Patent Publication Number: US-4545535-A

Title: Liquid metering and dispensing apparatus

Description:
This application is a continuation-in-part of my co-pending U.S. patent application Ser. No. 243,377 filed Mar. 13, 1981 and entitled LIQUID METERING AND DISPENSING APPARATUS, now U.S. Pat. No. 4,406,406, issued Sept. 27, 1983. 
    
    
     The present invention relates to a novel and improved apparatus for spraying plants with precisely-measured amounts of liquids such as herbicides, insecticides, fungicides or the like. The invention relates particularly to a prepackaged, disposable liquid dispensing system capable of supplying lawns, gardens, crops or the like with a treatment fluid in micro-dispensing amounts whenever treatment of the plants is required. 
     For dispensing insecticides or the like on plants, the chemical material is usually supplied in liquid or powdered form and is highly concentrated so that it must be diluted with water for application. The mixing of the concentrate with water presents serious difficulties to a home owner who finds it necessary to treat his lawn or garden, since it is burdensome and time consuming to mix the proper amount of concentrate, and very often too much mixture is made up, so that a large excess is wasted and must be discarded. It also often happens that the concentrate spills upon the hands of the user, causing burning or irritation of the skin, and leaving on the user&#39;s hands a highly toxic and dangerous chemical. In the instance where large areas of crops are to be sprayed with insecticides and the like, the chemical is supplied in highly-concentrated bulk form which must be diluted with large quantities of water. Normally, unskilled attendants are placed in charge of caring for the crops, so that the chemical concentrates are often spilled and wasted, and invariably the chemical is diluted in improper amounts, resulting in underdispensing of the chemical or an overdispensing which tends to damage the crops. 
     Recent studies have shown the desirability of supplying grass, plants, and other vegetation with chemical treatment liquids in micro-dispensing amounts, that is to say, minute amounts of chemical per million parts of water. Not only do these micro-dispensing amounts supply the optimum quantity of chemical to the vegetation, but they permit plant treatment to be accomplished simultaneously with the normal watering of the plants, by mixing the treatment chemical with the water supplied to a spray or sprinkler system. 
     An object of the present invention is to provide apparatus for dispensing liquid insecticide or similar liquid chemicals and applying it in micro-dispensing amounts to garden plants, crops or other vegetation. The apparatus includes a disposable cartridge containing the liquid in concentrated form and a reusable container for the cartridge containing means for metering the liquid chemical within the cartridge. The container is attachable to a source of water under pressure and is connected to a spray or sprinkler apparatus, with the liquid chemical being mixed with the stream of water flowing through the container. 
     An important feature of the micro-dispensing apparatus resides in the manner in which the liquid chemical is dispensed from the cartridge in a constantly fixed ratio to the flow of water passing from the pressure source to the sprinkler apparatus. To maintain this fixed flow ratio, the reusable container is provided with a metering head which is constructed to operate as a cavitating venturi to provide a high velocity cavitational area in the stream of water, thereby producing a vapor pressure of the water as a downstream bias for both the chemical liquid flow and the water flow. With common upstream pressure and common outlet pressure, the pressure differential between the chemical liquid flow and the water flow will always remain constant. The positive ratio control achieved by such cavitation produces a precise and accurate microdilution and achieves a fixed downstream pressure to assure accurate metering and proportioning of liquid chemical to main water flow at various inlet pressures. 
     U.S. Pat. Nos. 2,571,424 issued Oct. 16, 1951; 2,153,240 issued Apr. 4, 1939; 3,155,113 issued Nov. 3, 1964; and 3,720,230 issued Mar. 13, 1973 show spray devices of various constructions for feeding liquid concentrates in proportionate amounts to a stream of flowing water, but none of these devices are adaptable for micro-dispensing of liquid concentrate for plant treatment, and none provides a precise pre-selected mixing ratio of the liquid concentrate regardless of the pressure of the water stream provided. 
     Another object of the invention is to provide apparatus for dispensing liquid insecticides or similar liquid concentrates which is capable of automatically mixing with flowing water any desired liquid at a ratio of minute parts per million and within a range, for example, between 200 parts of liquid concentrate per million parts of water, and 4,000 parts per million (PPM). The apparatus is capable of feeding liquid at the same PPM ratio throughout the life of the contained disposable cartridge, and regardless of variations in the water pressure employed or of water pressure changes occuring during operation of the apparatus. 
     Another object of the invention is the provision of apparatus of the type described which is capable of receiving prepackaged disposable cartridges containing liquid concentrate, which cartridges are capable of being re-capped after partial use, so that they may be stored for subsequent re-use. The apparatus is adapted to receive cartridges of different size and content volume so that the same dispenser apparatus may be used to dispense one pint of liquid concentrate or more, or as little as one ounce of concentrate. The cartridge itself is constructed to dispense its contained liquid precisely at a pedetermined PPM ratio, and in such a manner that the selected PPM ratio cannot be adjusted or accidentally varied by the user. 
     Still another object of the invention is the provision of apparatus of the character described which may be used either with a hand-held nozzle, attached to a sprinkler system for lawns and gardens, or incorporated within a central watering system for crops or the like. 
     Another object of the invention is the provision of apparatus of the type described in which the disposable cartridge is prepackaged with liquid concentrate at the manufacturing plant and may be easily inserted into and removed from the dispenser portion of the apparatus without the user contacting the liquid concentrate contained within the cartridge. 
     A further object of the invention is the provision of apparatus of the character described in which the reusable dispenser portion of the apparatus is composed of a small number of parts and may be manufactured economically at such a reasonable cost that the apparatus is well within the purchasing range of the average home owner. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided liquid concentrate diluting and feeding apparatus comprising a dispenser member and a pressure-sensitive container for the liquid concentrate removably mounted on said dispenser member and having a rigid nozzle provided with an outlet orifice. The dispenser member comprises a metering head with hollow casing removably mounted thereon. The metering head has a water inlet opening adapted to be connected to a source of water, a water outlet opening adapted to be connected to liquid discharge means, and a water conduit interconnecting said water inlet and outlet openings and comprising a through bore having an upstream end communicating with said inlet opening, a downstream end communicating with said outlet opening, and a mixing area between said ends. 
     The metering head also has an auxiliary fluid conduit communicating with the upstream end of said through bore and with the interior of said hollow casing, an opening communicating with the interior of the mixing area of said water conduit, and means for mounting said pressure-sensitive container on said metering head with said nozzle orifice communicating with said mixing area through said opening and with said pressure-sensitive container suspended within said hollow casing. 
     In use, water under pressure from the water source flows in a first path through the water conduit and through the mixing area thereof to the water outlet opening, and in a second path to the interior of the hollow casing, thereby filling the latter and applying head pressure to the pressure-sensitive container to feed the contained liquid concentrate therein through the outlet orifice into said mixing area. The outlet orifice in the rigid nozzle is precisely dimensioned with a diameter sufficiently small to dispense liquid concentrate to mix with the stream of water flowing in said fist path through the mixing area in a precise pre-selected micro-dispensing amount of parts of concentrate per million parts of water supplied by said water source. 
     The through bore is formed with a restricted passageway of reduced diameter upstream of the mixing area and proportioned to operate as a cavitating venturi to issue a high velocity stream of water from the restricted passageway, with a vena contracta surrounded by a low pressure vapor space. The restricted passageway is so located relative to the mixing area that said low pressure vapor space extends over the nozzle outlet orifice with the latter discharging into said vapor space, and providing a fixed flow ratio between the water main head pressure and the water vapor pressure for both the water flow and liquid concentrate flow, regardless of variations in the pressure of the water supplied by the water source. 
     Additional objects and advantages of the invention will become apparent during the course of the following specification when taken in connection with the accompanying drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a metering and dispensing assembly made in accordance with the present invention, showing the assembly connected to a water faucet; 
     FIG. 2 is an enlarged sectional view of the dispensing member of the assembly, and of a portion of the disposable cartridge attached thereto; 
     FIG. 3 is an enlarged fragmentary sectional view of the mixing chamber portion of the dispensing member shown in FIG. 2; 
     FIG. 4 is a sectional view of the disposable cartridge shown in FIG. 1; 
     FIG. 5 is a partial sectional view of the cartridge, similar to FIG. 4, but showing the cartridge nozzle portion in exploded condition; 
     FIG. 6 is a sectional view of another embodiment of disposable cartridge which may be used in the assembly of the invention; 
     FIG. 7 is a partial sectional view of the cartridge of FIG. 6 showing the nozzle portion in exploded condition; 
     FIG. 8 is an enlarged sectional view similar to FIG. 2, showing the flow characteristics of the stream of water passing through the dispensing member and the vapor space surrounding the stream; and 
     FIG. 9 is an enlarged sectional view similar to FIG. 8, but illustrating an alternate embodiment in which the water flow conduit of the dispensing member is formed as a cavitating venturi. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring in detail to the drawings, and in particular to FIG. 1, there is illustrated a fluid dispensing assembly made in accordance with the present invention, and generally comprising a dispenser member 10 to which a disposable cartridge 12 is removably attached. The dispenser member is illustrated as a manually-operable hand-held sprinkler device and is attached to the outlet end of a garden hose 11 connected to a supply of water under pressure, for example, a water tap or faucet 14. 
     The fluid dispensing assembly is adapted to spray plants, trees and other forms of growing vegitation with beneficial treatment materials in liquid form, for example, a fertilizer, insecticide, or the like, diluted with water in exact proportions. The insecticide or other treatment material is supplied in concentrated form within the disposable cartridge 12, and when the latter is attached to dispenser member 10 and the faucet is opened to cause a stream of water to flow through the dispenser member, the insecticide contained in the cartridge is injected into the water stream in precise constant amounts, in a manner to be presently described. 
     The dispenser member 10 includes a fluid metering head 16 in the form of a rectangular block which is preferably made of a rigid plastic. The head has a longitudinal main bore 18 extending centrally therethrough and constituting a conduit for the flow of water from the faucet 14 and hose 11. The bore 18 is of a non-uniform diameter which provides a high pressure side 20 and a low pressure side 22, with an intermediate mixing chamber 32 of smaller diameter at said low pressure side. A first auxiliary fluid conduit in the form of a bore 24 extends perpendicularly to the main bore 18 and communicates therewith at the high pressure side thereof. A second auxiliary fluid conduit in the form of a bore 26 is also perpendicular to main bore 18 and communicates with the latter at the low pressure side thereof. The bore 26 is sized and shaped to receive and mount the disposable cartridge 12, as will be presently described. 
     The disposable cartridge 12 comprises a pressure-sensitive container 28, which, in the embodiment shown in FIGS. 1-4, is in the form of a piston cylinder containing a liquid chemical. The cylindrical container is preferably made of a transparent rigid plastic, and has at one end a plug or nozzle 30 which is externally threaded as shown at 31 for removable reception within the bore 26 which is internally threaded. The nozzle 30 has a frusto-conical end portion, in the tip of which is a small orifice 40. When the container 28 is mounted in operative position, as shown in FIG. 2, the orifice 40 communicates with the interior of the main bore 26 for injecting the liquid content of the container 28 into the stream of water flowing through the main bore 26. At its bottom end, the container 28 is formed with a large inlet opening 41, and a slidable piston 38 overlies this inlet opening 41. 
     The cartridge 12 is supplied as a disposable unit and contains a liquid chemical to be dispensed. The liquid chemical is preferably an insecticide or fungicide, but may also be a fertilizer or other liquid for treating growing plants. 
     The dispenser member 10 also includes an enclosed cylindrical pressure casing or jar 34 having a threaded top end 35. The fluid metering head 16 is formed with an integral depending annular flange 37 which is internally threaded for removably receiving the threaded top end of the pressure casing 34. When the pressure casing 34 is screwed into mounted position on the flange 37, it surrounds and encloses the cartridge 12 and also communicates with the auxiliary fluid conduit 24, as shown in FIG. 2. 
     When the dispensing assembly is to be used, the tap 14 is opened so that water under pressure flows through the hose 11 to the metering head 16. Water under pressure thus enters the bore 18 and flows from the high pressure side 20 to the low pressure side 22 via the mixing chamber 32. Upon entering the bore 18, a portion of the water stream is diverted into the interior 36 of the pressure casing 34 by way of the first auxiliary fluid conduit 24 which constitutes a water duct, so that water fills the interior of the pressure casing 34, surrounds the cartridge 12, and enters the lower portion of the pressure sensitive container 28 thereof through the inlet opening 41. As will be appreciated, water under pressure flows from the fluid conduit 24 into the entire interior of pressure casing 34, and exerts an upwardly-directed fluid pressure upon the piston 38 of the piston cylinder 28 so that the pressure of the liquid chemical within the cylinder 28 is substantially identical to the surrounding pressure in the interior of casing 34, and thus equal to the pressure in the high pressure side of the main bore 18. 
     The fluid pressure exerted on the chemical within the cylinder 28 causes the chemical to be discharged therefrom via the orifice 40 in nozzle 30, which, in mounted position, is located within the second fluid passage 26, the latter constituting a chemical duct. Orifice 40 of discharge nozzle 30 communicates with the interior of the mixing chamber 32 and injection of the liquid chemical into the latter is at a predetermined rate depending upon the size of the orifice 40, as will presently be described. 
     Liquid flowing from the high pressure side 18 to the low pressure side 22 follows a flow path through the mixing chamber 32 where the contents of the cylinder 28 are admixed with the water passing through the chamber. In this manner, liquid chemical is forced from the piston cylinder 28 due to the differential pressure described, and is picked up by the water which passes through the axial bore 18. The liquid chemical, subsequently, is admixed and dissolved within the water and discharged from the metering head 16 via a spray nozzle 42. A manually-operable and trigger-actuated on/off valve mechanism, designated generally by the reference numeral 44, allows water to be supplied to the metering head 16. The actuating mechanism is seen to comprise the usual valve trigger 46 and a hand-gripping portion 48 having a configuration similar to a gun butt. A fitting 50 is provided to detachably connect the actuating mechanism 44 to the metering head 16. 
     To aid in the formation of a high and low pressure section in the main bore 18, a thick flow restrictor plate 52 is provided at the upstream end 54 of the bore 18, which arrangement is shown in detail in FIG. 2. The restrictor 52 is adapted to form a resistance to flow in the bore 18 and causes a substantial pressure drop between the pressure on the upstream side of the restrictor and the pressure on the down stream side toward bore end 56. The pressure at the upstream side of the restrictor 52 is referred to herein as the &#34;higher pressure&#34; and the pressure at the downstream side as the &#34;lower pressure&#34;. The flow restrictor 52 is designed to allow quantity of flow per unit time to flow therethrough at a rate proportional to the pressure differential across the restrictor. The restrictor may be of any desired design to allow such proportional flow but in its preferred form is an annular disc of a suitable material as, for instance, a plastic having a central aperture 58. The restrictor 52 may, if desired, be anchored against longitudinal movement along bore 18 in the downstream direction by any desired means such as by a shoulder 60 in the bore 18. 
     As shown in FIG. 3, between the restrictor 52 and the mixing chamber 32, is a helical flow path 62 for the water passing from the aperture 58 of the restrictor 52 toward the low pressure side of the bore 18. The flow path 62 is formed by an outer substantially cylindrical shell 64 having one or more helical grooves 66 formed in its inner periphery. The grooves 66 extend like threading from the top to bottom of shell 64 and have a predetermined cross section. Adapted to fit in the chamber defined within the shell 64 is a block 68. The block 68 is shaped to abut the inner periphery of shell 64 with a fit which is sufficiently tight to avoid any leakage between block 68 and shell 64, except for the passage provided by the helical grooves 66. One end 70 of the groove 64 is aligned with the aperture 58 of the restrictor 52. The second opposite end 72 terminates at the opening 74 through which the flow path 62 communicates with the mixing chamber 32. In this manner, the water flowing along the helical groove or grooves are forced to follow a substantially sinuous passage which, upon exiting from the flow path 62, creates whirling and turbulence of the water in mixing chamber 32. This turbulence has been found to facilitate the admixing of the liquid chemical and water supplied to the mixing chamber 32. In addition, the whirling or turbulent water provides a back pressure within the mixing chamber 32 to ensure that the quantity of liquid chemical flow into the chamber 32 is kept constant and is maintained at the desired proportionate ratio. 
     At the discharge or downstream end 56, the bore 18 is formed with a flow restriction in the form of aperture 76. The diameter of aperture 76 is identical to that of aperture 58 in restrictor 52 and, likewise, its flow output is proportional to that of aperture 58. The bore end 56 is internally threaded at 78, FIG. 2, to receive the spray nozzle 42. 
     It will be apparent that the orifice 58 in the restrictor 52, with its downstream side submerged in water, is in effect a cavitating orifice which provides a classical cavitating venturi effect. This produces a low pressure area surrounding the high velocity central liquid core which is bounded by the vena contracta of the stream, as will be explained in greater detail hereafter. 
     The main bore 18, at its inlet end 80 at the high pressure side 16, is internally threaded to receive the fitting 50 of the water-actuating mechanism 44. Further provided in this bore end is a compressible O-ring 82, or similar sealing element to position the mechanism in the bore in a leak-proof manner. 
     As is clearly shown in FIGS. 2, 4 and 5, the nozzle or plug 30 which is screw-mounted within the chemical duct 26, fits snugly and precisely within the defining surfaces of this duct. The socket or duct 26 has an inner configuration identical to the outer configuration of the nozzle 30. The nozzle 30 is formed with upper tapering walls 84, externally threaded side walls 86, and a base in the form of a shoulder flange 88. The chemical discharge orifice 40 extends through the tapering walls 84 and in the arrangements shown, communicates with the interior of the mixing chamber 32 and with the interior of the pressure-sensitive container 28. The nozzle 30 is threadably received or screw mounted in the bore which defines the chemical duct 26. To mount the nozzle 30 in a leak-proof manner, an O-ring 90 is provided and compressed between the shoulder 88 of the nozzle and the lower housing surface 92 of the metering head 16. 
     The base or shoulder flange 88 of nozzle 30 is provided with external screw threading 94 which is sized to fit internal threading 96 bordering a central opening 98 in the top wall 100 of the pressure-sensitive container 28. In assembly of the disposable cartridge 12 as a unitary structure, the nozzle 30 is sealingly secured to the container 28 by means of an O-ring 102 which is seated within the opening 98. When the nozzle 30 is screwed tightly into mounted position, the O-ring 102 is compressed between the under surface 104 of shoulder flange 88 and a shoulder 106 underlying the opening 98. 
     The piston 38, which affords compression of the liquid chemical contained in cylindrical container 28, is of the usual type and is provided with conventional compression rings 107 (FIG. 4). 
     FIGS. 6 and 7 show a second embodiment of the invention in which the disposable rigid piston-cylinder 28 is replaced by a pressure-sensitive container in the form of a compressible bag 108 made of a thin, pliable material such as, for instance, rubber or a plastic material. The compressible bag 108 is positioned in upright position within an outer container 28a. The container 28a is identical to the container 28 shown in FIGS. 1 and 3, with the exception that water pressure exerted on the piston 38 is, in the arrangement shown, applied to the collapsible bag 108. For this purpose, water entering the interior 36 of pressure casing 34 via the water duct 24 flows under pressure into the cylindrical container 28a by way of an opening 110 in lower end 112 of receptacle 28a, FIG. 6. The bag 108 is placed under high pressure from the high pressure side 20 of the bore 18 and, as a result, the liquid chemical contained within the compressible bag 108 is deformed until the pressures of the chemical and water are substantially equal. At this time, the liquid chemical is discharged from the bag 108 via orifice 40 in nozzle 30. The chemical discharge, as in the first embodiment, is in the mixing chamber 32 where the chemical liquid is picked up and admixed with the water flowing through the main bore 18. 
     The manner in which the compressible bag 108 is detachably secured to the nozzle 30a is as follows. The bag 108 at its upper end 114, is initially extended through the opening 98a in the upper wall 100a of the container 28a. Subsequently, an annular sleeve member 116 is inserted through the opening 98a and in a manner such that the upper flange portion 118 of the sleeve 116 is seated in abutting engagement against the inwardly directed shoulder 106a formed in the bore 98a in the upper wall 100a of the container 28a. In this condition, the neck portion of the sleeve 116 extends downwardly into the bag 108 while the upper bag portion 114 is sandwiched between the sleeve flange 118 and the shoulder 106a within the bore 98a, FIG. 6. Presently, an O-ring 120, or similar sealing element, is positioned onto the flange portion 118. Finally, the container 28a is threadably secured to the shoulder flange 88 of the nozzle 30. The second O-ring 90 or similar sealing element is positioned and sandwiched between the shoulder flange 88 of the nozzle 30a and the under surface 92 of the metering head 16. 
     Once the pressure-sensitive container 28 of the first embodiment or the collapsible and compressible bag 108 of the second embodiment are detachably secured to the nozzle 30, the pressure casing 34 is detachably connected to the lower surface 92 of the metering head 16 by means of the internally-threaded depending annular flange 37, the inner diameter of which is slightly larger than the outer diameter of upper threaded end 122 of the casing 34 so as to provide a snug fit therewith. To provide a leak-proof sealing condition between the pressure casing 34 and the lower surface 92 of the metering head 16, an O-ring 124 is seated between the top end of the casing 34 and the bottom wall 92 of metering head 16. 
     In use of the fluid dispensing device, the user need only remove the casing or receptacle 34 from the metering head 16 and mount on the latter a chemical-filled disposable cartridge 12 by screwing the cartridge nozzle 30 into the threaded fluid passage 26 of the metering head. The cartridge may be either the piston-type shown in FIG. 4 or the collapsible bag type shown in FIG. 6. After the cartridge is mounted, the casing 34 is replaced on the metering head by means of its screw threading, the attached water tap 14 is turned on, and the trigger 46 is depressed to cause the flow of water from the tap to enter the metering head. The entering water separates into two paths of flow, one portion thereof passing through the water duct 24 to fill the interior of the receptacle 34, and the remaining portion passing through the main bore 18 and the chemical mixing chamber 32 to the nozzle 42. 
     As previously indicated, the high pressure water flow eminating from the high pressure side 20 of the main bore 18, and flowing into the receptacle 34 via the water duct 24, instantaneously exerts a pressure upon the liquid chemical contained either within the piston cylinder 28 or within the compressible bag 108. The pressure applied to the piston 38 (FIGS. 1 and 4), or to the compressible bag 108 (FIG. 6), by the water flow at the high pressure region 20 of bore 18 is greater than the pressure at the low pressure region 22, so that the liquid chemical in container 28 or in bag 108 is ejected in a steady, regulated flow from the cartridge nozzle orifice 40 into the mixing chamber 32 where it is thoroughly mixed with the swirling water passing through the mixing chamber and the mixture is then discharged through the metering head nozzle 42. This arrangement, together with the cavitating venturi effect within water duct 24, provides precise metering of the liquid chemical, the latter being dispensed in a quantity and at a ratio which is exactly proportional to the amount of water which is flowing through the metering head main bore 18. The mixing ratio is determined solely by the size of the orifice 40 in the nozzle 30 of the disposable cartridge 12, and is always constant, regardless of the pressure of the water flowing from tap 14. Thus, while the water pressure may vary from time to time, the ratio of the chemical liquid dispensed to the volume of water flowing through the mixing chamber 32, is always precisely the same. 
     As indicated previously, the internal structure of the fluid metering head 16 is such that the orifice 58 in the restrictor 52 serves as a cavitating venturi which operates to maintain a constant ratio of the amount of water flowing through the metering head main bore 18 to the amount of liquid chemical dispensed from the receptacle 34 through cartridge nozzle orifice 40, so that the liquid chemical is metered in an amount which is exactly proportional to the amount of water flowing through the main bore 18 and is always constant regardless of the pressure of the water flowing from the tap 14. 
     The construction and principle of operation of a cavitating venturi are well-known and abundantly described in available literature, for example in the article entitled &#34;Rocket Applications of the Cavitating Venturi&#34; by L. N. Randall, at pages 28-31 of the ARS Journal, January-February, 1952. The principle of operation of a cavitating venturi has been expressed as follows: As the pressure drop across a conventional Venturi is increased, a point is reached at the throat at which substantially all of the upstream head is converted into velocity head. The only static head remaining is that of the fluid vapor pressure. If, under these conditions, the upstream head is maintained constant, a further increase of the pressure drop obtained by decreasing the downstream pressure cannot result in increased flow. This unique characteristic of a cavitating venturi has been used advantageously in liquid propellant rocket flow control. 
     As explained in the foregoing article, it has been found that thick-plate orifices, although not having the conventional venturi configuration, are, in fact capable of operating as cavitating venturis, even when having sharp entrances. The aperture 58 in the flow restrictor 52 constitutes such a thick-plate orifice and cooperates with the helical flow path 62 and mixing chamber 32 to provide a medium-grade cavitating venturi. The high velocity stream issuing as the vena contracta from the downstream side of aperture 58 is surrounded by a low pressure vapor area, and as regain occurs in the enlarged mixing chamber 32, the local head is increased, reabsorbing the vapor as the fluid slows down. 
     FIG. 8 illustrates the flow of water through the aperture 58 in the flow restrictor 52 which acts as the thick-plate cavitating venturi. In this view, the constricted cavitating region or vena contracta of the water is designated by reference numeral 200, and the vapor space surrounding this region is designated 202. This vapor is a shroud surrounding a solid jet of liquid moving at high velocity in the cavitating region 200. P 1  designates the head pressure of the water in the upstream end 54 of the bore 18 as the water is received from the tap 14. P 2  designates the water pressure in the region of the throat or vena contracta, P 3  designates the downstream submergence or region pressure, and P 4  designates the external ambient pressure. Since any small orifice with its downstream side submerged in a pressurized fluid can cavitate, the flow through the orifice 58 is a linear function of the square root of P 1  minus P 2  rather than the square root of P 1  minus P 3 . 
     At the throat or vena contracta 200, the water reaches its maximum velocity and minimum pressure P 2 . The aperture 58 is made sufficiently small so that under conventional water main pressures, the water emitted from the downstream side of aperture 58 has been accellerated to such a high velocity that the water cavitates, achieving a change of state from liquid to gas, and P 2  becomes equal to the vapor pressure of the water. The stepped configuration of the conduits 61, 32 and 56 approximate the standard configuration of a cavitating venturi. 
     Since the helical flow path 62 is immediately adjacent to the downstream side of the inlet aperture 58, the vena contracta and the vapor zone surrounding the high velocity fluid stream emerging from the aperture 58 result in a low pressure cavitational area within the mixing chamber 32. This low pressure area is at the vapor pressure of the inlet water stream, approximately 12.79 m.m. of mercury (0.25 psia@15° C.). The physical design of the stepped chambers 62 and 32 is such that the flared portion of the vena contracta 200 overlies the orifice 40 of nozzle 30, and the latter discharges into the vapor 202 which is at the vapor pressure of water. It will thus be seen that the flow through the nozzle orifice 40 is controlled by the differential between the water main head pressure P 1  and the water vapor pressure P 2 . Similarly, the flow through the restrictor aperture 58 is controlled by the differential between the same pressures. Thus, since the pressure governing both flows is biased against the constant P 2  vapor pressure of the water, a fixed downstream pressure is provided to assure accurate metering of chemical concentrate to water flow at various inlet pressures. The maintenance of this flow ratio is therefore fixed and constant and will not vary, regardless of changes in the head pressure. 
     The provision of the helical flow path 62 immediately downstream of the restrictor aperture 58 assures the breakaway of water vapor from the high velocity vena contracta 200 existing at this point, thus prolonging the local cavitation which takes place in this area. This assures the existance of a uniformly low pressure in this area, maintaining the common variable pressure differential across both chemical and diluent flow orifices for a full range of operating flow rates. 
     Assuming that the ratio of flows through orifices 40 and 58 is exactly equal to the orifice areas (disregarding minor variations in orifice coefficients of discharge), then this ratio will be equal to the ratio of the squares of the respective orifice diameters. If, for example, the diameter of aperture 58 is 0.125 inch and the diameter of orifice 40 is 0.005 inch, then the ratio of areas and the ratio of flows will be 
     
         R=(0.125.sup.2 /0.005.sup.2)=625 to 1 
    
     and the resulting dilution from a 10% (100,000 PPM) chemical solution will be 0.000160 or 160.0 PPM. It is assumed that the flow through orifice 40 has a coefficient of discharge (C D ) of 0.63, equal to that of the aperture 58. In the event the specific gravity of the chemical concentrate is other than 1.00, the value of the specific gravity assumed for water, the flow of the chemical through orifice 40 can be tailored upwardly from the assumed coefficient of discharge of 0.63 by use of a conical entry or a rounded approach. 
     While the aperture 58 in restrictor 52, and the outlet aperture 76 are shown as of approximately the same size, and are located so that they must pass identical flow rates, these facts would imply identical pressure drops across both apertures. This, however, is far from the fact. Aperture 58, having a diameter of 0.125 inch, has a sharp edge which provides a C D  approximately equal to 0.63. On the other hand, the outlet aperture 76, of the same diameter, employs a conical approach which provides a C D  approximately equal to 0.83. 
     FIG. 9 shows a modified embodiment of metering head 10a in which the metering orifice 58 is replaced by a true cavitating venturi 204, resulting in its coefficient of discharge being improved to approximately 0.045 from 0.63. The cavitating venturi 204 has a conical upstream approach section 206 defining an angle of approximately 75° and merging with a smooth blend into a throat portion 208 having a length of approximately 0.125 inches. The throat section 208 in turn merges into a conical regain section 210 having an included angle of approximately 24°. To maintain the same flow as the embodiment of FIG. 8 the orifice area is reduced in the same ratio as the increase in orifice coefficients of discharge, that is the diameter of the venturi throat 208 is reduced to 0.102 inch from the 0.125 inch diameter of aperture 58. The cavitating region or vena contracta 200 of the water flow is elongated in this embodiment and the orifice 40 of nozzle 30 communicates with a large vapor pressure area 202 which is at the vapor pressure of water. 
     The true cavitating venturi 204 will remain in cavitation as long as the downstream backpressure P 3  does not exceed approximately 85-90% of the upstream pressure P 1 . The outlet orifice 76a of the spray nozzle 42a is therefore formed with a high C D  rounded approach and is provided with such a diameter that it will pass the same flow as the venturi 204 with a pressure drop of less than (0.85×P 1 ). Since the nozzle orifice 76a must meet the flow condition with a lower differential pressure, it is obvious that the nozzle diameter must be larger than the venturi throat diameter. It is also obvious that the coefficient of discharge of the nozzle orifice 76a can never be as high as that of the venturi 204 and the nozzle operating differential pressure when discharging to atmosphere must always be of a lower value than the venturi operating differential pressure. Therefore the nozzle design is such that its pressure (P 3  -P 4 ) is limited to 0.85 (P 1  -P vapor) to assure venturi cavitation at all times. 
     The embodiment of FIG. 9 has the advantage of assured cavitation over a larger range of operating pressures, as well as providing high velocity stream impingement mixing so that a mixer structure such as the helical flow path 62 of FIG. 8 is not required for mixing. 
     The dispenser apparatus of the present invention is particularly adapted to dispense a liquid-chemical mixture in a &#34;micro-dispensing&#34; proportion, that is in a ratio of small unit parts of liquid chemical per million parts of water. Thus, for some micro-dispensing applications, it may be essential to provide a mixture consisting of 200 parts of liquid chemical per million parts of water. In this case, the orifice 40 in the cartridge nozzle 30 is precisely dimensioned with a suitable small diameter. In other applications, a high ratio of, for example, 2000 p.p.m. (parts per million) of liquid chemical may be required for the mixture, in which case the nozzle of the cartridge is provided with a corresponding larger diameter outlet orifice. Since the cartridges are replaced and disposable, they may be made commercially available with a variety of different size nozzle orifices, so that the manufacturer may select the proper cartridge orifice in relation to the parts-per-million dispensing rate required by the chemical. 
     It will be appreciated that, after a spraying operation where liquid chemical still remains in the cartridge 12, the partially-filled cartridge may be removed from the metering head 16 and stored at a remote location for future use. For this purpose, the cartridge may be supplied with a screw cap (not shown) which is removably attachable to the threaded cartridge nozzle 30 to cover over the same and prevent spilling or evaporation of the contained liquid chemical. It will also be appreciated that the liquid chemical is confined within the interior of the cartridge 12, so that the latter may be freely handled by the user in insertion, removal or replacement thereof, without danger of the contained chemical contacting the user&#39;s hands. This is of particular importance where the contained concentrated chemical is toxic, caustic or the like. 
     The pressure casing 34 is preferably in the form of a jar made of glass or transparent plastic. The cartridge outer container 28, 28a is also made of transparent material, preferably plastic. Thus, the interior of the container 28 may be viewed by the user through the transparent walls, to check periodically upon the amount of liquid chemical remaining, and the cartridge may be replaced immediately when its contained chemical becomes depleted. 
     In the embodiment shown herein, the fluid dispensing assembly is illustrated as being of the hand-held, manually-operable sprinkler type. It is to be understood, however, that the assembly may be incorporated in an automatic sprinkler system, in which case the metering head 16, instead of having the built-in nozzle 42, would be connected by a hose to a remote sprinkler. The dispensing assembly could also be incorporated in a commercial sprinkler system of the type used to spray crops, and consisting of large numbers of sprinklers arranged in rows. In either instance, the micro-dispensing chemical proportioning and mixing capabilities of the dispensing assembly will be as effective as that heretofore described. 
     While preferred embodiments of the invention have been shown and described, it is obvious that numerous omissions, changes and additions may be made in such embodiments without departing from the spirit and scope of the invention.