Patent Document

RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. application Ser. No. 10/984,076, entitled “Liquid Level Sensor,” filed Nov. 8, 2004, which is hereby incorporated by reference in its entirety. The present application also relates to and claims priority from U.S. Provisional Application No. 60/641,393, entitled “Remote Heater Shutoff,” filed on Jan. 4, 2005, which is also hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Embodiments of the present invention generally relate to a deicing system configured for use with a fluid receptacle, such as a livestock water tank, and more particularly, to a deicing system that operates to deactivate a heating element by detecting changes in capacitance and/or resistivity. 
     Heating or deicing systems have been used to maintain unfrozen areas in water tanks for livestock, fish ponds, and the like. A typical deicing system includes a heater coil that may operate at a high output, such as 1500 Watts. The heat from the coil is transferred to water contained within the tank to keep it from freezing. Many tanks employed for this purpose are metallic, plastic, or other such materials. 
     If the heater coil continues to operate when the water level in the tank recedes to a point in which the heater coil directly contacts a surface of the tank, the temperature of the heater coil may heat the surface of the tank to a point in which it is dangerous to touch. In fact, the heater coil may cause the surface of some tanks to melt, or ignite. 
     Typically, thermostats are connected to the heater coil via a thermal path, and operate to deactivate the heater coil if the heater coil gets too hot, as it would if the water level in the tank drops such that the heater coil is exposed to air. If the thermostat is faulty, or only a portion of the heater coil is exposed to air, the heater coil may become extremely hot and present a potential fire hazard. For example, if a portion of the heater coil is exposed to air, but the portion connected to the thermostat is submerged in liquid, the thermostat may not detect the increased temperature of the portion exposed to the air. The exposed portion may contact the surface of the tank and pose the hazards mentioned above. 
     Thus, a need exists for a safer and more reliable system and method of operating and deactivating a deicing system within a fluid receptacle. 
     SUMMARY OF THE INVENTION 
     Certain embodiments of the present invention provide a deicing system configured to heat fluid within a fluid receptacle to prevent ice from forming. The system includes a sensing unit and a heating element. The sensing unit is configured to detect a change in capacitance and/or resistivity. The heating element is configured to heat the fluid within the fluid receptacle. The sensing unit operates to deactivate the heating element when the sensing unit detects the change in capacitance and/or resistivity. The change in capacitance and/or resistivity is the change that results from at least a portion of the system being exposed to water and then to air. 
     Certain embodiments of the present invention also provide a method of deactivating a heating element of a deicing system positioned within a fluid receptacle. The method includes detecting a change in at least one of capacitance and resistivity, and deactivating the heating element based on the detecting step. 
     Certain embodiments of the present invention also provide a deicing system that includes a flotation member configured to provide buoyancy, a heating element, and a capacitor plate. The heating element has a first length and a first width defining a first outer perimeter. The capacitor plate may be secured to at least one of the flotation member and the heating element. The capacitor plate has a second length and a second width defining a second outer perimeter. The first outer perimeter of the heating element does not extend past a length-width envelope of the second outer perimeter of the capacitor plate. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates an isometric simplified representation of a deicing system within a fluid receptacle according to an embodiment of the present invention. 
         FIG. 2  illustrates a lateral view of a deicing system according to an embodiment of the present invention. 
         FIG. 3  illustrates a schematic representation of a control unit according to an embodiment of the present invention. 
         FIG. 4  illustrates a lateral cross-sectional view of a heater coil according to an embodiment of the present invention. 
         FIG. 5  illustrates a simplified circuit diagram of a level sensing circuit, according to an embodiment of the present invention. 
         FIG. 6  illustrates a top isometric simplified representation of a heating element, according to an embodiment of the present invention. 
     
    
    
     The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings, certain embodiments. It should be understood, however, that the present invention is not limited to the arrangements and instrumentalities shown in the attached drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an isometric simplified representation of a deicing system  10  within a fluid receptacle  12  according to an embodiment of the present invention. The fluid receptacle  12  includes lateral walls  14  integrally formed with a base  16 . A fluid retention cavity  18  is defined between the lateral walls  14  and the base  16 . The fluid receptacle  12  may be a birdbath, a livestock water tank, or various other structures that are designed to retain fluid. Fluid, such as water  20 , may be retained within the fluid receptacle  12 . 
     The deicing system  10  may include a flotation member  22  connected to a heating element  24  (such as a heater coil), a capacitor plate (not shown in  FIG. 1 ) and an insulated cord  26  that supplies power from a power source  28 , such as a standard wall outlet, to the deicing system  10 . The deicing system  10  may also include a sensing unit  30 , such as a microchip having a central processing unit, which is programmed to control operation of the deicing system  10 , in communication with the capacitor (as discussed below). Alternatively, the deicing system  10  may not include a flotation member  22 . Optionally, the sensing unit  30  may be remotely located from the deicing system. Additionally, instead of the cord  26  and the power source  28 , the deicing system  10  may be battery or solar powered. 
     The flotation member  22  may be an air-filled bladder, Styrofoam, rubber, or various other materials that provide buoyancy to the deicing system  10 . The flotation member  22  ensures that the deicing system  10  floats within the fluid receptacle  12  when a sufficient amount of water  20  is retained therein. 
     The heating element  24  operates to heat the water  20  within the fluid receptacle  12  so that the water  20  does not freeze. As discussed below, when fluid recedes, evaporates, drains, or is otherwise removed from the fluid receptacle  12  such that fluid is no longer proximate the capacitor (not shown in  FIG. 1 ), the sensing unit  30  acts to deactivate the heating element  24  so that the heating element  24  does not damage the fluid receptacle  12 , or otherwise pose a safety hazard. 
       FIG. 2  illustrates a lateral view of the deicing system  10 . As mentioned above, the deicing system  10  includes the flotation member  22 , the capacitor plate  32 , and the heating element  24 . A conduit  34  passes through the flotation member  22  and allows power to be delivered from the cord  26  to the heating element  24 . The sensing unit  30  is shown on top of the flotation member  22 . However, the sensing unit  30  may be disposed within the flotation member  22 , on or within the capacitor plate  32  or a separate support, or mounted to the heating element  24 . 
       FIG. 3  illustrates a schematic representation of the sensing unit  30 . The sensing unit  30  includes a main body  31  housing a processor  33  operatively connected to a sensing circuit  35 , which may include the capacitor plate  32 . 
     Referring again to  FIG. 2 , the capacitor plate  32  is secured to an underside of the flotation member  22 . For example, the capacitor plate  32  may be fastened to the flotation member  22 , or a plate, sheet or the like that supports the flotation member  22 , through adhesives or bonding material. Optionally, the capacitor plate  32  may secure to the flotation member through screws, bolts, or the like. 
     As shown in  FIG. 2 , the capacitor plate  32  is approximately the same length and width as the heating element  24 . In particular, the capacitor plate  32  may have the same, or larger, perimeter as the heating element  24 , such that the length and width dimensions of the capacitor plate  32  are similar, or exactly the same, as that of the heating element  24 . Further, the capacitor plate  32  may be positioned directly above, and in alignment with, the heating element  24  so that, in general, the outer boundaries of the heating element  24  do not extend past the outer X-Y plane (relative to the deicing system  10 ) envelope of the capacitor plate  32 . That is, the footprint of the capacitor plate  32  is the same, or roughly the same, as that of the flotation member  22 . 
       FIG. 6  illustrates a top isometric simplified representation of the heating element  24 . As shown in  FIG. 6 , the heating element  24 , much like the capacitor plate  32  (shown in  FIG. 2 ), has a length “l”, a width “w”, and a height “h”. The outer length-width envelope of the heating element  24  is defined by plane AB, plane BC, plane CD, and plane DA, which extend through respective sides of the heating element  24 . 
     Referring again to  FIG. 2 , a post  36  extends from, or through, the capacitor plate  32  and secures the heating element  24  to the capacitor plate  32  and/or the flotation member  22 . Various other structures may be used to secure the heating element  24  to the flotation member  22 , such as lateral clamps, locks, or the like. The heating element  24  may be secured to a support bracket that also secures to the flotation member  22 . 
     As shown in  FIG. 2 , a clearance gap  38  is defined between the capacitor plate  32  and the heating element  24 . Fluid, such as air or water, may flow through the gap  38  between the capacitor plate  32  and the heating element  24 . Alternatively, the heating element  24  may be directly attached to the capacitor plate  32  such that the capacitor plate  32  directly abuts a top or bottom surface of the heating element  24 . 
       FIG. 4  illustrates a lateral cross-sectional view of the heater coil  24 . The heating element  24  includes a metal sheath  40  having a central core  42  that may be packed with Magnesium Oxide (MgO), which may serve as a thermal conductor and electrical insulator. An electrical insulating compound  44  is disposed within the central core  42 . Additionally, a heater wire  46  is disposed within a central portion of the electrical insulating compound  44 . Instead of using a separate capacitor plate, such as capacitor plate  32  (shown, for example, in  FIG. 2 ), the metal sheath  40  of the heating element  24  may be used as a capacitor. 
     Referring to  FIGS. 1–4 , presence of water around the heating element  24  may be detected by measuring capacitance. The capacitance of a plate, or set of plates, is determined through equation (1):
 
 C=Kε   0   L   (1)
 
where C is the value of capacitance, K is the dielectric constant, ε 0  is the permittivity constant with a value of 8.85×10 −12  Farads/meter, and L depends upon the geometry of the capacitor and has dimensions of length. For a parallel plate capacitor, L has the value A/d, where A is the area of each plate and d is the distance between the plates. The dielectric constant K depends upon the material adjacent to the capacitor. For instance, air has a dielectric constant of 1, Pyrex glass has a dielectric constant of 4.5, while polystyrene plastic has a dielectric constant of 2.6. The dielectric constant of water is 78. Thus, the capacitance changes by a factor of 78 for a plate capacitor, depending on whether air or water is the dielectric material. Such change in capacitance provides a determination as to whether water or air is contained in a receptacle on which the capacitor is mounted.
 
     The relationship between capacitance, charge and voltage is given by equation (2), set forth below:
 
 q=CV   (2)
 
where q is the charge on the capacitor and V is the voltage across the terminals. A change in capacitance is seen as a change in charge for a given voltage when the dielectric material changes.
 
       FIG. 5  illustrates a simplified circuit diagram  50  of the sensing unit  30  (shown in  FIGS. 1–3 ), according to an embodiment of the present invention. The circuit diagram  50  is simplified for the sake of clarity. The sensing unit  30  shown in  FIG. 3  may include a circuit including the elements shown in  FIG. 5 . Moreover, the sensing unit  30  may include additional electrical components, such as additional resistors, capacitors, and the like. 
     In order to observe a change in capacitance, one may connect a capacitor  52 , such as the capacitor plate  32  (shown in  FIG. 2 ) or the metal sheath  40  (shown in  FIG. 4 ), in series with a resistor  54  to form an RC circuit. Connecting the circuit to a battery  56  causes current to flow through the resistor  54  and charge the capacitor  52 . Instead of the battery  56 , the RC circuit may be connected to a source of alternating current, such as provided through a standard wall outlet. 
     The rate at which the charge on the capacitor  52  increases is given by equation (3):
 
 q=CV   0 (1 −e   −1/RC )  (3)
 
where q is once again the charge, V 0  is the battery voltage, R is the resistance of the resistor, and C is the capacitance of the capacitor. The value RC is called the time constant of the circuit. Because, V=q/C, the voltage V C  across the capacitor is given by equation (4) set forth below:
 
 V   C   =V   0 (1 −e   −1/RC )  (4)
 
Thus, the voltage across the capacitor  52  increases exponentially as a function of time. By replacing C in the equation (4) with a higher value, such as would occur with an increase in the dielectric constant, the time constant RC changes accordingly, and the capacitor  52  therefore takes longer to charge. The change in the time constant is directly proportional to the change in capacitance. A detected change in capacitance may be used to deactivate the heating element  24 .
 
     The presence of air and water around the heating element  24  may, alternatively, be detected by sensing changes in resistivity. The relationship between voltage, current, and resistance is given by equation (5), which is Ohm&#39;s Law:
 
 V=IR   (5)
 
where V is the voltage, I is the current in amperes, and R is the resistance in ohms. For a resistor, however, the resistance can be expressed in terms of resistivity “p” by equation (6):
 
 R=pA/L   (6)
 
where A is the cross-sectional area of the resistor, and L is the length of the resistor. The resistivity, in turn, is dependent upon temperature, as given by equation (7):
 
 p=p   0 [1+α( T−T   0 )]  (7)
 
where T is the temperature, p 0  is the resistivity at temperature T 0 , and α is the temperature coefficient of resistivity. For a material such as nickel, the temperature coefficient of resistivity is approximately 6.0×10 −3 K −1 . Using T 0 =298K (room temperature) and T=506K (the temperature at which paper ignites) with respect to equation (7), p=(2.2)p 0 . Thus, the resistivity more than doubles as the resistor is heated to 506K. However, for a resistor that is submerged in water at room temperature, the water absorbs the heat from the resistor, and the resistance is R=p 0 L/A. If, however, part of the resistor (e.g., 1/20 of the length) is protruding from the water and allowed to heat up to 506K, the resistance increases, such as by six percent (6%). This change in resistance may be detected and used to deactivate the heating element  24 .
 
     Referring to  FIGS. 1–5 , the sensing unit  30  may be mounted to the exterior of a container, or within a non-metallic container in which the dielectric constant of the capacitor plate  32  is determined by the presence of air or water adjacent to the capacitor plate  32 . Optionally, the sensing unit  30  may be separated from the capacitor plate  32  by a non-metallic substance. 
     As shown in  FIG. 2 , for example, the capacitor plate  32  is secured to the underside of the flotation member  22 . As the water  20  within the fluid receptacle  12  decreases, the deicing unit  10  may come to rest against an obstacle that tips the deicing unit  10  such that part of the heating element  24  is exposed to air. Because the footprint of the capacitor plate  32  is approximately the same as that of the heating element  24 , if a portion of the heating element  24  is exposed to air, a corresponding portion of the capacitor plate  32  will also be exposed to air. For example, if an end  60  of the heating element  24  is exposed to air, an end  62  of the capacitor plate  62  will also be exposed to the air. Optionally, if the metal sheath  40  of the heating element  24  is used as the capacitor plate, changes in capacitance are detected in a similar way. As shown in  FIG. 4 , the metal sheath  40  surrounds the heater wire  46 . 
     Whether the capacitor plate  32  or the metal sheath  46  is used to detect capacitance, the sensing unit  30  receives a capacitance change signal from the capacitor plate  32 , thereby sensing a change in capacitance, as discussed above. The processing unit  33  (shown in  FIG. 3 ) may then send a deactivation signal to the heating element  24  based on the sensed change in capacitance, thereby turning the heating element  24  off. 
     Alternatively, the resistance of the metal sheath  40  of the heating element  24  itself may be measured by the sensing unit  30  to determine if the resistivity of any section of the sheath  40  or the heating element  24  has increased, thereby signaling a hot spot. As discussed above, the resistivity may be determined by measuring the value of resistance with the known value when the deicing system  10  is operating properly. The sensing unit  30  may sense an increase in resistance, and operate to deactivate the heating element  24  in response to the sensed increase in resistance. 
     Thus, embodiments of the present invention provide a safe and reliable system and method of deactivating a deicing system within a fluid receptacle. Embodiments of the present invention provide a deicing system that automatically deactivates a heater coil based on a detected physical change, such as a change in capacitance or resistivity. Further, embodiments of the present invention do not rely on a thermostat in order to deactivate the heater coil. 
     While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.

Technology Category: 5