 86 Ohms. The elbow  250  acts as a coaxial wave guide, with an elbow housing  355  serving as the coaxial wave guide outer conductor. The elbow housing  355  is formed from a conducting material, and may be fashioned by machining a conducting material, for example, a solid piece of aluminum, or may be molded from a conducting material, for example, by injection molding. 
         [0038]    The sensor facing portion  305  of the elbow  250  terminates with a coaxial cable connector  310 . The probe facing portion  395  of the elbow  250  connects to the elongated portion  110 . The cutaway schematic diagram in  FIG. 3  shows the conductive elements of the elongated portion  110 , including an outer conductor  380 , and a center conductor  370 . A probe hollow area  390  separates the outer conductor  380  from the center conductor  370 . The probe hollow area  390  is contiguous with an elbow hollow area  330  within the elbow  250 , so that the contents of the vessel, for example, fuel or air, may occupy the probe hollow area  390  and the elbow hollow area  330 . The center conductor  370  is held in place by a center conductor support  365 , which positions the center terminal conductor  360  at a point substantially equidistant from the outer conductor  380 . The center conductor  370  ends inside the elbow  250  at center terminal conductor  360 . The elbow  250  may be attached to the outer conductor  380 , for example, with a collar  391  and screws  392 , so that the elbow housing  355  is in electrical communication with the outer conductor  380 . 
         [0039]    Internally, the coaxial cable connector  310  attaches to a terminal pin  340 . The terminal pin  340  provides the center conducting connection between the coaxial cable connector  310  and a center terminal conductor  360 , whereas the elbow housing  355  serves as an outer conductor. The solder joint  350  is the connection point between the terminal pin  340  and the center terminal conductor  360 . When assembled, the terminal pin  340  is inserted through a dielectric insert  320 . The dielectric insert  320  is fashioned from a solid dielectric material having a known dielectric constant, such as Teflon®. The dielectric insert  320  substantially fills the sensor facing portion  395  of the elbow  250 , and serves to prevent fluid ingression from the vessel, for example, fuel or air, from reaching the sensor facing portion  305  of the elbow  205 . Further details of the dielectric insert  320  are disclosed below. 
         [0040]    The impedance Z for a waveguide having an inner conductor with diameter d inner  and an outer conductor having a diameter d outer  is shown by 
         [0000]    
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     
                       60 
                       
                         ɛ 
                       
                     
                      
                     
                       ln 
                        
                       
                         ( 
                         
                           
                              
                             outer 
                           
                           
                              
                             inner 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Eq 
                     . 
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ∈ is the dielectric constant of the material between the inner conductor and the outer conductor. The dielectric constant for air is different from the dielectric constant for fuel, so, for example, if fuel partially fills a waveguide there will be an impedance transition for a signal traversing the waveguide at the physical location where the waveguide fill changes from air to fuel. For example, TDR may be used to detect this impedance transition in a fuel probe within a fuel tank to determine the fuel level within the probe, which is assumed to correspond to the fuel level within the fuel tank. 
         [0041]    Accordingly, the impedance within the elbow  250  may change at different locations along the elbow  250  depending upon the diameter of the inner conductor at that location, the diameter of the outer conductor at that location, and the dielectric constant of the material between the inner conductor and the outer conductor at that location. At different locations along the elbow  250  the inner conductor may be the terminal pin  340 , the solder joint  350 , the center terminal conductor  360 , and the center conductor  370 . The elbow housing  355  forms the outer conductor. The dielectric insert  320  has a known dielectric constant. The rest of the elbow  250  may be filled with, for example, air or fuel, or a first portion may be filled with air and a second portion may be filled with fuel. 
         [0042]    As discussed previously, there may be concern regarding electromagnetic energy sparking between the inner conductor and the outer conductor along the signal path of the elbow  250 . The probability of a spark arcing across the gap between the inner conductor and the outer conductor depends upon both the distance between the inner conductor and the outer conductor, and the fill material between the inner conductor and the outer conductor. For example, where the fill material is a solid dielectric, such as Teflon®, the probability of a spark is reduced compared to if the fill material is air. Similarly, the probability of a spark across a larger spark gap is less than the probability of a spark across a smaller spark gap. In general, a minimum spark gap on the order of 0.190 inches may be sufficient to prevent a spark from energy caused by a lightning strike conducted along the TDR signal path. 
         [0043]      FIG. 4  is a detail of the terminal pin  340 . The terminal pin  340  is formed of a conducting material, for example, nickel or aluminum. The terminal pin  340  serves as the center conductor to the outer conductor of the elbow housing  355  ( FIG. 3 ). The terminal pin  340  has several sections. Most of the sections are generally cylindrical in shape, and the diameter of each section may be determined to minimize reflection of the interrogation pulse and to set the desired impedance at different locations within the elbow  250  ( FIG. 3 ). The center terminal connector  348  of the terminal pin  340  connects to the center terminal conductor  360  of the elongated portion  110  ( FIG. 3 ) at a solder joint  350  ( FIG. 3 ). It should be noted that the solder joint may connect the center terminal connector  348  to the center terminal conductor  360  ( FIG. 3 ) using hot solder, cold solder, or by using another connection method suitable for physically and electrically connecting conductors, as familiar to persons having ordinary skill in the art. 
         [0044]    The terminal pin  340  joins the terminal connector  360  ( FIG. 3 ) at an angle, the angle for example in the range of 45 degrees to 135 degrees. While the center terminal connector  348  in  FIG. 4  is generally fork shaped, the center terminal connector  348  may be formed in other configurations to facilitate other connection methods to the center terminal conductor  365  ( FIG. 3 ). The center terminal connector  348  connects to a terminal pin barrel  342 . The terminal pin barrel  342  has a generally constant diameter, sized according to Eq. 1, above, with the terminal pin barrel diameter as d inner  and the elbow housing  355  diameter as d outer . 
         [0045]    The terminal pin barrel  342  is adjacent to a conical section  343 . The conical section has a wider radius at the terminal pin barrel  342  end and a narrower radius at the insert core  345  end. The dimensions of the conical section  343  are discussed below in relation to the dielectric insert  320  ( FIG. 3 ). At its widest point, the diameter of the conical section  343  may be narrower than the diameter of the terminal pin barrel  342 . At its narrowest point, the diameter of the conical section  343  is wider than the diameter of an insert core  345 . When the terminal pin  340  is assembled with the dielectric insert  320  ( FIG. 3 ), the conical section  343  is located generally within a rear recess  328  ( FIG. 5 ) of the dielectric insert  320  ( FIG. 3 ). 
         [0046]    The insert core  345  of the terminal pin  340  is adjacent to the narrow end of the conical section  343 . When the terminal pin  340  is assembled with the dielectric insert  320  ( FIG. 3 ), the insert core sits inside a center bore  326  ( FIG. 5 ) of the dielectric insert  320  ( FIG. 5 ). A protruding end  347  connects to the insert core  345 . When the terminal pin  340  is assembled with the dielectric insert  320  ( FIG. 3 ), the protruding end  347  extends from the end of the dielectric insert  320  ( FIG. 3 ) and protrudes outward where it may be connected with the coaxial cable connector  310  ( FIG. 3 ). The protruding end  347  is generally cylindrical and has a smaller diameter than the diameter of the insert core  345 . The protruding end  347  terminates at chamfer  346 . The chamfer  346  tapers from the diameter of the protruding end  347  so it may form an electrical connection with the coaxial cable  210  ( FIG. 2 ) at the coaxial cable connector  310  ( FIG. 3 ). 
         [0047]      FIG. 5  is a schematic diagram of a detail of the dielectric insert  320 . As mentioned above, the dielectric insert  320  acts to prevent ingression of liquid or gas into the sensor facing portion  305  ( FIG. 3 ) of the elbow  250  ( FIG. 3 ), and further provides a region within the elbow  250  having a known impedance. The impedance of the segment of the signal path traversing the dielectric insert  320  may be matched to the coaxial cable  210  ( FIG. 2 ) to smooth the signal transition between the hollow arm  220  ( FIG. 2 ) and the elbow  250  ( FIG. 2 ). In addition, the presence of the solid dielectric obviates concern about sparking through the solid dielectric material across the gap between the elbow housing  355 , serving as the outer conductor of a coaxial waveguide, and the terminal pin  340 , serving as the center conductor of a coaxial waveguide. 
         [0048]    The dielectric insert  320  is positioned within the sensor facing portion  305  ( FIG. 3 ) of the elbow  250  ( FIG. 3 ), adjacent to the elbow hollow area  330  ( FIG. 3 ). The dielectric insert  320  may have an insert barrel  322  and a bevel section  323 . The bevel section  323  tapers from a wide end, having a diameter substantially equal to the diameter of the insert barrel  322 , to a narrow end toward the sensor facing portion  305  ( FIG. 3 ) of the elbow  250  ( FIG. 3 ). The bevel section  323  serves to reduce the area of the dielectric material adjacent to the coaxial cable connector  310  ( FIG. 3 ), and to provide more elbow  250  ( FIG. 3 ) surface area for attaching the coaxial cable connector  310  ( FIG. 3 ) to the elbow  250  ( FIG. 3 ) with coaxial cable connector screws  302 . It should be noted that alternative embodiments of a dielectric insert  320  may omit the beveled section  323 . 
         [0049]    The insert barrel  322  is generally cylindrically shaped on the outer surface. Toward the probe facing end of the insert barrel  322 , there is a rear recess  328 . The rear recess is inset from the end of the barrel, and has a flat bottom surface and a beveled lip  325 . The angle of the beveled lip  325  is substantially the same as the taper angle of the conical section  343  ( FIG. 4 ) of the terminal pin  340  ( FIG. 4 ). The rear recess  328  maintains a suitable spark gap to reduce the likelihood of a high energy signal from igniting inflammable gasses or liquids in the vicinity of the elbow  250  ( FIG. 2 ). Similarly, the matching tapers of the lip  325  and the terminal pin conical section  343  allow for both reduction of reflections at the impedance transition between the dielectric insert  320  and the elbow hollow area ( FIG. 3 ) as per Eq. 1, and setting up proper geometry to maintain the appropriate spark gap as the diameter of the terminal pin  340  increases from the insert core  345  to the terminal pin barrel  342 . As described previously, an appropriate spark gap may be maintained by having a minimum spacing of approximately 0.2 inches between all inner conductors and outer conductors. 
         [0050]    In addition, the taper on the lip  325  provides a slope to allow any accumulated fluid, for example, fuel, to drain out from the rear recess  328 . The center bore  326  facilitates insertion of the insert core  345  ( FIG. 4 ) of the terminal pin  340  ( FIG. 4 ). The center bore starts at the rear recess  328 , passes substantially through the center of the insert barrel  322  and the bevel  323 . 
         [0051]      FIG. 6  shows the terminal pin  340  inserted into the dielectric insert  320  as assembled. The insert core  345  sits within the center bore  326 . The center bore may be only marginally larger in diameter than the diameter of the insert core  345  to prevent fluid from entering the center bore  326  when the terminal pin  340  is inserted. The sensor facing end of the terminal pin conical section  343  abuts the surface of the dielectric insert rear recess  328 . The terminal pin conical section  343  may be positioned tightly against the floor of the rear recess  328  to seal the center bore  326  against fluid. The terminal pin protruding end  347  extends outward from the dielectric insert bevel  323  toward the sensor facing end of the elbow  250  ( FIG. 3 ). The terminal pin protruding end  347  inserts into the coaxial cable connector  310  ( FIG. 3 ) to electrically couple the terminal pin  340  to the center conductor in the element connected to the elbow  250 , for example, a hollow arm with an internal coaxial cable, or a sensor unit directly connected to the elbow. 
         [0052]    The insert core  345  conducts the interrogation pulse through the dielectric insert  320 , while the conical section  343 , the terminal pin barrel  342  and the center terminal connector  348  conduct the interrogation signal through the elbow hollow area  330  ( FIG. 3 ). An impulse reflection will generally occur at the transition between the dielectric insert  320  and the elbow hollow area  330  ( FIG. 3 ), as the impedance of the signal path changes from a first impedance, for example, 50 Ohms matching the impedance of the coaxial cable  210  ( FIG. 2 ) to a second impedance, for example, 86 Ohms matching the elongated portion  110  ( FIG. 2 ). The length of the terminal pin barrel  342  may be relatively long to distinguish between first reflections of the interrogation pulse at the transition between the dielectric insert  320  and the elbow hollow area  330  ( FIG. 3 ), from second reflections of the interrogation pulse at the angle connection at the solder joint  350  ( FIG. 3 ). It may be similarly desirable for the terminal pin barrel  342  length to be sized based upon the shape of the interrogation pulse being generated. For example, compared to a step function with a fast rise time, an interrogation pulse having a step function with a slow rise time may cause a relatively long settling time of the reflected signal, making it difficult to distinguish between reflections from closely spaced reflection points. Therefore the specific geometry of the terminal pin  340  may be tailored to suit the type of interrogation pulse being used in the TDR system. 
         [0053]      FIG. 7  shows a second embodiment of a TDR probe with an elbow  250 . Under the second embodiment, the elbow  250  connects the sensor unit  130  to the elongated portion  110 . In the second embodiment, the elbow  250  may internally connect to the sensor unit  130  with a coaxial connector (not shown). The elbow  250  of the second embodiment transitions the second impedance of the elongated portion  110  to the first impedance of the internal coaxial connector (not shown) at the sensor unit  130 . 
         [0054]      FIG. 8  shows a third embodiment of a TDR probe with two elbows  250 A,  250 B. A first hollow arm  220 A connects the sensor  130  to a first elbow  250 A. A second hollow arm  220 B connects the first elbow  250 A to a second elbow  250 B. The second elbow  250 B connects to a elongated portion  110 . A probe configured with multiple hollow arms  220  and elbows  250  may be used, for example, to connect an elongated portion  110  to a sensor  130  by routing the hollow arms  220  and elbows  250  around objects or components in a confined space, for example, an aircraft wing. 
         [0055]    It should be noted that while  FIG. 8  depicts two elbows  250 A,  250 B that connect to hollow arms  220 A,  220 B and/or probes  110  at 90 degrees, there is no objection to using elbows  250  with different or dissimilar angles, for example, 80 degrees and 110 degrees. Similarly, there is no objection to a TDR probe having more than two elbows  250 , for example, three, four, or more elbows  250 . While arms  220  are depicted as straight, there is no objection to a curved hollow arm  220  ( FIG. 2 ). 
         [0056]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.