WO1999067604A1

WO1999067604A1 - Monitoring liquid level in a container with a capacitive transducer

Info

Publication number: WO1999067604A1
Application number: PCT/US1999/014359
Authority: WO
Inventors: John L. Shipley; Mark R. Eggett; Randy L. Borgstrom
Original assignee: Cordant Technologies, Inc.
Priority date: 1998-06-24
Filing date: 1999-06-24
Publication date: 1999-12-29
Also published as: AU4717799A

Abstract

A method and system for measuring the quantity of a liquid in an enclosure, the enclosure containing a gaseous medium above the liquid, the liquid having a first dielectric constant, ε, and the gaseous medium having a second dielectric constant ε0, different from ε, the measurement being performed by; installing a sensor capacitor having two plates separated by a dielectric gap and a compensator capacitor having two plates separated by a dielectric gap in the enclosure with an orientation such that the sensor capacitor plates extend vertically within the enclosure and are separated horizontally by the dielectric gap, and liquid in the enclosure enters the dielectric gap to a level identical to the level of liquid in the enclosure, the compensator capacitor dielectric gap being completely filled with liquid.

Description

TITLE OF THE INVENTION

MONITORING LIQUID LEVEL IN A CONTAINER WITH A

CAPACΓTΓVE TRANSDUCER

BACKGROUND OF THE INVENTION The present invention relates to the monitoring of fluid levels in a container. The invention is particularly concerned with monitoring the levels of electrically non-conductive liquids in situations which require a high degree of reliability and accuracy. These requirements apply, for example, to the monitoring of volatile and explosive liquids, such as liquid propane, mixtures of propane and butane as well as other mixtures, gasoline, diesel fuels, etc., in stationary, portable, or vehicle mounted containers.

As in the case of any consumable liquid, it is usually necessary or at least desirable to obtain an indication of the quantity of liquid remaining in a storage container. Frequently, the successful performance of a process depends on the ability to monitor the quantity of remaining liquid with a high degree of accuracy. In addition, in the case of certain types of liquids, such as volatile liquids, it can be extremely important to prevent overfilling of the container.

Such an indication is most commonly provided by mechanical fill sensors employing floats. Systems of this type must have a number of moving parts and have been found to not be highly reliable or durable.

It has also been proposed in the art to monitor the level and/or the quantity of liquid in a container by a capacitive system which utilizes the difference between the dielectric constant of the liquid to be monitored and the dielectric constant of the air or the gaseous environment in the container above the liquid. In such systems, a sensor capacitor is formed by two parallel plates of electrically conductive material disposed with a generally vertical orientation in the container. The plates are spaced apart in the horizontal direction to provide a gap which is in communication with the liquid. The plates constitute capacitor plates and the gap between the plates constitutes a dielectric gap. Liquid within the container rises to the same level in that gap as in the remainder of the tank, so that a portion of the gap between the two plates is filled with the liquid, while the gaseous environment above the liquid is present between the plates in the remainder of that gap. For any non- conductive liquid having a dielectric constant different from the dielectric constant of the environment above the liquid, the capacitance between the two plates will vary as a function of the proportion of the total volume of the space between the plates that is filled with the liquid. Most frequently, the liquid will have a higher dielectric constant than the gaseous environment. Sensors of this type are disclosed, for example, in U.S. Patent No. 5,052,223, which issued to Regnault et al. on October 1, 1991.

Capacitive sensors offer a number of inherent advantages, including the fact that they do not contain any moving parts and are durable and reliable. In addition, they can be made of materials which do not contaminate, and are not corroded by, liquids to be monitored.

The capacitance of a sensor capacitor of the type described above depends not only on the proportion of the total volume of the space between the plates that is filled with the liquid, but also on the dielectric constant of that liquid. However, the dielectric constant of the liquid can vary with changes in temperature, pressure, liquid density and/or mixture proportions, resulting in erroneous liquid level indications. Therefore, it has previously been proposed to combine a sensor capacitor with a reference, or compensation, capacitor whose dielectric space is completely filled with the liquid whose level is to be sensed. The capacitance of the compensation capacitor will be a function of the dielectric constant of the liquid and can be used to at least partially eliminate level sensing errors resulting from dielectric constant variations. Sensors of this type are disclosed in U.S. Patent Nos. 4, 924,702, which issued to Park on May 15, 1990; 5,001 ,927 which issued to LaCava et al. on March 26, 1991; and 5,005,409 which issued to Hochstein on April 9, 1991.

However, the capacitive sensing systems proposed to date have certain inherent inaccuracies and even known systems which employ a compensation capacitor are incapable of fully correcting for the effects of dielectric constant variation.

Furthermore, traditional capacitive measuring systems are not well suited for measuring the quantity or level of volatile liquid mixtures, particularly because the dielectric constant of such mixtures can vary considerably whenever the supply of liquid in the container is replenished.

In addition known systems for preventing overfilling of containers generally employ mechanical stop-fill valves that are prone to malfunction, thereby creating a substantial safety risk in the case of volatile liquids.

BRIEF SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a novel capacitive measuring method and system which alleviate the above shortcomings by providing a measurement which is substantially completely independent of changes in the dielectric constant of the liquid being monitored, thus allowing accurate measurement of liquids under changing temperature conditions and accurate measurement of liquid mixtures whose compositions may vary due to periodic replenishment with quantities having differing proportions of ingredients.

The above and other objects are achieved, according to the present invention, by a method for measuring the quantity of a liquid in an enclosure, the enclosure containing a gaseous medium above the liquid, the liquid having a first dielectric constant, ε, and the gaseous medium having a second dielectric constant So, different from ε, the method comprising: providing a sensor capacitor having two plates separated by a dielectric gap and a compensator capacitor having two plates separated by a dielectric ap; installing the sensor capacitor in the enclosure with an orientation such that the plates of the sensor capacitor extend vertically within the enclosure and are separated horizontally by the dielectric gap, and allowing liquid in the enclosure to enter the dielectric gap so that liquid in the sensor capacitor dielectric gap is at a level corresponding to the level of liquid in the enclosure; installing the compensator capacitor in the enclosure so that the compensator capacitor dielectric gap is completely filled with liquid; measuring the sensor capacitor capacitance, Cs, and the compensator capacitor capacitance, Cc; and producing an indication of the value of a ratio between a first term representing the difference between the measured value of Cs and a known capacitance value, C_os, which the sensor capacitor has when the sensor capacitor dielectric gap is completely filled with the gaseous medium, and a second term representing a difference between the measured value of Cc and a known capacitance value, C_oc, which the compensator capacitor has when the compensator capacitor dielectric gap is completely filled with the gaseous medium, wherein the ratio is proportional to the quantity of liquid in the enclosure.

Objects according to the invention are further achieved by a system for measuring the quantity of a liquid in an enclosure, the enclosure containing a gaseous medium above the liquid, the liquid having a first dielectric constant, ε, and the gaseous medium having a second dielectric constant εo, different from ε, the system comprising: a sensor capacitor having two plates separated by a dielectric gap and a compensator capacitor having two plates separated by a dielectric gap, the sensor capacitor being installed in the enclosure with an orientation such that the sensor capacitor plates extend vertically within the enclosure and are separated horizontally by the dielectric gap and so that liquid in the enclosure can enter the dielectric gap to be at a level identical to the level of liquid in the enclosure, and the compensator capacitor being completely filled with the same liquid as that in the enclosure; means for measuring the sensor capacitor capacitance, Cs, and the compensator capacitor capacitance, Cc; and means connected to the measuring means for producing an indication of the value of a ratio between a first term representing the difference between the measured value of Cs and a known capacitance value, C_os, of the sensor capacitor when the sensor capacitor dielectric gap is completely filled with the gaseous medium, and a second term representing a difference between the measured value of Cc and a known capacitance value, C_oc, of the compensator capacitor when the compensator capacitor dielectric gap is completely filled with the gaseous medium, wherein the ratio is proportional to the quantity of liquid in the enclosure.

It is to be understood that the statement that the capacitor plates extend vertically means the plates extend in a direction which has at least a vertical component and the statement that the capacitor plates capacitors are separated horizontally means that the direction between the plates has at least a horizontal component.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING FIG. 1 is an elevational, cross-sectional view of one embodiment of a capacitor level sensor according to the present invention. FIG. 2 is a diagram of one embodiment of a measuring circuit according to the invention.

FIGs. 3A and 3B are block diagrams illustrating components or functional equivalents of portions of the circuit shown in FIG. 2.

FIG. 4 is an elevational, cross-sectional detail view of a second embodiment of a system according the invention.

FIG. 5 is a pictorial perspective view of a modified form of construction of one component of a system according to the invention.

FIG. 6 is a simplified, elevational, cross-sectional view of a second modified form of construction of the component shown in FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

One preferred embodiment of a fluid level sensor according to the invention is illustrated in FIG. 1. This sensor includes an annular, preferably circular, upper body tube 2 welded to a flange 4. Flange 4 supports the fluid level measuring system from a cover or lid of an enclosure (not shown), typically a container or tank, in which the liquid whose level is to be measured is stored. Tube 2 extends downwardly from flange 4. The lower end of tube 2 is welded to the upper end of an annular capacitor tube 6 which extends vertically downwardly from tube 2. Preferably, tube 6 has a length such that the lower end of tube 6 will be in contact, or at least immediate proximity, with the bottom wall of the enclosure. Tubes 2 and 6 are also joined together by an annular coupling piece 8 which extends radially inwardly from tubes 2 and 6 and is provided with a through bore concentric with the longitudinal axes of tubes 2 and 6.

A first tubular insulator member 10 is mounted in the through bore in coupling piece 8 and covers a portion of the upper surface and the entirety of the lower surface of coupling piece 8, as well as the wall defining the through bore. A large diameter bolt 14 extends through the bore in piece 8 and is electrically insulated from coupling piece 8 by insulator member 10.

A further annular tube 18 is concentrically disposed within tube 6 and rests on a shoulder formed at the periphery of an electrically conductive support plate 22. The lower end of bolt 14 is threaded, as indicated by dashes, and is screwed into a threaded through bore formed in support plate 22. When bolt 14 is screwed into the threaded bore in support plate 22, tube 18 is clamped between support plate 22 and insulator member 10. Tube 18 is spaced radially inwardly from tube 6 by an annular dielectric gap 24 and is centered relative to tube 6 by the peripheral shoulder of support plate 22 and a downwardly extending rim at the periphery of insulator member 10 . Bolt 14 is a tubular bolt having a through bore which is lined by a second tubular insulator member 26 which has a thickness extending radially across a portion of the head, and extends downwardly to a point near the lower end of the shank of bolt 14.

A smaller diameter bolt 28 extends downwardly through insulator member 26 and a through bore in support plate 22. The through bore in support plate 22 receives a third tubular insulator member 32. Member 32 includes an interior axially extending part that lines a portion of the shank of bolt 28, a radially outwardly extending part that abuts against the lower surface of support plate 22, and a peripheral part which surrounds a peripheral part of support plate 22 and is interposed between support plate 22 and tube 6.

As is shown in FIG. 1, insulator member 26 extends over substantially the entire length of bolt 14. The space between the lower end of insulator member 26 and the upper end of the interior axially extending part of insulator member 32 is filled by two sealing rings and an insulating spacer member interposed between the sealing rings. The sealing rings, in cooperation with insulator members 26 and 32, act to prevent seepage of any liquid into the space between bolts 14 and 28, which seepage can have an undesired influence on the capacitance of the sensor capacitor. Below insulator member 32 there is provided a cup-shaped part 36 having a central bore which receives the shank of bolt 28 and is held in place on bolt 28 by a nut 38. Cup-shaped part 36 is spaced from the inner wall of tube 2 by a second dielectric gap 40.

All of the components illustrated in FIG. 1 are made of electrically conducting material, with the exception of insulator members 10, 26 and 32, the sealing rings and the insulating spacer member interposed between the sealing rings, which are made of electrically insulating material. Any electrically conducting and electrically insulating materials may be used, with the only significant requirement being that parts that will be exposed to liquid in the enclosure be made of materials that will not be corroded by or contaminate the liquid. All of the insulator members employed in embodiments of the invention may be made of nylon or other suitable electrical insulating material.

Support plate 22 is screwed onto the lower end of large diameter bolt 14, so that the assembly composed of support plate 22 and tube 18 is supported by bolt 14, while cup-shaped part 36 is supported by nut 38. Tightening of nut 38 not only maintains part 36 in place, but also clamps insulator member 32 against support plate 22. Tube 18 and the portion of tube 6 which faces tube 18 form a sensor capacitor, while the tubular, vertically extending peripheral wall of part 36 and the portion of tube 6 which faces part 36 form a compensator capacitor. The medium within dielectric gap 24 constitutes the dielectric of the first- mentioned capacitor and the medium in dielectric gap 40 constitutes the dielectric of the compensator capacitor. The radial dimension, di, of dielectric gap 24 can be the same as or different than the radial dimension, d₂, of dielectric gap 40. If dj or d₂ varies around the circumference of the associated capacitor, the capacitance will be determined by its average value.

Electrical connections to the capacitors can be made via flange 4 and bolts 14 and 28, which are conductively connected to tube 18 and part 36, respectively.

When the illustrated system is installed in an enclosure, liquid in the enclosure will fill the entirety of gap 40 and will enter gap 24, for example via passages 42 defined between insulator member 32 and tube 6, to rise to the same level as the liquid in the enclosure surrounding tube 6. The vertical dimensions of tube 18 and part 36 are selected to assure that the lowest acceptable level of liquid in the enclosure will be at least as high the lower extremity of gap 24 and the highest acceptable level of liquid will be not higher than the upper extremity of dielectric gap 24. Tube 6 is provided with one or more passages 46 located at the upper end of space 24 to allow escape of gas from space 24 as that space is filled with liquid.

If the system is installed so that the bottom edge of tube 6 rests on the 5 bottom of the enclosure, tube 6 may be provided with radial passages 44 to permit flow of liquid into dielectric gaps 40 and 24.

In an assembly according to the present invention, as shown in FIG. 1 , tube 18 is electrically insulated from part 36. In addition, insulator member 32 o provides a reliable fluid seal between spaces 24 and 40, on the one hand, and the region enclosed by tube 18.

All electrical connections are conveniently made at points located near the top of the assembly, which are out of contact with the liquid being 5 monitored. Preferably, the heads of bolts 14 and 28 will be used as connectors for tube 18 and part 36, respectively. In addition, only relatively low voltages need be applied to tube 18 and part 36. The combination of physical isolation between the electrical connections and the liquid and relatively low voltages eliminates the liquid level measuring assembly as an explosion risk. In o addition, all of the metal components may be plastic coated to provide electrical isolation. The capacitance, Cs, of the capacitor formed by tubes 6 and 18 and the dielectric in the gap 24 can be expressed, to a good approximation, as follows:

where: ε is the dielectric constant of the liquid in the container, or tank, in which the assembly of FIG. 1 is installed; εo is the dielectric constant of the gaseous medium in gap 24 above any liquid present in that space; h is the height of liquid in gap 24, i.e., above openings 42; w is the width of gap 24, in this case the width being the circumference of gap 24; dj is the radial thickness of gap 24; and i is vertical height of gap 24.

Equation (1) is not the exact equation for a cylindrical capacitor, but is a sufficiently accurate approximation of its capacitance value.

Equation (1) can be simplified as follows:

and equation (2) can be rearranged as follows:

The second term on the right-hand side of equation 3 represents C_os.

Therefore, if

Δε = ε - £o ,

then substitution of that expression into equation (3) yields:

Cs = Kι4ε Λ + C_0S (4),

where Ki = wh/di.

By a similar development, one can derive:

Cc = K₂ziε + C_oc (5),

where K₂ = A/d₂,

A is the product of the circumference and vertical height, or the area,

of gap 40,

d? is the radial thickness of gap 40, and ^oc ⁼ Δ ₀ d₂ ^~

If the value of h were calculated as a function of C_s/C_c, this would not allow for full correction of changes in dielectric constant because

Therefore, the influence of the offset terms C_os and C_oc would prevent the dielectric constant factor from being eliminated from the calculation.

On the other hand, if the calculation were based on:

the dielectric constant factor would be eliminated from the calculation result. The liquid level calculation performed according to the present invention is based on the latter equation.

FIG. 2 is a block diagram of one exemplary embodiment of a circuit for producing a liquid level indication in accordance with the present invention. It is to be understood that the illustrated circuit is presented solely by way of example and that many other circuit implementations would be readily apparent to those skilled in the art after having been informed of the novel features of the present invention.

In the arrangement shown in FIG. 2, the capacitor formed by tube 18 and the associated portion of tube 6 provides capacitance C_s and the capacitor formed between part 36 and the associated portion of tube 6 provides capacitance C_c. Each of those capacitors is connected in series with a respective current limiting resistor between a source of a fixed voltage and ground. Tube 6 provides the grounded plate of each capacitor.

Both plates of each capacitor, i.e., tubes 6 and 18 and part 36, are connected to a sensor circuit 50 which may be constituted, for example, by an application specific integrated circuit (ASIC). Circuit 50 is operatively associated with a microcontroller 52 which supplies control signals to circuit 50 and generates measurement information signals. For purposes of the present invention, microcontroller 52 would typically be provided with a ROM storing a control program, a RAM for storing reference values and measured values, a clock providing timing signals and appropriate logic circuitry.

Sensor circuit 50 is composed of two structurally and functionally identical parts, each associated with a respective one of capacitors C_s and C_c. Operation of circuit 50 is controlled in part by a command signal produced by controller 52 and supplied via a control line 54. Line 54 is connected directly to the upper part of circuit and indirectly to the lower part via an inverter 56. When the command signal has the form of a square wave, a positive voltage will be applied to the upper circuit part during one half cycle of the square wave and a positive voltage will be applied to the lower circuit part during the following half cycle of the square wave. Each half of circuit 50 has an output connected to an input of controller 52 via an OR gate 58 and produces an output pulse train only during the square wave half cycle when a positive command signal voltage is being supplied to that circuit half.

When a circuit half is receiving a positive command voltage, it generates a succession of square wave pulses each having a duration proportional to the capacitance of the respective capacitor Cs or Cc. These pulses are supplied via gate 58 to controller 52.

In controller 52, a predetermined number of the square wave pulses from one circuit half defines a time frame during which clock pulses generated in controller 52 are counted to produce a count number representative of Cs- C₀s or Cc-C_oc- The count number representative of Cs-C_os is applied to a memory 60 and the count number representative of Cc-C_oc is supplied to a memory 62. These count numbers are supplied to a digital divider 70 which produces a binary signal output representing the level, h, of the liquid in the enclosure.

The output from divider 70 may be supplied to a display device 72 and to a digital comparator 74 which also receives a digital signal, Ref, representing the maximum allowable value of h. When h reaches the maximum allowable value, comparator 74 can generate a signal for activating a flow shut off device 78 of any suitable type.

One exemplary configuration of each half of circuit 50 is illustrated in FIG. 3A.

The components within each circuit half include a comparator 80 having an input connected to the point of connection between its associated capacitance Cs or Cc and its respective current limiting resistor. The input of comparator 80 is additionally connected to a switch 82 having one terminal connected to ground. The output of comparator 80 is connected to a counter 84 which is operated to function as a pulse rate divider. A control logic 86 has an input connected to line 54 and is connected to control the operation of comparator 80, counter 84 and switch 82.

Comparator 80 is configured and controlled to operate, in cooperation with switch 82, in a manner to produce a series of pulses each having a duration determined by the capacitance being measured. Prior to the start of a measuring cycle, switch 82 is closed so as to discharge the capacitor. When the control signal on line 54 goes positive, it acts to open switch 82 so that the capacitor can begin charging. The output of comparator 80 remains low until the voltage across the capacitor reaches a predetermined threshold value, which is a selected percentage of V_rcf. When that upper threshold is reached, the output of comparator 80 goes high and acts to close switch 82. As a result, the capacitor begins to discharge and the voltage across the capacitor decreases toward ground. The capacitor voltage is allowed to decrease until it reaches a second threshold, lower than the first-mentioned threshold, at which time the output voltage from comparator 80 goes low and switch 82 again opens. Thus, the output of comparator 80 is in the form of a series of pulses and because the time required for the capacitor to charge and discharge when supplied with a fixed voltage is proportional to its capacitance, the duration of each pulse is proportional to that capacitance.

However, in view of the capacitance levels existing in sensors of the type contemplated by the present invention, the duration of a single pulse is generally insufficient to allow an accurate capacitance determination to be made. Therefore, according to the present invention, each capacitance measurement is based on the duration of a series of these pulses. In the embodiment illustrated herein, counter 84 is connected to act as a pulse repetition rate divider. The output of comparator 80 is applied to the count input of counter 84 and counter 84 counts the leading edge of each positive pulse appearing at the output of comparator 80. The output state of counter 84 undergoes a transition each time eight pulses have been delivered from comparator 80. The resulting output pulse train from counter 84 is delivered to controller 52 via gate 58. At the end of the positive voltage phase of the command signal on control line 54, switch 82 is closed and operation of comparator 80 and counter 84 are halted.

FIG. 3B illustrates one possible form of construction of the components of controller 52 which provide measurement values that are applied to divider 70. It is to be understood that FIG. 3B, like FIG. 3A, can be considered to be a conceptual diagram of the effective circuit established in microcontroller 52 and that microcontroller 52 can be formed from a conventional programmable gate array.

Operation of the components in microcontroller 52 is controlled by a control logic 90 which generates the command signal on line 54. The pulses conducted via gate 58 are supplied to a pulse rate divider 92 which, in effect, counts a selected number of transitions in the pulse train being received via gate 58. At the start of operation, divider 92 is preset to a previously determined count number and then counts down each pulse received via gate 58. Divider 92 produces an output which maintains a switch 94 in its closed state from the start of a measuring operation until counter 92 has counted down to zero. The start of a measuring operation coincides with, and is controlled by, each transition in the command signal on line 54.

A clock pulse generator 96 is coupled to an up/down counter 98 via switch 94. Clock pulse generator 96 produces a train of clock pulses at a known, high rate. When switch 94 is closed, clock pulses from generator 96 are supplied to, and counted by counter 98. Counter 98 is controllable to be preset to an initial count stored in a respective storage region of a memory 100. For this purpose, counter 98 has a setting input connected via a switch 102 to a selected location of memory 100 under control of control signals from control logic 90. The locations of memory 100 store count numbers representing previously determined values for the offset capacitances C_oc and C_os, respectively.

The count output from counter 98 is supplied to a respective one of memories 60 and 62, depending on which capacitance calculation is being performed, under control of control signals from logic 90. These control signals are applied to a switch 104 which is connected to the count output of counter 98 and is selectively connectable to either one of memories 60 and 62. Switch 104 may also have a third position in which it is disconnected from both memories 60 and 62.

Counter 98 is switched between its down counting state and its up counting state by a counter control 108. Each procedure for deriving an indication of the capacitance of one of the capacitors is initiated by a positive or negative transition in the control signal on line 54. In the present example, a positive transition begins the process for providing an indication of the value capacitance Cs and a negative transition beings process for providing an indication of the value of capacitance Cc. Each procedure proceeds in the same manner, so that only one procedure sequence will be described.

At the beginning of procedure, control logic 90 sets a predetermined value for n in counter 92, closes switch 94, enables counter 98, if this is necessary, and operates switch 104 to connect the output of counter 98 to the read input of a respective one of memories 60 and 62. In addition, control logic 90 operates switch 102 to connect the appropriate location of memory 100 to the set inputs of counter 98 so as to preset a count in counter 98 corresponding to the stored value of the appropriate offset capacitance term.

The value for n stored in counter 92 can be the same for both capacitors or can have slightly different values for measurement of the two capacitors. It is preferred that these values be as close as possible to one another and they are selected on the basis of a desire to maximize each final count produced by counter 98. With the system in the state described above, counter 92 responds to each transition of the square wave conducted via gate 58 by counting down one step and continues this operation until n transitions have been counted. After n transitions have been counted, counter 92 has a zero output state. Until counter 92 reaches the zero output state, switch 94 is closed and counter 98 counts the pulses produced by clock pulse generator 96.

Counter 98 is initially set to perform a down counting operation until it reaches an output count of zero. Upon reaching an output count of zero, counter control 108 switches counter 98 to its up counting mode and subsequent pulses from clock pulse generator 96 are counted up until switch 94 is opened when the count output from counter 92 reaches the zero state.

On the next transition of the command signal on line 54, counter 92 is reset to an initial count corresponding to the desired value for n and counter control 108 is operated to switch counter 98 back to its down count mode, while switches 102 and 104 are switched to their other positions and switch 94 is again closed.

It will be understood from the above that the time during which switch

94 is closed in the course of a given capacitance determining operation is proportional to the present value of the capacitance being sensed. During the period that switch 94 is closed, counter 98 counts down by an amount represented by the associated offset capacitance and does not begin counting up until it has counted down to a zero value. It thus follows that the final count produced by counter 98 at the time that switch 94 is opened will be indicative of the current value of the capacitance being measured minus its associated offset term. Thus, memory 60 will store a count which is proportional to Cs-C_os while memory 62 will store a count proportional to Cc-

*— oc-

Before the start of the capacitance measuring operation, appropriate values for C_os and C_oc are stored in memory 100. This is achieved by initially setting the values stored in the locations of memory 100 to values of zero and performing the above-described operations while the container in which the level sensor is disposed is empty. During this operation, under control of logic 90, successive count outputs from counter 98 will be stored in respectively locations of memory 100. After this initial operation has taken place, control logic 90 operates to prevent memory 100 from subsequently reading the output of counter 98.

Finally, delivery of the data stored in memories 60 and 62 to divider 70 can be controlled by logic 90 to occur at times when new data is not being read into either one of those memories. While FIGs. 3A and 3B illustrate circuits containing discrete components, it will be understood that many other implementations are possible. Simply by way of example, all of the signal processing operations could be performed on a digital basis in a programmed digital computer, or analog circuitry could be employed to carry out all of the calculations on the basis of voltages representing the various capacitance values.

The invention can be employed for monitoring the level of virtually any non-conductive liquid having a higher dielectric constant than the gaseous medium above the liquid and can be used in fuel tanks containing, for example, propane, gasoline, diesel fuel, or combinations thereof, which tanks can be installed in a vehicle or can be any type of stationary, storage tank.

Because of the contribution made by the compensator capacitor, the liquid level measurement will automatically compensate for changes in the dielectric constant of the liquid due, for example, to temperature, pressure, liquid density and mixture proportion changes.

FIG. 4 is an elevational, cross-sectional, detail view of the lower portion of an embodiment of a level sensor according to the invention, provided with a preferred embodiment of the compensator capacitor. According to this embodiment, the compensator capacitor comprises a plurality of outer capacitor plate elements 122 and a plurality of inner capacitor plate elements 124. All elements 122 and 124 have an annular, circularly cylindrical form with a center axis concentrically aligned with the center axis of bolt 28. Each outer plate element 122 has a generally cup- shaped form and includes a radially extending horizontal portion and a longitudinally extending vertical portion, the latter being aligned with sensor capacitor plate 6. Each inner plate element 124 is a flat, radially extending horizontal plate element.

Plate elements 122 are separated from plate elements 124 and from bolt 28 by a plurality of insulating spacers 126. Spacers 126 also have an annular, circularly cylindrical form with an axis concentrically aligned with the longitudinal axis of bolt 28 and a generally L-shaped cross-section. Inner plate elements 124 are spaced both longitudinally and radially from associated plate elements 122 to create dielectric spaces which will be filled with liquid contained in the tank in which the level sensor is installed.

The lower end of the vertically extending part of each outer plate element 122 is provided with a recess 128. In addition, the horizontally extending part of each outer plate element 122 is provided with one or more circumferentially spaced vertical bores 130 and insulating member 32 is provided with one or more circumferentially spaced vertical bores 132 that are aligned with bores 130 in the uppermost outer plate element 122 when all of the components of the level sensor are assembled together.

Recesses 128 and bores 130 provide flow paths for the flow of liquid from the interior of the tank into the dielectric gaps of the compensator capacitor formed by plate elements 122 and 124, and bores 132 provide flow paths for the flow of that liquid into dielectric gap 24 of the sensor capacitor.

Plate elements 122 are connected to ground via cylinder 6, while plate elements 124 are con ductively connected to bolt 28. In the illustrated embodiment, the inner edges of elements 124 are threaded and screwed into bolt 28 to assure good conductive contact with bolt 28. With this arrangement, the compensator capacitor is constituted, in effect, by a plurality of capacitors connected together in parallel, thus providing a satisfactory capacitance level in a structure having a small vertical height. Because of the small vertical height of the compensator capacitor of this embodiment, the lower end of the sensor capacitor can be placed relatively close to the bottom of the tank. As a result, the sensor capacitor can provide a level indication for virtually all fill levels of the tank in which the device is installed.

If the tank is made of conductive material and is grounded, the lowermost plate element 122, which will typically be grounded, may be elongated vertically to assure that bolt 28 remains out of contact with the tank. When a level sensor having the form shown in FIG. 1 or FIG. 4 is connected to the circuit of FIG. 2 and is installed in an enclosure having the form of a cylinder with a vertical longitudinal axis, the output signal from divider 70 of the circuit shown in FIG. 2 will be proportional to both the 5 height of liquid in the enclosure and the volume, or quantity, of liquid in the enclosure. However, if the enclosure has a shape which is non-uniform over its height, e.g., a cylinder having a horizontal longitudinal axis, a sphere, etc., the output signal from divider 70 will still be proportional to the height of liquid in the enclosure, but will no longer be proportional to the volume of o liquid therein. In many instances, it is desirable to provide an output signal which is representative of quantity, or volume, of liquid in the enclosure, rather than, or in addition to, liquid height.

An indication of liquid quantity can be obtained by applying the output5 signal from divider 70 to a function generator 134, as shown in FIG. 2.

Function generator 134 is configured to modify the height indication signal according to a function representative of the geometry of the storage region within the enclosure in order to produce an output signal representative of liquid quantity, which signal can be supplied to display 72 along with the o liquid level height indication output from divider 70. The construction of such a function generator is well within the capabilities of those skilled in the art. In addition, according to further embodiments of the invention, an indication of the volume, or quantity, of liquid in such an enclosure can be directly obtained from divider 70 of the circuit of FIG. 2 by appropriately configuring the plates of the sensor capacitor so that variations in the capacitance of that capacitor in response to changes in the liquid level is a function of the shape of the enclosure. More specifically, if the sensor capacitor capacitance is conceptualized as a succession of differential capacitances, each differential capacitance being the contribution to the total capacitance of a thin horizontal slice of the capacitor structure, each slice having the same height, the sensor capacitor can be shaped so that the capacitance value contributed by each differential capacitance is proportional to the area of a horizontal cross-section of the interior of the enclosure at the height of that differential capacitance. This can be achieved by appropriate variation of one, or a combination, of the width and thickness of the dielectric gap between capacitor plates.

For example, for monitoring the volume of liquid in an enclosure having the form of a circular cross-sectioned cylinder with a horizontal longitudinal axis, the sensor capacitor could be configured in either one of the ways shown in FIGs. 5 and 6. In FIG. 5, the sensor capacitor includes two circular plates 136 and 137 having a common horizontally extending axis of symmetry 138. Capacitor plates 136 and 137 are separated by a dielectric gap having a constant thickness. The diameters of plates 136 and 137 could be slightly smaller than the internal diameter of the enclosure, with axis 138 located at the same height as the longitudinal axis of the enclosure. However, the longitudinal axis 138 need not be aligned with, or parallel to, the longitudinal axis of the enclosure and can, in fact, be oriented at right angles to that longitudinal axis to facilitate mounting of the capacitor assembly in the enclosure.

In the embodiment shown in FIG. 6, capacitor plates 139 and 140 may have a constant horizontal width but are curved so that the thickness of the dielectric gap therebetween varies in a manner which is inversely proportional to the variation of the horizontal cross-section of the interior of the enclosure with changes in the vertical distance between each horizontal cross-section plane and the lowest point within the enclosure. A horizontal axis of symmetry 141 of plate 139 and 140 may be located in the same manner as axis 138 of the capacitor shown in FIG. 5.

It will be appreciated that the number of variations are possible. For example, the solutions illustrated in FIGs. 5 and 6 could be combined by providing vertically extending elliptical capacitor plates with vertical major axes, and by curving those plates so that the thickness of the resulting dielectric gap is a minimum at the location of the horizontal center axis of the capacitor. Any combination of variable gap width and variable gap thickness which causes w/d to vary with height according to a function which is the same as the function describing the horizontal cross-sectional area of the interior of the enclosure as a function of height would allow the desired result to be achieved.

5 According to another interesting possibility for monitoring the quantity of liquid in an enclosure having a spherical storage space, a circularly cylindrical sensor capacitor having the cross-sectional shape shown in FIG. 6 could be employed. Such a capacitor would have a vertical longitudinal axis, i.e., would have a configuration similar to that shown in FIG. 1, with the o capacitor plates being curved toward one another in the manner shown in FIG.

6.

While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made5 without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.

The presently disclosed embodiments are therefore to be considered in 0 all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

WHAT IS CLAIMED IS:

1. A method for measuring the quantity of a liquid in an enclosure, the enclosure containing a gaseous medium above the liquid, the liquid having a first dielectric constant, ╬╡, and the gaseous medium having a second dielectric constant, ╬╡o, different from ╬╡, said method comprising: 5 arranging a sensor capacitor comprising at least two sensor capacitor plates in the enclosure with an orientation such that the sensor capacitor plates extend vertically in the enclosure and are separated horizontally by a sensor capacitor dielectric gap, and allowing a portion of the liquid in the enclosure to enter the sensor capacitor dielectric gap and rise to a level in the sensor o capacitor corresponding to the level of the liquid in the enclosure; arranging a compensator capacitor comprising at least two compensation capacitor plates in the enclosure so that the compensator capacitor plates are separated by a compensator capacitor dielectric gap, which is completely filled with liquid; 5 measuring sensor capacitor capacitance, Cs, and compensator capacitor capacitance, Cc; and producing an indication of the value of a ratio between a first term representing the difference between the measured value of Cs and a known capacitance value, C_os, of the sensor capacitor when the sensor capacitor 0 dielectric gap is completely filled with the gaseous medium, and a second term representing a difference between the measured value of Cc and a known capacitance value, C_oc, of the compensator capacitor when the compensator capacitor dielectric gap is completely filled with the gaseous medium, wherein the ratio is proportional to the quantity of liquid in the enclosure.

2. The method of claim 1 wherein the first and second terms represent respective time periods.

3. The method of claim 1 wherein said step of producing an indication comprises: producing a first count signal representative of the first term and a second count signal representative of the second term; and mathematically dividing the first count signal by the second count signal to produce a digital quotient signal representing the ratio between the first and second terms.

4. The method of claim 3 wherein: said step of producing a first count signal comprises producing a preliminary pulse signal having a duration proportional to C_os and a second preliminary pulse signal having a duration proportional to Cs, and giving the first count signal a value equal to the difference between the durations of the first and second preliminary pulse signals; and said step of producing a second count signal comprises producing a third preliminary pulse signal having a duration proportional to C_oc and a fourth preliminary pulse signal having a duration proportional to Cc, and giving the second count signal a value equal to the difference between the durations of the third and fourth preliminary pulse signals.

5. The method of claim 4 wherein: said step of producing a first count signal comprises presetting a counter to a count value representative of the first preliminary pulse signal and causing the counter to count clock pulses during the second preliminary pulse signal in order to cause the counter to produce the first count signal; and said step of producing a second count signal comprises presetting a counter to a count value representative of the third preliminary pulse signal and causing the counter to count clock pulses during the fourth preliminary pulse signal in order to cause the counter to produce the second count signal.

6. A system for measuring the quantity of a liquid in an enclosure, the enclosure containing a gaseous medium above the liquid, the liquid having a first dielectric constant, ╬╡, and the gaseous medium having a second dielectric constant ╬╡o, different from ╬╡, said system comprising: a sensor capacitor comprising at least two plates separated by a sensor capacitor dielectric gap and a compensator capacitor comprising at least two sensor capacitor plates separated by a compensation capacitor dielectric gap, said sensor capacitor being installed in the enclosure with an orientation such that the sensor capacitor plates extend vertically within the enclosure and are 0 separated horizontally by the sensor capacitor dielectric gap and so that a portion of the liquid in the enclosure can enter the sensor capacitor dielectric gap to be at a level corresponding to the level of liquid in the enclosure, and said compensator capacitor being completely filled with the same liquid as that in the enclosure; 5 means for measuring sensor capacitor capacitance, Cs, and compensator capacitor capacitance, Cc; and means connected to said measuring means for producing an indication of the value of a ratio between a first term representing the difference between the measured value of Cs and a known capacitance value, C_os, of the sensor o capacitor when the sensor capacitor dielectric gap is completely filled with the gaseous medium, and a second term representing a difference between the measured value of Cc and a known capacitance value, C_oc, of the compensator capacitor when the compensator capacitor dielectric gap is completely filled with the gaseous medium, wherein the ratio is proportional to the quantity of 5 liquid in the enclosure.

7. The system of claim 6 wherein the first and second terms represent respective time periods.

8. The system of claim 6 wherein said means for producing an indication comprise: means for producing a first count signal having a duration representative of the first term and means for producing a second count signal 5 having a duration representative of the second term; and means for mathematically dividing the first count signal by the secondcount signal to produce a digital quotient signal representing the ratio between the first and second terms.

9. The system of claim 7 wherein: said means for producing a first count signal comprises means for producing a first preliminary pulse signal having a duration proportional to C_os and a second preliminary pulse signal having a duration proportional to Cs, 5 and for giving the first count signal a value equal to the difference between the durations of the first and second preliminary pulse signals; and said means for producing a second count signal comprises means for producing a third preliminary pulse signal having a duration proportional to C_oc and a fourth preliminary pulse signal having a duration proportional to Cc, o and for giving the second count signal a value equal to the difference between the durations of the third and fourth preliminary pulse signals.

10. The system of claim 9 comprising a counter and wherein: said means for producing a first count signal comprises means for presetting said counter to a count value representative of the first preliminary pulse signal and means for causing said counter to count clock pulses during 5 the second preliminary pulse signal in order to cause the counter to produce the first count signal; and said means for producing a second count signal comprises means for presetting said counter to a count value representative of the third preliminary pulse signal and means for causing the counter to count clock pulses during o the fourth preliminary pulse signal in order to cause the counter to produce the second count signal.

11. The system of claim 6 wherein: said compensator capacitor is disposed below said sensor capacitor when said system is installed in the enclosure, and said two plates of said compensator capacitor include: 5 a first plate having a plurality of first plate elements, each first plate element having a vertically spaced, horizontally extending portion; and a second plate having a plurality of vertically spaced, horizontally extending second plate elements which alternate with said first plate element portions in the vertical direction, o whereby each second plate element is separated from at least one first plate element portion by a respective dielectric gap section.

12. The system of claim 1 1 wherein: said sensor capacitor has a cylindrical form with a vertically extending central axis and said compensator capacitor has a circular form with a vertically extending central axis.

13. The system of claim 12 wherein said sensor capacitor plates include a first plate and a second plate enclosed by said first plate, and said system further comprises a first vertically extending, electrically conducting bolt which is conductively connected to said first plate of said sensor capacitor to provide an electrical connection to said first plate of said sensor capacitor.

14. The system of claim 13 wherein said first bolt is a tubular bolt having a vertically extending passage, and said system further comprises a second vertically extending, electrically conducting bolt which extends through said vertically extending passage in said first bolt, is electrically insulated from said first bolt and is conductively connected to said second plate of said compensator capacitor to provide an electrical connection to said second plate of said compensator capacitor.

15. The system of claim 13 wherein said first plate of said compensator capacitor is in physical contact with, and conductively connected to, said first plate of said sensor capacitor.

16. The system of claim 11 wherein said first plate of said compensator capacitor is provided with passages for flow of liquid to be measured into said sensor capacitor dielectric gap.

17. The system of claim 6 for use in an enclosure having a bottom surface and a storage region with a horizontal cross-sectional area which varies as a first function of vertical distance above the bottom surface, wherein said sensor capacitor is shaped to have a differential capacitance which varies as a second function of vertical distance above the bottom surface, and the second function is substantially equal to the first function.