Abstract:
A method and a system for detecting the level of fluid in a container is provided: The level meter includes first, second and third generally parallel probes being positioned within the container. The first probe includes a first portion that is at least partially covered by an insulating material, and a second generally exposed end. The second probe is generally free of the insulating material and the third probe provides an electrical ground. An electronic unit is operable to measure electrical characteristics of the first and second probes to determine an amount of the second probe which is exposed to a fluid within the container.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates generally to measurement devices, and more particularly, to an impedance level meter for liquids in tanks. 
   BACKGROUND OF THE INVENTION 
   Level metering is used in a vast number of applications to monitor the level of liquid, gas or other material in a container. In the gas industry, for example, a widely used level measuring device is float level meter. This type of meter requires the installation of a float inside the tank. The float is connected to the body of the meter by a metal arm. The arm allows the position of the interface between liquified gas and gas which is in a gaseous state to be monitored. The movement of the float is translated to a rotational displacement by the arm. The displacement of the arm requires quite a bit of space, making it difficult to use this type of level meter in small tanks (e.g., gas grill and portable tanks). Float level meters are also not effective in portable applications because of the sensitivity of the sensors to movement. Furthermore, with regard to vertical portable LP gas tanks, the small diameter of the containers makes it difficult or impossible to use float level meters. 
   SUMMARY OF THE INVENTION 
   The present invention provides a level meter and method of level metering for materials contained in tanks that substantially eliminates or reduces at least some of the disadvantages and problems associated with the previous level meters and methods. 
   In accordance with a particular embodiment of the present invention, a level meter includes first, second and third generally parallel probes that are positioned within a container. The first probe includes a first portion that is covered by an insulating material, and a second generally exposed end. The second probe is generally free of insulating material. The third probe provides an electrical ground. An electronic unit is also provided that is operable to measure electrical characteristics of the first and second probes, and liquid adjacent exposed areas of the probes and ground, to determine a length of the second probe which is exposed to a fluid within the container. 
   In accordance with another embodiment of the present invention, a method for detecting a level of fluid in a container includes installing first, second and third generally parallel probes within a container. In this embodiment, the first probe also includes a first portion at least partially covered by an insulating material, and a second generally exposed end. The second probe is generally free of the insulating material and the third probe is an electrical ground. The method further includes measuring electrical characteristics of the first and second probes to determine a length of the second probe that is exposed to a fluid within the container, using an electronic unit. 
   Technical advantages of particular embodiments of the present invention include an electronic measuring device that may be used to measure the level of practically any fluid in a container, using the electrical properties of the fluid to generate a frequency signal containing information about the level, in its duty-cycle. Such an electronic measuring device does not require calibration to be used in different fluids, and in different applications. 
   Another technical advantage of particular embodiments of the present invention is a level meter that is versatile and able to easily transmit its output by a coaxial cable or radio frequency, Furthermore, the output of the level meter can be converted to a digital number or an analog level directly. 
   Yet another technical advantage of particular embodiments of the present invention is a level meter which may employ various types of sensing elements. For example, the conductors (e.g., probes) can be metal rods, or ribbons of printed circuit materials etched and processed as PC boards. The conductors and related electronics may be placed on a one-piece probe or may be separated using a connector. 
   Still another technical advantage of particular embodiments of the present invention includes a level meter sensor that is immune to changes in temperature or fluid characteristics. The ratio-metric operation of the sensor provides this immunity to temperature and/or other characteristics of the fluid. 
   Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some or none of the enumerated advantages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram illustrating a level meter installed within a container, in accordance with a particular embodiment of the present invention; 
       FIG. 2  illustrates a wiring diagram suitable for use with the level meter of  FIG. 1 , in accordance with a particular embodiment of the present invention; 
       FIG. 3  illustrates further details regarding a wiring diagram suitable for use with the level meter of  FIG. 1 , illustrating various aspects of the present invention; 
       FIG. 4  illustrates a level meter having a digital readout, in accordance with a particular embodiment of the present invention; 
       FIG. 5  illustrates a printed circuit board (PCB) ribbon having probes etched thereupon, in accordance with another embodiment of the present invention; and 
       FIG. 6  illustrates an example system and method for detecting the level of fluid in a container, in accordance with a particular embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a level meter  1 , in accordance with a particular embodiment of the present invention. Level meter  1  includes three elongate rod probes  4 ,  5  and  6 . Probes  4 ,  5  and  6  each extends into a tank, or container  2 . Container  2  is partially filled with a fluid  3 . Reference number  9  designates the level of fluid  3  within container  2 . Level meter  1  may be used to determine the level  9  of fluid  3  within container  2 , in accordance with aspects of the present invention. 
   Rod probe  4  is partially covered with an insulator  12 , which protects and insulates portions of rod probe  4  that are adjacent to insulator  12 . Rod probe  4  also includes an uninsulated portion  7 , at a lower end of rod probe  4 . Rod probe  6  is uninsulated along its entire length. 
   Rod probe  5  provides a ground. In various embodiments of the present invention, metal rod probes  4 ,  5  and/or  6  may be anodized rods. 
   In accordance with the teachings of the present invention, uninsulated portion  7  of rod probe  4  is used as a reference, and level  9  of fluid  3  in container  2  is determined by calculating how much of non-insulated rod probe  6  is covered by fluid  3 , compared with the length of uninsulated portion  7  of rod probe  4 . This ratio-metric determination of the level  9  of fluid  3  makes level meter  1  independent of temperature and other fluid characteristics. 
   Level meter  1  includes a metal body cap  8  that is installed adjacent the top of tank  2 . Rod probes  4 ,  5  and  6  pass through metal body cap  8 , and the anodization provides the necessary insulation between rod probes  4 ,  5  and  6 . In this embodiment, the respective tips  4   a ,  5   a , and  6   a , of rod probes  4 ,  5  and  6  may be sanded to provide conductivity thereto. 
   Level meter  1  also includes a plastic electronic unit  39 . Electronic unit  39  is connected to rod probes  4 ,  5  and  6  using spring connectors  34 ,  35  and  36 , respectively. Spring connectors  34 ,  35  and  36  are mounted directly on a PC board  37 . A liquid crystal display  38  is also coupled with electronic unit  39 . The liquid crystal display  38  provides alphanumeric information to an operator through a generally transparent portion of electronic unit  39 . 
     FIG. 2  illustrates the operation of level meter  1 , in accordance with a particular embodiment of the present invention. As illustrated in  FIG. 2 , ground rod probe  5  is coupled to the ground of an electronic circuit, or to any other stable voltage. Rod probes  4  and  6  are electrically coupled to the time setting input of monostable multivibrators  13  and  14 , respectively. Each monostable multivibrator  13  and  14  includes an associated trigger  22  and  21 , respectively. Triggers  21  and  22  are electrically coupled to outputs  15  and  16  of multivibrators  13  and  14 , respectively. The circuit formed by these components of level meter  1  generates the type of waveforms illustrated in FIG.  2 . 
   Waveforms  18 ,  19  and  20  of  FIG. 2  illustrate three level “conditions.” Waveform  19  illustrates a fifty percent fluid level. Waveform  18  illustrates a ninety percent fluid level. Waveform  20  illustrates a ten percent fluid level. In accordance with this embodiment of the present invention, the reference rod probe (rod probe  4 ) should be “wet,” since measurements are unreadable below that level. 
   Rod probes  4 ,  5  and  6 , of the illustrated embodiment, are each made of a metal that is treated chemically to make rod probes  4 ,  5  and  6  partially insulated. In a particular embodiment, rod probes  4 ,  5  and  6  may be made of aluminum. In this embodiment, anodizing provides a strong electric insulation from solid materials, but liquids may penetrate the anodization. This will lower the impedance between the liquid surrounding the rod probe, and the center of the rod. 
   Since rod probes  4 ,  5  and  6  are anodized, they may penetrate metal body cap  8  without short circuiting with the other rod probes or metal body cap  8 . Outer diameters of the anodized rod probes are equal to the inner diameter of holes through metal body cap  8 . In accordance with a particular embodiment of the present invention, the holes may be expanded using heat to allow the rod to penetrate through the cap. However, when the temperature dissipates, a very tight seal and electric insulation is provided. In  FIG. 2 , a separator  11  is used to maintain distance between rod probes  4 ,  5  and  6 . 
   The level sensing technology employed within the teachings of the present invention uses a differential measuring technique to relate height of a fluid  3  inside a container  2  with the impedance between a sensing rod probe  6  and a ground rod probe  5 . The impedance is measured and compared with the impedance between the ground rod probe  5 , and the small exposed, or uninsulated portion  7  of the reference rod probe  4 . Again, reference rod probe  4  is highly insulated along its entire length, except for a small exposed area (uninsulated portion  7 ). An output pulse width modulation is generated to provide information regarding how much of sensing rod probe  6 ,is covered by the fluid  3 , expressed as a multiple of the length of uninsulated portion of rod probe  4 . In some embodiments, the reference rod may be eliminated for example, if the container is metallic and the fluid impedance is low. 
   In accordance with a particular embodiment of the present invention, the reference height (e.g., uninsulated portion  7  of rod probe  4 ) must be covered by fluid in order for level meter  1  to function properly. Accordingly, no measures may be taken below the lowest level of insulator  12 . However, these techniques are very inexpensive to employ and provide reliable level metering. A single level meter incorporating the technology described herein may be used to measure the level of any liquid in a container, without changing any components, configuration or other aspects of the level meter. 
   In accordance with another aspect of the present invention, a small constant impedance between rod probes  4 ,  5  and/or  6  may be caused by parasitic electrical elements that cannot be completely eliminated during the manufacturing process of level meter  1 . Straight capacitances generated by the insertion of rod probes  4 ,  5  and/or  6  through metal end cap  8  may be compensated for, using the teachings of the present invention. One such technique is described with reference to FIG.  3 . 
     FIG. 3  illustrates a circuit  10  that is operable to generate pulse width modulated signals that contain information regarding the level  9  of fluid  3  in container  2 . Electronic NAND gates of  FIG. 3  have Schmidt trigger inputs. It is desirable that such gates be free of input protecting diodes. Otherwise, an unbalance of charge interchange between rod probes  4  and  6  may cause molecular migration. This would have a tendency to alter the electrical properties in the surface of rod probes  4  and  6 . As illustrated in  FIG. 3 , reference rod probe  4  and sensing rod probe  6  are electrically coupled to respective inputs of corresponding ultra high impedance NAND gates  70  and  71 , respectively. 
   Following along circuit  10 , NAND gates  70  and  71  are electrically coupled by capacitors  26  and  25 , respectively, to the outputs of NAND gates  27  and  28 , respectively. These outputs can vary from approximately ground to a predetermined supply voltage. For example, if the output of NAND gate  27  is at +V, a current flow by capacitor  26  will charge at a speed proportional to the impedance between reference rod probe  4  and ground rod probe  5 . When the level at the input of NAND gate  70  reaches the low input level threshold, the output of NAND gate  70  goes high. This makes the output of NAND gate  27  low. The high to low transition of NAND gate  27  described above, is used to trigger a second monostable circuit. 
   A small capacitor  24  and resistor  26  convert the high to low output step from NAND gate  27  to a small pulse that occurs at the input of NAND gate  28 . The output of NAND gate  28  goes high and a charge process of capacitor  25  is initiated. The charge process proceeds at a pace established by the impedance between sensing rod probe  6  and ground rod probe  5 . 
   When the voltage at NAND gate  71  input reaches the low threshold level, the output of NAND gate  71  will go high and a new period at the output of NAND gate  27  is initiated. This is due to the fact that the monostable multivibrator formed by NAND gate  27  and NAND gate  70  will be retriggered by the high to low-level transition of NAND gate  28 . 
   In accordance with a particular embodiment of the present invention, capacitors  25  and  26  should be very small and have a very high internal resistance. If the frequency of isolation is low, an almost completely transient RC circuit can be considered during the charge-discharge process of capacitors  25  and  26 . If the output frequency is very high, capacitance and inductance of the liquid are important. However, these conditions should not affect the duty cycle information of the output. 
   High-speed discharge circuitry for capacitor  26  may be used to insure that the voltage across capacitor  26  will be zero when the trigger signal from NAND gate  28  arrives. The charge time of capacitor  26  should always be larger than the charge time of capacitor  25 . Capacitor  26  may be discharged more quickly if the output of NAND gate  27  can swing below ground during a small time frame. 
     FIG. 4  illustrates a level meter  1 , in accordance with another embodiment of the present invention. Level meter  101  includes a two-digit display  30 . Display  30  is used to provide information (readings) to an operator of level meter  101 . Level meter  101  also includes an output connector  31  for coaxial cable. In the illustrated embodiment, output connector  31  comprises a 75 Ohms output connector. A protection lid  33  is provided to protect level meter  101  and its components from the elements (rain and sun), and ambient environmental conditions. 
   In alternative embodiments of the present invention, connector  31  may be modified to communicate with a remote location using a medium other than coaxial cable. For example, twisted pair, ethenet or other type of conductor may be used to transmit information to a remote location. Similarly, level meter  101  may employ radio frequency (RF) signals to communicate information to a remote location. 
     FIG. 5  illustrates an embodiment of the present invention, wherein rod probes  4 ,  5  and  6  are replaced by a printed circuit board (PCB) ribbon  51 . PCB ribbon  51  includes three conductors  50  that extend approximately parallel with one another along PCB ribbon  51 . In the illustrated embodiment, conductors  50  are each separated by a distance of one to five millimeters. Therefore, in this embodiment, most of the electronic circuits of the level meter may be placed upon the same printed circuit board ribbon. 
   In  FIG. 5 , conductors  50  are placed, printed and etched upon PCB ribbon  51 . Pulse width modulation circuitry  52  is also imprinted on the same PCB ribbon  51 . Using printed conductors instead of rods may provide advantages, within the teachings of the present invention. For example, only one conductor  53  must extend through metal body cap  8 . This can be very useful if the pulse width modulated output is intended to be sent to a remote monitoring device. 
   The power supply for the inner circuitry of PCB ribbon  51  and output signal can share the same wire, in the embodiment of FIG.  5 . PCB ribbon  51  also includes a ground wire  54 . Ground wire  54  may be connected to the electronics reference to the metal body of the sensor. 
     FIG. 6  illustrates portions of level meter  1 , in accordance with a particular embodiment of the present invention. Reference probe  4  of  FIG. 6  is covered by insulation material  12 . Rod probe  4  includes an exposed area, or uninsulated portion  7 . The length of uninsulated portion  7  should be covered by the liquid  3  at all times, and will be considered the reference height. The exposed conducting material of uninsulated portion  7  is used to measure the electrical impedance with respect to ground rod probe  5 . This impedance depends on the capacitance, resistance and inductance between the reference rod probes&#39;  4  exposed area (uninsulated portion  7 ) and ground. All of these factors are affected by the electrical properties of liquid  3 , the geometry of reference sensing electrodes (rod probe  4  and ground rod probe  5 ), the wet surfaces, and the temperature. 
   In the illustrated embodiment, the level  9  of liquid  3  is approximately a distance h above the bottom of rod probe  4 , as shown in FIG.  6 . For purposes of this example, it is assumed that height h is equal to three times the length of uninsulated portion  7  of rod probe  4 . Therefore, impedance approximately equal to one third of that measure in reference rod probe  4  can be measured between sensing rod probe  6  and ground rod probe  5 . The output frequency  60  contains the information regarding how many times the reference length (h/3 in the illustrated embodiment) is covered by liquid along the sensing rod. 
   Although embodiments of the invention and their advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention, as defined by the appended claims.