Patent Publication Number: US-2022221321-A1

Title: Remote measuring liquid level sensor for intermediate bulk container applications

Description:
FIELD OF INVENTION 
     The present application relates to externally located liquid level sensors for remotely measuring liquid levels in nonmetallic bulk containers. Such a measuring system is particularly suitable for measuring liquid levels in containers used for storing hazardous or corrosive liquid materials, as well as liquids that cannot come into contact with the sensor itself, as may be used for example in diesel exhaust fluid applications and other intermediate bulk container applications. 
     BACKGROUND OF THE INVENTION 
     One example of a liquid level sensor is a capacitive liquid level sensor. One type of capacitive liquid level sensor operates by measuring a mutual capacitance between two conductors, such as between two metallic plates or electrodes. In the absence of liquid, the two conductors have an intrinsic capacitance, and the capacitance changes from the intrinsic capacitance in the presence of a liquid nearby. In particular, the presence of liquid near the two conductors causes the capacitance between the two conductors to vary, and thus the capacitive liquid level sensor operates by measuring a change in this capacitance and determining a liquid level based on such change in capacitance. Another type of capacitive liquid level sensor utilizes a single conductor that has an intrinsic capacitance with the surrounding environment. This capacitance, referred to as “self-capacitance”, also will change based on the presence of a liquid nearby and thus likewise can be used to determine a liquid level. A capacitive liquid level sensor can detect a liquid through a non-conductive barrier, such as through a plastic container wall. Common capacitive sensors further may include a capability to electronically transmit, and preferably wirelessly transit, liquid level measurements to another electronic device for easy monitoring. 
     In many industrial applications, plastic containers are preferred for storing hazardous and corrosive liquid because the plastic containers do not degrade due to the corrosive properties of the liquid. In addition, to prevent exposure to the hazardous liquid, it is desirable to minimize the need for opening the container, such as to check the liquid level. For such applications, a capacitive level sensor is particularly suitable because the liquid level can be detected from outside the plastic container through the container wall. In conventional configurations, a capacitive sensing strip may be taped or otherwise adhered to an outer surface of the container. In some applications, however, for additional safety the plastic container further is contained within an outer cage, which is configured essentially as a frame that renders direct contact with the container more difficult. For example, diesel exhaust fluid (DEF) often is stored in plastic containers further isolated using an outer cage. In DEF and similar applications, therefore, the installation of a capacitive sensor to an outer surface of the plastic container is rendered difficult by presence of the cage, and therefore enhanced capacitive sensor configurations are needed. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present application include a liquid level sensor system that can be attached to the outside wall of a nonmetallic bulk container, and that measures with high resolution a level of liquid inside the container using capacitance changes at the sensor system. The sensor system transmits capacitance measurements and/or the determined liquid level wirelessly to an external electronic device, either through a direct wireless communication or indirectly via the Internet, for easy monitoring of the liquid level over said Internet connection or using an application on a mobile communication device. The sensor system has a capacitive sensing element that is configured as a continuous capacitive sensing element that is highly conformable to the outside surface of the container. The sensing element is shielded so that the sensing element only reacts to liquid inside the container, and thus will not react to external influences like rain, humidity, moisture, and like environmental conditions. The sensing element also has a hard outer component for durability. The sensing element can fit within the cage of an intermediate bulk container (IBC), and therefore the sensing element is particularly suitable for liquid level sensing of diesel exhaust fluid (DEF) containers and for comparable applications in which liquid containers contain hazardous liquid and are located in harsh environments. 
     In exemplary embodiments, the sensor system includes a capacitive sensing element that includes a conductive strip that is highly conformable to the outer surface of the container, and a capacitance associated with the conductive strip changes at the same rate as the liquid level, such as DEF, inside a nonmetallic IBC. The sensing element is placed between a cage and the container, with the sensing element being held against the outer surface of the container with a compression assembly having components that attach to the cage. 
     In exemplary embodiments, the sensing element includes a metal channel, such as a U-shaped aluminum channel, that houses a ribbon of foam material. At least one conductive strip is applied to the foam ribbon oppositely from the metal channel, and thus the metal channel and conductive strip with the foam ribbon material therebetween form a capacitor. The sensor system further includes sensor electronics, and an electrical connection is applied from the sensor electronics to the conductive strip. The sensor electronics further includes a wireless transmitter for wirelessly transmitting sensor information from the sensor electronics to an external electronic device for monitoring the liquid level. The sensor system further includes an electronic controller for receiving a capacitance measurement and determining a liquid level based on the capacitance measurement. The at least one conductive strip may include a plurality of conductive strips of different lengths to permit sensing the liquid level through different zones of the container. 
     In exemplary embodiments, the sensing system further includes a compression assembly for fixing the sensing element to an outer cage while maintaining the sensing element applied against an outer surface of a container located within the cage. The compression assembly may include a channel frame having a base and opposing sides perpendicular to the base to form a channel that fits over a cage bar and receives the sensing element. The channel frame sides have a plurality of cutouts that are positioned in use to extend around cross bars of the cage, and the compression assembly further includes a plurality of clamps that secure the channel frame to the cage bars at the cutouts. The compression assembly further includes a plurality of springs located within the channel that extend from the base of the channel frame to provide an outward bias from the base. In use, the sensing element is placed within the channel over the springs such that the springs are compressed, and the outward bias of the springs presses the sensing element against the outer surface of the container. Because of the nature of the conductive strip and foam ribbon, the sensing element is highly conformable to the outer surface of the container when pressed by the springs. 
     In exemplary embodiments, the compression assembly includes a compression plate, and a bar retainer that is attached to a first end of the compression plate and includes a recessed retainer that is configured to receive a first cross bar of the cage. A wire clip includes a plurality of clip segments that form a bar channel that is configured to receive a second cross bar of the cage different from the first cross bar received by the bar retainer, wherein the wire clip is configured to clip the compression plate to the second cross bar. The compression assembly further includes a spring that is positioned on the compression plate, wherein the spring when compressed asserts an outward bias from the compression plate to press the sensing element against the outer surface of the container. Because of the nature of the conductive strip and foam ribbon, the sensing element is highly conformable to the outer surface of the container when pressed by the spring. 
     These and further features of the present invention will be apparent with reference to the following description and attached drawings. In the description and drawings, particular embodiments of the invention have been disclosed in detail as being indicative of some of the ways in which the principles of the invention may be employed, but it is understood that the invention is not limited correspondingly in scope. Rather, the invention includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto. Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing depicting an exemplary sensor system positioned on a container and cage in accordance with embodiments of the present application. 
         FIG. 2  is a close-up view of a portion of  FIG. 1 . 
         FIG. 3  is drawing depicting an exemplary embodiment of a capacitive sensing element in accordance with embodiments of the present application. 
         FIG. 4  is a drawing depicting a first stage of assembling the sensing element of  FIG. 3 . 
         FIG. 5  is a drawing depicting a second stage of assembling the sensing element of  FIG. 3 . 
         FIG. 6  is a drawing depicting a third stage of assembling the sensing element of  FIG. 3 . 
         FIG. 7  is a drawing depicting a fourth stage of assembling the sensing element of  FIG. 3 . 
         FIG. 8  is a drawing depicting an exemplary compression assembly in accordance with embodiments of the present application. 
         FIG. 9  is a drawing depicting a front view of the compression assembly of  FIG. 8 . 
         FIG. 10  is a drawing depicting a side view of the compression assembly of  FIG. 8 . 
         FIG. 11A  is a drawing depicting a side view of a channel frame component of the compression assembly in isolation, and  FIG. 11B  is a drawing depicting an edge view of the channel frame of  FIG. 11A . 
         FIG. 12A  is a drawing depicting a first side view of one of the clamps of the compression assembly in isolation, and  FIG. 12B  is a drawing depicting a second side view of the clamp from a perpendicular viewpoint relative to  FIG. 12A . 
         FIG. 13  is a drawing depicting one of the compression springs of the compression assembly in isolation. 
         FIG. 14  is a drawing depicting an exploded view of the exemplary sensor system relative to a container and cage, which illustrates the manner by which the sensor system is applied. 
         FIG. 15  is a drawing depicting a first view of another configuration of an exemplary compression assembly for use in the sensor system in accordance with embodiments of the present application. 
         FIG. 16  is a drawing depicting a second view of the compression assembly of  FIG. 15  from an opposing viewpoint relative to  FIG. 15 . 
         FIG. 17  is a drawing depicting another configuration of an exemplary sensor system which includes a sensing element and a plurality of compression assemblies of  FIGS. 15 and 16 . 
         FIG. 18  is a drawing depicting a close-up view of a portion of  FIG. 17  illustrating a portion of the sensor system in the region of a first compression assembly. 
         FIG. 19  is a drawing depicting a close-up view of a portion of  FIG. 17  illustrating a portion of the sensor system in the region of a second compression assembly. 
         FIG. 20  is a drawing depicting a close-up view of a portion of  FIG. 17  illustrating a portion of another sensor system in the region of a third compression assembly that is a variation on the compression assembly of  FIGS. 15 and 16 . 
         FIG. 21  is a drawing depicting another sensor system which includes a sensing element and another alternative configuration of a compression assembly, and a close-up view illustrating a portion of the sensor system in the region of the compression assembly. 
         FIG. 22  is a drawing depicting another sensor system which includes a sensing element and another alternative configuration of a compression assembly, and a close-up view illustrating a portion of the sensor system in the region of the compression assembly. 
         FIG. 23  is a drawing depicting an alternative configuration of an exemplary sensing element including multiple conductive strips. 
         FIG. 24  is a graphical depiction of the manner by which the use of multiple conductive strips of different lengths may be employed to provide a more informative indication of the liquid level in the container 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present application will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. It will be understood that the figures are not necessarily to scale. 
     Embodiments of the present application include a liquid level sensor system that can be applied to an outer surface of a nonmetallic bulk container, and that measures with high resolution a level of liquid inside the container using capacitance changes at the sensor system. The sensor system transmits capacitance measurements and/or the determined liquid level wirelessly to an external electronic device, either through a direct wireless communication or indirectly via the Internet, for easy monitoring of the liquid level over said Internet connection or using an application on a mobile communication device. The sensor system has a capacitive sensing element that is configured as a continuous capacitive sensing element that is highly conformable to the outside surface of the container. The sensing element is shielded so that the sensing element only reacts to liquid inside the container, and thus will not react to external influences like rain, humidity, moisture, and like environmental conditions. The sensing element also has a hard outer component for durability. The sensing element can fit within the cage of an intermediate bulk container (IBC), and therefore the sensing element is particularly suitable for liquid level sensing of diesel exhaust fluid (DEF) containers and for comparable applications in which liquid containers contain hazardous liquid and are located in harsh environments. 
       FIG. 1  is a drawing depicting an exemplary sensor system  10  positioned on a container and cage in accordance with embodiments of the present application, and  FIG. 2  is a close-up view of a portion of  FIG. 1 . The sensor system  10  is employed to measure a liquid level of a liquid contained in a nonmetallic container  12  that is housed within an outer cage  14 . In general, the sensor system  10  includes a capacitive sensing element  16  and a compression assembly  18 , whereby the compression assembly  18  provides attachment of the sensing element  16  to the cage  14  while applying the sensing element  16  to an outer surface of the container  12 . The sensor system  10  further includes sensor electronics  20  (see  FIG. 1 ), which as further detailed below includes a wireless transmitter for wirelessly transmitting sensor information from the sensor electronics to an external electronic device by any suitable wireless interface technology for efficient liquid level monitoring. 
       FIG. 3  is drawing depicting an exemplary embodiment of the capacitive sensing element  16  in isolation, and  FIGS. 4-7  depict exemplary stages of assembling the sensing element  16 . In exemplary embodiments, the capacitive sensing element  16  includes at least one conductive strip, and a capacitance associated with the at least one conductive strip changes at the same rate as the liquid level, such as DEF, inside a nonmetallic IBC. The sensing element is placed between a cage and the container, with the sensing element being held against the container with a compression assembly having components that attach to the cage, as further detailed below. 
     Referring to  FIGS. 3-7 , the sensing element  16  includes a metal channel  22 , such as a U-shaped aluminum channel, that houses a ribbon of foam material  24 . At least one conductive strip  26  is applied to the foam ribbon  24  oppositely from the metal channel  22 , and thus the metal channel  22  and conductive strip  26  with the foam ribbon  24  therebetween form a capacitor. The conductive strip  26  is made of a material suitable for conforming to the outer surface of the container to which the sensing element is to be applied. Accordingly, the strip configuration combined with a suitable conductive material composition renders the conductive strip  26  both flexible and stretchable for precisely conforming to the outer surface of the container. Suitable materials for the conductive strip  26  include, for example, a conductive fabric such as a nickel fabric tape, or a conductive paint material such as carbon black or silver painted onto a silicone rubber base. The foam material  24  may be any suitable rubber material that is compressible and flexible to aid conformance of the conductive strip  26  with the outer surface of the container. For outdoor applications in particular, a compressible hydrophobic material is preferred to avoid absorption of moisture. Ethylene vinyl acetate (EVA) foam is a suitable foam material that meets such properties. The metal channel  22  is a rigid and hard component that provides both physical durability and shielding of the sensing element  16  from harsh environmental conditions. 
     For assembly, the U-shaped metal channel  22  is cut to a desired size, and the foam ribbon  24  is inserted into the channel  22  and fixed flush within the channel using any suitable means, such as using an adhesive. The conductive strip  26  is then applied centrally to a face of the foam ribbon  24  opposite from a face of the foam ribbon  24  that is fixed against the metal channel  22 . As referenced above, the conductive strip may be applied to the foam ribbon by any suitable means, such as by using an adhesive or painting a conductive strip onto the foam ribbon. Initially during assembly, as illustrated particularly in  FIG. 6 , a portion of the metal channel  22  remains exposed, and a printed circuit board (PCB)  28  is fixed to the metal channel  22 , for example using a double-sided tape. An end  30  of the conductive strip  26  extends from the foam ribbon  24  and connects to the PCB  28  to provide electrical connection to the conductive strip  26 . The PCB  28  includes suitable circuitry to measure a capacitance of the sensing element  16 . A first electrical connector  34  on the PCB  28  and a second electrical connector  36  are connected to each other via wiring  38 , and the second electrical connector  36  plugs into the sensor electronics  20  to provide the electrical connection between the sensor electronics  20  and the sensing element  16 . Additional segments of the foam ribbon material  24  are then applied to cover the initially exposed portion of the metal channel  22  including the PCB  28 , resulting in the assembled configuration depicted in  FIG. 3 . 
     The sensor electronics  20  includes embedded electronics for electrical communication with the sensing element  16 . For example, the sensor electronics  20  includes a battery to power the sensing element. A substantially low power level is suitable, and thus long battery life is achieved. The PCB  28  includes capacitance reading circuitry that can electronically communicate capacitance measurement values to the sensor electronics  20 . The sensor electronics  20  further includes a wireless transmitter for wirelessly transmitting sensor information from the sensor electronics to an external electronic device, which can include transmitting capacitance measurements and/or associated liquid level values as read by the PCB capacitance reading circuitry. For example, a capacitance measurement may be rendered by the PCB and communicated to the sensor electronics, which is then wirelessly transmitted to an external electronic device that can determine a liquid level based on the capacitance measurement. Any suitable wireless communication technology may be employed. For example, sensor information may be transmitted from the sensor electronics directly to the external electronic device, such as by Bluetooth or other short-range direct wireless communication. Additionally or alternatively, sensor information may be transmitted from the sensor electronics indirectly to the external electronic device over the Internet or other wireless network, such as by WiFi, a cellular network, or other comparable networked wireless communication. The sensor electronics further may include GPS tracking capabilities for tracking container locations as may be desirable in inventory tracking systems. 
     As referenced above, the sensor system  10  further includes the compression assembly  18  that provides attachment of sensing element  16  to the outer cage while applying the sensing element  16  against an outer surface of the container.  FIG. 8  is a drawing depicting an exemplary compression assembly  18  in accordance with embodiments of the present application, with  FIGS. 9 and 10  respectively depicting front and side views of the compression assembly  18  of  FIG. 8 . In general, the compression assembly  18  includes a channel frame  42 , a plurality of clamps  44 , and a plurality of compression springs  46 . The channel frame  42 , one of the clamps  44 , and one of the compressions springs  46  are depicted respectively in isolation in  FIGS. 11-13 . The components of the compression assembly may be made of any suitable rigid material, such as various metal and plastic materials, as are commonly used for clamping type attachments. 
       FIG. 11A  illustrates a side view of the channel frame  42  in isolation, and  FIG. 11B  illustrates an edge view of the channel frame  42  of  FIG. 11A . In exemplary embodiments, the channel frame  42  includes a base  48  and opposing sides  50  and  52  that extend perpendicularly from the base  48  to form a bar channel  54  that receives one of the longitudinal cage bars and the sensing element  16 , as further detailed below. The channel frame sides  50  and  52  have a plurality of cutouts  55  that are positioned in use to extend around cross bars of the cage, as also further detailed below. 
       FIG. 12A  illustrates a first side view of one of the clamps  44  in isolation, and  FIG. 12B  illustrates a second side view of the clamp  44  from a perpendicular viewpoint relative to  FIG. 12A . In general, the plurality of clamps  44  secure the channel frame  42  to the cage bars. As illustrated first in  FIG. 12A , each of the plurality of clamps  44  may be configured as a U-shaped clamp having a clamp base  56  and opposing arms  58  and  60  that define a frame receiving space  62  therebetween. In addition as illustrated in  FIG. 12B , each of the arms  58  and  60  (only one arm is shown from the side viewpoint of  FIG. 12B ) has a bar-receiving cutout  64 . The bar-receiving cutout may be rounded or otherwise shaped commensurately with a cage bar cross-sectional shape. As further detailed below, the combination in the configuration of the clamp  44  of the frame receiving space  62  and bar receiving cutout  64  permits the clamp  44  to interact with and secure against both the channel frame  42  and the cage bars. 
       FIG. 13  illustrates one of the compression springs  46  in isolation. Referring to  FIG. 13  is combination with  FIGS. 8-10  illustrating the overall compression assembly  18 , the plurality of compression springs  46  are located within the frame channel  54  of the channel frame  42 , whereby the compression springs  46  extend from the base  48  of the channel frame  42  to provide an outward bias from the base. As further detailed below, when in use the sensing element  16  is placed over the bar channel  54  and thus over the springs  46  such that the springs are compressed, and the outward bias of the springs  46  presses the sensing element  16  against the outer surface of the container. Although in exemplary embodiments the compression springs  46  are illustrated as coil springs, other suitable spring configurations such as leaf springs or comparable may be employed. 
       FIG. 14  is a drawing depicting an exploded view of the sensor system  10  relative to a container  12  and cage  14 , which illustrates the manner by which the sensor system  10  is applied. A typical cage  14  includes longitudinal bars  70  and transverse cross bars  72 . The sensing element  16  is positioned against an outer surface  74  of the container  12 , with the conductive strip  26  positioned against the outer container surface  74 . As referenced above, with such positioning the metal channel  22  faces outward relative to the outer container surface and thus provides both physical durability and shielding to the sensing element  16  from harsh environmental conditions. The sensing element extends longitudinally along the outer container surface  74 , i.e., from a location near the bottom of the container towards the top of the container. As also referenced above, the sensing element  16  is thereby configured as a continuous capacitive sensing element that is highly conformable to the outer surface of the container, and the liquid level is determined based on capacitance changes that are measured along the sensing element. 
     For securing the sensing element  16 , the sensing element  16  is aligned with one of the longitudinal cage bars  70   a . Referring to the components of the compression assembly  18  (see  FIGS. 8-12  also), the channel frame  42  is positioned such that the longitudinal cage bar  70   a , i.e., the cage bar with which the sensing element  16  is aligned, is received within the bar channel  54 . In addition, the channel frame  42  is positioned such that the cage cross bars  72  extend through the cutouts  55  of the channel frame  42 . The compression springs  46  further are located to extend outward from the bar channel  54  at positions between the cutouts  55 , i.e. between the cage cross bars  72 . The clamps  44  also are fixed to the channel frame  42 . In particular, initially the clamps are clamped to the base  48  of the channel frame  42  outward relative to the bar channel  54 , and thus the base  48  of the channel frame  42  is received within the frame receiving spaces  62  of the clamps  44 . Once the clamps  44  are clamped onto the channel frame  42 , the clamps may be slid downwardly along the channel frame  42  until the cage cross bars  72  respectively are received within the bar receiving cutouts  64 . In this manner, the compression assembly is clamped to the cage in a secured fashion. In addition, once the clamps  44  are applied, the compression springs  46  are compressed, and the outward bias of the compression springs  46  presses the sensing element  16  against the outer surface of the container  12  to hold the sensing element against the outer surface of the container  12 , and more specifically with the at least one conductive strip pressed against the outer surface of the outer in a highly conformable manner. 
     The resultant assembled configuration is illustrated in  FIGS. 1 and 2  referenced above. The sensor electronics  20  of the sensor system  10  may be fixed to the container  12  at any suitable location, with the top of the container being a suitable location as illustrated in  FIG. 1 . In addition, any suitable mechanism, such as for example adhesives or mechanical fastening elements, may be used to fix the sensor electronics to the container. Variations of the sensor system  10  may be employed to accommodate different container and cage configurations for different applications and circumstances. The sensing element and channel frame of the compression assembly may be sized and shaped for any size container and cage. Relatedly, the channels and cutout spaces defined by the components of the compression assembly that receive the various cage bars also may be sized, shaped, and spaced apart as warranted to accommodate any cage configuration. 
     A potential drawback of the compression assembly  18  is that the positioning of the cutouts  55  in the channel frame  42 , which are formed at the time of initial manufacturing, is fixed. As a result, a given compression assembly  18  would be configured at the time of manufacture to accommodate a given spacing specifically of the transverse cage cross bars  72 . Although cages tend to come in given configurations, there can be some differences in the cage bar spacing among cages for different sized containers and/or different applications. As referenced above, the cutout spaces may be spaced apart as warranted to accommodate any cage configuration, but this occurs at the time of manufacture and thus a given compression assembly  18  is suitable only for a particular cage bar spacing. Subsequent embodiments provide alternative configurations of the compression assembly to be more versatile in accommodating essentially any cage bar spacing. 
     Subsequent figures depict alternative configurations of a compression assembly that provides attachment of the sensing element  16  to the outer cage while applying the sensing element  16  against an outer surface of the container. In particular,  FIG. 15  is a drawing depicting a first view of an exemplary compression assembly  80  in accordance with embodiments of the present application, with  FIG. 16  depicting a second view of the compression assembly  80  from an opposing viewpoint relative to  FIG. 15 . In general, the compression assembly  80  includes a compression plate  82 , a bar retainer  84 , a wire clip  86 , and a compression spring  88 . Similar to the previous embodiment, the components of the compression assembly  80  may be made of any suitable rigid material, such as various metal and plastic materials, as are commonly used for clamping type attachments.  FIGS. 15 and 16  depict one compression assembly  80 , and multiple iterations of the compression assembly  80  may be provided along the length of the container to apply the entire sensing element  16  securely to the outer surface of the container. 
     As seen in  FIGS. 15 and 16 , the compression plate  82  may be configured as a thin rectangular or other suitably shaped plate that provides a support structure for the other components of the compression assembly  80 . The bar retainer  84  is attached to a first end of the compression plate  82 , and a second end of the compression plate  82  opposite from the first end may be a free end. In the example of  FIGS. 15 and 16 , the bar retainer  84  is a separate component that defines a slot  90  that receives the first end of the compression plate  82 . The bar retainer  84  may be made of metal or plastic. Alternatively, the compression plate  82  and the bar retainer  84  alternatively may be configured as a single integral component. The bar retainer  84  includes a recessed retainer  92  that is configured to receive a first one of the cross bars of the cage, as further detailed below. In this example, the recessed retainer is formed as two prongs effectively configured as a claw that receives and grips one of the cage cross bars. The recessed retainer  92  may be shaped as is suitable for any particular cage design to be commensurate with the associated cage bar cross-sectional shape, such as for example round cross-sectional cage bars versus square or rectangular cross-sectional cage bars. 
     The wire clip  86  includes a plurality of clip segments that are configured to secure the compression plate  82  to the cage toward the second end of the compression plate  82  opposite from the bar retainer  84 . The wire clip  86  forms a bar channel that is shaped to receive a second one of the cage cross bars different or opposite from the first cross bar received by the bar retainer. In a particular example in which the cage bars have a rectangular cross section, the wire clip  86  includes a cross segment  94  (see particularly  FIG. 15 ), which in use lays across the width of the compression plate  82 . Opposing longitudinal segments  96  extend perpendicularly from the cross segment  94 , such that in use the opposing longitudinal segments  96  extend essentially parallel to or in the direction of the longitudinal sides of the compression plate  82 . Opposing transverse segments  98  extend perpendicularly from the longitudinal segments  96 , and opposing retention segments  100  extend perpendicularly from the opposing transverse segments  98  so as to be parallel to and spaced apart from opposing longitudinal segments  96 . In this manner, the opposing longitudinal segments  96 , the opposing transverse segments  98 , and the opposing retention segments  100  form a bar channel  102  that receives one of the cross bars of the cage, as further detailed below. The example of the wire clip  86  of  FIGS. 15 and 16  is therefore particularly suitable for rectangular cross-sectional cage bars as the segments of the wire clip  86  form a rectangular bar channel  102 . It will be appreciated that the segments of the wire clip  86  may be arranged to define or form a bar channel to accommodate any corresponding cage bar cross-sectional shape, such as for example rounded or other bar cross-sectional shapes. 
     The compression spring  88  may be configured comparably as the compression springs  46  of the previous embodiment. The compression spring  88  may be secured to the compression plate  82  approximately midway between the first and second ends of the compression plate  82  using any suitable fastener mechanism, such as for example a bolt or screw fastener. In the depicted example, the compression spring  88  may be positioned against the compression plate  82  with a spring retainer  104  (see particularly  FIG. 16 ) and fixed with a bolt  106 , although again any suitable fastener mechanism may be employed. The compression spring  88  is located and fixed to the compression plate  82 , whereby the compression spring  88  extends from the compression plate  82  to provide an outward bias from the compression plate when the spring is compressed. As further detailed below, when in use the sensing element  16  is placed parallel to the compression plate  82  and thus over the compression spring  88  such that the spring is compressed, and the outward bias of the spring  88  relative to the compression plate  82  presses the sensing element  16  against the outer surface of the container. As referenced above in connection with the previous embodiment, although in exemplary embodiments the compression spring  88  is illustrated as a coil spring, other suitable spring configurations such as leaf springs or comparable may be employed. 
       FIG. 17  is a drawing depicting a sensor system  10   a  which includes the sensing element  16  and a plurality of compression assemblies  80  of  FIGS. 15 and 16 .  FIG. 17  illustrates the sensor system  10   a  as positioned relative to a container  12  and cage  14 , which illustrates the manner by which the sensor system  10   a  is applied to the container and cage.  FIG. 18  depicts a close-up view of a portion of  FIG. 17  illustrating a portion of the sensor system  10   a  in the region of a first compression assembly  80   a , and  FIG. 19  depicts a close-up view of a portion of  FIG. 17  illustrating a portion of the sensor system  10   a  in the region of a second compression assembly  80   b . As referenced above in connection with the previous embodiment, a typical cage  14  includes longitudinal bars  70  and transverse cross bars  72 . The sensing element  16  is positioned against an outer surface  74  of the container  12 , with the at least one conductive strip  26  positioned against the outer container surface  74 . The sensing element extends longitudinally along the outer container surface  74 , i.e., from a location near the bottom of the container towards the top of the container. As also referenced above, the sensing element  16  is thereby configured as a continuous capacitive sensing element that is highly conformable to the outer surface of the container, and the liquid level is determined based on capacitance changes that are measured along the sensing element. 
     For securing the sensing element  16  in the embodiment of sensor system  10   a  including the compression assembly  80 , the sensing element  16  positioned to run along the container  12  essentially parallel to and between two adjacent longitudinal cage bars  70   a  and  70   b . Referring to the components of the compression assemblies  80   a  and  80   b  (see also  FIGS. 15 and 16 ), as to each compression assembly the bar retainer  84   a / 84   b  is connected to a respective transverse cross bar  72 . The claw configuration of the prongs of the bar retainer holds the compression plate to the cross bar. In this example in particular, the bar retainer  84   a  of the first compression assembly  80   a  is secured to a first cross bar  72   a , and the bar retainer  84   b  of the second compression assembly  80   b  is secured to a second cross bar  72   b . Further as to each compression assembly, the compression springs  88   a / 88   b  extend from the compression plate  82   a / 82   b  in a direction toward the container  12  to bias the sensing element  16  against the outer surface of the container. With the bar retainer claw gripping one of the cross bars, the compression plate can be rotated into position against second cross bar different from the first cross bar, i.e., the compression plate  82   a  is rotated to be against a third cross bar  72   c  and the compression plate  82   b  is rotated to be against a fourth cross bar  72   d . Once positioned in this manner, the wire clip  86   a / 86   b  is slid down about the associated cross bar to clip the respective compression plate  82   a / 82   b  to the associated cross bars  72 , with the cross bar  72  being positioned in the bar channel formed by the wire clip  86  with the cross segment  94  locked against the outer width of the compression plate as shown also in  FIGS. 15 and 17 . In this example in particular, the wire clip  86   a  of the first compression assembly  80   a  clips the compression plate  82   a  of the first compression assembly  80   a  to the third cross bar  72   c , and the wire clip  86   b  of the second compression assembly  80   b  clips the compression plate  82   b  of the second compression assembly  80   b  to the fourth cross bar  72   d.    
     In this manner, the compression assemblies  80   a  and  80   b  are clipped in a secured fashion to the cage  14 . In addition, once the clips  86  are applied, the compression springs  88  are compressed, and the outward bias of the compression springs  88  relative to the compression plates  82  presses the sensing element  16  against the outer surface of the container  12  to hold the sensing element against the outer surface of the container  12 , and more specifically with the at least one conductive strip pressed against the outer surface of the outer in a highly conformable manner. As shown in  FIG. 19 , an optional press plate  89  may be employed between the spring  88  and the sensing element  16  to improve the compression force applied by the spring to the sensing element. The press plate  89  may be a plastic or metal disc. In the particular example of  FIGS. 18 and 19 , the use of two compression assemblies  80   a  and  80   b  is sufficient given the container size to effectively apply the entire sensing element  16  to the container surface, and any suitable number of compression assemblies may be employed depending upon a particular size of container and associated length of sensing element. In addition, this configuration that employs the wire clip configuration may be used regardless of cage bar spacing as the clip can be applied anywhere along the compression plate. 
     Variations of the compression assembly  80  of  FIGS. 15-19  may be employed. For example,  FIG. 20  depicts a close-up view of a portion of  FIG. 17  illustrating a portion of another sensor system  10   b  in the region of a third compression assembly  180  that is a variation on the compression assembly  80  of the previous embodiment. The compression assembly  180  bears similarity to the previous embodiment, and includes at least the following variations. In this exemplary embodiment, the compression assembly  180  includes a modified compression plate  182  that is non-planar. In particular, the compression plate  182  includes a planar portion  184  that is secured to cage cross bars  72   e  and  72   f  in a manner comparably as described above, and an angled portion  186  that extends at an angle from the planar portion  184  toward the container  12 . To accommodate cage bars of varying spacing, a relatively longer length of compression plate may be used to ensure the compression plate spans at least two cage cross bars, and thus depending upon the cage bar spacing there may be an excess portion of the compression plate. By configuring the compression plate  182  with two portions extending in a direction angled toward the container, safety is enhanced because the excess portion of the compression plate is less likely to snag on an operator or other equipment. In this manner, the compression plate  182  can accommodate a wider range of cage cross bar spacings, with any excess plate portion being bent around an associated cross bar as shown in  FIG. 20  for enhanced safety. 
       FIG. 21  is a drawing depicting another sensor system  10   c  which includes the sensing element  16  and another alternative configuration of a compression assembly  280 , and a close-up view illustrating a portion of the sensor system  10   c  in the region of the compression assembly  280 . The compression assembly  280  bears similarity to the previous embodiment, and includes at least the following variations. Compression assembly  280  includes an alternative configuration of the bar retainer, denoted in  FIG. 21  as bar retainer  284 , and of the wire clip, denoted in  FIG. 21  as wire clip  286 . In this example, the bar retainer  284  includes a three-pronged claw configuration including two outer prongs  284   a  and  284   b , and an opposing central prong  284   c  positioned on an opposite side of the cross bar relative to the outer prongs. In use, therefore, the outer prongs  284   a  and  284   b  are on an opposite side of the cage bar relative to the central prong  284   c , and thus this three-pronged claw configuration provides a positive gripping force to better secure the bar retainer  284  to the associated transverse cross bar  72   g.    
     In the above embodiments including the wire clip  86 , the bar channel  102  is formed such that the wire clip  86  extends essentially over the top of the cage cross bar  72 . In the configuration of the wire clip  286 , the wire clip  286  has a cross segment  294  that lies against the compression plate comparably as the cross segment  94  of the wire clip  86 . The wire clip  286 , in contrast to the previous embodiment, has a securing segment  296  that wraps around the associated transverse cross bar  72   h , and opposing bent segments  298  that bend around the compression plate  82  being essentially parallel to the cross segment  294 . This configuration of the wire clip  286  provides a stronger fixation to the cage cross bar. To assemble the sensor system in place, the wire clip  286  is first positioned around the cross bar  72   h . The compression plate is then slid upward through the wire clip  286 , and then the compression plate may be pressed down onto the cross bar  72   g  with the bar retainer claw  284  gripping the cross bar  72   g  to achieve the position shown in  FIG. 21 . This embodiment also includes the optional press plate  89  located between the spring and the sensing element. 
       FIG. 22  is a drawing depicting another sensor system  10   d  which includes the sensing element  16  and another alternative configuration of a compression assembly  380 , and a close-up view illustrating a portion of the sensor system  10   d  in the region of the compression assembly  380 . The compression assembly  380  bears similarity to the previous embodiment, and includes at least the following variations. Compression assembly  380  includes an alternative configuration that does not use a wire clip. Instead, the compression assembly  380  includes a compression plate  384  that includes a first bar retainer  386  positioned at a first end of the compression plate  384 , and a second bar retainer  388  positioned at a second end of the compression plate  384  opposite from the first end. The first bar retainer  384  may be configured as pronged or claw shaped bar retainer similarly as in previous embodiments to grip a first cross bar  72   i , and the second bar retainer  388  may be configured as a bent portion of the compression plate that bends around a second cross bar  72   j . For assembly, the compression plate is slid into position from top to bottom such that the first bar retainer  386  catches the cross bar  72   j , and until the second bar retainer  388  grips the cross bar  72   i . This embodiment also includes the optional press plate  89  located between the spring and the sensing element. 
     In connection with the first embodiment, the sensing element  16  includes a single conductive strip  26  that is applied to the foam ribbon  24  oppositely from the metal channel  22 , and thus the metal channel  22  and conductive strip  26  with the foam ribbon  24  therebetween form a capacitor. Such configuration of the sensing element may be used in combination with any configuration of compression assembly. In addition, an alternative configuration of the sensing element may employ a multiple or plurality of conductive strips of different lengths that are applied to the foam ribbon  24  oppositely from the metal channel  22 , and thus the metal channel  22  and conductive strips with the foam ribbon  24  therebetween form multiple capacitors of different sizes. The use of a multiple or plurality of conductive strips of different lengths can improve the accuracy of the liquid level measurement. Such configuration of the sensing element employing multiple conductive strips also may be used in combination with any configuration of compression assembly. 
       FIG. 23  is a drawing depicting an alternative configuration of an exemplary sensing element  16   a . In the embodiment of  FIG. 23 , sensing element  16   a  includes a plurality of conductive strips of different lengths that are applied to the foam ribbon  24  oppositely from the metal channel (omitted from  FIG. 23  for simplicity of illustration). In this particular example, three conductive strips  26   a ,  26   b , and  26   c  of different lengths are applied to foam ribbon  24 , although it will be appreciated any suitable number of conductive strips may be employed. Conductive strip  26   a  has a first length that corresponds to a 100% level detector in that conductive strip  26   a  extends along substantially the full length of the sensing element. Accordingly, conductive strip  26   a  is positioned to measure a liquid level over the entire length of the sensing element. Conductive strip  26   b  has a second length different from the first length, which in this example corresponds to a 90% level detector in that conductive strip  26   b  extends along about 90% of the length of the sensing element as measured from the top of sensing element. Accordingly, conductive strip  26   b  is positioned to measure a liquid level over 90% of the length of the sensing element, which in other words detects liquid level when the container is from about entirely full emptying down to about 10% full. Conductive strip  26   c  has a third length different from the first and second lengths, which in this example corresponds to a 10% level detector in that conductive strip  26   c  extends along about 10% of the length of the sensing element as measured from the top of sensing element. Accordingly, conductive strip  26   c  is positioned to measure a liquid level over 10% of the length of the sensing element, which in other words detects liquid level when the container is from about entirely full emptying down to about 90% full. The configuration of three conductive strips represents an example, and additional conductive strips and/or conductive strips of different percentage lengths may be employed as may be suitable for any particular application. 
       FIG. 24  is a graphical depiction in the manner by which the use of multiple conductive strips of different lengths may be employed to provide a more informative indication of the liquid level in the container or barrel. Moving along the horizontal axis corresponds to the barrel progressively emptying, and the vertical axis indicates the signal strengths of the signals being received off of the respective conductive strips based on the capacitance measurements. At a given liquid level, the signal strength measured off of the conductive strips is higher the longer the length of the conductive strip. Starting at the left portion of the graph of  FIG. 24 , with the barrel full all three conductive strips generate a maximum signal. As the barrel is emptied, the liquid level first moves through Zone  1  and the signal strengths measured from each of the conductive strips decreases in a linear fashion as the barrel is emptied from full. As referenced above, conductive strip  26   c  corresponds to a 10% level detector in that conductive strip  26   c  extends along about 10% of the length of the sensing element as measured from the top of sensing element, which in other words detects liquid level when the barrel is from about entirely full emptying down to about 90% full. Accordingly, when the liquid level in the barrel falls below 90% full, shown as the transition from Zone  1  to Zone  2 , the signal strength measured off of conductive strip  26   c  goes flat as conductive strip  26   c  is no longer in contact with a portion of the barrel commensurate with the liquid content. The signal strength measured off of conductive strip  26   c  will remain flat through the course of further emptying of the barrel because conductive strip  26   c  will remain spaced away from the vicinity of the liquid content. 
     Similar measurements are observed as to the other conductive strips as the barrel is further emptied through Zones  2  and  3 . In particular, as the barrel is emptied further, the liquid level next moves through Zone  2  and the signal strengths measured from each of conductive strips  26   a  and  26   b  decreases in a linear fashion as the barrel is emptied further (again, the signal from conductive strip  26   c  has gone flat). As referenced above, conductive strip  26   b  corresponds to a 90% level detector in that conductive strip  26   b  extends along about 90% of the length of the sensing element as measured from the top of sensing element, which in other words detects liquid level when the barrel is from about entirely full emptying down to about 10% full. Accordingly, when the liquid level in the barrel falls below 10% full, shown as the transition from Zone  2  to Zone  3 , the signal strength measured off of conductive strip  26   b  also goes flat as conductive strip  26   b  now is no longer in contact with a portion of the barrel commensurate with the liquid content. The signal strength measured off of conductive strip  26   b  from here on will remain flat through the course of further emptying because conductive strip  26   b  will remain spaced away from the vicinity of the liquid content. 
     Similarly, as the barrel is emptied further, the liquid level next moves through Zone  3  and the signal strength measured from conductive strips  26   a  decreases in a linear fashion as the barrel is emptied further (again, the signal from both conductive strips  26   b  and  26   c  have gone flat). As referenced above, conductive strip  26   a  corresponds to a 100% level detector in that conductive strip  26   a  extends along essentially the entire length of the sensing element, which in other words detects liquid level when the barrel is from about entirely full emptying down to about entirely empty. Accordingly, when the liquid level in the barrel falls to empty at the end of Zone  3 , the signal strength measured off of conductive strip  26   a  also goes flat as conductive strip  26   a  now is no longer in contact with a portion of the barrel commensurate with the liquid content. In other words, when the barrel is empty, the signal strengths of each of the conductive strips is at a minimum. 
     In this manner, the use of multiple or a plurality of conductive strips of different lengths provides a more informative indication of liquid level in the container or barrel. In particular, the relative signal strengths provide an indication of the zone of the container in which the current liquid level is present based on which signal strengths have gone flat. In the depicted example, three conductive strips of different lengths indicate three zones of liquid level in addition to full and empty. The use of additional conductive strips of different lengths would result in a commensurate addition of more zones of liquid level to enhance the liquid level measurement. 
     In the example depicted in the figures, the sensor systems include a unitary sensing that extends along the container surface essentially without interruption. In other embodiments of the sensor system, the sensing element and the compression assembly may be divided into separate segments that each is separately fixed to the container and cage. The separate segments may be daisy chained to each other whereby the segmented sensing element components are electrically connected to each other by intervening wires with a single electrical connection to the sensor electronics, or each segmented sensing element can be wirelessly connected, such as by a Bluetooth or comparable wireless connection, to the sensor electronics. Segmented configurations provide a more modular design that may accommodate different cage configurations, but also may require more effort to install as each segment is installed individually and may require more power to operate. 
     The described configuration of the sensor system results in many advantages and efficiencies in monitoring liquid levels in nonmetallic containers for hazardous liquids, such as for example may be employed in DEF and other intermediate bulk container applications. The remote monitoring of liquid containers allows for efficient logistics of distributors of these liquids and chemicals by allowing distributors to know when and where customers need product. The external monitoring further allows for continuous, high resolution capacitance measurements for monitoring the hazardous or corrosive liquid levels without opening the container, which prevents material contamination and harmful exposure to personnel. The sensor system including the particular configuration of the compression assembly provides for easy installation and application of the sensing element to the outer surface of the container located within the cage in a highly conformable manner. Installation, therefore, does not require a factory install, and the sensor system can be installed aftermarket without strong technical expertise and can be easily uninstalled at any time to be reused on another container. The sensor electronics provides efficient, battery powered operation of the sensing element with long-life battery operation without needing charge or replacement, and further permitting the wireless communication of sensor information to external electronic devices which also can include GPS tracking. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.