Patent Publication Number: US-2021181009-A1

Title: Measuring fluid level in tank with complex geometrical shape

Description:
TECHNICAL FIELD 
     This disclosure relates the measurement of the level of a fluid in a vessel having an irregular shape. 
     BACKGROUND 
     Fluid containers may be used, for example to supply oil to various components of an engine. To achieve the necessary fluid volume the containers may be formed in unconventional shapes so as to permit installation in confined and possibly inaccessible locations. A particular application is in a gas turbine engine. 
     Gas turbine engines may include a compressor, a combustor and a turbine. Typically, the compressor is an air compressor rotating on a longitudinal shaft of the engine to provide air for the combustion cycle. The air is provided to the combustor along with fuel where combustion occurs to create a high pressure, high temperature flow, which is provided to the turbine. The turbine may provide mechanical torque to the shaft and provides exhaust gas that creates thrust. The gas turbine engine typically includes bearings, such as shaft bearings that allow the shaft to rotate. Such bearings may be lubricated by bearing oil. The bearing oil may be distributed to one or more bearings from an oil sump(s). Seals may be used to stop leaking of the bearing oil around the shaft or other rotating parts of the gas turbine engine. An oil scavenge system may return bearing oil to the oil sump(s). The location and cross-section and overall configuration of such an oil tank may be constrained by the geometry of the turbine and associated air guiding structures. Other liquid storage reservoirs may encounter space restrictions where the disclosed technology may be useful. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments may be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views. 
         FIG. 1  illustrates a longitudinal cross-sectional view of an example of a gas turbine engine, with an example location of an oil reservoir; 
         FIG. 2  is a simplified transverse cross-section view (looking forward) of a portion of an example gas turbine engine of  FIG. 1  illustrating the example location of the oil reservoir; 
         FIG. 3  A is a transverse cross-sectional view of an arcuate vertical support adapted to guide a float having a compatible cross section and radius of curvature; 
         FIG. 3B  is a transverse cross-sectional view of a float body having a cross section compatible with the support of  FIG. 3A ; 
         FIG. 3C  is a longitudinal cross-sectional view of the float body of  FIG. 3B  where the radius of curvature conforms to the radius of curvature of the vertical support; 
         FIG. 4A  is an example of a structure having a plurality of capacitive-sensors, where several different linear dimensions are used; 
         FIG. 4B  is an elevation and cross-sectional view of a float body similar to that of  FIG. 3B  having a metal plate for forming a capacitor with the capacitive sensors of  FIG. 4A ; 
         FIG. 4C  is an elevation view showing the relationship of the float assembly with respect to the structure of  FIG. 5A  as the float moves along the structure; 
         FIG. 5A  illustrates the conceptual relationship between an actual fluid level in a reservoir and the output of the capacitive sensor array as the fluid level changes; 
         FIG. 5B  illustrates the relationship between the oil gauge output of  5 A and the actual quantity of oil in the reservoir; 
         FIG. 6A  is an example of a structure having a plurality of optical retroreflective sensors, where several different linear dimensions are used for the separation between sensors; 
         FIG. 6B  is an elevation and cross-sectional view of a float body similar to that of  FIG. 4B  having a reflective strip extending along a vertical dimension thereof; and 
         FIG. 6C  is an elevation view showing the relationship of the float assembly with respect to the structure of  FIG. 6A  as the float moves along the structure. 
     
    
    
     DETAILED DESCRIPTION 
     In an example, the vessel may be an oil reservoir that is located between the inner portion of a fan duct and the outer housing of the turbine portion of a gas turbine engine. Other locations are not intended to be excluded. The location of the oil reservoir may be constrained by the other aspects of the engine design such that direct access to the oil reservoir is either difficult or not feasible without disassembly. Often the design requirements for ancillary equipment may be constrained by the requirements of the remainder of the power plant. Oil may be added to the reservoir using a filler pipe or pressurized oil supply to make up for oil consumed during operation. To do this, the level of oil in the reservoir needs to be determined, and the oil re-supply operation should not result in overfilling of the reservoir. 
     In another example, the oil reservoir may be formed with a pair of arcuate opposing sides so as to increase the angular length of the oil reservoir when the reservoir is located, for example, to approximately conform with the radius of curvature of an interior part of the engine assembly. The device used to measure the oil level over a significant portion of the volume of the oil reservoir may have an arcuate shape as well so as to permit the length dimension of the device to be incorporated into the oil reservoir without interference with the walls thereof. The oil reservoir also be known as an oil tank, or similar name. 
     In an example, oil level measuring device, or oil gauge, is described, substantially conforming to the to the curvature of the reservoir itself and comprises a arcuate support structure, a float adapted to be guided along the support structure, and an electrical quantity sensor. This structure is intended to represent a situation where the cross sectional area of the reservoir varies along the height or vertical direction thereof, so that the relationship of the quantity of fluid in the reservoir and a height of the fluid in the reservoir may not be linear. However, this aspect is not intended to exclude a measuring device having a straight profile in the height dimension. 
       FIG. 1  is a cross-sectional view of an example of a gas turbine engine  100 . The gas turbine engine  100  may, for example, supply power to and/or provide propulsion of an aircraft. Examples of the aircraft may include a helicopter, an airplane, an unmanned space vehicle, a fixed wing vehicle, a variable wing vehicle, a rotary wing vehicle or the like. In other examples, the gas turbine engine  100  may be utilized in a configuration unrelated to an aircraft such as, for example, an industrial application, an energy application, a power plant, a pumping set, a marine application (for example, for naval propulsion), a weapon system, a security system, a perimeter defense or security system. 
     The gas turbine engine  100  may take a variety of forms in various embodiments. Although depicted in the example of  FIG. 1  as a ducted axial-flow engine with multiple spools, in some forms the gas turbine engine  100  may have additional or fewer spools and/or may be a centrifugal or mixed centrifugal/axial flow engine. In some forms, the gas turbine engine  100  may be a turboprop, a turbofan, or a turboshaft engine. Furthermore, the gas turbine engine  100  may be an adaptive cycle and/or variable cycle engine. Other variations are also contemplated. Other engine types may also employ a fluid tank where remote quantity measurement is desired. 
     The gas turbine engine  100  may include an air intake  102 , multistage axial-flow compressor  104 , a combustor  106 , a multistage turbine  108  and an exhaust  110  concentric with a central axis  112  of the gas turbine engine  100 . The multistage axial-flow compressor  104  may include a fan  116 , a low-pressure compressor  118  and a high-pressure compressor  120  disposed in a fan casing  122 . The multistage turbine  108  may include a high-pressure turbine  128  and a low-pressure turbine  132 . 
     A low-pressure spool includes the fan  116  and the low-pressure compressor  118  driving the low-pressure turbine  132  via a low-pressure shaft  144 . A high-pressure spool includes the high-pressure compressor  120  driving the high-pressure turbine  128  via a high-pressure shaft  148 . In the illustrated example, the low-pressure shaft  144  and the high-pressure shaft  148  are disposed concentrically in the gas turbine engine  100 . In other examples, other shaft configurations are possible. 
     During operation of the gas turbine engine  100 , fluid received from the air intake  102 , such as air, is accelerated by the fan  116  to produce two air flows. A first air flow, or core air flow, travels along a first flow path indicated by dotted arrow  138  in a core of the gas turbine engine  100 . The core is formed by the multi-stage axial compressor  104 , the combustor  106 , the multi-stage turbine  108  and the exhaust  110 . A second air flow, or bypass airflow, travels along a second flow path indicated by dotted arrow  140  outside the core of the gas turbine engine  100  past outer guide vanes  142 . 
     The first air flow, or core air flow, may be compressed within the multi-stage axial compressor  104 . The compressed fluid may then be mixed with fuel and the mixture may be burned in the combustor  106 . The combustor  106  may include any suitable fuel injection and combustion mechanisms. The resultant hot, expanded high-pressure fluid may then pass through the multi-stage turbine  108  to extract energy from the fluid and cause the low-pressure shaft  144  and the high-pressure shaft  148  to rotate, which in turn drives the fan  116 , the low-pressure compressor  118  and the high-pressure compressor  120 . Discharge fluid may exit the exhaust  110 . 
     The first air flow  138  and the second air flow  140  are coaxial and are confined and separated from each other by a structure comprising the fan casing  122  and an outer compressor case  160  and the outer case  162  of the multi-stage compressor  118 , 120 . A void  170  may exist between the compressor case  160  and the outer case  162  where auxiliary equipment such as an oil reservoir  10 , shown in cross section, may be provided, using this otherwise empty space. 
     In an aspect,  FIG. 2  is a simplified transverse cross-section view (looking forward) of a portion of an example gas turbine engine of  FIG. 1  illustrating a the location of the oil reservoir  10  with respect to the outer compressor case  162  and the outer case  162  of the engine core, which may the multi-stage compressor or high pressure turbine  128 . In this example, the oil reservoir  10  is disposed along an arcuate side portion of the void  170  created by the walls  160  and  162 . Details of the mounting arrangement are not shown as they depend on the engine specific design. However, the oil reservoir  10  may be fixedly attached to a wall  160  or  162 . 
     The reservoir  10  may comprise arcuate surfaces opposing the walls  160  and  162  where the radius of curvature of the walls of the reservoir are selected to conform to the general radius of curvature of the void  170 , facilitating installation of a reservoir  10  of a desired capacity in a confined space. The radius of curvature may vary as part of the detailed design of the oil reservoir  10 , taking into account the required fluid volume, the shape of the oil measurement device  15 , mounting and fluid feeding arrangements, the location other equipment such as a sight glass  42  or a camera device  40 , if any. 
     An oil level sensor assembly may comprise a float assembly  15 , captivated to an oil sensor assembly  20  having an arcuate support structure  21  so that the float assembly  15  may slide freely along at least a portion of the length of the arcuate support structure  21 , which may be attached to an inner wall  16  of the oil reservoir  10 . 
     In an example, the oil measuring device  20 , which may be termed an oil gage, fluid level sensor or the like, may be comprised of the central arcuate supporting structure  21 , shown in cross-section in  FIG. 3A , which may be a rod, a column or a beam which may be solid or hollow, and having a shape in cross section that cooperates with a similarly-shaped complimentary surface of an aperture in a float  22  so as to rotationally captivate the float  22  to the central supporting structure  21 . As shown in  FIG. 3A , the rod  21  has a protuberance or tongue  23  which renders the rod  21  rotationally asymmetric.  FIG. 3B  is an example of a cross section of a float  22  compatible with the rod  21 , where the central opening  25  of the float  22  is sized and dimensioned to slide without binding to the rod  21  when moving in a direction parallel to the axis of the rod  21 . A gap between the inner surface of the central opening  25  and the outer surface  29  of the rod  21  is sized to permit fluid to penetrate the gap to provide lubrication. 
     The central axis  27  of the aperture  25  of the float has a radius of curvature determined by that of the rod  21  so that, the float  22  may move vertically in the oil reservoir  10  along the rod  21  without binding. The float  22  may be a solid such as a suitable plastic or similar material, composite material or a hollow structure fabricated from a metal or the like, that has an effective specific gravity that is less than that of the liquid to be measured, such that a top surface of the float  22  protrudes above that of the corresponding liquid. The length of the float  22  in the direction parallel to the direction of motion is selected to contain the element that is sensed to determine the position of the float  22  along the rod  21 , and is sufficient to provide the needed buoyancy. 
     In an aspect, one or more elements  30  whose position may be sensed may be mounted on or embedded within the float  22  and positioned such that the element to be sensed opposes a plurality of sensor elements disposed along at least a portion of the length of the rod  21 . The sensing elements may be in a structure  32  that lies outside the periphery of the float  22 , or is inside the rod  21  or on the surface  29  thereof. When the sensing elements are disposed in a structure outside the surface  29  of the float  22  on an arcuate structure  32  generally conforming to the rod, a spacing sufficient to avoid binding or deleterious viscous effects is selected. Since the sensor does not need to be in contact with the element to be sensed, this spacing is a matter of design, taking account of the particular sensing technique and fluid properties. 
     So long as the inner surface  28  of the float  22  and the outer surface  29  of the rod  21  conform such that a float  22  does not bind to the rod  21 , the specific details of this aspect of the structure may be selected based on other considerations. The cross-section of the central opening  25  may be a tongue  24  as shown, an oval, a rectangle, a square or the like. Further, the upper and lower extremities of the central opening  25  may be relieved with respect to the rod  21  so as to minimize frictional forces. 
     Examples of the element to be sensed are magnetic field from a permanent magnet, a capacitance, a reflected light, or a ferrous metal. The gauge may permit the oil quantity to be measured either in-flight or on-the-ground over a desired range of fill levels so as to determine the rate of consumption of oil and to monitor the filling or re-filling of the tank. 
     In another example, the oil reservoir may have a shape that permits the use of a sensor device that has a straight vertical aspect, but the sensor may be inclined to the vertical so as to extend along the vertical dimension of the oil reservoir and provide oil level measurements for the desired range of fill levels of the tank, and to provide information on oil consumption rates, or the like. 
     In each example, the relationship between sensor output indications and the fill level, in liquid measure, may be non-linear, but monotonic. Further, the operational use and manufacturing cost of the oil level sensor may benefit from an oil level measurement device where the sensitivity to oil level change differs along the vertical dimension of the device. This may be combined with a computer-aided linearization technique where quantitative measurements are needed, such as near the top or the bottom of the reservoir. The non-linear characteristic may be a result of the differing cross-sectional area of the reservoir along in the vertical dimension thereof, or a non-linear effective spacing of sensing elements to reduce the number of sensing elements to lower the cost or increase the reliability of the device. 
     Such measurements may be used to guide servicing personnel in ramp-level maintenance so as to avoid overfilling of reservoir, to indicate excessive oil consumption in flight and to provide a low-fluid-level alert as part of avionics health measurement. In circumstances where the oil may be replenished in flight from another source, the measuring device may be used to alert the flight crew to the necessity to perform the operation, or perform and control the operation automatically. 
     The position of the float  22  along the vertical dimension of the supporting structure  21  may be sensed by magnetic techniques as discussed above. 
     In another example of sensing the level of the float in the liquid, a capacitive sensor assembly may be used. This may be facilitated by mounting one or more metal elements on or near the surface of the vertical portion of the float so as to change the capacitance of capacitors incorporated into the structure  50 . The electronic elements needed for sensing may be provided on circuit board  51  forming the capacitors. 
       FIGS. 4A-4C  illustrate an example of a capacitive sensor  50  suitable for use in the present context. For simplicity, the geometry is shown as generally planar and linear; however, one may appreciate that the geometry of the surfaces of the sensor elements may be curved, to conform to the overall geometry of the reservoir such as the arcuate shaped reservoir  10 . In the context of  FIGS. 2 and 3A -C, the float,  61  of  FIG. 4B  and  FIG. 4C  corresponds to the float  22  of  FIG. 2 ,  FIG. 3B  and  FIG. 3C , and a similar correspondence is found between the sensor support structures ( 32 ,  51 ), and the aperture in the float ( 25 ,  62 ), respectively, at least in functional terms. 
     A capacitive oil-level-measurement device may comprise sensor support structure  51 , which may be a printed wiring assembly or the like, in whole or in part, having mounted thereon or embedded therein a plurality of pairs of metal plates,  55 ,  56 ,  57  each pair of the plurality of pairs may be of the same physical dimensions; alternatively, the physical dimensions of the metal plates may vary along the length of the substrate  51 . Several examples of differently sized metal plates are shown in  FIG. 5A , and the length of each pair may be determined based on the measurement accuracy requirements. One or more electronic circuits  64  may be provided having a capacitance measurement capability. Such a capacitive measurement device may be provided with a DC power supply, or circuitry to convert an AC power source to DC to operate the circuitry configured to measure the capacitance of pairs of plates  55 ,  56 ,  57  and to provide an indication of the value of the capacitance with respect to a predetermined threshold value for each of the pairs of plates. When a float  61  is disposed as in  FIG. 4B  and  FIG. 4C , as an example, the vertical location the float will change with respect to the pairs of metal plates  55 ,  56 ,  57 . The float  61  may have a metallic plate  58  or conductive surface which may be, for example, a conductive tape or plating, having a vertical dimension d 1  attached to or embedded in the float and disposed so as to conform to any curvature of the substrate  51  and separated from the substrate  51  by a distance d 4  by the remaining structural elements (not shown) and perhaps by ridges on the float  61  or the substrate  51 . Adjacent pairs of metal plates  55 , and  56  or  56  and  57 , for example, are separated in a vertical direction by a distance d 2  and by a horizontal distance d 3 . The dimension d 1  of the plate  58  is greater than any dimension d 2 , and the dimension d 5  of the plate  58  is greater than the separation d 3  between pairs of plates on the substrate  51  so that the plate  58  overlaps a pair of capacitive elements (e.g.  57 ). 
     Where the term “vertical” is used, the direction is that in which the surface of a fluid in the reservoir changes in response to a change of fluid quantity in the reservoir, and may represent a motion or direction along a support  32  having a radius of curvature. 
     The dielectric constant of typical lubricating oil between about 2.1 and about 2.8 and the dielectric constant of typical substrates used in the manufacture of printed circuit boards is between 2.1 and 4.5 whereas the dielectric constant of air is 1.0. 
     In an example, the configuration of the pairs of metal plates  55 ,  56 ,  57  is such that the edges of the plates oppose each other and the plates lie in a common plane, rather than the conventional arrangement of a capacitor where the flat surfaces of the metal plate would oppose each other, separated by a small distance. In the present circumstance, a capacitance exists between each of the pairs of plates that is a consequence of the fringing electric field between the two plates. In most electronic circuits this capacitance is considered undesirable and a design usually minimizes the “fringing capacitance” with respect to the desired design value. In more complex electronic circuits reducing the fringing capacitance also has the effect of minimizing potential spurious resonance effects at frequencies that are remote from the design frequency of the circuit, or minimizes coupling of energy between unrelated parts of an electronic circuit. Here, the fringing capacitance is a small capacitance that is measured when the sensed plate  58  is not in proximity to the corresponding pair of plates (e.g.,  56 ) on the substrate  51 . 
     In this example, the float  61  is provided with a metal plate  58  disposed a distance d 4  from the substrate  51  upon which the pairs of plates  55 ,  56 ,  57  are positioned. When the plate  58  fully opposes a pair of plates such as  55 , the combination pair of plates  55  and the metal plate  58  on the float  61  forms a capacitor C having a capacitance substantially greater than the fringing capacitance between two adjacent plates on the substrate  51  and, from an equivalent circuit viewpoint, connected in parallel with the fringing capacitance. The capacitance measuring circuit  64  may measure capacitance by determining, for example, a change in impedance, or resonant frequency, of an electronic circuit utilizing the arrangement of the pair of metallic plates such as  56  and the metallic plate  58  as a capacitive circuit element. 
     A threshold capacitance value may be established for each of the pairs of metal plates  55 ,  56  and  57  so that when the metal plate  58  on the float  61  opposes one or more of the pairs of metal plates  55 ,  56 ,  57 , the change in capacitance is detected and reported through an interface  59 . This may be a switch closure indication (that is, a binary signal) or the uppermost of multiple simultaneous switch closures as a value associated with the position of the float  61 . Since the height d 1  of the plate  58  on the float  61  is greater than the vertical distance d 2  between adjacent pairs of longitudinally disposed sensing elements when the fluid level is transitioning in height between the adjacent plates there is no loss of sensing capability. 
     Where the accuracy of measurement of oil level is intended to be low, the capacitive plates may have the configuration shown as  57 , where the height d 1  of the metal plate  58  on the float  61  is less than the height L 3  of the metal plates  57 . So, when the plate  58  on the float opposes the pair of metal plates  57 , the capacitance change is detected only on the measuring circuit associated with the pair of plates  57  corresponding to the coincidence in height of the plates, except during a transition state between adjacent vertically disposed pairs of plates (e.g.,  56 ,  57 ). The resolution of this measurement is correspondingly low and an indicated level may not change until the float level changes in height so as to oppose either a higher or lower pair of plates. Provided that the height d 1  of the metal plate  58  on the float  61  is at least greater than the vertical separation distance d 2  between adjacent metal plates, the measurement sequence is continuous. The height of the metal plate d 1  may be greater than the height (e.g., L 1 ) of some of the smaller dimensioned pairs of plates so that more than one of the smaller dimensioned plates detects a capacitive change in an overlapping manner. A data processing algorithm may then determine the value to be displayed. In such an instance, for example, the uppermost pair of plates so activated represents the height of the fluid near the top of the reservoir and the lowermost pair of plates represents the height of the fluid near the bottom of the reservoir. This description should not be construed as limiting the device to a configuration where the plates  55 ,  56  and  57  have different vertical dimensions. 
       FIG. 5A  shows an example output for a hypothetical case where the amount of oil in the reservoir is linearly proportional to the height of the float, and the ratio of the vertical lengths L 1 , L 2 , L 3  is 1:2:3. The state of each of the capacitive sensors is shown, where a solid line indicates an output. The state during the transition from adjacent sensors is not shown, but is at least either of the two adjacent states depending on the processing algorithm. The measured output is generally a stair-step function approximating the fluid level with the granularity of the measurement dependent on the relative dimensions of the capacitive elements of the sensor and the sensed element on the float. 
     In the circumstance where the volume of oil in the reservoir is not linearly related to the height of the float, such as where the cross-section of the reservoir varies with height, a conversion factor between output indications and oil quantity may be adjusted in accordance with a predetermined factor in each case. For more accurate measurements, the temperature of the oil may be measured using an electronic sensor and a further correction made. 
       FIG. 5B  shows a representative relationship between the gauge output reading (abscissa) and the actual fluid amount (ordinate). The individual sensed readings may be converted into numerical fluid quantities for reporting or display or may be associated with literal terms meant to prompt servicing action. Where the oil reservoir is automatically replenished, the indicated actions may be initiated without operator intervention. 
     Other capacitive sensor arrangements may be made, where a complimentary approach employing apertures in a metal surface instead of the metal surface on a dielectric surface may be employed. In another aspect, a single vertical columnar arrangement of metal single plates  55 ,  56 ,  57 , rather than the pairs of plates shown in  FIG. 5A , may be employed and the capacitance between pairs of adjacent metal plates (e.g.  56 ,  57 ) in a vertical direction may be used. In this instance the capacitance is measured pairwise in the vertical direction. 
     In still another aspect, the location of the float may be sensed using other proximity sensing techniques such as a photoelectric retroreflective sensor, a discrete capacitive sensor, an inductive type proximity sensor, or the like. A representative example of such an arrangement  70  is shown in  FIG. 6A-C . The individual sensors  71  are mounted on a support  72  such that the sensitive portion of the sensor  71  opposes a portion  76  of the float  77  to be sensed. The portion to be sensed,  76 , may be, for example, one of a diffuse or miniature retro-reflector array, which may be a tape or other strip of reflective material in the case of a photoelectric sensor, a metal strip, or a plurality of magnetic layers, each of the sensed elements extending over a vertical length L 5 . The spacing between the sensors  71  may be a variable distance such as d 5 , d 6  where the length of the sensed element  76  is at least greater than the maximum vertical distance between adjacent sensors. This ensures that at least one of the sensors  71   a ,  71   b ,  71   c , . . . is activated at all times that the level of the float  77  is within the range of the measuring device  70 . 
     In a similar manner to the embodiment of  FIG. 4 , the granularity of the measurement is governed by the maximum and the minimum spacing distances that have been selected. Where more than one of the sensors  71   a - n  are activated at one time, the sensed float level may be represented by the uppermost of the activated sensors  71  when the float is above the low sensitivity region and the lowermost of the activator sensors  71  when the float is below the low sensitivity region. The displayed oil level may then be corrected for any non-linear relationship between the detected oil level and the quantity of oil represented by the corresponding level of the float. 
     The subject-matter of the disclosure may also relate, among others, to the following: 
     1. In an aspect a device for measuring the level of a fluid in a container, comprises: 
     a structure having a plurality of sensing elements mounted along a vertical direction thereof; and 
     an object to be sensed, constrained to move along the vertical direction of the structure in response to a change in fluid level and spaced apart therefrom, 
     each sensing element capable of detecting the object to be sensed when a sensed portion of the object is disposed so as to oppose the sensing element; and the object to be sensed is configured to be sensed by at least one sensing element. 
     2. The device of aspect 1, wherein the status of each sensing element is determined by a processor and the processor executes a program stored in a non-volatile computer readable medium to: 
     determine that one or more of the sensors has sensed the sensed portion of the object; and 
     select a location value to be output, 
     wherein the location value to be output is a vertical location of the sensor when a single sensor has sensed the object to be sensed; or, when more than one sensor has sensed the object, the location value is a vertical location of the sensor that has sensed the object and is closest to an end of the structure. 
     3. The device of aspect 1, wherein a relationship between a location along the vertical direction and a quantity of fluid is determined for a reservoir in which the structure having the sensors is fixedly mounted, and a location value to be output is converted to a fluid quantity using a predetermined relationship between a sensor output of the sensing element and a fluid quantity at the location of the sensing corresponding to the location value output. 
     4. The device of aspect 1, wherein the relationship between the location value output by the sensing element and a quantity of fluid in the reservoir is determined and stored in the non-volatile memory. 
     5. The device of aspect 2, the discrete sensor further comprises:
         a plurality of pairs of metal plates arranged on or near a surface of the structure, where the structure is a non-conducting material and the object to be sensed has a metallic strip constrained to be movably disposed opposite the plurality of discrete sensors and having a length extending in the transverse direction of the structure.       

     6. The device of aspect 5, wherein the discrete sensor comprises pairs of metal plates spaced apart in the vertical direction of the structure by a variable linear spacing; and a capacitance value of each of the pairs of metal plates is measured by a processor; and 
     the processor configured to execute computer-readable instructions stored in a non-volatile memory to determine a state of the pair of metal plates based on a comparison of measured capacitance value with a capacitance value threshold for each of the pairs of metal plates. 
     7. The device of aspect 2, wherein a plurality of metal plates is arranged in a column along the vertical direction of the structure by a variable spacing; and the processor measures a capacitance of adjacent metal plates in the column; and 
     a state of each of the pairs of adjacent metal plates is determined by the processor based on a comparison of the measured capacitance of each pair of the adjacent metal plates with a capacitance threshold for each adjacent pair of metal plates in the column. 
     8. The device of aspect 3, wherein the discrete sensor is a retro-reflective optical sensor and the element to be sensed is a diffuse reflector or a retroreflector strip. 
     9. The device of aspect 8, wherein the element to be sensed is a linear strip adhered to the object to be sensed. 
     10. In another aspect. a method of determining the level of a fluid in a reservoir, comprises:
         providing a structure having a plurality of discrete sensing elements mounted along a vertical direction;   providing an object to be sensed, constrained to move along the vertical direction of the structure and spaced apart therefrom, the object having a smaller specific gravity than the specific gravity of the fluid; and   a processor configured to execute a program stored in a non-volatile computer-readable medium,   wherein a linear spacing between the adjacent discrete sensing elements along the vertical direction is varied between an upper distance limit and a lower distance limit; and a sensed portion of the object to be sensed has an extent in the vertical direction that is at least as great as the upper distance limit;       

     the method further comprises: 
     determining, by the processor, that one or more of the discrete sensing element has sensed the sensed portion of the object; and 
     outputting a location value representing a vertical position of the sensed portion of the object, 
     wherein the location value is a location of the discrete sensor when the sensed portion of the object is sensed by a single sensor, or the location value is the location of the discrete sensor having the smallest linear spacing to an adjacent sensor and closest to an end of the support when the sensed portion of the object is sensed by more than one sensor. 
     11. The method of aspect 10, further comprising: 
     determining a relationship between the location value and a volume quantity of the fluid in the reservoir; and 
     converting the location values to engineering units to be output. 
     12. The method of aspect 10, further comprising determining a relationship between the location value and the quantity of the fluid in the reservoir; and 
     converting the location value to alphanumeric indications. 
     13. The method of aspect 10, further comprising:
         setting a predetermined maximum level of fluid permitted in the reservoir and a predetermined minimum level of fluid permitted in the reservoir; and   the processor configured to control adding fluid to the reservoir when the minimum level of fluid is determined and to cease adding fluid to the reservoir when a maximum level of fluid is determined.       

     14. The method of aspect 10, wherein the sensor further comprises:
         a plurality of pairs of metal plates arranged on or near a surface of the structure non-conductive structure, and where the object to be sensed comprises a metal strip constrained to move in a vertical direction corresponding to a level of the fluid, dimensioned such that when the metal strip is disposed opposite the pair of metal plates, the metal strip opposes both of the plates of the pair of plates.       

     15. The method of aspect 10, wherein the sensor comprises pairs of metal plates spaced apart in a direction transverse to the vertical direction of the structure by the variable linear spacing; and a capacitance value of each of the pairs of metal plates is measured by a processor; and 
     the processor configured to execute computer-readable instructions stored in a non-volatile memory to determine a state of the pair of metal plates based on a comparison of measured capacitance value with a capacitance value threshold for each of the pair of metal plates. 
     16. The method of aspect 10, wherein the sensor comprises metal plates spaced apart in a column in the vertical direction of the structure spacing; and a capacitance value of each of the pairs of metal plates in the column, taken as a pair, is measured by a processor; and 
     the processor configured to execute computer-readable instructions stored in a non-volatile memory to determine a state of the pair of metal plates based on a comparison of measured capacitance value with a capacitance value threshold for each of the pair of metal plates. 
     To clarify the use of and to hereby provide notice to the public, the phrases “at least one of &lt;A&gt;, &lt;B&gt;, . . . and &lt;N&gt;” or “at least one of &lt;A&gt;, &lt;B&gt;, &lt;N&gt;, or combinations thereof” or “&lt;A&gt;, &lt;B&gt;, . . . and/or &lt;N&gt;” are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, . . . and N. In other words, the phrases mean any combination of one or more of the elements A, B, . . . or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. Unless otherwise indicated or the context suggests otherwise, as used herein, “a” or “an” means “at least one” or “one or more.” 
     While various embodiments have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible. Accordingly, the embodiments described herein are examples, not the only possible embodiments and implementations.