Patent Publication Number: US-2015075278-A1

Title: Fluid level detector

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 12/850,015, filed Aug. 4, 2010, now U.S. Pat. No. 8,850,883 issued Oct. 7, 2014, which is a continuation of U.S. patent application Ser. No. 12/106,371, filed on Apr. 21, 2008, now U.S. Pat. No. 7,770,447 issued Aug. 10, 2010, which is a continuation of U.S. patent application Ser. No. 11/032,976, filed on Jan. 10, 2005, entitled “Fluid Level Detector,” now U.S. Pat. No. 7,360,417 issued Apr. 22, 2008, the entire disclosures of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to fluid level detectors for use with various sized containers. More particularly, the present disclosure relates to a fluid level detector including a piezoelectric element that may be used to determine the presence or absence of a fluid within the container to which the fluid level detector is attached. 
     BACKGROUND OF THE INVENTION 
     The use of piezoelectric materials in fluid level sensors is known. An existing design includes two piezoelectric sensor elements mounted opposite each other on the inside of a container. The sensor elements are both mounted at the level of interest. A first sensor element functions as a transmitter and is electrically excited with a voltage pulse or continuous frequency such that it transmits an acoustic signal. The second sensor element functions as a receiver of the transmitted acoustic signal. When both sensor elements are immersed in a fluid, the acoustic signal generated by the first sensor propagates through the fluid and is detected by the second sensor element, thereby indicating the presence of fluid at the level of the sensor elements. In the presence of air, the acoustic signal is not detected by the second sensor element, indicating that fluid is not present at the level of interest. 
     As noted, existing fluid level sensors often require intimate contact between the sensor elements and the fluid being detected. As well, because the sensor elements are typically mounted inside the container, the structural integrity of the container must be breached to install the sensor elements. As such, the container must usually be empty, or at least not have fluids at or above the level of interest, when the sensor elements are being installed. 
     SUMMARY OF THE INVENTION 
     The present disclosure recognizes and addresses the foregoing considerations, and others, of prior art constructions and methods. Accordingly, it is an object of the present disclosure to provide an improved fluid level detector. 
     This and other objects are achieved by a transducer for use in a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The transducer includes a piezoelectric element that outputs an ultrasonic signal in response to an input electrical signal and a lens with an upper portion and a lower portion. The piezoelectric element is coupled to the upper portion of the lens so that, when the lens is disposed adjacent the outer surface of the wall such that the lens is intermediate the piezoelectric element and the wall, the lens focuses the ultrasonic signal toward the wall. 
     Another embodiment of the present disclosure includes a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The fluid detector includes a piezoelectric element that outputs a first ultrasonic signal in response to an input electrical signal and a lens with an upper portion and a lower portion. The piezoelectric element is coupled to the upper portion of the lens so that, when the lens is disposed adjacent the outer surface of the wall such that the lens is intermediate the piezoelectric element and the wall, the lens focuses the first ultrasonic signal toward the wall so that the first ultrasonic signal enters the wall. The fluid detector further includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The fluid detector further includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal. 
     Yet another embodiment of the present disclosure includes a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The fluid detector includes a polymer piezoelectric element that outputs a first ultrasonic signal in response to an input electrical signal and a polymer lens with an upper portion and a lower portion. The piezoelectric element is coupled to the upper portion of the lens so that, when the lens is disposed adjacent the outer surface of the wall such that the lens is intermediate the piezoelectric element and the wall, the lens focuses the first ultrasonic signal toward the wall so that the first ultrasonic signal enters the wall. The fluid detector further includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal. 
     A further embodiment of the present disclosure includes a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The fluid detector includes a housing, an electrical signal source, and a piezoelectric element disposed in the housing that outputs a first ultrasonic signal in response to an input electrical signal provided by the signal source. A spring is included having an electrically conductive element, wherein the spring is electrically coupled between the electrical signal source and the piezoelectric element so that the spring conducts the input electrical signal between the electrical signal source and the piezoelectric element. The spring is also disposed in the housing in operative communication with the piezoelectric element so that, when the housing is disposed adjacent the outer surface of the wall, the spring biases the piezoelectric element operatively toward the outer surface of the wall so that the first ultrasonic signal is directed to the wall. The fluid detector further includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal. 
     Another embodiment of the present disclosure includes a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The fluid detector includes a housing, an electrical signal source, and a piezoelectric film element disposed in the housing that outputs a first ultrasonic signal in response to an input electrical signal provided by the electrical signal source. The piezoelectric film element has a top side and a bottom side. A lens with an upper portion and a lower portion is included, and the upper portion of the lens is coupled to the bottom side of the piezoelectric film element. A first spring with an electrically conductive element is electrically coupled between the electrical signal source and the top side of the piezoelectric film element so that the first spring conducts the input electrical signal between the electrical signal source and the piezoelectric film element. The first spring is also disposed in the housing in operative communication with the piezoelectric film element so that, when the housing is disposed adjacent the outer surface of the wall, the first spring biases the piezoelectric film element operatively toward the outer surface of the wall so that the first ultrasonic signal is directed to the wall. A second spring having an electrically conductive element is electrically coupled to the bottom side of the piezoelectric film element. The fluid detector also includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal. 
     Yet another embodiment of the present disclosure includes a fluid detector for determining a presence of a fluid within a container, the container having a wall with an outer surface and an inner surface. The fluid detector includes a housing having a base and a piezoelectric element disposed in the housing that outputs a first ultrasonic signal in response to an input electrical signal. An adhesive layer is disposed between the base and the outer surface of the wall, thereby securing the housing to the wall when the housing is disposed in a position adjacent the wall so that the first ultrasonic signal is directed to the wall. The fluid detector also includes an ultrasonic detector that, when disposed in a predetermined position adjacent the outer surface of the wall, receives a second ultrasonic signal from the wall that results from the first ultrasonic signal and that is affected in a predetermined manner by presence or absence of fluid at the inner surface of the wall. The ultrasonic detector generates an output electrical signal corresponding to the second ultrasonic signal. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the fluid level detector and, together with the description, serve to explain the principles of the fluid level detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the fluid level detector, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the accompanying figures, in which; 
         FIG. 1  is an exploded, perspective view of a fluid level detector in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a side view of the assembled fluid level detector as shown in  FIG. 1 ; 
         FIG. 3  is a bottom view of the assembled fluid level detector as shown in  FIG. 1 ; 
         FIG. 4A  is an exploded, perspective view of a sensor assembly of the level detector as shown in  FIG. 1 ; 
         FIG. 4B  is a top perspective view of the assembled sensor assembly as shown in  FIG. 4A ; 
         FIG. 5A  is a side, cross-sectional view of the sensor assembly as shown in  FIG. 4B , taken along line  5 A- 5 A; 
         FIG. 5B  is a side, cross-sectional view of the sensor assembly as shown in  FIG. 4B , taken along line  5 B- 5 B; 
         FIG. 6  is a detailed, partial cross-sectional view of the sensor assembly as shown in  FIG. 5B ; 
         FIG. 7A  is a side, cross-sectional view of the fluid level detector as shown in  FIG. 3 , along line  7 A- 7 A; 
         FIG. 7B  is a side, cross-sectional view of the fluid level detector as shown in  FIG. 7A , positioned adjacent a container wall; 
         FIGS. 8A and 8B  are electrical schematics of an electronic module for use with a fluid level detector as shown in  FIG. 1 ; 
         FIGS. 9A and 9B  are electrical schematics of an excitation circuit for use in an electronic module as shown in  FIGS. 8A and 8B ; and 
         FIGS. 10A and 10B  are graphical representations of signals processed by the electronic module as shown in  FIGS. 8A and 8B  and  9 A and  9 B. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the fluid level detector according to the disclosure. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the fluid level detector, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation, not limitation, of the fluid level detector. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present fluid level detector without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalence. 
     Referring now to  FIG. 1 , a fluid level detector  100  includes a bottom housing  110 , a top housing  130 , a wiring harness  140 , and a sensor assembly  160  ( FIGS. 4A and 4B ). As described in more detail below, sensor assembly  160  comprises an ultrasonic transducer. Bottom housing  110  includes a generally cylindrical upper wall  112  and a disc-shaped base  118 . Upper wall  112  defines a generally cylindrical central bore  114  disposed about the longitudinal center axis of bottom housing  110 . Upper wall  112  further includes an annular groove  116  extending inwardly from its outer surface and an annular lip  117  extending outwardly from its outer surface. Base  118  defines an aperture  120  ( FIG. 7A ) that it is in communication with the central bore  114  and a bottom surface  122  that is configured for abutment with a container wall  104  ( FIG. 7B ). Aperture  120  is transverse to the longitudinal center axis of central bore  114  and has a diameter that is smaller than the diameter of central bore  114 . 
     Top housing  130  includes a top portion  136  and a substantially cylindrical wall  132  extending downwardly therefrom. Cylindrical wall  132  is configured to slidably receive upper wall  112  of bottom housing  110 . An annular groove  133  ( FIGS. 7A and 7B ) extends outwardly into the cylindrical wall  132  of top housing  130  and is located and configured such that when upper wall  112  of bottom housing  110  is adequately inserted into cylindrical wall  132 , annular lip  117  on upper wall  112  is firmly seated in annular groove  133 , thereby securing bottom housing  110  and top housing  130  together. A wiring harness receptacle  134  for slidably receiving wiring harness  140  is formed in cylindrical wall  132 . Preferably, both bottom housing  110  and top housing  130  are formed from molded polymers such as, but not limited to acrylonitrile butadiene styrene. However, it should be appreciated that any suitable material could be utilized. 
     Prior to assembling bottom housing  110  and top housing  130 , an O-ring  150  is positioned in annular groove  116  of upper wall  112 . O-ring  150  serves to prevent dirt, debris, and humidity from entering fluid level detector  100  after bottom and top housings  110  and  130  are assembled. 
     Referring now to  FIGS. 4A and 4B , transducer  160  includes a lens  162 , a piezoelectric element  170 , a conductive sleeve  180 , an insulative disc  186 , a tab contact  190 , and a conductive pad  194 . Lens  162  is generally cylindrical in shape and includes an annular ledge  168  disposed between upper and lower portions of lens  162  and extending radially from the main body of the lens. The upper portion of lens  162  includes a domed surface  164  having a conductive layer  163  ( FIG. 6 ) formed thereon. The lower portion of lens  162  defines a contact face  166  that abuts a container wall  104  ( FIG. 7B ) when fluid level detector  100  is installed for operation. Preferably, lens  162  is formed of a material exhibiting low acoustic loss and an acoustic impedance similar to that of piezoelectric element  170  and container wall  104 . Preferably, lens  162  is constructed of polystyrene, REXOLITE, or other similar materials. As well, an example of a suitable material for conductive layer  163  is copper, although it should be appreciated that many materials exhibit suitable electrical conductivity and could be utilized. 
     Piezoelectric element  170  is preferably a flexible piezoelectric film element (preferably a suitably processed polyvinylidene fluoride copolymer (PVDF)) having a top surface  172  and a bottom surface  174 . A first electrode layer  176  ( FIG. 6 ) and a second electrode layer  178  ( FIG. 6 ) are formed on top and bottom surfaces  172  and  174 , respectively. First and second electrode layers  176  and  178  are isolated electrically from each other by piezoelectric film element  170 . Because piezoelectric film element  170  is flexible, it conforms to the shape of domed surface  164  of lens  162  when secured thereto. 
     Conductive sleeve  180  is substantially cylindrical and includes an inwardly depending lip  182  at the top end and an edge  184  at the bottom end that is configured to abut annular ledge  168  of lens  162  when transducer  160  is assembled. Further, the inner diameter of conductive sleeve  180  is slightly larger than the outer diameter of the upper portion of lens  162  such that lens  162  is partially insertable into conductive sleeve  180 . Preferably, conductive sleeve  180  is formed of stainless steel, or other similarly conductive materials. 
     Insulative disc  186  defines a central aperture  188  that is configured to receive a portion of tab contact  190 . The outer diameter of insulated disc  186  is slightly less than the inner diameter of conductive sleeve  180  such that insulative disc  186  can be disposed inside conductive sleeve  180 , adjacent inwardly depending lip  182 . Disc  186  is formed of any material suitable for the purpose of insulating conductive sleeve  180  from tab contact  190 , preferably a polymer such as, but not limited to, acrylonitrile butadiene styrene, for example marketed under the name CYOLAC MG94 by GE Plastics. 
     Tab contact  190  includes a portion that is insertable into central aperture  188  of disc  186  and a planar surface  192  having a diameter greater than that of central aperture  188 . As such, planar surface  192  prevents the passage of tab contact  190  through central aperture  188 . Preferably, tab contact  190  is formed of nickel plated brass. However, other similarly electrically conductive materials are acceptable. A conductive pad  194  is comprised of foam with a nickel plating and has an outer diameter such that it is at least partially insertable into central aperture  188  of insulative disc  186 . Although plated, conductive pad  194  remains pliant and thereby facilitates electrical contact of conductive pad  194  with both piezoelectric film element  170  and tab contact  190 . After transducer  160  is assembled, piezoelectric film element  170  is disposed between domed surface  164  of lens  162  and conductive pad  194  ( FIG. 6 ). 
     As previously noted, and referring also to  FIG. 6 , piezoelectric film element  170  includes first and second electrode layers  176  and  178  formed respectively on top and bottom surfaces  172  and  174  of piezoelectric film element  170 . During assembly, piezoelectric film element  170  is adhesively secured to domed surface  164  such that second electrode layer  178  and conductive layer  163  are adjacent to and in electrical contact with each other, as best shown in  FIG. 6 . Preferably, piezoelectric film element  170  is secured to domed surface  164  with cynoacrylate, although other adhesives, such as silver filled epoxies are acceptable. When securing piezoelectric film element  170  to domed surface  164 , electrical contact is maintained between second electrode layer  178  and conductive layer  163  by mechanical contact. 
     First and second electrode layers  176  and  178  are formed by plating opposing sides of piezoelectric film element  170  with a combination of platinum and gold, and conductive layer  163  is formed on domed surface  164  from copper. These materials are merely provided as examples of suitable coatings, although it should be noted that other similarly conductive materials can be used in other embodiments. Preferred piezoelectric film element  170  is a PVDF film as available from Ktech, Inc., 1300 Eubank Blvd., SE, Albuquerque, N. Mex., 87123-3336. Although embodiments are envisioned wherein multiple transducers  160  are used in combination to detect the presence of fluids, preferred embodiments utilize a single transducer  160  wherein lens  162  and piezoelectric film element  170  not only transmit acoustic signals, but also act as an ultrasonic receiver for detecting return signals. 
     Next, insulative disc  186  is secured inside conductive sleeve  180  adjacent inwardly depending lip  182 . As noted, conductive sleeve  180  is comprised of stainless steel, and insulative disc  186  is formed of a polymer. Insulative disc  186  is secured to conductive sleeve  180  adjacent inwardly depending lip  182  by dimpling conductive sleeve  180  such that it grips insulative disk  186 . However, various methods, such as gluing or tacking, are acceptable for use with various other embodiments. Tab contact  190  is inserted into central aperture  188  of insulative disc  186 , and conductive pad  194  is secured to the bottom portion of contact tab  190  by a conductive, pressure-sensitive adhesive (not shown). As such, conductive pad  194  extends downwardly from tab contact  190  and into the interior of conductive sleeve  180 . 
     Conductive sleeve  180  is passed over the upper portion of lens  162  until bottom edge  184  of conductive sleeve  180  abuts lens annular ledge  168 . Once positioned, conductive sleeve  180  is dimpled about lower edge  184  so that it grips lens  162 . So positioned, conductive pad  194  is in mechanical and electrical contact with first electrode layer  176  of piezoelectric film element  170 , as best seen in  FIG. 6 . 
     Referring to  FIG. 7A , an adhesive layer  152  for securing fluid level detector  100  to a container wall is secured to bottom surface  122  of bottom housing  110 . Preferably, adhesive layer  152  comprises a layer of double-sided tape having a pressure-sensitive adhesive on both sides. Double-sided tape layer  152  has the same diameter as bottom surface  122  of base  118  and has an aperture  152  formed at its center. Aperture  152   a  has a diameter at least equal to that of aperture  120  ( FIGS. 7A and 7B ). After securing double-sided tape layer  152  to bottom surface  122 , a non-stick, peel-away film is adhered to the side of the double-sided tape layer  152  opposite to that which is secured to bottom surface  122 . The peel-away film (not shown) is a solid piece of film similar to those typically found on stickers and double-sided tape, that is not removed until fluid level detector  100  is to be installed. As such, the peel-away film inhibits debris, dust, and moisture from entering the housing by way of aperture  120  prior to the use of fluid level detector  100 . 
     A coupling layer  156  is adhered to contact face  166  ( FIG. 5A ) of lens  162 . As shown, the diameter of coupling layer  156  is substantially similar to the diameter of contact face  166  and slightly less than the diameter of aperture  152   a  formed in the double-sided tape layer  152 , as best seen in  FIG. 3 . Desirable materials for coupling layer  156  are capable of being formed in thin sections to minimize acoustic losses, able to conform to surface irregularities, exhibit low acoustic loss, have acoustic impedances that closely match those of most polymers, and are non-aqueous in nature to facilitate extended periods of use. Examples of suitable coupling materials include: urethane, neoprene, thixotropic glycerin, and high-temperature grease, although it should be appreciated that other suitable materials can be utilized. 
     Next, transducer  160  is slidably received in central bore  114  of bottom housing  110 . Inward motion of transducer  160  is limited by the peel-away surface (not shown), such that coupling layer  156  lies in the same plain as double-sided tape layer  152 . Note, the greatest outside diameter of transducer  160  is sized such that transducer  160  readily slides within central bore  114  ( FIG. 1 ). 
     Referring to  FIGS. 1 ,  7 A and  7 B, after positioning transducer  160  in central bore  114 , top housing  130  is secured to bottom housing  110 . Top housing  130  includes a pair of electrical contacts secured therein. For ease of description, only first electrical contact  196  is shown. Preferably, first electrical contact  196  includes a base portion  196   a , a male electrode  196   b , and a spring  196   c . Base portion  196   a  is securely held by portions of top housing  130 . Male electrode  196   b  extends from base portion  196   a  and into wiring harness receptacle  134  of top housing  130 . Spring  196   c  extends from base portion  196   a  and makes contact with tab contact  190  of transducer  160 . Spring  196   c  can be of any suitable configuration, such as a leaf spring or coil spring, that biases the transducer into operative contact with the container. Although spring  196   c  can be either metallic or non-metallic, preferred embodiments include metal springs such that the spring itself is an electrically conductive element. 
     The biasing element of the second electrical contact (not shown) depends inwardly from top housing  130  and makes mechanical and electrical contact with conductive sleeve  180  of transducer  160 . The second electrical contact also has a male electrode extending outwardly into wiring harness receptacle  134 . Engagement of annular groove  133  by annular lip  117  maintains bottom housing  110  and top housing  130  in the assembled position. O-ring  150  is disposed in annular groove  116  between bottom and top housings  110  and  130  and helps maintain the structural integrity of fluid level detector  100 . 
     Operation 
     Prior to use, and still referring to  FIGS. 7A and 7B , fluid level detector  100  is first applied to a wall  104  of the container containing the liquid to be monitored. Preferred embodiments of the present fluid level detector  100  are used to monitor fluid levels in polymer containers or containers constructed of other similar materials having maximum wall thicknesses of approximately 0.150 inches (″). For such containers, preferred embodiments of fluid detector  100  include piezoelectric film elements  170  measuring approximately 0.230″ long, 0.125″ wide, and 0.004″ thick, and constructed of PVDF. However, these dimensions can be varied for container walls of varying thicknesses. 
     As is known in the art, materials attenuate acoustic energy as the energy passes there through. Moreover, acoustic energy at higher frequencies is attenuated over shorter distances within a given material than is acoustic energy at lower frequencies. As noted above, preferred embodiments of fluid level detector  100  include a piezoelectric film element  170  composed of a PVDF, which has a relatively high natural operating frequency of approximately 10,000,000 Hz (10 MHz). Thus, preferred embodiments of fluid detector  100  are typically used on polymer containers having maximum wall thicknesses of up to 0.150″ so that adequate return signals exist for fluid detection, as discussed hereafter. 
     Increasing the size (length by width) of piezoelectric film element  170  permits fluid level detector  100  to be used with greater wall thicknesses since a greater amount of wall material is required to attenuate the larger amount of acoustic energy that is generated. This requires a corresponding increase in the domed top surface of lens  162  to accommodate the larger piezoelectric film and focus its resulting acoustic signals. The amount of acoustic energy generated by the transducer can also be increased while maintaining the size of both the domed top surface of lens  162  and piezoelectric film element  170  by “stacking” multiple film elements. By stacking multiple piezoelectric film elements atop each other and electrically connecting them, either in parallel or in series, the amount of acoustic energy generated will be the cumulative amount of that energy generated by each piezoelectric film. 
     Fluid level detector  100  can also be used with greater wall thicknesses when piezoelectric film element  170  is composed of piezoelectric materials with lower natural frequencies since the generated acoustic energy travels farther through the same materials than does the high frequency acoustic energy before being detrimentally attenuated. Moreover, for a given wall thickness, acoustic energy at lower frequencies provides larger return signals than does acoustic energy at higher frequencies. 
     Prior to installing fluid level detector  100  on the container wall, the point on container wall  104  corresponding to the desired level of detection is determined. The installer then removes the peel-away surface (not shown) disposed on the bottom face of double-sided tape layer  152 . With the peel-away surface removed, force exerted on transducer  160  by springs  196   c  (only one is shown) urges transducer  160  along central bore  114  such that contact face  166  of lens  162  extends beyond double-sided tape layer  152 , as shown in  FIG. 7A . Outward motion of transducer  160  caused by springs  196   c  ceases when annular ledge  168  abuts the inwardly depending ledge of bottom housing  110  that defines aperture  120 . Note, contact face  166  and coupling layer  156  extend slightly beyond double-sided tape layer  152  such that full contact between transducer  160  and the container wall  104  is possible. 
     Fluid level detector  100  is then pressed firmly against container wall  104  at the desired location. Double-sided tape layer  152  secures fluid level detector  100  to container wall  104 , as shown in  FIG. 7B . As fluid level detector  100  is pressed against container wall  104 , contact between container wall  104  and contact face  166 , by way of coupling layer  156 , urges transducer  160  back inside central bore  114 . Springs  196   c  maintain pressure on transducer  160 , thereby ensuring proper positioning of contact face  166  adjacent the outer surface  104   a  of container wall  104 . As such, springs  196   c  serve to bias piezoelectric film element  170  operatively toward outer surface  104   a  of wall  104 . 
     By biasing piezoelectric film element  170  operatively toward wall  104 , springs  196   c  bias the piezoelectric film element  170  either directly against wall  104  or against intermediate components, such as lens  162  and coupling layer  156  in the embodiment shown. These components in turn couple the acoustic signal generated by piezoelectric film element  170  to wall  104 . Although in the latter case springs  196   c  still bias piezoelectric film element  170  in the direction of wall  104 , it is possible that in other arrangements springs  196   c  will bias piezoelectric film element  170  in a direction other than toward wall  104 , yet still into coupling elements such that the acoustic signal generated by piezoelectric film element  170  is nevertheless coupled to the wall. In such arrangements, springs  196   c  are said to bias piezoelectric film element  170  operatively toward the wall although they do not bias piezoelectric film element  170  directionally toward the wall. 
     Wiring harness  140  includes female electrode receptacles  142  (only one is shown). Wiring harness  140  is slidably received inside wiring harness receptacle  134  of top housing  130  such that female electrode receptacles  142  are connected with male electrodes  196   b . Electronic signals to and from fluid level detector  100  may now be transmitted to the detector as desired. 
     In preferred embodiments, the input electrical signal to fluid level detector  100  is a ten volt peak-to-peak (ground to +10 volts) pulse lasting approximately 50 nanoseconds, or a twenty-four volt (+12v to −12v) square wave lasting approximately 100 nanoseconds, (hereafter, “excitation signal”). As shown in  FIGS. 7A and 7B , the excitation signal is applied across opposing top and bottom sides  172  and  174  of piezoelectric film  170  by way of two independent electrical paths. In the illustrated embodiment, the first electrical path is as follows: from first electrical contact  196  to tab contact  190  by way of spring  196   c ; from tab contact  190  to conductive pad  194 ; and from conductive pad  194  to first electrode layer  176  formed on top surface  172  of piezoelectric film element  170  (as shown in  FIG. 6 ). The second path is as follows: from the second electrical contact (not shown) to conductive sleeve  180  by way of the spring, from conductive sleeve  180  to conductive layer  163  disposed on domed surface  164 ; and from domed surface  164  to second electrode layer  178  formed on bottom surface  174  of piezoelectric film element  170  (as shown in  FIG. 6 ). 
     Application of the excitation signal creates vibrations in piezoelectric film element  170 . PVDF is used for piezoelectric film element  170  because of its inherently low acoustic impedance and natural frequency of approximately 10 MHz. Typical acoustic impedance values for PVDF range from 2.5×10 6  to 3.0×10 6  Rayleighs (2.5 to 3.0 Mrayls), making the piezoelectric film desirable for transmitting acoustic signals into walls of similar-impedance polymer containers with minimal losses. Most polymers have impedance valves of between about 1.5 and 3.0 Mrayls. 
     Because piezoelectric film element  170  is secured to domed surface  164 , vibrations of piezoelectric film element  170  create pressure fluctuations in the material of lens  162 . As previously noted, lens  162  is preferably constructed of a polystyrene or other like material such that the acoustic impedance of lens  162  will be substantially similar to that of piezoelectric film element  170  and that of wall  104  of the container. Substantially similar acoustic impedance values for the various materials facilitate the passage of acoustic energy as each transmitted signal propagates into the adjacent materials. Preferably, acoustic impedance values of the materials used to construct piezoelectric film element  170 , lens  162 , and container wall  104  are within 2.5 Mrayls of each other. Thus, each acoustic signal generated by the excitation of piezoelectric film element  170  propagates from one component to the next with acceptable energy loss, insuring effective operation of fluid detector  100 . 
     The generated pressure fluctuations propagate through lens  162  until they reach contact face  166 . As shown in  FIGS. 5A and 5B , in a preferred embodiment, each point along domed surface  164  is equidistant from a focal point (f 1 ) located on contact face  166  of lens  162 , this distance being the radius of curvature of the domed surface  164 . Lens  162  thereby focuses a maximum amount of pressure fluctuation at focal point (f 1 ) as the acoustic signal travels from domed surface  164  toward contact face  166 . It should be noted that focal point (f 1 ) need not be located on contact face  166  of lens  162 . For example, preferred embodiments have focal points (f 1 ) located on inner surface  104   b  of container wall  104 , as shown in  FIG. 7B , provided the width of both coupling layer  156  and container wall  104  are known. Similarly, focal points (f 1 ) may be selected that are located within lens  162 , wall  104 , or the fluid to be detected. 
     The piezoelectric element of the preferred embodiments is a piezoelectric film element  170 . As is known in the art, acoustic output power of piezoelectric films is generally less than that of piezoelectric ceramics in response to comparable input signals. By focusing the pressure fluctuations transferred from piezoelectric film element  170  to lens  162  at focal point (f 1 ), however, lens  162  transfers the relatively lower pressure across the surface of the piezoelectric film element to a higher pressure at focal point (f 1 ). Lens  162  thereby delivers a sufficiently strong acoustic signal to container wall  104  to facilitate operation of fluid detector  100 . 
     Lens  162  also functions as an acoustic standoff. More specifically, lens  162  is dimensioned such that reflected acoustic signals are not received at piezoelectric film element  170  until after piezoelectric film element  170  has ceased vibrating in response to application of the excitation pulse. Lens  162  facilitates operation of fluid level detector  100  by insuring that reflected signals for determining the presence or absence of fluids are not received until after the transmission phase of piezoelectric film element  170  has subsided. 
     In the preferred embodiment, the radius of curvature of domed surface  164  of lens  162  is measured from focal point (f 1 ) on inner surface  104   b  of wall  104  ( FIG. 7B ) and is approximately 0.40″. The preferred radius of curvature takes into account the height of lens  162  (0.250″ in the preferred embodiment), the thickness of coupling layer  156 , and the thickness of wall  104 . Considerations for the height of lens  162  can include adequate distance to accomplish acoustic standoff for the frequency of the acoustic signal being used and potentially the overall dimensions of fluid level detector  100 . For example, greater lens heights can be required to provide sufficient acoustic standoff for lower frequency acoustic signals used with containers having greater wall thicknesses to avoid detrimental attenuation. 
     Although, ideally, no acoustic impedance mismatch would exist at the respective interfaces between the materials of interfaces piezoelectric film element  170 , lens  162 , and container wall  104 , there will normally be at least slight impedance mismatches. Transmission of the acoustic signal will therefore be affected as it passes from one material to the next. For example, an impedance mismatch likely exists at the boundary of contact face  166  and outer surface  104   a  of container wall  104 , where lens  162  and wall  104  are operatively coupled to allow transmission of acoustic signals therebetween. The impedance mismatch between the materials of lens  162  and container wall  104  causes a portion of the energy of the acoustic signal to be reflected back through lens  162 , eventually reaching piezoelectric film element  170 . The first reflection of acoustic energy causes vibration of piezoelectric film element  170 . In response, piezoelectric film element  170  creates an electrical signal across its electrodes that is sent to an electronics module, as discussed hereafter, as a single pulse or echo. That portion of the acoustic signal that is not reflected at contact face  166  of lens  162  continues to propagate into the next material layer. 
     Transmission of acoustic signals from one material to the next is facilitated when acoustic impedances of the materials are matched and when the two abutting surfaces are in full contact. Coupling layer  156  is therefore preferably disposed between contact face  166  and outer surface  104   a  of container wall  104  and is preferably composed of a material that is sufficiently pliant to accommodate surface irregularities between contact surface  166  and outer surface  104   b , thereby preventing the formation of air pockets between the abutting surfaces that would otherwise degrade propagation of acoustic signals. 
     The material of coupling layer  156  is also chosen to minimize the effects of any acoustic impedance mismatch between the materials of lens  162  and container wall  104 . It is expected that fluid level detector  100  will be used to detect fluid levels in containers constructed of various polymers having acoustic impedance valves in the range of 1.5 to 3 Mrayl. In the event the acoustic impedance of the container wall is not sufficiently matched to the acoustic impedance of lens  162 , a coupling material can be used to improve the impedance match. For example, the acoustic impedance of coupling layer  156  is preferably between the acoustic impedance values of lens  162  and the container wall such that the acoustic signal encounters the overall mismatch incrementally rather than all at once. This facilitates transfer of acoustic energy from lens  162  to coupling layer  156  and from coupling layer  156  to the container wall. 
     The coupling layer, although chosen to enhance acoustic impedance matching, results in two interfaces at which a slight mismatch nevertheless occurs—between lens  162  and coupling layer  156  and between coupling layer  156  and the container wall. The two interfaces result in two reflected signals when the acoustic signal from the piezoelectric element passes through the coupling layer to the wall. As indicated above, the reflected acoustic energy travels back through lens  162  and eventually causes vibration of piezoelectric film element  170 . Preferably, however, coupling layer  156  is sufficiently thin that the second reflection, i.e. due to the coupling layer  156 \container wall  104  interface, arrives at the piezoelectric film at substantially the same time as does the first reflected signal, i.e. due to the lens  162 /coupling layer  156  interface, and for purposes of this discussion, the two reflections are considered to be a single reflection. As described in more detail below, the electronics module is configured to disregard this combined reflection. 
     The remainder of the acoustic signal that has not been reflected at the above noted material interfaces propagates into and through container wall  104  until reaching inner surface  104   b , at which point a third reflection of acoustic energy occurs. The amplitude of the reflected acoustic energy is largely dependent upon the size of the acoustic impedance mismatch that occurs between the material of wall  104  and the material disposed in the container opposite fluid level detector  100 . 
     Air has an approximate acoustic impedance of 407 rayls. Most polymers have acoustic impedances of between 1.5 to 3.0 Mrayls. Thus, a large acoustic impedance mismatch occurs at the inner surface  104   b  of wall  104  when air is located within the container opposite fluid level detector  100 . Thus, the overwhelming majority of energy of the acoustic signal will be reflected at inner surface  104   b  when the container&#39;s liquid level falls below the position at which detector  100  is attached to the wall. Water, on the other hand, has an approximate acoustic impedance of 1.48 Mrayls, notably closer to the values of acoustic impedances for most polymers, and the reflected energy from an interface between inner surface  104   b  and water is therefore small as compared to the reflected energy when air is present. Most fluids have acoustic impedance values similar to that of water, meaning they have essentially the same effect on the acoustic signal as does water. Therefore, when water or other fluid is present opposite fluid level detector  100 , the overwhelming majority of acoustic energy is transmitted from container wall  104  into that fluid, where it eventually dissipates. 
     The third reflected signal (i.e. due to the interface of the inner wall surface and air or liquid) propagates back through container wall  104 , coupling layer  156 , and lens  162  until it reaches piezoelectric film element  170 . As before, the third reflected signal causes vibration of piezoelectric film element  170 , resulting in an electrical signal being created across the film&#39;s electrodes and sent to the electronic module. The voltage of the signal created by piezoelectric film element  170  is proportional to the amount of energy reflected at inner surface  104   b  of wall  104 . Accordingly, a large voltage signal received at the electronic module indicates that air or other gas is present in the container at the level of fluid level detector  100 . Conversely, a small voltage signal received at the electronic module indicates that a fluid is present in the container opposite fluid level detector  100 . 
     Operation of the electronics module will now be discussed with respect to  FIGS. 8 through 10 . Referring initially to  FIGS. 8A and 8B , a dual wire bundle  141  output from wiring harness  140  ( FIG. 1 ) includes a ground wire  200  and a signal wire  202  that carries both the input electrical signal to the transducer and the output electrical signal corresponding to the reflected ultrasonic signal. Wire  202  electrically connects through the wiring harness to the spring  196   c  ( FIG. 7A ) that contacts tab contact  190 , whereas wire  200  electrically connects through the wiring harness to conductive sleeve  180  ( FIG. 7A ) by the second spring (not shown). Dual wire bundle  141  extends from fluid level detector  100  to a printed circuit board remote from the detector and upon which the circuitry shown in  FIGS. 8A and 8B  is disposed. 
     A processor  204  (in a preferred embodiment, a four megahertz single chip microcontroller) disposed on the printed circuit board controls an excitation circuit  206 , a detection circuit  208  and a blanking period generator circuit  210  through the output of high or low signals (for example, +5 volts or ground) on a trace  212 . Generally, this system alternates between an excitation mode, in which excitation circuit  206  provides an input electrical signal to detector  100 , and a detection mode, in which detection circuit  208  receives and notifies the microprocessor of signals corresponding to acoustic echoes from a gas interface at the inner container wall opposite the detector. In a preferred embodiment, the microprocessor triggers the excitation mode once per second such that the electronics module checks the output of fluid level detector  100  for liquid level approximately once per second, although the timing can vary as desired. 
     Immediately prior to the excitation mode, the output of microprocessor  204  on trace  212  is low such that input  214  to a NAND gate  218  is low. The low signal at  212  maintains an input  216  high through a switch such as a MOSFET  215 . The low signal at  214  results in a low signal on line  202  such that no excitation signal is provided to the piezoelectric film of fluid level detector  100 . At the beginning of the excitation mode, however, the microprocessor&#39;s output to trace  212  goes high, thereby immediately bringing input  214  high. Since input  216  is normally high, this causes the output of NAND gate  218  to go low. An inverter  220  changes the low signal to high at an input  222  to a comparator  224  that level-shifts the signal to +10 volts at  226 . A capacitor  228  AC-couples the excitation signal, which passes through a diode  230  to input line  202  and, then, to the electrodes driving the piezoelectric film. 
     Diodes  230  and  232  isolate the return signal from the excitation circuit. As described in more detail below, the return signal on line  202  generated by vibrations of the piezoelectric film in fluid level detector  100  are of relatively low power such that the return signal is insufficient to activate diode  232 . 
     The duration of the high portion of the input electrical signal on line  202  is defined by the RC time constant of a resistor  234  and a capacitor  236 . More specifically, the high signal on trace  212  does not immediately cause MOSFET  215  to bring input  216  low. Instead,  216  goes low when capacitor  236  charges sufficiently to activate MOSFET  215 . When input  216  goes low, the output of NAND gate  218  goes high, causing the outputs of inverter  220  and comparator  224  to go low. In the illustrated embodiment, the RC time constant of register  234  and capacitor  236  is approximately 50 nanoseconds. Thus, the duration of the input electrical signal pulse output by excitation circuit  206  is approximately 50 nanoseconds. 
     Blanking period generator circuit  210  defines the duration of the excitation mode. Immediately prior to the excitation mode, when the signal on trace  212  is low, an input  238  to a NAND gate  240  is low. Thus, an input  242  to a NAND gate  244  is high, and the value of an output  246  from NAND gate  244  therefore depends upon the signal at an input  248 . Input  248  is the output of detection circuit  208  and, as described in more detail below, is in either a high or low state, depending upon whether detection circuit  208  has received a sufficiently strong signal on line  202 . When detection circuit  208  detects such a signal, the detection circuit places a high signal on trace  248 . This causes NAND gate  244  to transition from high to low at  246 , thereby notifying microprocessor  204  that a signal has been received indicating that fluid level detector  100  has detected air or other gas on the opposite side of the container wall from the detector. 
     Accordingly, as long as trace  242  remains high, NAND gate  244  passes the detection signal from detection circuit  208  to the microprocessor. This condition exists during the detection mode, which is therefore defined by the time period during which either the signal on trace  212  is low or an input at  250  is low. 
     Again referring to the time immediately prior to the excitation mode, the signal on trace  212  is low. Output  242  of NAND gate  240  is therefore high, and NAND gate  244  therefore gates the output of detection circuit  208  to microprocessor  204 . When microprocessor  204  drives the signal on trace  212  high, however, input  238  to NAND gate  240  immediately goes high. Input  250  to NAND gate  240  is normally high during detection mode, and so NAND gate  240  drives trace  242  to a low signal. This causes output  246  of NAND gate  244  to be high regardless of the signal from detection circuit  208  on trace  248 . Accordingly, changes on line  202  caused by return signals detected by fluid level detector  100  have no effect on output  246 , and microprocessor  204  therefore does not receive signals on  246  indicating that such return signals have occurred. In other words, the transition to the high signal on trace  212  starts a period during which blanking period generator  210  blocks detection circuit  208  from reporting detection of a return acoustic signal by fluid level detector  100 . This condition is the excitation mode. 
     The duration of the excitation mode is defined by the RC time constant of a resistor  252  and a capacitor  254 . A MOSFET  256  normally maintains the signal on trace  250  high when the signal on  212  is low. When  212  goes high, the RC network  252 / 254  prevents the new high signal from immediately driving the signal on trace  250  low. When capacitor  254  eventually charges sufficiently to cause MOSFET  256  to drive the signal on trace  250  low, the low signal causes NAND gate  240  to drive the signal on trace  242  high regardless of the high signal on  238 . Thus, NAND gate  244  again passes the output of detection circuit  208  to microprocessor  204 , and the system has returned to detection mode. Microprocessor  204  thereafter drives the signal on trace  212  low, thereby resetting excitation circuit  206  prior to triggering the next excitation mode on the one second interval. 
     In a preferred embodiment, the RC time constant of resistor  252  and capacitor  254  defines the duration of the excitation mode to 5.0×10 −6  seconds (5 ms). This period may vary as desired, however, for example depending upon characteristics of fluid level detector  100  and the timing of signals it is likely to detect. For example, and referring also to  FIG. 10A , the excitation period (indicated at  258 ), and therefore the RC time constant of resistor  252  and capacitor  254 , should be sufficiently long so that blanking period generator circuit  210  blocks the responses of detector circuit  208  to both the input electrical signal generated by excitation circuit  206  and to signals returned on line  202  as a result of ringing of the piezoelectric film following the excitation signal. 
     As described above, the input electrical signal from excitation circuit  206  is a 10 volt pulse lasting approximately 50 nanoseconds. Detection circuit  206  detects this relatively large signal as it is being output onto line  202 . Furthermore, the piezoelectric film in fluid level detector  100  vibrates for some period of time after the end of the 50 nanosecond pulse. This ringing of the film creates a signal across the film&#39;s electrodes that is returned to the detection circuit over line  202 . Thus, during excitation mode, detection circuit  206  sees a relatively large signal, indicated at  260  in  FIG. 10A , that would otherwise cause the detection circuit to incorrectly send a signal to microprocessor  204  indicating an acoustic echo had been received. Because blanking period generator  210  maintains a low signal on trace  242  during the excitation mode, however, NAND gate  244  does not gate this signal to the microprocessor, which therefore sees no false echo report during this period, as indicated at  262  in  FIG. 10B . To assure that detection circuit  208  does not report a false echo, the RC time constant defined by resistor  252  and capacitor  254  should be established so that blanking period generator  210  blocks signals detected by detection circuit  208  for a period longer than the time during which signals resulting directly from the input electrical signal (i.e. not from an acoustic echo following the input electrical signal) are expected to be sufficiently high that detector circuit  208  would otherwise incorrectly provide a signal to microprocessor  204  indicating an acoustic echo had been received. 
     Detection circuit  208  is comprised of a pair of amplifier stages  264  and  266 , an AC coupling capacitor  268 , and a comparator  270 . Because signals generated by the piezoelectric film in fluid level detector  100  are of relatively low power, for example on the order of 1 to 2 millivolts, amplifier stages  264  and  266  apply an approximately four hundred times gain to the signal received from fluid level detector  100  over line  202 . Comparator  270  then compares the amplified signal to a predetermined voltage level defined by divider resistors  272  and  274  at  276 . The voltage level at  276  is preferably set so that signals generated by acoustic echoes from the transducer and the outer wall of the container are ignored, while the stronger signals resulting from the container&#39;s inner wall surface and air trigger a change in the detector circuit&#39;s output. 
     As described above, the acoustic echo from the interface between the lens and coupling material, and between the coupling material and the container wall outer surface, is weaker than an acoustic echo resulting from an air interface with the container inner wall surface. The first echo therefore results in weaker vibrations in the piezoelectric film than does the second echo, and the first echo therefore generates a lower voltage signal on line  202 . Accordingly, the first echo results in an amplified signal at the input  278  to comparator  270 , indicated at  280  in  FIG. 10B , having a lower voltage level than an amplified signal, indicated at  282 , that results from an acoustic echo from the air interface. The voltage level, indicated at  284 , defined by the divider is set higher than the expected level of the first amplified signal (and also higher than the expected level of an amplified signal resulting from an echo from a liquid interface at the container&#39;s inner wall surface) but less than the expected level of the second amplified signal. Accordingly, comparator  270  remains low upon receipt of a signal resulting from an acoustic echo from the container wall&#39;s outer surface (or from a liquid interface at the container wall&#39;s inner surface) but outputs a high signal on trace  248  upon receipt of a signal corresponding to an acoustic echo from the interface between the container wall&#39;s inner surface and air. Because the electronic module is now in detection mode, the signal on trace  242  to NAND gate  244  is high. Thus, the transition of the signal on trace  248  from low to high upon receipt of an acoustic echo from an air interface drives the output signal from NAND gate  244  on trace  246  from high to low. During the detection mode, this transition notifies microprocessor  204  that detector  100  has detected a condition at which fluid level inside the container has fallen below the level of the detector. Microprocessor  204  then outputs a signal indicating this condition on a line  286  to an output circuit  288  that drives a notification device such as a lamp, audible device or other peripheral device. Alternatively, or additionally, microprocessor  204  can communicate with a remote processor through an RS-232 circuit  290 . 
     In one preferred embodiment, microprocessor  204  repeatedly checks the signal on trace  246  and does not change the state of its output until detecting a change on trace  246  at five consecutive reads. This inhibits false responses due to jitter in the digital circuitry. 
     In a still further preferred embodiment, and referring to  FIGS. 8 and 9 , excitation circuit  206  is replaced by an excitation circuit  292  to provide a square wave electrical signal rather than a pulse. Excitation circuit  292  is comprised of an H-bridge circuit  294  controlled by a timing circuit  296 . H-bridge circuit  294  applies a square wave to fluid level detector  100  across lines  200  and  202  that varies between −12 volts and +12 volts. 
     Prior to the excitation mode, the signal on trace  212  is low. This causes timing circuit  296  to de-activate the H-bridge circuit such that no input electrical signal is provided to the detector. When microprocessor  204  applies a high signal to trace  212  at the beginning of the excitation mode, however, the high signal immediately turns on switches  298  and  300  (which may be, for example, high speed MOSFET&#39;s or bipolar transistors), thereby applying a 12 volt signal from the power source to the H-bridge. An approximate two nanosecond delay caused by a resistor  302  and switch  304  allows the H-bridge circuit to power up through switches  298  and  300  before the occurrence of subsequent transitions. 
     The activation of switch  304  turns on switches  306  and  308  (again which, for example, may be MOSFET&#39;s or bipolar transistors) in the H-bridge, thereby applying a +12 volt signal to the piezoelectric film across lines  200  and  202 . Meanwhile, capacitors  310  and  312  charge through resistors  314  and  316 , respectively. Capacitor  312  charges first, thereby turning on switch  318 . This grounds the gate of switch  304  and thereby turns off switches  306  and  308 . 
     The time constant defined by resistor  314  and capacitor  310  (approximately 50 nanoseconds) is such that, at this point, a switch  320  turns on, thereby turning on switches  322  and  324  through switches  326  and  328 . This applies a −12 volt portion of the square wave across lines  200  and  202 . Microprocessor  204  then (approximately 50 nanoseconds later) drives the signal on trace  212  low, thereby deactivating the H-bridge circuit. 
     In the configuration of the blanking period generator circuit shown in  FIGS. 8A and 8B , the low signal on trace  212  also returns the electronic module to receive mode. In the event that ringing in the piezoelectric film following the end of the square wave does not result in a return signal of sufficient magnitude to require blocking of the detection circuit by the blanking period generator, or if the microprocessor is programmed to ignore such a response, this is acceptable. In the event, however, that it is desired to block the detection for a certain period of time in order to block signals resulting from ringing in the piezoelectric film following the input electrical signal, blanking period generator circuit  210  is preferably modified so that the output of NAND gate  244  remains high for a sufficiently long period after the end of the input electrical signal. 
     While one or more preferred embodiments of the fluid level detector have been described above, it should be understood that any and all equivalent realizations of the fluid level detector are included within the scope and spirit thereof. For example, the piezoelectric film and lens can be replaced by one or more piezoelectric ceramic elements. Because ceramic elements produce stronger electrical signals, amplification in the electronics module can be reduced or eliminated. Thus, the depicted embodiments are presented by way of example only and are not intended as limitations on the fluid level detector. It should be understood that aspects of the various one or more embodiments may be interchanged either in whole or in part. Therefore, it is contemplated that any and all such embodiments are included in the present disclosure as may fall within the literal or equivalent scope of the appended claims.