Abstract:
A transducer for determining the liquid level within a container that is subjected to at least partial solidification at or below a freezing temperature is provided. The transducer includes a mounting head adapted for connection to the container, a liquid level sensor adapted for extending into the container from the mounting head; and a heating unit extending through the mounting head. The heating unit is constructed of at least one tubular member that surrounds or encircles the liquid level sensor. Heated fluid from a fluid source is circulated through the at least one tubular member for heating the contents of the tank at least within the vicinity of the liquid level sensor. A fluid withdrawal tube can also be in close proximity to the at least one tubular member so that the contents of the tank surrounding the heating unit can be removed even when the remaining tank contents are in a frozen state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/330,969 filed on May 4, 2010, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to liquid level transducers, and more particularly to liquid level transducers having heating arrangements for heating the surrounding material to be measured. 
         [0003]    Transducers for measuring liquid level are often used in vehicles, industrial equipment as well as other mobile and stationary systems and components. The electrical output of such transducers changes in response to a change in the liquid level being measured and is typically in the form of a change in resistance, capacitance, current flow, magnetic field, and frequency. These types of transducers may include variable capacitors or resistors, optical components, Hall Effect sensors, strain gauges, ultrasonic devices, reed switch arrays, and so on. 
         [0004]    No matter what transducer type is used, the tank level measurement is most successful when the material being measured is in a liquid state as opposed to a semi-solid or frozen state. Although many fuels have a freezing point well below the operating temperature range of most vehicles and equipment, other fluids are subjected to freezing such as engine coolant and diesel exhaust fluid (DEF). DEF is especially problematic since it is used in vehicles equipped with Selective Catalytic Reduction (SCR) systems. DEF is a solution that typically comprises purified water and approximately 32.5 percent urea and is used to reduce nitrogen oxide (NOx) emissions from diesel-powered vehicles into nitrogen, water and carbon dioxide (CO 2 ). The DEF is kept in a tank on the vehicle and is automatically accessed during vehicle operation to reduce emissions. A liquid level transducer is often associated with the tank to indicate a level of the DEF to an operator or other observer. Unfortunately, the DEF can freeze when subjected to low temperature conditions and thus cannot be accurately measured or extracted from the tank until it is changed to a liquid state. 
         [0005]    Prior art solutions have been inadequate in addressing these problems in a satisfactory manner. It would therefore be desirous to provide a heating arrangement associated with the liquid level transducer and/or liquid withdrawal or supply tubes of DEF tanks or the like so that the level of DEF can be more quickly ascertained and accessed during freezing conditions. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one aspect of the invention, a transducer for determining the level of contents within a container wherein the contents are subjected to solidifying below a freezing temperature is provided. The transducer includes: a mounting head adapted for connection to the container; a liquid level sensor adapted to extend into the container from the mounting head; and a spiral-shaped heating unit comprising a first elongate tube extending through the mounting head. The first elongate tube is formed with at least one coil that surrounds at least a portion of the liquid level sensor and is adapted to circulate heating fluid therein to thereby heat the contents of the container at least in the vicinity of the liquid level sensor. 
         [0007]    In accordance with a further aspect of the invention, a transducer for determining liquid level within a container includes: a mounting head adapted for connection to the container; a liquid level sensor adapted for extending into the container from the mounting head; and a heating tube extending through the mounting head. The heating tube has: first and second upright segments connected via a first lower bend; a third upright segment connected to the second upright segment via an upper bend; and a fourth upright segment connected to the third upright segment via a second lower bend. The first and fourth upright segments are adapted for fluid connection to a fluent heating source for heating the contents of the container. 
         [0008]    In accordance with yet another aspect of the invention, a transducer for determining the level of contents within a container wherein the contents are subjected to solidifying below a freezing temperature is provided. The transducer includes: a mounting head adapted for connection to the container; a liquid level sensor adapted for extending into the container from the mounting head; and a heating unit extending through the mounting head and into the container. The heating unit has an outer tube and an inner tube extending inside and along a length of the outer tube. The outer and inner tubes are in fluid communication such that heating fluid is adapted to flow from one of the outer and inner tubes to the other of the outer and inner tubes to thereby heat the contents of the container. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The following detailed description of the preferred embodiments of the present invention will be best understood when considered in conjunction with the accompanying drawings, wherein like designations denote like elements throughout the drawings, and wherein: 
           [0010]      FIG. 1  is a front isometric view of a liquid level transducer with a heat transfer unit connected to a tank in accordance with on embodiment of the present invention; 
           [0011]      FIG. 2  is a side elevational view thereof; 
           [0012]      FIG. 3  is a view similar to  FIG. 1  with the tank removed; 
           [0013]      FIG. 4  is a view similar to  FIG. 3  with a mounting plate removed to reveal the details of an upper portion of the liquid level transducer; 
           [0014]      FIG. 5  is top isometric view of the liquid level transducer; 
           [0015]      FIG. 6  is a view similar to  FIG. 5  with a housing portion removed to reveal the details of the upper portion of the liquid level transducer; 
           [0016]      FIG. 7  is a view similar to  FIG. 6  with a transfer block removed to reveal more details of an end portion of the liquid level transducer; 
           [0017]      FIG. 8  is an enlarged bottom isometric view of the liquid level transducer; 
           [0018]      FIG. 9  is a bottom perspective view of the liquid level transducer; 
           [0019]      FIG. 10  is a front elevational view of a liquid level transducer with a heat transfer unit in accordance with a further embodiment of the present invention; 
           [0020]      FIG. 11  is a rear elevational view thereof; 
           [0021]      FIG. 12  is a top perspective view thereof; 
           [0022]      FIG. 13  is a side perspective view of an upper portion of the liquid level transducer of  FIG. 10 ; 
           [0023]      FIG. 14  is a left side elevational view of an upper portion thereof; 
           [0024]      FIG. 15  is a right side elevational view of an upper portion thereof; 
           [0025]      FIG. 16  is a front elevational view of a lower portion of the liquid level transducer of  FIG. 10 ; 
           [0026]      FIG. 17  is left side elevational view of the lower portion thereof; 
           [0027]      FIG. 18  is a top schematic view of a tank and the heat transfer unit of  FIG. 10  for comparing the size of the opening with the heat transfer unit; 
           [0028]      FIG. 19  is a front isometric view of a liquid level transducer with a heat transfer unit connected to a tank in accordance with a further embodiment of the present invention; 
           [0029]      FIG. 20  is a sectional view of a heat transfer unit in accordance with yet another embodiment of the invention; 
           [0030]      FIG. 21  is a sectional view thereof taken along line  21 - 21  of  FIG. 20 ; 
           [0031]      FIG. 22  is a front elevational view of a liquid level transducer with heat transfer unit in accordance with a further embodiment of the invention; and 
           [0032]      FIG. 23  is an enlarged view of a portion of the heat transfer unit of  FIG. 22 . 
       
    
    
       [0033]    It is noted that the drawings are intended to depict only exemplary embodiments of the invention and therefore should not be considered as limiting the scope thereof. It is further noted that the drawings are not necessarily to scale. The invention will now be described in greater detail with reference to the accompanying drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    Referring now to the drawings, and to  FIGS. 1 and 2  in particular, a liquid level transducer  10  according to an exemplary embodiment of the present invention is illustrated. The liquid level transducer  10  preferably extends into a container  11 , such as a fuel tank, oil reservoir, radiator, brake fluid chamber, or any other container for holding and/or transporting a liquid (not shown). In accordance with one preferred application of the invention, the transducer  10  is particularly useful for liquids that have a tendency to freeze at lower temperatures, such as diesel exhaust fluids (DEF) in a NOx emissions control system. Such fluids can include, but are not limited to, water, urea, ammonia, and combinations thereof. 
         [0035]    With additional reference to  FIGS. 3-5  and  9 , the transducer  10  preferably includes a mounting head  14 , an elongate sensing probe  12  extending through the mounting head  14  and downwardly therefrom, a heating unit  16  extending through the mounting head  14  and bending around the sensing probe  12 , and a fluid supply tube  18  extending through the mounting head  14  and along a substantial length of the heating unit  16 . 
         [0036]    As best shown in  FIGS. 2-4  and  6 - 8 , the sensing probe  12  preferably senses liquid level in a linear direction and, in accordance with one preferred embodiment of the invention, includes an outer sensor tube  22  with an upper end  24  that extends through the mounting head  14  and a lower end  26  with a support block  28 . A float  30  is preferably cylindrically-shaped and includes a central bore  32  (shown in  FIG. 8 ) that is sized to receive the sensor tube  22  so that the float slides freely therealong. The support block  28  preferably holds the heating unit  16 , the lower end  26  of the sensor tube  22 , and preferably serves as a lower resting position for the float  30  in the event of a very low level or empty tank condition. A printed circuit board (PCB—not shown) is positioned within the sensor tube  22  and preferably extends along a substantial length thereof. A plurality of reed switches (not shown) are located along the length of the PCB. The reed switches are responsive to one or more magnets (not shown) located in the float  30  for creating a liquid level signal in a well-known manner as the float rides along the sensor tube  22  in response to a change in liquid level within the tank. Although not shown, insulating material, such as heat-shrink tubing, potting material, and so on, is preferably located between the PCB and the sensor tube  22  to insulate and protect the reed switches and other components against shock, vibration, and other harsh conditions to which the transducer  10  may be exposed. Potting material (not shown) may also be located at the upper end  24  of the sensor tube  22  to provide strain relief for the electrical wires  40  ( FIGS. 3 ,  4 ,  6  and  7 ) that extend between the PCB and an electrical connector  42 , as well as vibration protection for the PCB and its interface with the wires. 
         [0037]    It will be understood that the sensor tube  22  can also contain other sensors besides liquid level, in particular temperature, which would provide information to the heating circuit for controlling circulation of the heating fluid through the heating unit  16 . Wiring connections and any circuitry required for the sensing is preferably located within the sensor tube and inside a sealed compartment within the mounting head  14  and sensor tube connection. 
         [0038]    Although a reed-switch-type probe has been shown and described, it will be understood that the present invention is not limited thereto. It will be understood that other linear-type liquid level measurement sensors can be used, including but not limited to, capacitance, heated wire, ultrasonic, optical, and so on, as well as non-linear-type sensors such as resistance-type pivoting float arms. 
         [0039]    As best shown in  FIGS. 6-8 , the heating unit  16  preferably includes a single piece or length of tube that is bent into the tortuous shape as shown and includes a first upper segment  44  that extends generally horizontally and is fluidly connected to a fluent heat source (not shown) such as such as engine coolant, oil, hot exhaust gases and so on, in order to provide constant or selective intermittent circulation of heating fluid to warm the contents of the tank  11  ( FIG. 1 ). A first upright leg  46  extends generally vertically downwardly from the first upper segment  44  and is connected to a second generally vertically extending upright leg  48  via a first lower generally U-shaped bend  50  extending therebetween. A third generally vertically extending upright leg  52  is in turn connected to the second leg via an upper generally U-shaped bend  54 . Likewise, a fourth generally vertically extending upright leg  56  is in turn connected to the third leg  52  via a second lower generally U-shaped bend  58  that is vertically higher than the first bend  50 . The first and second lower U-shaped bends  50 ,  58  are preferably connected to the support block  28  for providing stability at the lower end of the transducer  10 . The fourth leg  56  is in turn fluidly connected to a second generally horizontally extending upper segment  60 , which is in turn fluidly connected to the fluent heat source. 
         [0040]    In order to eliminate the need for an internal tank restraint and provide greater structural integrity for the transducer  10 , the sensor tube  22  and fluid supply tube  18  are preferably securely connected to the heating tube  16  and to each other via clips  62  and  64 . However, it will be understood that the parts can be connected together through any well-known connection means, including but not limited to, adhesives, welding, other types of mechanical fastening, and so on. 
         [0041]    A substantial portion of the fluid supply tube  18  preferably extends adjacent to the first leg  46  of the heating tube  16 . However, it will be understood that the supply tube  18  can alternatively be located adjacent to the fourth leg  56 . The supply tube  18  preferably includes a generally horizontally extending upper segment  68  that extends through the mounting head  14 . The supply tube  18  is adapted for connection to a pump (not shown) or the like in a well-known manner for delivering liquid from the tank  11  to a remote location. The supply tube  18  preferably extends to an empty level position inside the tank adjacent to the lower U-shaped bends  50 ,  58 . If desired, a filter (not shown) can be located at the lower end of the supply tube  18  inside the tank. 
         [0042]    The tortuous shape of the heating tube  16  is particularly advantageous since the four upright legs  46 ,  48 ,  52  and  56  increase the amount of heating tube surface area installed in the tank and create a space or volume  66  within the tank  11  that is more quickly heated than the surrounding area. When the heating tube carries warm fluid, such as engine coolant, the heat transferring from the heating tube is used to thaw or prevent freezing of the tank contents surrounding the sensor as well as the supply tube  18  located within the space  66 . Increasing the amount of surface area of the heating tube  16  increases the amount of heat transfer in a given amount of time. This reduces the potential for freezing of the tank contents in the area of the sensor and supply tubes at lower temperatures and causes quicker thawing of the contents at a given temperature than if the heating tube  16  were constructed with less segments. 
         [0043]    As shown in  FIGS. 3-6  and  9 , the mounting head  14  preferably includes a cover  70  connected to a mounting plate  72  which is in turn connected to the tank  11  ( FIG. 1 ). The cover  70  together with the mounting plate  72  create a hollow interior through which the segments  44 ,  60  of the heating tube  16  and the segment  68  of the supply tube  18  preferably extend. A transfer block  74  is secured to the mounting plate  72  and includes passages for receiving the heating tube and supply tube segments, as well as an opening for receiving the electrical wires  40  and connector  42 . A valve assembly  76  extends into the transfer block  74  and is in fluid communication with the segment  44  of the heating tube and the fluent heating source (not shown). 
         [0044]    Referring now to  FIGS. 10 and 11 , a liquid level transducer  110  in accordance with a further exemplary embodiment of the invention is illustrated. The liquid level transducer  110  preferably extends into a container (not shown), such as a fuel tank, oil reservoir, radiator, brake fluid chamber, or any other container for holding and/or transporting a liquid (not shown). In accordance with one preferred application of the invention, the transducer  110  is particularly useful for liquids that have a tendency to freeze at lower temperatures, such as diesel exhaust fluids (DEF) in a NOX emissions control system. Such fluids can include, but are not limited to, water, urea, ammonia, and combinations thereof. 
         [0045]    With additional reference to  FIG. 12-15 , the transducer  110  preferably includes a mounting head  114 , an elongate sensing probe  112  extending through the mounting head  114  and downwardly therefrom, a helically-shaped heating unit  116  extending through the mounting head  114  and spiraling around the sensing probe  112 , a fluid supply tube  118  extending through the mounting head  114  and along a substantial length of the heating unit  116 , and a liquid return tube  120  extending through the mounting head  114 . 
         [0046]    As best shown in  FIGS. 10 ,  11 ,  13 ,  16  and  17 , the sensing probe  112  preferably senses liquid level in a linear direction and, in accordance with one preferred embodiment of the invention, includes an outer sensor tube  122  with an upper end  124  that extends through the mounting head  114  and a lower end  126  with a stop flange  128 . A float  130  is preferably cylindrically-shaped and includes a central bore  132  (shown in hidden line in  FIG. 16 ) that is sized to receive the sensor tube  122  so that the float slides freely therealong. The stop flange  128  provides a lower resting position for the float  130  in the event of a very low level or empty tank condition. 
         [0047]    A printed circuit board (PCB)  134  is positioned within the sensor tube  122  and preferably extends along a substantial length thereof. A plurality of reed switches (not shown) are located along the length of the PCB  134 . The reed switches are responsive to one or more magnets (not shown) located in the float  130  for creating a liquid level signal in a well-known manner as the float rides along the sensor tube  122  in response to a change in liquid level within the tank. 
         [0048]    Insulating material  136 , such as heat-shrink tubing, potting material, and so on, is preferably located between the PCB  134  and the sensor tube  122  to insulate and protect the reed switches and other components against shock, vibration, and other harsh conditions to which the transducer  110  may be exposed. Potting material  138  ( FIG. 13 ) is located at the upper end  124  of the sensor tube  122  to provide strain relief for the electrical wires  140  and vibration protection for the PCB  134  and its interface with the wires. A potting grommet  142  is received over the PCB  134  for limiting the height of the potting material during assembly and curing. A cushion  133  ( FIG. 17 ) is preferably located with the sensor tube  122  and surrounds the PCB  134  below the stop flange  138  for providing further protection against vibration and undesired forces that may otherwise be present on the PCB during shipping, installation and/or operation. The sensor tube  122  can also contain other sensors besides liquid level, in particular temperature, which would provide information to the heating circuit for controlling circulation. Wiring connections and any circuitry required for the sensing is preferably located within the sensor tube and inside a sealed compartment above the mounting head  114  and sensor tube connection. 
         [0049]    Although a reed switch type probe has been shown and described, it will be understood that the present invention is not limited thereto. Other linear-type liquid level measurement sensors can be used, including but not limited to, capacitance, heated wire, ultrasonic, optical, pivoting float arm, and so on, as well as non-linear-type sensors such as resistance-type pivoting float arms. 
         [0050]    As best shown in  FIGS. 10 ,  11  and  14 - 17 , the heating unit  116  is preferably in the form of a single, elongate tube with a first leg  144  and a second leg  146  and a generally U-shaped bend  148  extending therebetween. The first and second legs  144  and  146  include straight upper segments  150  and  152 , respectively, that extend through the mounting head  114 . The upper ends of the segments  150 ,  152  are adapted for connection to supply and return conduits (not shown) of a fluent heat source, such as such as engine coolant, oil, hot exhaust gases and so on, in order to provide constant and/or intermittent circulation of heating fluid to warm the contents of the tank (not shown). In order to eliminate the need for an internal tank restraint and provide greater structural integrity for the transducer  110 , the sensor tube  122  is preferably securely connected to the heating tube  116 . When the sensor tube and heating tube are constructed of metallic material, such as stainless steel, the parts are preferably welded together. However, it will be understood that the parts can be connected together through any well-known connection means, including but not limited to, adhesives, ultrasonic welding, mechanical fastening, and so on. 
         [0051]    A substantial portion of the fluid supply tube  118  is preferably connected to the first leg  144  of the heating tube  116  and thus spirals around the sensing probe therewith. However, it will be understood that the supply tube  118  can alternatively be connected to the second leg  146 . The supply tube  118  preferably includes a straight upper segment  154  that extends through the mounting head  114 . The supply tube  118  and return tube  120  are adapted for connection to a pump or the like in a well-known manner for delivering liquid from the tank (not shown) on which the transducer is mounted to a remote location and returning unused liquid back into the tank. 
         [0052]    The extension of the fluid supply and return tubes into the tank can be inside of the helical heating tube  116  or parallel on the same diameter. The fluid return tube does not have to extend far into the tank, but can if desired. The supply tube  118  preferably extends into the tank to the empty level inside the tank adjacent the U-shaped bend  148 . If desired, a filter (not shown) can be located at the lower end of the supply tube  118  inside the tank. 
         [0053]    As shown in  FIGS. 10 and 18 , the helical configuration of the heating tube  116  is especially advantageous in that the helical coil can be made larger in diameter than the mounting head  114  ( FIG. 10 ) and the opening  156  ( FIG. 18 ) in the tank wall  158  to which the transducer  110  is mounted. As shown in  FIG. 18 , the major or outside diameter C of the heating tube  116  is larger than the diameter A of the tank opening  156 , which is in turn larger than the minor or inside diameter B of the heating tube  116 . By way of example, and in accordance with a preferred embodiment of the invention, the maximum major diameter C can be calculated as follows: 
         [0000]        C=A +( A−B ) 
         [0054]    For a 5-inch tank opening A and a 2.5-inch minor diameter B, the major diameter C of the heating tube  116  is approximately 7.5 inches, a significantly larger heating tube area that the contents of the tank will be exposed to over prior art solutions. 
         [0055]    As shown in  FIG. 10 , the distance or spacing  160  between adjacent coils is preferably greater than a thickness of the tank wall  158  ( FIG. 18 ) to which the transducer  110  will be mounted so that the thickness of the tank wall at the tank opening  156  can be cleared during the installation process. In this manner, the transducer  110  can be screwed into a tank opening  156 , preferably with the float lifted to the upper portion of the sensing tube  122  just below the mounting head  114 , with the tank opening being much smaller in diameter than the outside diameter of the coils of the helically-shaped heating tube  116 . With a larger diameter helically-shaped heating tube  116 , the amount of heater tubing surface area installed in the tank is significantly increased. When the coil carries warm fluid, such as engine coolant, the heat transferring from the coil is used to thaw or prevent freezing of the tank contents surrounding the sensor as well as the supply and return tubes. Increasing the amount of surface area of the heater tubing increases the amount of heat transfer in a given amount of time. This reduces the potential for freezing of the tank contents in the area of the sensor and supply tubes at lower temperatures and causes quicker thawing of the contents at a given temperature than if the coils of the heating tube  116  were constructed with a smaller diameter. 
         [0056]    Referring now to  FIG. 19 , a liquid level transducer  180  in accordance with yet another embodiment of the invention is illustrated. The liquid level transducer  180  preferably extends into a container  11  and preferably includes a mounting head  14 , an elongate sensing probe  12  extending through the mounting head  14  and downwardly therefrom with a float  30  movable along the length of the probe  12  as previously described, a first or inner heating unit  16  extending through the mounting head  14  and bending around the sensing probe  12 , a second or outer heating unit  116  spiraling around the inner heating unit  16  and a fluid supply tube  18  extending through the mounting head  14  and along a substantial length of the heating unit  16 . The inner and outer heating units are similar in construction to the heating units previously described, with the inner heating unit  16  being sized to slip through the tank opening and the outer heating unit  116  having an outer diameter, as previously described, that is larger than the tank opening so that the liquid level transducer  180  turned or twisted through the tank opening to install the transducer in the tank. With this arrangement, the inner and outer heating units provide more surface area for thawing or warming the fluid to be measured at an increased rate without increasing the overall size of the liquid level transducer so that it can fit within a standard tank opening. 
         [0057]    Referring now to  FIGS. 20 and 21 , a lower portion of a liquid level transducer  190  in accordance with a further embodiment of the invention is illustrated. The liquid level transducer  190  preferably includes a sensor tube  192  located within a heating unit which preferably includes an inner heating fluid return tube  194  which is in turn located within an outer heating fluid supply tube  196 . The sensor tube  192  is preferably connected to the outer supply tube  196  via a connector  198  that preferably includes a hub  200  that preferably encircles and connects to the inner return tube  194  and spokes  202  that extend radially outwardly from the hub  200  and connect to the outer supply tube  196 . A lower end  204  of the outer supply tube  196  preferably tapers toward the sensor tube  192  to create an internal chamber  206  that communicates with both the inner return tube and outer supply tube. In operation, heating fluid from a fluid source (not shown) such as previously described, is directed down into the outer supply tube  196 , as shown by arrows  208 , to thereby heat the outer tube and the contents within the tank in the vicinity of the outer tube, and then up into the inner return tube  194 , as shown by arrows  210 ,  212 , where it exits the transducer  190 . It will be understood that the inner tube  194  can alternatively receive heating fluid and the outer tube  196  can function as the fluid return conduit without departing from the spirit and scope of the invention. 
         [0058]    The inner return tube  194  and/or outer supply tube  196  can be constructed of stiff or flexible material. In accordance with one preferred embodiment of the invention, the inner tube  194  is constructed of a flexible material that is compatible to the heating fluid such as rubber, polyurethane, vinyl, and so on, while the outer tube  196  is constructed of a more rigid or stiff material such as stainless steel, aluminum, other metals, and so on. However, it will be understood that the inner and outer tubes can be constructed of any suitable materials without departing from the spirit and scope of the invention. 
         [0059]    The sensor tube  192  preferably houses a liquid level probe such as a reed-switch-type probe as previously shown and described. However, it will be understood that the present invention is not limited thereto as other linear-type liquid level measurement sensors can be used, including but not limited to, capacitance, heated wire, ultrasonic, optical, and so on, as well as non-linear-type sensors such as resistance-type pivoting float arms 
         [0060]    Referring now to  FIGS. 22 and 23 , a liquid level transducer  220  in accordance with yet another embodiment of the invention is illustrated. The transducer  220  preferably includes a mounting head  222 , an elongate sensing probe  224  extending through the mounting head  114  and downwardly therefrom, and a helically-shaped heating unit  226  extending through the mounting head  222  and spiraling around the sensing probe  224 . 
         [0061]    The sensing probe  224  is preferably similar in construction to the sensing probe  112  with float  130  as previously described. The heating unit  226  preferably includes an inner heating fluid return tube  228  located within an outer heating fluid supply tube  230 . The inner tube  228  is preferably constructed of a flexible material that is compatible to the heating fluid such as rubber, polyurethane, vinyl, and so on, while the outer tube  230  is constructed of a more rigid or stiff material such as stainless steel, aluminum or other metals, so that the heating unit  226  can be shaped in a quick and easy manner during manufacture through simple bending operations. A lower end  231  of the outer heating unit is sealed so that the heating fluid remains in the heating unit during use. The heating unit  226  in this embodiment is easier to manufacture and requires less material than the spiral heating tube previously described with reference to  FIGS. 10 and 19  since the heating unit does not need to spiral back up as in the previous embodiments. As described in the  FIG. 19  embodiment, the inner tube  228  of the present embodiment can alternatively receive heating fluid and the outer tube  230  can function as the fluid return conduit without departing from the spirit and scope of the invention. 
         [0062]    In accordance with one preferred embodiment of the invention, the heating unit  226  has an outer diameter that is larger than the tank opening, as previously described with respect to  FIG. 18 . In accordance with another preferred embodiment of the invention, the heating unit  226  has an outer diameter that is smaller than the tank opening so that the liquid level transducer  220  can be installed straight into the tank without the need to twist the transducer. 
         [0063]    Rods  232  and  234  or other support structure can extend between the mounting head  222  and a lower base member  236  to provide added support to the liquid level transducer  220 . 
         [0064]    It will be understood that the term “preferably” as used throughout the specification refers to one or more exemplary embodiments of the invention and therefore is not to be interpreted in any limiting sense. 
         [0065]    It will be further understood that the term “connect” and its derivatives refers to two or more parts capable of being attached together either directly or indirectly through one or more intermediate members. In addition, terms of orientation and/or position as may be used throughout the specification denote relative, rather than absolute orientations and/or positions. 
         [0066]    It will be further understood that terms of orientation and/or position as may be used throughout the specification, such as upper and lower, horizontal and vertical, inner and outer, and so on, refer to relative rather than absolute orientations and/or positions. 
         [0067]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.