System and method for measuring volume of liquid in a chamber

Methods and systems for measuring the volume of a liquid in a subject chamber, such as the mixing chamber in a system for mixing and delivering drugs, for example. A volume of a control gas in communication with the subject chamber is isolated, such that the total volume of the control gas and the liquid is held constant, and the initial pressure of the control gas is measured. Then, a known volume of liquid is moved into or out of the subject chamber. This known volume may be exchanged between the subject chamber and a measurement chamber, in order to determine this volume. The final pressure of the control gas is also measured after the reference volume has been removed from or added to the chamber and while the volume of control gas remains otherwise isolated. The volume of liquid in the chamber may then be measured based on (a) the initial pressure, (b) the final pressure, (c) the reference volume and (d) the volume of gas in the subject chamber when the subject chamber is empty of liquid.

TECHNICAL FIELD
 The present invention relates to systems and methods for measuring the
 volume of liquid in a chamber.
 SUMMARY OF THE INVENTION
 The invention includes methods for measuring the volume of a liquid in a
 subject chamber, such as the mixing chamber in a system for mixing and
 delivering drugs, for example. In a preferred method, a volume of a
 control gas in communication with the subject chamber is isolated, such
 that the total volume of the control gas and the liquid is held constant,
 and the initial pressure of the control gas is measured. Then, a known
 (i.e., measured or measurable) volume of liquid--referred to herein as a
 reference volume--is moved into or out of the subject chamber. In a
 preferred method, the reference volume is exchanged between the subject
 chamber and a measurement chamber, such as a delivery chamber, where the
 reference volume is measured. The final pressure of the control gas is
 also measured after the reference volume has been removed from or added to
 the chamber and while the volume of control gas remains otherwise
 isolated. The volume of liquid in the chamber may then be measured based
 on (a) the initial pressure, (b) the final pressure, (c) the reference
 volume and (d) the volume of gas in the subject chamber when the subject
 chamber is empty of liquid.
 A system for measuring the volume of a liquid in the subject chamber,
 according to a preferred embodiment of the invention, includes an
 isolatable volume of a control gas in communication with the subject
 chamber such that the total volume of the subject-chamber control gas and
 the liquid in the subject chamber may be held constant; a port through
 which liquid may flow into and out of the subject chamber; and a transfer
 system that causes the reference volume of the liquid to flow through the
 port. The system also includes a controller, which controls the transfer
 system. The controller also receives data from a pressure sensor in
 communication with the isolatable volume and from a delta-volume system
 that measures the reference volume. The controller also calculates the
 volume of liquid in the subject chamber based on the pressure data, the
 reference volume and the volume of gas in the subject chamber when the
 subject chamber is empty of liquid.
 In a preferred embodiment, the transfer system causes the reference volume
 of the liquid to flow between the subject chamber and a measurement
 chamber, and the delta-volume system includes means for measuring the
 change in volume of a control gas in the measurement chamber when the
 reference volume flows through the port. Preferably, this means for
 measuring the change in volume of the control gas includes an acoustic
 volume-measurement system, which applies vibrations to the control gas in
 the measurement chamber and measures the response of the control gas to
 the applied vibrations. Preferably, the acoustic volume-measurement system
 calculates the reference volume based on a resonant frequency of the
 control gas in the second chamber.
 The transfer system preferably includes one or more pressure adjusters for
 adjusting the pressure of a control gas. In a preferred embodiment, the
 pressure adjuster includes means for adding or removing a control gas, to
 or from the subject chamber and/or the measurement chamber. The pressure
 adjuster preferably includes positive- and negative-pressure reservoirs,
 which urge control gas into or out of the chambers.
 A preferred embodiment of a system, incorporating the present invention,
 for mixing and delivering medical liquids to a patient includes a
 solvent-supply port; a solute-supply port; an outlet; a measurement (or
 delivery) chamber; a mixing chamber; a valve system which controls flow
 among the solvent-supply port, the solute-supply port, the outlet, the
 measurement chamber and the mixing chamber; an isolatable volume of a
 control gas in communication with the mixing chamber such that the total
 volume of the control gas and the liquid in the mixing chamber may be held
 constant; a pressure sensor in communication with the isolatable volume; a
 pressure adjuster which urges fluid into or out of the mixing chamber; a
 delta-volume system which measures changes in volume of liquid in the
 measurement chamber; and a controller, which (i) controls the valve system
 and the pressure adjuster so as to cause a reference volume of liquid to
 flow between the mixing chamber and the measurement chamber, (ii) receives
 pressure data from the pressure sensor before and after the reference
 volume of liquid has flown between the mixing chamber and the measurement
 chamber, (iii) receives volume data from the delta-volume system, and (iv)
 calculates the volume of liquid in the mixing chamber based on (a) the
 pressure data, (b) the reference volume and (c) the volume of gas in the
 mixing-chamber when the mixing chamber is empty of liquid.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
 In a system that automatically mixes and delivers intravenous drugs to a
 patient and that uses a mixing chamber and a delivery chamber--and in
 particular, in the system described in U.S. patent application for
 "System, Method and Cassette for Mixing and Delivering Intravenous Drugs,"
 Ser. No. 09/137,025 filed Aug. 20, 1998 (which application is incorporated
 herein by reference along with its counterpart, International Application
 No. PCT/US98/17313)--it is desirable at times to know the volume of liquid
 in the mixing chamber. FIG. 1 is a schematic depicting such a system. The
 system includes a port 22 for connecting to an IV fluid source. This IV
 fluid is used as a solvent for mixing drugs. Another port 26 connects the
 system to a source of a liquid medication, which may or may not be diluted
 with the IV fluid before delivery to the patient. The system also accepts
 vials 11a-11c, which may contain liquid medication or powdered solute. The
 system permits mixing a solution from powdered solutes contained in vials
 11a-11c. Valve 31a controls the flow of the solvent into the system, while
 valves 31c-31f control flow to and from the sources of liquid and powdered
 solute. The system also preferably includes a vent 24, which is controlled
 by valve 31g and which may be used to equalize pressure in the system.
 The system includes a delivery chamber 27 and a mixing chamber 28. A
 manifold 45 connects the chambers 27, 28, the ports 22, 26, the vials
 11a-11c and the vent 24. Valve 31b controls flow between the delivery
 chamber 27 and the manifold 45. Valves 32 and 33 control the flow of IV
 solution from the delivery chamber 27 to the conduit 23 leading to the
 patient. Because the mixing chamber 28 will often contain some gas in
 addition to liquid, the mixing chamber is preferably provided two mouths:
 a top mouth 47 for permitting gas to be drawn from the mixing chamber and
 a bottom mouth 48 for permitting liquid to be drawn from the mixing
 chamber. Flow through the top and bottom mouths 47, 48 is controlled
 respectively by valves 31h and 31i.
 The delivery chamber 27 measures the amount of solvent drawn from the IV
 fluid port 22. Typically several boluses of solvent are drawn and urged to
 the mixing chamber 28. When the desired amount of solvent is collected in
 the mixing chamber 28, the solvent is then mixed with the solute. For
 powdered solutes, the solvent is urged back and forth between the mixing
 chamber 28 and one of the vials 11a-11c, in order to dissolve the powder
 fully. Once the solute and solvent are mixed, a bolus of the mixture is
 urged from the mixing chamber 28 to the delivery chamber 27, where the
 volume of this mixture bolus may be measured. This bolus may be delivered
 to the patient directly. Alternatively, the mixture may be diluted further
 by the introduction of additional solvent from port 22 into the delivery
 chamber 27, after which the diluted mixture may be delivered to the
 patient.
 Both the mixing chamber 28 and the delivery chamber 27 are preferably
 provided with means for urging liquid out of the chamber and drawing
 liquid into the chamber. As discussed in greater detail below, such means
 preferably adjust the pressure of a control gas in order to force liquid
 into or out of the chamber.
 The delivery chamber 27 is provided with an accurate system for measuring
 the volume of liquid being drawn into or expelled from the delivery
 chamber. Such a system is described in U.S. Pat. No. 5,349,852 for a "Pump
 Controller Using Acoustic Spectral Analysis" to Kamen et al. and its
 progeny (U.S. Pat. Nos. 5,526,844; 5,533,389 and 5,575,310), which are
 incorporated herein by reference. These patents describe acoustic
 volume-measurement systems, which use sound waves to determine the volume
 of gas in a chamber. Changes in the volume of gas in the delivery chamber
 indicate opposite and equal changes in the volume of liquid in the
 delivery chamber. As shown in U.S. Pat. No. 5,349,852, positive- and
 negative-pressure reservoirs are used to urge fluid out of and draw fluid
 into the chamber. The positive- and negative-pressure reservoirs are
 connected through the acoustic volume-measurement system to the chamber.
 (See, for example, FIGS. 5-8 of U.S. Pat. No. 5,349,852.)
 In the system shown in FIG. 1 of the present application, an acoustic
 volume-measurement system is preferably used to calculate changes in the
 volume of gas--and accordingly, changes in the volume of liquid--in the
 delivery chamber 27. Through use of the procedure described hereinbelow,
 the acoustic volume-measurement system associated with the delivery
 chamber 27 can be used to calculate the volume of liquid in the mixing
 chamber 28. It will be appreciated that by employing a single acoustic
 volume-measurement system to measure the volume of both the delivery
 chamber 27 and mixing chamber 28 significant cost savings can be obtained.
 FIG. 2 is a schematic representing the arrangement of some of the
 components of the FIG. 1 system. Many of the components of the FIG. 1
 system are not shown in FIG. 2 so as to permit a clearer description of a
 preferred embodiment of the procedure for using the acoustic
 volume-measurement system of the delivery chamber 27 to measure the amount
 of liquid in the mixing chamber 28. For instance, the entire manifold 45
 and valve 31b are not shown, even though they connect the bottom mouth 48
 of the mixing chamber 28 to the delivery chamber 27. Only valve 31i is
 shown controlling flow between the two chambers. It will be appreciated
 that valve 31b may be actuated in lieu of or in conjunction with valve
 31i. Moreover, it will be appreciated that the present invention may be
 carried out with a wide variety of different components and arrangements.
 Mixing chamber 28 preferably has a larger volume than the delivery chamber
 27 in order to permit a sufficient amount of solvent to be injected into
 the vials to dissolve the powdered solute. The mixing chamber 28 includes
 a membrane 59 that separates the chamber into a section that contains
 liquid 51 and a section that contains a control gas 55 (preferably air).
 The section that contains liquid 51 may also contain some gas 52. The
 total volume (V.sub.TOTAL) contained by the mixing chamber 28 (i.e., the
 sum of the volumes of the liquid 51, the control gas 55 and the gas 52 on
 the liquid side of the membrane 59) is constant and is known.
 The pressure of the control gas 55 in the mixing chamber 28 may be adjusted
 by a pressure adjuster. One of several different means of adjusting the
 pressure may be used, some of which are discussed below in connection with
 FIGS. 6-8. The pressure adjuster is in fluid communication with the mixing
 chamber 28 through a mouth 10 and a valve 12. The valve 12 may be closed
 in order to isolate the control gas 55 in the mixing chamber 28 from the
 pressure effects of the pressure adjuster. A pressure transducer 49
 measures the pressure of the control gas 55 in the mixing chamber 28. A
 controller 62 receives pressure data from the transducer 49, as well as
 volume information from the acoustic volume-measurement system. The
 controller 62 also controls the pressure adjusters and the various valves
 12, 31a-31i, 32 and 33. The controller also calculates the volume of
 liquid in the mixing chamber 28 as described below.
 Like the mixing chamber, the delivery chamber 27 also has a membrane 60,
 which separates the chamber into a section that contains a liquid 53 and a
 section that contains a control gas 56. Since the presence of the gas
 bubble in the liquid section of the delivery chamber may in some instances
 affect the accuracy of the acoustic volume-measurement system, and since
 it is not desirable to deliver a gas bubble to the patient, any liquid
 that is detected to have a gas bubble in the delivery chamber 27 is urged
 back to either the mixing chamber 28 or the IV fluid source (item 22 in
 FIG. 1)--or to one of the sources of solute (such as the vials 11a-11b).
 The air-detection systems described in U.S. Pat. No. 5,349,852 or in U.S.
 Pat. No. 5,641,892 for an "Intravenous-Line-Air-Detection System" to
 Larkins et al. may be used to detect the presence of a gas bubble in the
 liquid 53 in the delivery chamber 27. The delivery chamber 27 also has a
 mouth 50, which leads to the acoustic volume-measurement system and a
 pressure adjuster. It will be appreciated that separate mouths may be used
 in the delivery chamber 27 for the pressure adjuster and the acoustic
 volume-measurement system.
 FIG. 3 is a flow chart of a preferred method for measuring the volume of
 liquid in the mixing chamber 28 of the system shown in FIG. 2. In step 1,
 the control gas 55 in the mixing chamber 28 is isolated. (FIGS. 4 and 5
 show the state of the FIG. 2 system during steps 2 and 3 respectively of a
 volume-measurement cycle represented by FIG. 3.) As shown in FIG. 4,
 valves 31h, 31i and 12 are closed in order to initially isolate the mixing
 chamber 28. After the mixing chamber 28 is isolated, a pressure reading
 (P.sub.INITIAL)from pressure transducer 49 is made (step 2). Once this
 pressure reading has been made, valve 31i is opened and a volume of
 liquid--referred to herein as a "reference volume" (V .sub.REF)--is
 exchanged between the mixing chamber 28 and the delivery chamber 27 (step
 3). As shown in FIG. 5, valve 31h remains closed during step 3 in order to
 prevent the gas 52 on the liquid side of the membrane 59 from escaping out
 of the mixing chamber 28. Valve 12 also remains closed during step 3 so
 that the control gas 55 remains isolated and is affected only by the
 change in volume of the liquid 51 in the mixing chamber 28.
 FIGS. 4 and 5 show the reference volume moving from the mixing chamber 28
 to the delivery chamber 27 in step 3. Some of the different techniques for
 causing the transfer of liquid between the two chambers are discussed
 below. The method for calculating the volume of liquid in the mixing
 chamber may also be accomplished by moving a volume of liquid from the
 delivery chamber 27 to the mixing chamber 28. However, when the reference
 volume is moved from the delivery chamber 27 to the mixing chamber 28, the
 reference volume (V.sub.REF) has a negative sign in the formulas set forth
 below.
 As shown in FIG. 5, valve 32, which controls flow from the delivery chamber
 27 to the patient, remains closed during step 3, as the reference volume
 of liquid is transferred between the mixing chamber 28 and the delivery
 chamber 27. Valves 31a and 31c-31g, shown in FIG. 1, also remain closed
 during step 3. Thus, the transfer of the reference volume from the mixing
 chamber 28 to the delivery chamber 27 increases the volume of liquid in
 the delivery chamber by the amount V.sub.REF.
 After the reference volume has been transferred between the delivery
 chamber 27 and the mixing chamber 28 (step 3), two measurements are made:
 (i) the acoustic volume-measurement system is used to measure the change
 in the volume of liquid in the delivery chamber 27 (step 4), and (ii) the
 pressure transducer 49 is used to measure the final pressure (P.sub.FINAL)
 in the mixing chamber 28 (step 5). (Steps 4 and 5 may be performed at the
 same time, or step 5 may be performed before step 4.) The change in the
 volume of liquid in the delivery chamber 27 is the reference volume
 (V.sub.REF).
 The pressure data (P.sub.INITIAL, P.sub.FINAL) and the volume data
 (V.sub.REF) are read by the controller. The controller can then calculate
 (step 6) the volume (V.sub.INITIAL) of liquid in the mixing chamber 28 at
 the beginning of this measurement cycle based on P.sub.INITIAL,
 P.sub.FINAL and V.sub.REF, as well as V.sub.TOTAL --the total volume of
 the mixing chamber (i.e., the total volume of gas in the mixing chamber
 when the mixing chamber is emptied of liquid).
 The formula for calculating the initial volume of liquid is:
EQU V.sub.INITIAL =V.sub.TOTAL -(V.sub.REF /((P.sub.INITIAL P.sub.FINAL)-1))
 (1)
 To determine the volume (V.sub.FINAL) of liquid at the end of the
 measurement cycle, an additional calculation may be made using the
 formula:
EQU V.sub.FINAL =V.sub.INITIAL -V.sub.REF (2)
 These formulas may be derived from the following three formulas, which
 relate the volumes of liquid and gas and the pressures in the mixing
 chamber:
EQU P.sub.INITIAL V.sub.GAS,INITIAL =P.sub.FINAL V.sub.GAS,FINAL (3)
EQU V.sub.REF =V.sub.GAS,FINAL -V.sub.GAS,INITIAL =V.sub.LIQUID,INITIAL
 -V.sub.LIQUID,FINAL (4)
EQU V.sub.GAS,INITIAL =V.sub.TOTAL -V.sub.LIQUID,INITIAL (5)
 V.sub.LIQUID,INITIAL and V.sub.LIQUID,FINAL are equivalent to V.sub.INITIAL
 and V.sub.FINAL in formulas (1) and (2). Combining equations (3) and (4)
 provides
EQU V.sub.REF =((P.sub.INITIAL
 V.sub.GAS,INITIAL)/P.sub.FINAL)-V.sub.GAS,INITIAL,
 which maybe manipulated and combined with equation (5) as follows
EQU V.sub.GAS,INITIAL =V.sub.REF /((P.sub.INITIAL /P.sub.FINAL)-1)=V.sub.TOTAL
 -V.sub.LIQUID,INITIAL,
 which in turn can be manipulated to obtain equation (1).
 A variety of different systems may be employed in order to cause the
 exchange of the reference volume between the delivery chamber 27 and the
 mixing chamber 28. Both the delivery chamber 27 and the mixing chamber 28
 preferably have a pressure adjuster, which adjusts the pressure of the
 control gas so as to tend to urge fluid into or out of the chamber. The
 pressure adjuster is used primarily to move liquid into or out of the
 chamber from or to another component of the system shown in FIG. 1. For
 instance, the pressure adjuster may be used in the mixing chamber 28 to
 cause liquid to be repeatedly squirted into and drawn out of one of the
 vials 11a-11c in order to dissolve thoroughly the powdered solute in the
 vial. Also, the pressure adjuster may be used in the delivery chamber 27,
 for example, to urge IV solution to the patient, or to draw liquid into
 the delivery chamber from the mixing chamber or from the IV fluid source,
 or to urge bubble-containing liquid out of the delivery chamber into the
 mixing chamber or to the vent. It may also be desirable at times to use
 the pressure adjuster in the delivery chamber to adjust the pressure, so
 that the control gas in the delivery chamber is at an optimum pressure for
 measuring volume changes with the acoustic volume-measurement system. The
 two chambers preferably may make use of the same pressure adjuster,
 wherein fluid communication between each chamber and the pressure adjuster
 is controlled by a valving system; however, separate pressure adjusters
 may be used.
 FIGS. 6-8 show a variety of pressure-adjusting systems that may be used on
 the delivery and the mixing chambers 27, 28. Although FIGS. 6-8 show the
 pressure-adjusting systems connected to the mouth 10 leading to the mixing
 chamber 28, similar pressure-adjusting systems may of course also be
 connected to the delivery chamber 27, or as noted above, the same
 pressure-adjusting system may be connected through valves to both
 chambers. FIG. 6 shows a preferred pressure-adjusting system, which is
 similar to the pressure-adjusting system shown in FIGS. 5-7 of U.S. Pat.
 No. 5,349,852. The FIG. 6 pressure-adjusting system includes a
 positive-pressure reservoir 18, which contains control gas under positive
 pressure, and a negative-pressure reservoir 19, which contains control gas
 at a partial vacuum. Fluid communication between these two reservoirs 18,
 19 and the mouth 10 leading to the mixing chamber (or the mouth leading to
 the delivery chamber) is controlled by valves 12, 14 and 16. (It will be
 appreciated that valve 12 may be eliminated from this system, unless the
 same pressure-adjusting system is used for both chambers.) In order to
 increase the pressure of the control gas in the mixing chamber (so as to
 force fluid out of the mixing chamber, for instance), valve 16 is kept
 closed while valves 12 and 14 are opened (preferably, partially) until the
 desired pressure is reached in the mixing chamber. Similarly, in order to
 decrease the pressure of the control gas in the mixing chamber (so as to
 draw fluid into the mixing chamber, for instance), valve 14 is kept closed
 while valves 12 and 16 are opened (preferably, partially) until the
 desired pressure is reached in the mixing chamber. As noted previously, in
 using this pressure-adjusting system for the delivery chamber, the
 pressure-adjusting system may pass control gas through the acoustic
 volume-measurement system, as shown in FIGS. 5-7 of U.S. Pat. No.
 5,349,852, or the pressure adjuster and the acoustic volume-measurement
 system may have separate mouths leading to the delivery chamber.
 In a preferred method for causing a reference volume of liquid to be urged
 from the mixing chamber to the delivery chamber, the control gas in the
 mixing chamber is pressurized--to a pressure greater than the pressure in
 the delivery chamber--before the mixing chamber is fully isolated.
 Referring to FIGS. 2 and 6, valves 31h and 31i, which control the flow
 into the liquid-side of the membrane 59, are closed, and valves 12 and 14
 are opened in order to increase the pressure of the control gas 55 in the
 mixing chamber 28 by exposing the mixing chamber to the positive-pressure
 source 18. Once the mixing chamber 28 is pressurized, valve 12 (or 14 or
 both) is dosed so as to isolate fully the control gas 55. Pressurizing the
 mixing chamber 28 has the advantage of reducing the effect that membrane
 59 may have on the pressure read by transducer 49. After the initial
 pressure (P.sub.INITIAL) is read, valve 31i is opened, and since the
 mixing chamber 28 is at a higher pressure than the delivery chamber 27,
 some liquid will flow from the mixing chamber to the delivery chamber. In
 one embodiment, liquid may be allowed to flow between the mixing chamber
 28 and the delivery chamber 27 until equilibrium is reached; when
 equilibrium is reached, the final pressure (P.sub.FINAL) may be read by
 the transducer 49, and change in volume (V.sub.REF) of liquid 53 in the
 delivery chamber may be measured by the acoustic volume-measurement
 system. In a preferred embodiment, however, it is sometimes desirable to
 adjust the pressure of the control gas 56 in the delivery chamber 27 in
 order to optimize the accuracy of the acoustic volume-measurement system.
 It is, thus, preferable to close valve 31i after a volume (V.sub.REF) has
 been transferred from the mixing chamber 28 and the delivery chamber 27
 and before the final pressure (P.sub.FINAL) is read; the pressure in the
 delivery chamber 27 may then be adjusted and the acoustic
 volume-measurement system employed to determine the reference volume. Note
 that valve 12 (or its equivalent, e.g., valves 14 and 16) is kept close
 from the initial pressure reading (P.sub.INITIAL) until the final pressure
 reading (P.sub.FINAL), so as to keep the mixing chamber's control gas
 isolated throughout the measurement cycle. Valves 31h and 32 (as well as
 valves 31a and 31c-31g, shown in FIG. 1) are also kept closed, so that the
 increase in volume of liquid in the delivery chamber is equal to the
 decrease in volume of liquid in the mixing chamber, thus permitting the
 accurate measurement of the reference volume (V.sub.REF).
 FIGS. 7 and 8 show alternative pressure-adjusting systems. The
 pressure-adjusting system of FIG. 7 includes a pump 13 that is capable of
 introducing control gas (which, as noted previously, is preferably air)
 into the chamber or removing control gas from the chamber and that can
 prevent flow through the pump so as to isolate the control gas in the
 chamber. The pressure-adjusting system of FIG. 8 includes a piston 15 that
 moves back and forth to change the pressure of the control gas in the
 chamber. The pressure-adjusting system of FIG. 8 may, for instance, be
 used to pressurize the control gas 55 of the mixing chamber 28 at the
 start of the measurement cycle. By moving to the left in FIG. 8, the
 piston 15, of course, pressurizes the control gas 55. By remaining in that
 position, the piston 15 isolates the control gas 55.
 The reference volume may also be transferred between the chambers by
 adjusting the pressure of the control gas 56 in the delivery chamber 27.
 Once the control gas 55 in the mixing chamber 28 is isolated and the
 initial pressure is taken, valve 31i is opened, and the pressure adjuster
 acting through mouth 50 may apply a negative pressure to draw the
 reference volume into the delivery chamber 27 or may apply a positive
 pressure to urge the reference volume into the mixing chamber 28.
 Alternatively, other means for transferring the reference volume between
 the chambers may be used; for instance, a peristaltic pump may be located
 between the chambers; or valve 31i may control gravity-induced flow from
 the upper to the lower chamber. Alternatively, a piston/cylinder system
 may be substituted for the delivery chamber in this volume-measurement
 system, wherein the piston acts directly on the liquid. In such a system,
 the reference volume is known from the displacement of the piston.
 Although the invention has been described with reference to several
 preferred embodiments, it will be understood by one of ordinary skill in
 the art that various modifications can be made without departing from the
 spirit and the scope of the invention, as set forth in the claims
 hereinbelow.