Liquid level sensing apparatus

A fluid level gauge assembly for measuring a level of a fluid in a fluid container is provided. The fluid level gauge assembly includes a first coaxial cable section having a first end and a second end, the first coaxial cable section including a first center conductor, a first outer conductor and a solid dielectric between the first center and outer conductor; a second coaxial cable section having a first end and a second end, the second coaxial cable section including a second center conductor, a second outer conductor and an air dielectric between the second center and outer conductor; and a coupling between the second end of the first coaxial cable section and the first end of the second coaxial cable section. The second coaxial cable section being disposed in a fluid for determining a level of the fluid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus for precisely measuring the level of a liquid.

2. Description of the Related Art

Most vehicles, including cars, trucks aircraft and boats include fuel tanks and sensors for detecting the level of fuel in the fuel tank. One common fuel level sensor includes a float that will ride approximately on the surface of the liquid fuel in the fuel tank. Thus, the float will move vertically in the fuel tank as the amount of fuel in the tank varies. The float may be disposed to slide along a detector and the relative position of the float along the detector provides an indication of the level of fuel in the tank. Fuel level sensors that rely upon a float are mechanically complicated, costly and imprecise. Furthermore, a problematic fuel level float is very difficult to repair or replace without removing and disassembling a significant part of the fuel system of the vehicle.

Time domain reflectometers are used to measure the length of a coaxial cable and have been used to measure the level of a liquid. An example of the use of time domain reflectometers to measure a quantity of liquid is shown in U.S. Pat. No. 3,703,829. More particularly, U.S. Pat. No. 3,703,829 relates to a system that mounts a coaxial cable vertically in a tank. The coaxial cable has a center conductor, an outer conductor and an air dielectric between the center conductor and the outer conductor. Electrical signal pulses are generated and applied to the upper end of the center conductor of the coaxial cable. The signals travel down the center conductor and are reflected at the fluid surface. The volume of fluid in the tank is a function of the time delay between the application of the pulse and the receipt of the surface-reflected pulse. The time domain reflectometer technology of U.S. Pat. No. 3,703,829 can work well if the tank and time domain reflectometer can be designed concurrently and if space is not an issue. Most tank level gauges do not rely on time domain reflectometers and cannot be retrofitted easily for the type of time domain reflectometer disclosed in U.S. Pat. No. 3,703,829.

Accordingly, an object of the invention is to provide a fluid level gauge that enables time domain reflectometer technology to be retrofitted into an existing tank.

SUMMARY OF THE INVENTION

The invention relates to a fluid level gauge assembly that comprises a first coaxial cable section, a second coaxial cable section and a coupling between the first and second coaxial cable sections. The first coaxial cable section has a center conductor, outer conductor and a dielectric between the center and outer conductors. The dielectric of the first coaxial cable section preferably is a solid dielectric, such as Teflon. The first coaxial cable section has an outer end and an inner end. The outer end of the first coaxial cable section is configured for connection to a time domain reflectometer.

The second coaxial cable section of the fluid level gauge assembly has a center conductor, an outer conductor and an air dielectric between the center and outer conductors. The center conductor of the second coaxial cable section may be a unitary extension of the center conductor of the first coaxial cable section. The second coaxial cable section has a lower end disposed to be below the surface of the fluid for all levels of fluid and an upper end to be disposed above the surface of the fluid for all levels of fluid. A cap may be mounted to one end of the second coaxial cable section and may assist keeping the coaxial arrangement of the center and outer conductors of the second coaxial cable section. The cap has at least one opening to permit fluid or air to flow into the space between the center and outer conductors of the second coaxial cable section.

The coupling of the fluid level gauge assembly has a center conductor, an outer conductor and a dielectric extending therebetween. The center conductor of the coupling preferably is a unitary extension of the center conductor of the first coaxial cable section and preferably continues unitarily to define the center conductor of the second coaxial cable section. The outer conductor of the coupling connects the outer conductors of the first and second coaxial cable sections. At least part of the dielectric of the coupling is an air dielectric and communicates with the air dielectric of the second coaxial cable section. Additionally, the outer conductor of the coupling has at least one vent hole to provide communication between the air dielectrics of the coupling and the second coaxial cable section to the ambient surroundings. The coupling preferably is configured to permit the first and second coaxial cable sections to be aligned at an angle to one another.

The angle of the coupling and the lengths of the first and second coaxial cable sections are selected in view of the size and shape of the fluid container and the space available outside the container for electrical connection to the time domain reflectometer. Furthermore, the ability to choose the angle of the coupling and the lengths of the first and second coaxial cable sections enables the fluid level gauge assembly to be fit into an existing fluid system, such as an existing fuel system. Accordingly, the fluid level gauge assembly can be designed and dimensioned to replace a less efficient fluid level system, such as a system that relies upon a float.

The fluid level gauge assembly may be manufactured by providing a coaxial cable with a solid dielectric. Selected lengths of the outer conductor and the dielectric then are removed from one end of the coaxial cable to leave a selected length of the center conductor exposed. The exposed end portion of the center conductor then may be bent at a position near the remaining parts of the dielectric and the outer conductor. The coupling then may be mounted to surround part of the outer conductor and may be secured by brazing or the like. A tubular outer conductor then may be mounted over the exposed end portion of the center conductor and may be secured to the coupling by brazing or the like. An end cap may be mounted to the end of the tubular outer conductor opposite the coupling to maintain the concentric relationship between the exposed center conductor and the tubular outer conductor. The portion of the coaxial cable that does not have the outer conductor and the dielectric removed define the above-described first coaxial cable section. The exposed inner conductor and the tubular outer conductor define the above-described second coaxial cable section. The order of the manufacturing steps may be varied. For example at least part of the bending of the center conductor can be carried out after the coupling is positioned.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

A fluid level gauge assembly in accordance with the invention is identified by the numeral10inFIGS. 1 to 3. The fluid level gauge assembly10is used in this embodiment to measure the level of fuel F in a fuel tank T. However, the fluid level gauge assembly10can be used to measure levels of fluid in other environments, e.g., a fluid container.

The fluid level gauge assembly10includes a first coaxial cable section12that has opposite outer and inner ends14and16. The first coaxial cable section12also has an outer conductor18, a solid dielectric20and a center conductor22. The outer conductor18and the solid dielectric20extend from the outer end14to the inner end16of the first coaxial cable section12. The center conductor22extends from the outer end14of the first coaxial cable section12beyond the inner end16thereof. A coaxial connector24is mounted to the outer end14of the first coaxial cable section12and extends partly through a side wall of the tank T. The coaxial connector24can be connected to a time domain reflectometer (not shown).

The fluid level gauge assembly10further includes a coupling26with opposite first and second ends28and30. A conductive side wall32extends between the ends28and30of the coupling26and a nonlinear passage34extends through the coupling from the first end28to the second end30. The passage34includes a first section36adjacent the first end28of the coupling26and a second section38adjacent the second end30of the coupling26. In the illustrated embodiment, the first and second sections36and38of the passage34are aligned at a right angle. However, other angular alignments are possible. Portions of the first section36of the passage34are configured to telescope onto the outer conductor18at the inner end16of the first coaxial cable section12. Holes or apertures40pass through the coupling26. The center conductor22extends through the passage34and beyond the second end30of the coupling26.

The fluid level gauge assembly10further includes a conductive tube42that coaxially surrounds the center conductor22and telescopes into second section38of the passage34at the second end30of the coupling26. The conductive tube42then is brazed into engagement with the second end30of the coupling26. The conductive tube42and portions of the center conductor22therein define a second coaxial cable section44. In the illustrated embodiment, the second coaxial cable section44further includes a plurality of spaced apart supports46that extend substantially radially in from the conductive tube42to maintain the coaxial arrangement between the conductive tube42and the center conductor22.

The second coaxial cable section44further includes an end cap48, as shown inFIGS. 2,3and3A, to contribute to the coaxial arrangement of the conductive tube42and the center conductor22. The end cap48is formed with vent holes or apertures50for allowing movement of a fluid therethrough. In one embodiment, the end cap48is formed with a threaded aperture49configured for receiving the center conductor22. The end cap48is disposed over the lower end of the conductive tube42and twisted on until the center conductor is fully received into the threaded aperture49. Optionally, the end cap48will be secured by at least one rivet51to the conductive tube.

The fluid level gauge assembly10may be manufactured by providing the coaxial cable12with an outer end14, an outer conductor18, a solid dielectric20and a center conductor22. The coaxial connector24then is connected to the outer end14of the coaxial cable12. Selected lengths of the outer conductor18and the dielectric20then are removed from one end of the coaxial cable12to leave a selected length of the center conductor22exposed. The exposed end portion of the center conductor22is bent at a position near the remaining parts of the outer conductor18and the dielectric20. The coupling26then is mounted to surround part of the outer conductor18and is secured by brazing. The conductive tube42then is mounted over the exposed end portion of the center conductor22and is brazed to the coupling26. An end cap48is mounted to the end of the conductive tube42opposite the coupling26to maintain the concentric relationship between the exposed center conductor22and the conductive tube42.

The fluid level gauge assembly10is mounted in the tank T so that the coupling26is substantially adjacent the bottom wall of the tank T and so that the end of the conductive tube42remote from the coupling26projects up in the tank T. Fuel F in the tank T will flow through the holes40and into the air dielectric between the center conductor22and the conductive tube42. Air in the space between the center conductor22and the conductive tube42can escape through the holes50in the end cap48. The position of the fuel along the center conductor22will vary in accordance with the level of fuel in the tank T. Signals then are generated external of the tank T and are directed into the center conductor22via the connector24. The time domain reflectometer can determine the location of the interface between the fuel F in the gap between center conductor22and the conductive tube42.

The coupling26shown inFIG. 1, is a right angle coupling. However, other optional alignments are possible to meet the space limitations of a particular application. For example, an alternate coupling56is shown inFIGS. 4 and 5and has a passage58extending therethrough. The passage58has first and second sections60and62that are aligned to one another at an angle of about 100 degrees.

The invention has been described with respect to a preferred embodiment. However, it is apparent that various changes can be made without departing from the scope of the invention. Several such options are described below.

The illustrated embodiments have the coupling26at the bottom end of the fluid level gauge assembly10. However, the fluid level gauge assembly10can be mounted in an inverted orientation with the coupling26at the top66of the fluid container and the cap48at the lower or bottom end68, as illustrated inFIG. 6. In this embodiment, the end cap48is positioned slightly above the bottom of the fuel tank to accommodate a flow of fluid via vent holes50into the space between the outer conductive tube42and the center conductor22. Similar to the embodiments described above, the coupling26may be configured to provide other optional alignment angles to meet the space limitations of a particular application, as opposed to the 90 degree angle shown inFIG. 6.

The couplings26and56are shown as being of unitary construction. However, the couplings can be made of several parts that are assembled around the exposed parts of the center conductor22.

The center conductor22is shown as extending continuously through the fluid level gauge assembly10. However, the center conductor need not be unitary. For example, the coupling may be a coaxial connector, such as a right angle coaxial connector, that is connected to the first coaxial cable section12. The second coaxial cable section then may be connected to the coaxial connector that is used in place of the coupling26,56.