Patent Publication Number: US-2015082882-A1

Title: System for determining the level of a liquid in a container

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 61/882,968, filed Sep. 26, 2013, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to systems for determining the level of a liquid in a container and relates more particularly to a novel system for determining the level of a liquid in a container. 
     There are many situations in which it may be desirable to determine the level, i.e., quantity, of a liquid in a container. For example, the liquid in the container may be useful as a reactant or solvent in a chemical process, with volumes of the liquid periodically or continuously being withdrawn from the container to enable the chemical process to take place. Consequently, it may be desirable to know when the level of the liquid in the container falls below a predetermined threshold so that, for example, additional liquid may be added to the container or the container may be replaced with a container containing a greater quantity of liquid, thereby avoiding a disruption in the chemical process. In some cases, it may be possible to determine the level of a liquid in a container through some form of visual inspection, for example, by looking through the walls of the container or by looking through an opening in the container. However, in other cases, such a visual inspection may be undesirable or impractical, such as when the walls of the container may be too opaque to permit viewing the level of the liquid or when the container should not be opened and exposed to the ambient environment or vice versa. Moreover, a visual inspection may be inaccurate as it is dependent on the skill of the person performing the visual inspection. Furthermore, a visual inspection requires the physical presence of the person conducting the inspection in proximity to the container, which proximity may be dangerous or impractical. 
     For many of these reasons, a number of automated systems have been devised for determining the level of a liquid in a container. One such type of automated system involves the use of a pressure transducer that is submersible in the liquid and that is coupled through a cord to an externally-located control unit with a display. Such systems, however, cannot be used in certain hazardous or potentially hazardous environments where electrical current is not permitted. In addition, such systems typically do not take into account the specific gravity of the liquid in the container; consequently, a user must perform some type of conversion of the pressure reading to obtain a liquid level reading. Other types of automated systems involve the use of optical or mechanical sensors to detect liquid level. However, many of the optically-based systems cannot be used with opaque containers, and many of the mechanically-based systems cannot be attached to the containers or otherwise used in certain situations. 
     Moreover, in the case of existing automated systems, where there are a number of liquid containers, a separate automated system must be associated with each container. As can be appreciated, the need for a plurality of automated systems to handle a corresponding plurality of liquid containers can quickly lead to considerable expense. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel system for determining the level of a liquid in a container. 
     It is another object of the present invention to provide a system as described above that overcomes at least some of the shortcomings associated with existing systems for determining the level of a liquid in a container. 
     According to one aspect of the invention, there is provided a system for determining the level of a liquid in a container, the system comprising (a) a first fluid conduit, the first fluid conduit comprising a distal end and a first proximal end, the first fluid conduit being appropriately dimensioned so that, when the distal end is inserted into the container and is situated proximate to the bottom of the container, the first proximal end is situated outside of the container; (b) a pressure transducer, the pressure transducer being operatively coupled to the first proximal end of the first fluid conduit so as to emit a signal proportional in amplitude to the fluid pressure head in the container; (c) a processor operatively coupled to the pressure transducer to receive the signal emitted by the pressure transducer and to determine the liquid level in the container based on the signal; and (d) a display operatively coupled to the processor to display the determined liquid level in the container. 
     In another, more detailed feature of the invention, the first fluid conduit may be dimensioned so that, when the distal end is inserted into the container and is situated proximate to the bottom of the container, the first proximal end may be positioned outside the container at least one foot away. 
     In another, more detailed feature of the invention, the first fluid conduit may further comprise a second proximal end, and the system may further comprise a check valve and a gas displacement device. The check valve may comprise a first end and a second end, the first end of the check valve may be operatively coupled to the second proximal end of the first fluid conduit, and the second end of the check valve may be operatively coupled to the gas displacement device. 
     In another, more detailed feature of the invention, the display may comprise at least one of a bar graph array and a digital display. 
     In another, more detailed feature of the invention, the display may comprise a bar graph array. 
     In another, more detailed feature of the invention, the system may further comprise a printed circuit board, and each of the pressure transducer, the processor, and the display may be coupled to the printed circuit board. 
     In another, more detailed feature of the invention, the processor and the display may be connected wirelessly. 
     According to another aspect of the invention, there is provided a system for determining the level of a liquid in a container, the system comprising (a) a first fluid conduit, the first fluid conduit comprising a distal end and a first proximal end, the first fluid conduit being appropriately dimensioned so that, when the distal end is inserted into the container and is situated proximate to the bottom of the container, the first proximal end is situated outside of the container; (b) a pressure transducer, the pressure transducer being operatively coupled to the first proximal end of the first fluid conduit so as to emit a signal proportional in amplitude to the fluid pressure head in the container; (c) a processor operatively coupled to the pressure transducer to receive the signal emitted by the pressure transducer and to determine the liquid level in the container based on the signal; and (d) a user interface module operatively coupled to the processor, the user interface module comprising at least one user-accessible input control and at least one user-perceptible notification device. 
     In another, more detailed feature of the invention, the at least one user-accessible input control may comprise at least one of a sampling frequency control for adjusting the frequency at which liquid level measurements are processed by the processor and at least one calibration control for use in calibrating the processor. 
     In another, more detailed feature of the invention, the at least one user-perceptible notification device may comprise at least one of at least one visual notification device and at least one audible notification devices. 
     In another, more detailed feature of the invention, the at least one visual notification device may comprise at least one of a bar graph array and a digital display. 
     According to another aspect of the invention, there is provided a system for determining the level of a liquid in one or more of a plurality of containers, the system comprising (a) a plurality of fluid conduits, each of the fluid conduits comprising a distal end and a first proximal end and being appropriately dimensioned so that, when the distal end is inserted into a container and is situated proximate to the bottom of the container, the first proximal end is situated outside of the container; (b) a corresponding plurality of pressure transducers, each of the pressure transducers being operatively coupled to the first proximal end of a different one of the fluid conduits so as to emit a signal proportional in amplitude to the fluid pressure head in a corresponding container; (c) a processor operatively coupled to the pressure transducers to receive the signals emitted by the pressure transducers and to determine the liquid level in the containers based on the signals; and (d) a user interface module operatively coupled to the processor, the user interface module comprising at least one user-perceptible notification device for communicating the liquid level in at least one of the containers. 
     In another, more detailed feature of the invention, the system may further comprise a printed circuit board, and each of the pressure transducers, the processor and the user interface module may be coupled to the printed circuit board. 
     In another, more detailed feature of the invention, the user interface module and the processor may be connected wirelessly. 
     In another, more detailed feature of the invention, the user interface module may comprise a graphical user interface on a computer. 
     In another, more detailed feature of the invention, the user interface module may further comprise a channel selection control for selecting a desired one of the containers to be analyzed for liquid level. 
     According to another aspect of the invention, there is provided a system for determining the level of a liquid in one or more of a plurality of containers, the system comprising (a) a plurality of fluid conduits, each of the fluid conduits comprising a distal end and a first proximal end and being appropriately dimensioned so that, when the distal end is inserted into a container and is situated proximate to the bottom of the container, the first proximal end is situated outside of the container; (b) a valve assembly, the valve assembly comprising an output port and a plurality of input ports, each of the plurality of fluid conduits being operatively coupled to a different input port, the output port being selectively fluidly connectable to one of the input ports; (c) a pressure transducer, the pressure transducer being operatively coupled to the output port of the valve assembly so as to emit a signal proportional in amplitude to the fluid pressure head in a corresponding container fluidly coupled thereto; (d) a processor operatively coupled to the pressure transducer to receive the signal emitted by the pressure transducer and to determine the liquid level in a container based on the signal; and (e) a user interface module operatively coupled to the processor, the user interface module comprising at least one user-perceptible notification device for communicating the liquid level in at least one of the containers. 
     In another, more detailed feature of the invention, the user interface module and the processor may be connected wirelessly. 
     In another, more detailed feature of the invention, the user interface module may comprise a graphical user interface on a computer. 
     In another, more detailed feature of the invention, the user interface module may further comprise a channel selection control for selecting a desired one of the containers to be analyzed for liquid level. 
     In another, more detailed feature of the invention, the system may further comprise a printed circuit board, and each of the pressure transducers, the processor and the user interface module may be coupled to the printed circuit board. 
     Additional objects, as well as aspects, features and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts: 
         FIG. 1  is a simplified schematic diagram of a first embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being shown, for illustrative purposes, together with an exemplary container containing a quantity of a liquid; 
         FIG. 2  is a side view of the system of  FIG. 1 , the system being shown, for illustrative purposes, together with an exemplary container containing a quantity of a liquid; 
         FIG. 3  is a simplified schematic diagram of a second embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being shown, for illustrative purposes, together with an exemplary container containing a quantity of a liquid; 
         FIG. 4  is a simplified schematic diagram of a third embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being configured for use with a plurality of containers, each container containing a quantity of a liquid; 
         FIG. 5  is a side view of the system of  FIG. 4 , the system being shown, for illustrative purposes, together with an exemplary plurality of containers, each container containing a quantity of a liquid; 
         FIG. 6  is a simplified schematic diagram of a fourth embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being configured for use with a plurality of containers, each container containing a quantity of a liquid; and 
         FIG. 7  is a simplified schematic diagram of a fifth embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being configured for use with a plurality of containers, each container containing a quantity of a liquid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2 , there are shown a simplified schematic diagram and a side view of a first embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being represented generally by reference numeral  11 . For illustrative purposes, system  11  is shown together with a container C containing a quantity of a liquid L. 
     System  11  may comprise a first fluid conduit  13 , a pressure transducer  15 , a check valve  17 , a second fluid conduit  19 , a gas displacement device  21 , a user interface module  23 , a processor  25 , a printed circuit board  27 , a power supply  29 , and a housing  30 . 
     First fluid conduit  13  may be a flexible or rigid unitary tubular member made of a chemically-inert, non-porous, gas-impermeable material, such as a suitable polymer or stainless steel. First fluid conduit  13  may be branched proximate to its proximal end to yield a first proximal branch  31 , a second proximal branch  33 , and a distal branch  35 . Distal branch  35  of first fluid conduit  13  may have a suitable length such that its distal end  37  may be positioned inside a container, preferably near the bottom or at the bottom of the interior of the container, while its proximal end  39  may be located outside the container at a location that may be remote relative to the container, such as at least several inches away from the container and, in some cases, at least one or more feet away from the container and perhaps even at least dozens of feet away from the container. In this manner, all of the other components of system  11 , including all of the electrical components of system  11 , may be kept remote and/or isolated from the container, such as in a cabinet or even in another room. The maximum workable length of first fluid conduit  13 , as measured from a proximal end  41  of first proximal branch  31  to distal end  37  of distal branch  35 , may depend, in part, on the sensitivity of pressure transducer  15  and, in part, on the inner diameter of first fluid conduit  13 . In general, the greater the inner diameter of first fluid conduit  13 , the longer the maximum workable length. According to one embodiment, first fluid conduit  13  may comprise TYGON E-3603 flexible tubing (Saint-Gobain, Charny, France) and may have an inner diameter of approximately 3/32 inch and a length, as measured from proximal end  41  of first proximal branch  31  to distal end  37  of distal branch  35 , of approximately 80 feet. 
     Pressure transducer  15 , which may be a conventional pressure transducer, preferably of high sensitivity, may be operatively coupled to proximal end  41  of first proximal branch  31  in such a way as to sense the fluid pressure head of a container into which distal end  37  of distal branch  35  has been inserted and to emit an electrical signal proportional in amplitude to the magnitude of the fluid pressure head sensed thereby. 
     Check valve  17 , which may be a conventional one-way check valve, such as a one-way ball check valve, may be operatively coupled at one end to a proximal end  47  of second proximal branch  33  and may be operatively coupled at an opposite end to a distal end  49  of second fluid conduit  19 . Check valve  17  may serve to prevent a loss of the fluid pressure head via the leakage of air proximally from first fluid conduit  13  through second fluid conduit  19  and gas displacement device  21 . 
     Second fluid conduit  19  may be a flexible or rigid unitary tubular member made of a chemically-inert, non-porous, gas-impermeable material, such as a suitable polymer or stainless steel. Second fluid conduit  19  may be made of the same material as first fluid conduit  13  but need not be. A proximal end  51  of second fluid conduit  19  may be operatively coupled to an output end  53  of gas displacement device  21 . 
     Gas displacement device  21  may be a conventional air blower, air pump, fan, a compressed inert gas with a valve arrangement, or any other type of device that may be used to move air or another suitable gas. Gas displacement device  21  may be, but need not be, electrically powered. As will be discussed further below, gas displacement device  21  may be used, prior to each liquid level measurement, to blow air or an inert gas through secondary fluid conduit  19 , check valve  17 , second proximal branch  33 , and distal branch  35 , thereby clearing any debris from distal end  37  of distal branch  35  that may interfere with the measurement of the fluid pressure head. If desired, a particulate filter (not shown) may be positioned downstream of gas displacement device  21  to remove particulate matter from the output stream of gas displacement device  21 . 
     User interface module  23  may comprise one or more user-accessible input controls and one or more user-perceptible notification devices. For example, the one or more user-accessible input controls may comprise a sampling frequency control  61  for adjusting the frequency at which liquid level measurements are analyzed by processor  25 , a calibration start control  63  for directing processor  25  to initiate the calibration procedure for liquid L in container C, a calibration high control  65  for directing processor  25  to take a calibrating measurement when the level of liquid L in container C is at a desired maximum level, and a calibration low control  67  for directing processor  25  to take a calibrating measurement when the level of liquid L in container C is at a desired minimum level, which may or may not be when container C is empty. As can be appreciated, sampling frequency control  61  may be omitted if one does not wish to vary the sampling frequency, and calibration start control  63 , calibration high control  65  and calibration low control  67  may be omitted if one wishes to calibrate through firmware on processor  25 . 
     The one or more user-perceptible notification devices of user interface module  23  may comprise one or more visual or audible notification devices. Examples of visual notification devices may comprise one or more of a bar graph array  71  for depicting, through lighted segments, the level of liquid L in container C relative to the calibrated maximum and minimum levels, a digital display  73  for depicting numerically the percentage of liquid L present in container C relative to the calibrated maximum and minimum levels, an illuminable high level alert  75 , which may be, for example, an LED, for indicating when the determined level exceeds a certain set point level, which may be at the maximum level or near the maximum level (90%), and an illuminable low level alert  77 , which may be, for example an LED, for indicating when the level drops below a certain level set point, which may be at the minimum or near the minimum (e.g., 10% full). Other types of visual notification devices may include a high resolution display illustrating a bottle-type icon with a depiction of a liquid level therein. Such an icon may be accompanied by a percent full/level number. Examples of audible notification devices may comprise audible alarms for indicating when the level exceeds the maximum level or drops below a certain level set point. 
     Processor  25  may be operatively coupled to pressure transducer  15  and may be equipped with appropriate firmware to measure and to process the signals emitted from pressure transducer  15 . Because the mathematical relationship between the fluid pressure head and the liquid level height in a container is linear, once processor  25  has the calibrated values for the maximum and minimum levels, any signal from pressure transducer  15  can be converted by processor  25  into a liquid level reading by comparing the sensed fluid pressure head reading to the calibrated maximum and minimum level values. 
     Processor  25  may also be operatively coupled to user interface module  23  so that the one or more user-accessible input controls of user interface module  23  may be used to control the operation of processor  25  and so that the one or more user-perceptible notification devices of user interface module  23  may be controlled by processor  25 . 
     Processor  25  may additionally be operatively coupled to gas displacement device  21  so that processor  25  may control the actuation of gas displacement device  21 . 
     Pressure transducer  15 , user interface module  23  and processor  25  may be mounted on or otherwise coupled to printed circuit board  27 . Printed circuit board  27 , which may be a one-sided board or a two-sided board, may include a conductive trace for connecting processor  25  to each of pressure transducer  15  and user interface module  23  in the manner described above. 
     Power supply  29 , which may be coupled to those components of system  11  that require electrical power, may comprise a rechargeable or non-rechargeable battery mounted on or otherwise coupled to printed circuit board  27 . Alternatively, and as shown in the present embodiment, power supply  29  may comprise a low voltage wall plug type transformer, which may be mounted on or otherwise coupled to printed circuit board  27  and which may comprise a plug  79  adapted to be plugged into a convention AC wall outlet. If installed in a NEMA-type enclosure or DIN rail arrangement, an appropriate voltage terminal may be used in lieu of the aforementioned wall transformer. 
     Housing  30  may comprise a first portion  81  and a second portion  83 , each of which may be made of a rigid, durable material, such as a suitable polymer. First portion  81  and second portion  83  may be joined together by suitable means (not shown), such as screws, adhesives, and/or welding, to define a cavity  85 . Cavity  85  may be appropriately dimensioned to accommodate all of the components of system  11 , except for at least the distal portion of distal branch  35  of first fluid conduit  13 . Housing  30  may be shaped to include a window  87  through which user interface module  23  may be accessible. 
     Although not shown, one or more visual or audible notification devices of user interface module  23  may be located remotely and externally to housing  30  and may be coupled to processor  25  through an output connector coupled to printed circuit board  27  through one or more relays. 
     In use, one may first calibrate system  11 . This may be done, for example, by inserting distal end  37  of distal branch  35  into an empty container of the type to be used or by positioning distal end  37  of distal branch  35  above the liquid level of such a container containing liquid. Next, one may initiate the calibration procedure of system  11  by actuating calibration start control  63  and then actuating calibration low control  67 , causing processor  25  to take a fluid pressure head reading from pressure transducer  15 . Just prior to said reading being taken, gas displacement device  21  may cause air or a gas to be blown or expelled therefrom, thereby clearing any debris from distal branch  35 . Next, one may position distal end  37  of distal branch  35  at or near the bottom of the same type of container containing a maximum desired quantity of the particular type of liquid to be monitored and then may actuate calibration high control  65 , causing processor  25  to take a fluid pressure head reading form pressure transducer  15 . Again, just prior to said reading being taken, gas displacement device  21  may cause air or a gas to be blown or expelled therefrom, thereby clearing any debris from distal branch  35 . As can be appreciated, the order of the calibration high and calibration low readings may be taken in the reverse order from that disclosed above. Because there is a linear relationship between fluid pressure head and liquid level, the aforementioned high and low readings may be used by processor  25  to convert any future fluid pressure readings into liquid level readings. 
     With system  11  thus calibrated, one may use sampling frequency control  61  to select the frequency with which readings are to be taken. At whichever frequency is selected, gas displacement device  21  may clear debris from distal branch  35 , and a fluid pressure head reading from pressure transducer  15  may be sent to processor  25 . Processor  25  may compare the measured value to the calibrated values and, thereby, determine the liquid level. (If desired, a plurality of measured values taken over a short time span may be averaged and the average then used to determine the liquid level.) This information may then be transmitted from processor  25  to user interface module  23 , where it may be communicated visually and/or audibly by user interface module  23 . 
     Referring now to  FIG. 3 , there is shown a simplified schematic diagram of a second embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being represented generally by reference numeral  211 . For illustrative purposes, system  211  is shown together with container C containing a quantity of liquid L. 
     System  211  may be similar in most respects to system  11 , a principal difference between the two systems being that, whereas system  11  may comprise a user interface module  23 , system  211  may instead comprise a control module  213 , a display module  215 , a wireless data transmitter  217 , and a wireless data receiver  219 . 
     Control module  213  of system  211  may comprise one or more of the user-accessible input controls of user interface module  23 . Control module  213  may be associated with a housing  221  in much the same way that user interface module  23  is associated with housing  30 . Display module  215  may comprise one or more user-perceptible notification devices of user interface module  23 . Display module  215  may be externally and remotely located relative to housing  221 . Wireless data transmitter  217  may be associated with housing  221  and may be coupled to processor  25  whereas wireless data receiver  219  may be externally and remotely located relative to housing  221  and may be coupled to display module  215 . 
     System  211  may be used in much the same fashion as system  11 , except that wireless data transmitter  217  and wireless data receiver  219  enable display module  215  to be positioned remotely relative to housing  221 . 
     As can readily be appreciated, system  211  may be modified so that, instead of having processor  25  communicate wirelessly with display module  215 , display module  215  may be coupled to processor  25  through the combination of an output connector operatively coupled to printed circuit board  29  and a cable. 
     Referring now to  FIGS. 4 and 5 , there are shown a simplified schematic diagram and a side view of a third embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being represented generally by reference numeral  311 . For illustrative purposes, system  311  is shown together with a plurality of containers, each container containing a quantity of a liquid. A first container C 1  contains a quantity of a liquid L 1 , a second container C 2  contains a quantity of a liquid L 2 , and a third container C 3  contains a quantity of a liquid L 3 . Containers C 1 , C 2  and C 3  may be the same type of container or may be different types of containers, liquids L 1 , L 2 , and L 3  may be the same type of liquid or may be different types of liquid, and the quantities of liquids L 1 , L 2  and L 3  in containers C 1 , C 2 , and C 3 , respectively, at any given time, may be the same quantities or may be different quantities. 
     System  311  may comprise a first fluid conduit  313 , a second fluid conduit  315 , and a third fluid conduit  317 . Each of first fluid conduit  313 , second fluid conduit  315 , and third fluid conduit  317  may be similar in size and construction to distal branch  35  of system  11 . First fluid conduit  313 , which may comprise a proximal end  319  and a distal end  321 , may have distal end  321  positioned inside container C 1 , preferably near the bottom or at the bottom of the interior of container C 1 , while proximal end  319  may be located outside container C 1  at a location that may be remote relative to container C 1 . Second fluid conduit  315 , which may comprise a proximal end  323  and a distal end  325 , may have distal end  325  positioned inside container C 2 , preferably near the bottom or at the bottom of the interior of container C 2 , while proximal end  323  may be located outside container C 2  at a location that may be remote relative to container C 2 . Third fluid conduit  317 , which may comprise a proximal end  327  and a distal end  329 , may have distal end  329  positioned inside container C 3 , preferably near the bottom or at the bottom of the interior of container C 3 , while proximal end  327  may be located outside container C 3  at a location that may be remote relative to container C 3 . 
     System  311  may also comprise a valve assembly  331 . Valve assembly  331  may include three input ports  333 ,  335 , and  337  and one output port  339  and may be constructed so that fluid connection between output port  339  and any one of input ports  333 ,  335 , and  337  may be selectively made. Proximal end  319  of first fluid conduit  313  may be operatively coupled to input port  333 , proximal end  323  of second fluid conduit  315  may be operatively coupled to input port  335 , and proximal end  327  of third fluid conduit  317  may be operatively coupled to input port  337 . 
     System  311  may further comprise a fourth fluid conduit  341 . Fourth fluid conduit  341  may be similar in construction to first fluid conduit  13  of system  11 , albeit possibly shorter in length, and may be branched to yield a distal branch  343 , a first proximal branch  345 , and a second proximal branch  347 . A distal end  349  of distal branch  343  may be operatively coupled to output port  339  of valve assembly  331 . 
     System  311  may further comprise a pressure transducer  351 , which may be similar to pressure transducer  15  of system  11 . Pressure transducer  351  may be operatively coupled to a proximal end  351  of first proximal branch  345 . 
     System  311  may further comprise a check valve  361 , a fifth fluid conduit  363 , and a gas displacement device  365 . Check valve  361 , which may be similar to check valve  17  of system  11 , may be operatively coupled at one end to a proximal end  367  of second proximal branch  347  and may be operatively coupled at an opposite end to a distal end  369  of fifth fluid conduit  363 . Fifth fluid conduit  363 , which may be similar to second fluid conduit  19  of system  11 , may be coupled at a proximal end  371  to gas displacement device  365 . Gas displacement device  365  may be similar to gas displacement device  21  of system  11 . If desired, a particulate filter (not shown) may be positioned downstream of gas displacement device  365  to remove particulate matter from the output stream of gas displacement device  365 . 
     System  311  may further comprise a user interface module  381 . User interface module  381  may comprise one or more user-accessible input controls and one or more user-perceptible notification devices. For example, the one or more user-accessible input controls may comprise a channel selection control  383  for selecting a desired one of containers C 1 , C 2  and C 3  to be calibrated and/or analyzed in terms of liquid level, a sampling frequency control  385  for adjusting the frequency at which liquid level measurements are to be analyzed, a calibration start control  387  for initiating the calibration procedure for a selected container, a calibration high control  389  for causing a calibrating measurement to be taken when the liquid level in the selected container is at a desired maximum level, and a calibration low control  391  for causing a calibrating measurement to be taken when the liquid level in the selected container is at a desired minimum level, which may or may not be when the container is empty. As can be appreciated, sampling frequency control  385  may be omitted if one does not wish to vary the sampling frequency, and calibration start control  387 , calibration high control  389  and calibration low control  391  may be omitted if one wishes to calibrate through firmware on the system processor. 
     The one or more user-perceptible notification devices of user interface module  381  may comprise one or more visual or audible notification devices. Examples of visual notification devices may comprise one or more of bar graph arrays  393 - 1 ,  393 - 2 , and  393 - 3  for depicting, through lighted segments, the liquid levels in containers C 1  through C 3 , respectively, digital displays  395 - 1  through  395 - 3  for depicting numerically the liquid levels in containers C 1  through C 3 , respectively, expressed as percentages relative to the calibrated maximum and minimum levels, illuminable high level alerts  397 - 1  through  397 - 3 , which may be, for example, LEDs, for indicating when the liquid levels in containers C 1  through C 3 , respectively, exceed a certain set point level, which may be at the maximum level or near the maximum level (90%), and illuminable low level alerts  399 - 1  through  399 - 3 , which may be, for example LEDs, for indicating when the liquid levels in containers C 1  through C 3 , respectively, drop below a certain level set point, which may be at the minimum or near the minimum (e.g., 10% full). As can be appreciated, instead of including a plurality of bar graph arrays  393 - 1  through  393 - 3 , a plurality of digital displays  395 - 1  through  395 - 3 , a plurality of high level alerts  397 - 1  through  397 - 3 , and/or a plurality of low level alerts  399 - 1  through  399 - 3 , user interface module  381  may instead comprise a single bar graph array, digital display, etc., together with an indicator that conveys which container the results represent. Examples of audible notification devices may comprise audible alarms for indicating when the liquid level exceeds the maximum level or drops below a certain level set point. 
     System  311  may further comprise a processor  401 , which may be operatively coupled to pressure transducer  351  and may be equipped with appropriate firmware to measure and to process the signals emitted from pressure transducer  351 . Processor  401  may also be operatively coupled to user interface module  381  so that the one or more user-accessible input controls of user interface module  381  may be used to control the operation of processor  351  and so that the one or more user-perceptible notification devices of user interface module  351  may be controlled by processor  401 . Processor  401  may additionally be operatively coupled to gas displacement device  365  so that processor  401  may control the actuation of gas displacement device  365 . 
     Pressure transducer  351 , user interface module  381  and processor  401  may be mounted on or otherwise coupled to a printed circuit board  403 . Printed circuit board  403 , which may be a one-sided board or a two-sided board, may include a conductive trace for connecting processor  401  to each of pressure transducer  351  and user interface module  381  in the manner described above. 
     System  311  may further comprise a power supply  407 , which may be coupled to those components of system  311  that require electrical power. Power supply  407  may be similar to power supply  29  of system  11 . 
     System  311  may further comprise a housing  409 , which may be similar to housing  30  of system  11  and which may be used to accommodate all of the components of system  311 , except for at least the distal portions of first fluid conduit  313 , second fluid conduit  315  and third fluid conduit  317 . 
     System  311  may be calibrated in much the same way as system  11 , except that each of containers C 1  through C 3  may be calibrated independently or not, depending on whether the containers and the liquids contained therein are different or not. Channel selection control  383  may be used to select a particular container for calibration. Once system  311  has been calibrated, pressure transducer  351  may be fluidly connected sequentially to each of the containers via valve assembly  331  so that pressure transducer  351  may sense the fluid pressure head in each container and may emit a corresponding signal. The signals emitted by pressure transducer  351  may then be transmitted to processor  401 , and a liquid level determination for each container may be made by processor  401 . This information may then be sent to user interface module  381  to be communicated visually or audibly. This measuring procedure may be repeated at a desired frequency. 
     Referring now to  FIG. 6 , there is shown a simplified schematic diagram of a fourth embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being represented generally by reference numeral  511 . For illustrative purposes, system  511  is shown together with a plurality of containers, each container containing a quantity of a liquid. A first container C 1  contains a quantity of a liquid L 1 , a second container C 2  contains a quantity of a liquid L 2 , and a third container C 3  contains a quantity of a liquid L 3 . Containers C 1 , C 2  and C 3  may be the same type of container or may be different types of containers, liquids L 1 , L 2 , and L 3  may be the same type of liquid or may be different types of liquid, and the quantities of liquids L 1 , L 2  and L 3  in containers C 1 , C 2 , and C 3 , respectively, at any given time, may be the same quantities or may be different quantities. 
     System  511  is similar in some respects to system  311 , a principal difference between the two systems being that, whereas system  311  may comprise a single pressure transducer  351  that is selectively coupled to first fluid conduit  313 , second fluid conduit  315 , or third fluid conduit  317  through valve assembly  331 , system  511  may omit such a valve assembly and may instead comprise a plurality of pressure transducers  513 - 1  through  513 - 3 , wherein each of pressure transducers  513 - 1  through  513 - 3  is coupled to a different container C 1  through C 3 , respectively, through its own fluid conduit  515 - 1  through  515 - 3 , respectively. Another difference between the two systems may be that, whereas system  311  may comprise a single check valve  361  and a single gas displacement device  365 , system  511  may comprise a plurality of check valves  517 - 1  through  517 - 3  and a plurality of gas displacement device  519 - 1  through  519 - 3 , wherein each of check valves  517 - 1  through  517 - 3  is coupled at one end to a different gas displacement device  519 - 1  through  519 - 3 , respectively, and wherein each of check valves  517 - 1  through  517 - 3  is coupled at another end to a different fluid conduit  515 - 1  through  515 - 3 , respectively. Although not shown, a particulate filter may be positioned downstream of each of gas displacement devices  519 - 1  through  519 - 3  to remove particulate matter from the output streams of gas displacement devices  519 - 1  through  519 - 3 . 
     System  511  may be used in much the same way as system  311 , with a principal difference being that system  511  is configured so that a separate pressure transducer senses the fluid pressure head from each container, without requiring a valve assembly to switch the pressure transducer from one container to another. 
     Referring now to  FIG. 7 , there is shown a simplified schematic diagram of a fifth embodiment of a system constructed according to the present invention for determining the liquid level in a container, the system being represented generally by reference numeral  611 . For illustrative purposes, system  611  is shown together with a plurality of containers, each container containing a quantity of a liquid. A first container C 1  contains a quantity of a liquid L 1 , a second container C 2  contains a quantity of a liquid L 2 , and a third container C 3  contains a quantity of a liquid L 3 . Containers C 1 , C 2  and C 3  may be the same type of container or may be different types of containers, liquids L 1 , L 2 , and L 3  may be the same type of liquid or may be different types of liquid, and the quantities of liquids L 1 , L 2  and L 3  in containers C 1 , C 2 , and C 3 , respectively, at any given time, may be the same quantities or may be different quantities. 
     System  611  may be similar in most respects to system  511 , a principal difference between the two systems being that, whereas system  511  may comprise user interface module  381  disposed within housing  409 , system  611  may instead comprise a computer  613  externally located relative to housing  409 , computer  613  being coupled to processor  401  via a cable  615  and an output connector  617 . Computer  613  may be equipped with a graphical user interface that may be configured to provide input controls and notification devices analogous to those that may be present in user interface module  381 . 
     System  611  may further comprise a plurality of a plurality of particulate filters  621 - 1  through  621 - 3 , a different such particulate filter  621 - 1  through  621 - 3  being positioned downstream of each of gas displacement devices  519 - 1  through  519 - 3 . 
     It should be understood that the variations discussed above in the context of single-container systems, such as systems  11  and  211 , may be applied to multi-container systems, such as systems  311 ,  511  and  611 , and vice versa. For example, the multi-container systems disclosed herein may be modified for wireless transmission of the determined liquid levels to a remote location. In corresponding fashion, the single-container systems disclosed herein may be modified for integration with a computer. 
     Some of the positive attributes of the system of the present invention are that it is rugged and reliable, it includes a minimal number of parts, it is easy to calibrate and to use, and it can be used with opaque containers and in hazardous environments where electrical components are not permitted to be present. In addition, certain embodiments of the invention are adapted to measure the liquid level of a plurality of containers. 
     The embodiments of the present invention described above are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications to it without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.