Patent Publication Number: US-6340431-B2

Title: SPA chemistry monitor and treatment unit

Description:
This is a continuation of copending U.S. Ser. No. 09/179,908, filed Oct. 27, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to devices and methods for monitoring and regulating the water chemistry in a spa or hot tub or the like, to maintain the water quality at a clean, clear, and substantially sanitary condition. More specifically, this invention relates to a self-contained and free floating monitor unit for checking selected water chemistry parameters at programmable intervals, and for automatically responding to the monitored parameters by delivering one or more chemical additives to the water to maintain the spa water in a clean and hygienic state. 
     Therapeutic spas and hot tubs and the like are generally known in the art, and typically comprise an upwardly open structure adapted to contain a selected volume of water. Control means are normally provided for heating the water, and for circulating the water through air-water hydrotherapeutic massage jets. One or more persons can sit within the spa, partially immersed within the heated water, in positions so that the spa jets can be directed against the body to provide a therapeutic massage action. To maintain the spa water in a clean and sanitary condition, the circulating water is normally passed through a filter which removes and collects particulate matter. In addition, selected chemical agents, such as chlorine are periodically added to the spa water in prescribed amounts suitable for preventing growth of bacterial organisms, to maintain the water in a hygienic state. Other chemical agents such as a sanitizer, e.g., an oxidizer such as bromine, are also periodically added to the water. 
     In the past, periodic manual testing of the spa water has been required to determine the actual concentration of chemical agents therein, in order to determine whether one or more chemical agents should be added to the spa water to maintain a desired sanitary condition. In this regard, the pH level (a logarithmic function of hydrogen ion concentration) is commonly checked by the spa owner or by maintenance personnel at regular intervals, and chemical agents such as a selected base or acid are added to the water in appropriate amounts for respectively adjusting the pH level up or down as may be required to maintain the pH reading within a typical desired range of about 7.2-7.8. In addition, oxidation reduction potential (ORP) is also checked regularly, and a selected chemical sanitizer such as bromine is added to the water in an appropriate amount to maintain the ORP reading within a typical desired range of about 650-750 millivolts when the pH reading is within the above-stated desired range. Regular monitoring of the water chemistry, and regular addition of these chemical agents in the appropriate amounts, is essential to maintain the spa water in a clean and sanitary condition. However, manually conducted water chemistry testing and manual addition of the chemical agents has been highly subject to time interval variations and measurement inconsistencies which can have an adverse impact upon water quality. 
     For some swimming pool and spa installations, devices and systems have been developed for unattended delivery of certain chemical agents to the water over an extended period of time. For example, floating dispensers have been widely used in swimming pool and spa applications, wherein a quantity of a chemical such as chlorine or bromine is carried by a buoyant housing which has a variably opened gate for controlled release of the chemical on a substantially continuous basis and for an extended period of time. In such dispensers, the floating housing typically carries a supply of chemical-containing tablets in solid form, designed for rate-controlled dissolution according to the degree to which the gate is opened. While such dispensers beneficially maintain the chemical concentration, the delivery rate is still predicated upon periodic manual water chemistry testing and corresponding periodic gate adjustment to regulate the chemical delivery rate. Moreover, such floating dispensers have not been designed to accommodate delivery of other chemical agents, such as acids or the like when a reduction in pH level is indicated, or a sanitizer or oxidizer for regulating and maintaining the ORP level within a prescribed reading range. The need for and the addition of such other chemical agents has still relied upon periodic manual water chemistry testing and periodic manual addition of the chemical agents in selected amounts to the pool or spa water. 
     In other swimming pool and spa installations, substantially automated systems have been designed for integration in-line into the water filtration and circulation equipment for automatically analyzing the water chemistry at periodic intervals, and for responding to the chemistry readings to automatically add chemical agents to the water in prescribed amounts. See, for example, the automated pool water chemistry marketed by Polaris Pool Systems of San Marcos, Calif. under the product designation Watermatic. See also U.S. Pat. No. 5,019,250. Such automated systems, however, are relatively complex and are thus relatively costly, particularly with respect to the requisite plumbing modifications for installation into the filtration system of a preexisting pool or spa. Moreover, the complexity and resultant cost of such automated in-line chemical dispensers has been a significant deterrent to use thereof in stand-alone spas which do not share a water filtration system with an associated swimming pool. Accordingly, such automated chemical dispenser systems have not been widely used. 
     The present invention overcomes these problems and disadvantages by providing a compact and self-contained floater device for automatically analyzing spa water chemistry at regular programmable intervals, and for automatically responding to the water chemistry readings to add one or more chemical agents in appropriate amounts to maintain the spa water in clean and highly sanitary state. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, a water chemistry monitor unit is provided for automated monitoring and regulation of the water chemistry in a spa tub or hot tub or the like, to maintain the water in a clean and sanitary condition. The monitor unit comprises a relatively compact buoyant housing adapted to float freely within the spa water. The monitor unit includes a programmable controller coupled to one or more sensor electrodes for monitoring selected water chemistry parameters at timed intervals. The monitor unit responds to the parameter readings to deliver one or more chemical agents in appropriate amounts to maintain the monitored parameters within prescribed ranges consistent with maintaining the water in a clean and sanitary condition. 
     In the preferred form, the controller is mounted on-board within the housing of the monitor unit, and an exposed data entry panel is provided for programming the controller to activate the sensor electrodes at predetermined clock times to take readings of the water chemistry parameters. In the preferred form, the sensor electrodes are designed for reading the hydrogen ion concentration level (pH), and for reading oxidation reduction potential (ORP). The controller responds to the pH level reading to deliver an appropriate amount of a selected chemical agent such as a selected base, e.g., sodium carbonate to raise the pH level, or alternately to deliver an appropriate amount of a selected chemical agent such as muriatic acid or other selected acid solution to lower the pH level, for the purpose of maintaining the pH level of the spa water within a predetermined range typically on the order of about 7.2-7.8. In addition, the controller responds to the ORP reading to deliver a selected chemical agent such as a sanitizer or oxidizer, e.g., bromine, to maintain the ORP level of the spa water within a predetermined range typically on the order of about 650-750 millivolts when the pH level is within the above-stated prescribed range. 
     The chemical agents are carried by the housing of the monitor unit and appropriate amounts thereof are delivered automatically under regulation by the controller, in response to the monitored parameter readings. More particularly, in the preferred form, separate chemical agents such as a base and an acid in liquid form for respectively adjusting the pH level up or down are contained within individual and preferably pre-packaged containers or bottles adapted for mounting in an inverted position within individual and uniquely matingly shaped sockets formed in the unit housing. In the preferred form, an additional or third chemical agent in liquid form such as a strong oxidizer or shock agent or the like, typically a strong chlorine solution, may also be provided within an individual bottle for inverted mounted into a matingly shaped socket on the unit housing. These bottles each include a metering assembly mounted in the neck thereof for engaging an associated plunger of a solenoid actuator mounted on the housing of the monitor unit. In operation, in programmed response to the water chemistry readings, the controller actuates one or more of the solenoid actuators to displace the associated plungers through a selected number of advance and retract strokes, to deliver an appropriate number of discrete doses of the appropriate chemical agent or agents to the spa water. 
     In addition, in accordance with the preferred form, the chemical agent for regulating ORP level is provided in solid form such as soluble tablets or the like contained within a cartridge suspended from the housing of the monitor unit. The cartridge includes a perforated segment positioned in underlying relation to a rotary gate of perforated construction, wherein the rotary gate is movably positioned by the one or more solenoid actuators in accordance with the detected ORP level to permit or prevent spa water circulation into contact with the soluble tablets in the cartridge. In this manner the perforated segment on the cartridge can be opened or closed to control tablet dissolution and thereby regulate delivery of the associated chemical agent to the spa water. Alternately, the solenoid actuator may be movably positioned for variably opening and closing the perforated segment on the cartridge to variably adjust the rate of chemical addition in response to the ORP reading. 
     Other features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate the invention. In such drawings: 
     FIG. 1 is a schematic illustration depicting a typical spa or hot tub installation including a spa water circulation and filtration system, with a spa chemistry monitor unit embodying the novel features of the invention shown floating in the spa water; 
     FIG. 2 is an enlarged perspective view, shown partially in exploded form, illustrating the top and one side of the spa chemistry monitor unit shown in FIG. 1; 
     FIG. 3 is a bottom perspective view of the monitor unit; 
     FIG. 4 is a schematic diagram illustrating programmable operation of the monitor unit; 
     FIG. 5 is an exploded perspective view of the monitor unit; 
     FIG. 6 is a top plan view of the monitor unit, with chemical agent containers removed therefrom to illustrate mounting sockets for receiving and supporting said containers; 
     FIG. 7 is a perspective view of a portion of the monitor unit, with an upper portion of the monitor unit removed to reveal internal components thereof; 
     FIG. 8 is an exploded and partially fragmented side elevational view of a portion of a chemical agent container, depicting components of a metering assembly mounted in a neck of the container; 
     FIG. 9 is an exploded perspective view further illustrating components of the metering assembly from an inboard side thereof; 
     FIG. 10 is an exploded perspective view further illustrating components of the metering assembly from an outboard side thereof; 
     FIG. 11 is an enlarged fragmented vertical sectional view taken generally on the line  11 — 11  of FIG. 2, showing the metering assembly in a normal closed position; 
     FIG. 12 is an enlarged fragmented vertical sectional view similar to a portion of FIG. 11, but illustrating metering assembly engagement with a solenoid actuator mounted within the unit housing, with the solenoid actuator in an advanced position for dispensing a chemical agent from the container; 
     FIG. 13 is an exploded perspective view showing an alternative chemical agent container in the form of a cartridge for supporting a supply of soluble tablets; 
     FIG. 14 is an enlarged fragmented vertical sectional view of a portion of the monitor unit, depicting installation of the soluble tablet cartridge into the monitor unit; and 
     FIG. 15 is a perspective view similar to FIG. 7, and illustrating a movable gate in an open position for dissolution of the soluble tablets to dispense a chemical agent carried thereby to the spa water. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the exemplary drawings, a spa chemistry monitor unit referred to generally in FIG. 1 by the reference numeral  10  is provided for automatic or unattended monitoring of water chemistry parameters in a spa or hot tub  12  or the like, and for automatic or unattended dispensing of one or more chemical agents in appropriate amounts in response to parameter readings. The monitor unit  10  generally comprises a compact and free floating device adapted for simple placement into the spa water  14 . The monitor unit  10  includes an on-board controller  16  (FIG. 4) which is programmable to take specific water chemistry parameter readings at predetermined regular times by electronic activation of a pair of sensor electrodes  18  and  20  (FIGS. 4 and 5) disposed in direct contact with the spa water  14 . In response to these parameter readings, the controller  16  automatically dispenses one or more specific chemical agents carried by the monitor unit, in a dosage amount or rate of flow to maintain the spa water in a substantially optimized condition of clarity and hygiene. 
     The spa chemistry monitor unit  10  of the present invention is designed for substantially automated water quality monitoring and maintenance in a body of water such as a typical spa or hot tub  12  or the like. In this regard, the spa or hot tub  12  is depicted generally in FIG. 1 to comprise an upwardly open structure including a floor  22  joined to a surrounding and upstanding side wall  24  for containing the spa water  14 . The spa  12  is typically sized to accommodate one or more persons (not shown) in a partially immersed position, commonly to include a bench or seat  26  to allow such person or persons to sit comfortably within the water. A filtration system  28  is also normally provided to include a pump  30  for drawing water through a suction conduit  32  from one or more drains  34 , and for circulating such water through a filter canister  36  and a heater  38  prior to return flow to the spa  10  via one or more return conduits  40 . As is known in the art, the filter canister  36  includes a suitable filtration medium (not shown) for capturing and separating particulate from the spa water, whereas the heater  38  is designed to elevate the water temperature to a level typically in the range of about 95-105° F. The filtered and heated spa water  12  is normally recycled to the spa  10  through one or more hydrotherapy massage jet nozzles  42  mounted at spaced locations on the side wall  24  beneath the water line, wherein these massage jet nozzles  42  may also include means (not shown) for entraining air bubbles into the water stream to provide a vigorous air-water massage action. 
     The spa chemistry monitor unit  10  comprises a compact and free floating device for simple drop-in placement into the spa water  14 , as viewed in FIG.  1 . The monitor unit  10  is programmable for automatic or unattended operation to analyze the spa water chemistry on a regular basis, and to add specific chemical agents to the spa water in response to the actual water chemistry readings for the purpose of maintaining the water in a clean, clear and hygienic state. These chemical agents are carried in prepackaged containers mounted directly on the monitor unit  10  for appropriate engagement with actuator means to deliver the correct chemical agent or agents each in the correct dosage amount to maintain spa water quality. 
     As shown generally in FIGS. 2-3 and  5 , the monitor unit  10  comprises a main housing constructed from molded plastic or the like to include an upper housing plate  46  attached by screws or the like (not shown) to a lower housing base  48  in a manner capturing and retaining therebetween a buoyant float ring  50  (FIG. 5) of expanded foam material or the like. The upper housing plate  46  carries an upwardly exposed data entry panel or keyboard  52  to enable programming of the controller  16  (FIG. 4) for automated operation to take water chemistry readings. The upper housing plate  46  also supports a plurality of pre-packaged chemical agent containers, three of which are shown in the illustrative drawings in the form of inverted liquid-containing bottles  54 ,  55  and  56 . Each of these chemical agent bottles  54 - 56  carries a metering assembly  78  (FIGS. 8-10) in the mouth-forming neck thereof for engagement by an associated solenoid actuator  60  (FIGS. 5,  7  and  11 - 12 ) mounted on the housing base  48  to dispense the chemical agent in metered doses, all under the control of the controller  16 . An additional chemical agent in solid soluble form is carried within a cartridge  61  (FIGS.  7  and  13 - 15 ) mounted removably on the housing base  48  for engagement with a pair of solenoid actuators  62  to regulate contact of the soluble chemical agent with the spa water. A battery power pack  63  (FIGS. 3 and 5) is removably suspended from a battery terminal fixture  67  on the housing base  48  to provide a safe source of electrical power to operate the monitor unit  10 . The lower housing base  48  is perforated to permit spa water circulatory flow into and through the interior of said lower housing base. 
     The monitor unit  10  is depicted in schematic form in FIG.  4 . More particularly, the monitor unit  10  includes the controller  16  in the form of a microcontroller such as a small computer or computer chip powered by the battery pack  63 . The controller is coupled to the data entry panel  52  which includes appropriate keys (FIG. 2) for entering programming information. Specifically, the data entry panel  52  is utilized to enter information such as time of day, spa size in gallons of water, and a schedule for taking water chemistry readings. In addition, the data entry panel  52  may be used to program the target ranges for water chemistry readings together with associated chemical agent dosages to be added to the spa water in response to water chemistry readings, although such information may be pre-programmed into the controller  16  in the form of a “read only” memory. A visual display  64  such as a liquid crystal display (LCD) is associated with the data entry panel  52  and may include appropriate alpha-numeric characters for facilitated data entry. In the preferred form, the visual display  64  also includes indicators to indicate the operational state of the monitor unit  10 , such as an indicator  65  reflecting low battery pack power, and an indicator  66  reflecting that a water chemistry reading test is in progress. 
     Water chemistry readings are taken at programmed intervals by the monitor unit  10 , by means of the pair of sensor electrodes  18  and  20 . In the preferred form of the invention, the sensor electrode  18  is designed for reading the hydrogen ion concentration level of the spa water, normally expressed as a logarithmic function in the form of pH level. By contrast, the second sensor electrode  20  is designed for reading the oxidation reduction potential (ORP) of the spa water. These two sensor electrodes  18 ,  20  are coupled via a corresponding pair of amplifiers  68  and  70  to the controller  16 . As described above, the controller  16  is programmed to activate the sensor electrodes  18 ,  20  at predetermined times to take these water chemistry readings. 
     The controller  16  responds automatically to the detected water chemistry levels to deliver chemical agents in appropriate amounts to maintain the water chemistry levels within preselected reading ranges consistent with clear and sanitary water conditions. In this regard, a typical preferred pH level is in the range of about 7.2 to about 7.8. If the water chemistry reading reveals an actual pH level below this range, one of the solenoid actuators  60  is activated by the controller  16  to dispense a chemical agent such as a base solution (e.g., sodium carbonate) from the first bottle  54 , in one or more metered doses sufficient to adjust the pH level upwardly to within the prescribed range. Alternately, if the chemistry reading indicates an actual pH level above the prescribed range, a second one of the solenoid actuators  60  is activated by the controller  16  to dispense a second chemical agent such as a selected acid solution or the like (e.g., muriatic acid) from the second bottle  55 , again in an amount appropriate to adjust the pH level back to within the prescribed range. In the same general manner, if the ORP water chemistry reading indicates that the actual ORP level is below a typical preferred range of about 650-750 millivolts which is reflective of the parts-per-million (ppm) concentration of the sanitizing agent, the set of solenoid actuators  62  are appropriately activated by the controller  16  to open the cartridge  61  for ingress of spa water to dissolve the soluble chemical sanitizing agent (e.g., bromine tablets) contained therein. Conversely, if the actual detected ORP level is within or above the prescribed range, the solenoid actuators  62  are movably positioned by the controller  16  to close the cartridge  61  and thereby prevent further addition of the soluble chemical sanitizing agent to the spa water. In this regard, the ORP level reading is generally dependent upon the pH level which needs to be within the prescribed range as noted above, to obtain ORP readings which reliably indicate the need to dispense the sanitizing agent. 
     In the illustrative preferred form of the invention, the third liquid-containing chemical agent bottle  56  may carry an oxidizer or so-called shock chemical which is added to the spa water  14  in a selected amount at periodic, relatively long-term programmable intervals such as weekly in accordance with spa usage. The controller  16  is programmed to activate the solenoid actuator  60  associated with the bottle  56  at the programmed time to deliver the shock agent in the prescribed dose to the spa water. A typical shock agent for this purpose comprises a strong chlorine solution. In addition, or in the alternative, the controller  16  may be programmed to accommodate manually initiated addition of the shock chemical to the water upon appropriate manipulation of the data entry panel  52 . 
     Since the monitor unit  10  of the present invention is normally intended to be removed from the spa  12  when the spa is being used by one or more persons, a water sensor electrode  72  is also provided in the system. This water sensor electrode  72  is designed to detect whether the monitor unit  10  is floating within the spa water  14 , or whether the monitor unit has been temporarily removed from the spa. If the monitor unit is in the spa, the water sensor electrode  72  signals the controller  16  to proceed with water chemistry readings and appropriate additions of chemical agents in a normal manner. However, if the water sensor electrode  72  detects that the monitor unit has been removed from the spa, the controller  16  is signaled to postpone water chemistry readings and chemical agent additions until the monitor unit is again returned to the spa, as detected by the water sensor electrode  72 . 
     As shown in FIGS. 2-3, and  5 - 6 , the upper housing plate  46  of the monitor unit  10  includes a plurality of upwardly open and relatively shallow cup-shaped sockets  74  having a size and shape for respectively receiving and supporting the chemical agent bottles  54 - 56  in an inverted orientation. In this regard, in the illustrative preferred embodiment of the invention, three of these sockets  74  are provided in the upper housing plate  46  in a generally triangular array for substantially mated fit reception of the mouth-forming necks  76  of the chemical agent bottles  54 - 56 , wherein each bottle neck  76  incorporates a metering assembly  78  (FIGS. 8-12) for dispensing the associated chemical agent in discrete doses of predetermined measured volume to the surrounding spa water  14 . The metering assembly  78  is operated by a solenoid plunger tip or pin  80  forming a portion of the associated solenoid actuator  60  mounted on the lower housing base  48  (FIGS. 5,  7  and  11 - 12 ), with the plunger tip  80  protruding upwardly through a central port  82  formed within the bottom of the associated socket  74 . 
     In accordance with one aspect of the invention, each socket  74  further includes an array of ribs  84  in the bottom thereof (FIG. 6) for uniquely interfitting with matingly shaped ribs  86  (FIG. 10) on the associated chemical agent bottle. With this construction, each of the three chemical agent bottles  54 - 56  is adapted for respective cooperative or complementary installation into a specific and unique one of the three sockets  74 . In other words, the first bottle  54  containing, for example, a base solution for use in raising the pH level of the spa water, is adapted to fit only within a single one of the sockets  74 , such as the socket associated in FIGS. 2 and 6 by the adjacent Roman numeral legend “I” on the upper housing plate  46 . Similarly, the second bottle  55  containing an acid solution for use in lowering water pH level is adapted to fit only within a single one of the sockets, such as the socket associated in FIGS. 2 and 6 with the Roman numeral legend “II”. Finally, the third bottle  56  containing, for example, a selected shock solution is adapted to fit only within the remaining socket  74  identified in FIGS. 2 and 6 with the Roman numeral legend “III”. The controller  16  is pre-programmed to recognize the association of these individual sockets  74  with specific chemical agents to be dispensed to the spa water. The uniquely and matingly shaped interfit between the ribs  86  of the bottle necks  76  and the associated ribs  84  within the sockets  74  prevents undesirable loading of a socket  74  with a bottle containing the incorrect chemical agent. 
     The solenoid actuators  60  associated with the chemical agent bottles  54 - 56  are shown best in FIGS. 5,  7  and  11 - 12 . As shown, three such solenoid actuators  60  are provided on a mounting bracket  88  (FIG. 5) secured in a suitable manner to the lower housing base  48 . Each solenoid actuator comprises an armature  90  actuatable by the controller  16  to advance and retract a downwardly projecting core pin  92 . More particularly, when the armature  90  is energized, the core pin  92  is retracted upwardly into the armature  90 , whereas upon armature de-energization the core pin  92  returns to a normal downwardly extending position. The core pin  92  carries a plunger arm  94  which projects upwardly around the associated armature  90  and terminates at its upper end in the plunger tip  80  received through the central port  82  in the associated socket  74 , as previously described. Reciprocal motion of the core pin  92  within the armature  90  serves to reciprocate the plunger tip  80  through advance and retract strokes to operate the metering assembly  78  on a chemical agent bottle installed into the socket  74 . 
     The metering assembly  78  is shown in detail in FIGS. 8-12 with respect to the chemical agent bottle  54 , with the understanding that each chemical agent bottle  54 - 56  is equipped with a metering assembly  78  of the same or functionally similar construction. As shown, the metering assembly  78  comprises an outer cap  96  mounted as by threading onto the bottle neck  76 , wherein the outer cap further defines an external thread segment  98  which cooperates with the associated unique ribs  84  for unique mating seated reception into the single associated one of the sockets  74 . The outer cap  96  additionally includes a secondary inner thread segment  100  for threaded assembly with an inner cap  102  of generally cup-shaped geometry to define cooperatively a metering chamber  104  of predetermined known volume. A cylindrical shuttle valve  106  having a closed lower end (FIGS. 11-12) is movably carried between the outer and inner caps  96  and  102 , guided by guide posts  108  (FIG. 9) and biased by a spring  110  toward a normal lower position engaging an annular valve seat  112  on the outer cap  96  to close a dispense port  114  formed therein. A seal ring  116  is shown at the lower end of the shuttle valve  106  for engaging and sealing against the valve seat  112 . 
     In this normal spring-loaded lower position, as viewed in FIG. 11, an upper margin of the shuttle valve  106  is spaced downwardly below a metering port  118  formed in the inner cap  102 , to permit flow of the liquid chemical agent within the bottle  54  downwardly from the hollow bottle interior to fill the metering chamber  104 . In this regard, an open web  120  is shown installed within this metering port  118 , to include a central post  122  for supporting and retaining an upper end of the spring  110  which extends downwardly into and applies the normal downward biasing force to the shuttle valve  106 . 
     When the associated solenoid actuator  60  is activated by the controller  16  to dispense the chemical agent within the bottle  54 , the plunger tip  80  is advanced upwardly within the socket  74  to engage the closed lower end of the shuttle valve  106 . The plunger tip  80  moves the shuttle valve  106  through an upward stroke to an upper position within the metering chamber  104 , as viewed in FIG.  12 . In this upper position, the upper margin of the shuttle valve  106  is moved against the inner cap  102  in surrounding relation to the metering port  118  to isolate the metering chamber  118  from the bottle interior. At the same time, however, the lower closed end of the shuttle valve  106  is also moved upwardly from the valve seat  112  to permit chemical agent within the metering chamber  118  to flow downwardly through the dispense port  114 . The chemical agent thus exits the bottle for further flow downwardly through the central port  82  in the socket  74  (FIG. 6) to mix with the spa water  14  which substantially fills the lower housing base  48  when the monitor unit  10  is floated within the spa. Subsequent retraction of the plunger tip  80  enables the shuttle valve  106  to shift back to the lowered position as viewed in FIG. 11, whereupon the metering chamber  118  is allowed to re-fill with a second metered volume dose of the chemical agent. For any given dispense cycle, the controller  16  operates the solenoid actuator  60  to advance and retract the plunger tip  80  through a number of strokes sufficient to dispense an appropriate quantity of the chemical agent to the spa water. Importantly, in the event of failure of the associated solenoid actuator  60  for any reason or at any time, the use of the metering assembly  58  insures that the interior of the chemical-containing bottle is not open for direct and complete drainage dispensing of the entire bottle contents to the spa water. 
     The cartridge  61  containing the soluble chemical sanitizing agent such as bromine in tablet form is shown in FIG.  13 . As shown, this cartridge  61  conveniently comprises a compact cylindrical cartridge housing  124  adapted to be preloaded or pre-packaged with a stack of soluble tablets  126 . A lower end of this cartridge housing  124  is flared outwardly with a truncated conical shape which has a circumferential array of perforations  128  formed therein. A lower cap  130  is attached to and closes the lower end of the cartridge housing, wherein this cap  130  is shown bearing the Roman numeral designation “IV”. 
     The cartridge  61  is sized and shaped for quick and easy mounting into a central opening formed in a bottom wall  131  within the lower housing base  48 , as viewed in FIG.  15 . In this regard, the cartridge  61  may include radially outwardly projecting ears  129  on the lower cap  130  for suitable part-turn, quick-connect and quick-release attachment to the wall  131  of the housing base. When installed, the perforate segment  128  of the cartridge is positioned within a rotary gate  132  of truncated conical shape and mounted on the housing base  48  for back and forth rotation through a short arcuate or part-circumferential stroke, on the order of about  5 - 6  degrees. Such back and forth displacement of the rotary gate  132  is achieved by a push-pull action from the set of solenoid actuators  62  mounted on the housing base  48  by means of a pair of brackets  134 . These solenoid actuators  62  each include a plunger  136  (shown best in FIG. 5) protruding from an associated armature  138  for engaging opposite sides of a radially outwardly extending bearing lug  140  on the rotary gate  132 . Importantly, the rotary gate  132  has a generally truncated conical shape to closely overlie the perforated flared portion of the cartridge  61 , and the gate  132  also includes a circumferential array of perforations  142 . Back and forth positional adjustment of the rotary gate  132  thus effectively opens (FIG. 7) or closes (FIG. 15) the perforations  128  in the cartridge housing  124 , by respective alignment or misalignment of the gate perforations  142  therewith, to correspondingly regulate water contact with and the resultant dissolution rate of the soluble tablets  126 , all in response to operation of the solenoid actuators  62  under control by the controller  16 . 
     In the preferred form, with the soluble tablets  126  containing a bromine sanitizing agent for regulating spa water ORP level, the controller  16  may be programmed to take and ORP reading at relatively frequent intervals of about three hours. In response to the ORP reading as detected by the controller  16 , the controller  16  then operates the solenoid actuators  62  to displace the rotary gate  132  in a manner opening the perforations  128  in the cartridge housing  124  if the ORP level is below a target range, typically a low voltage reading on the order of about 650-750 millivolts when the pH level is within the prescribed range. Alternately, if the ORP reading is within or above this target range, which may be programmed into the controller  16 , the solenoid actuators  62  displace the rotary gate  132  in a manner closing the perforations  128  in the cartridge housing  124 . Alternately, if desired, the controller  16  can be programmed to modulate the degree to which the cartridge housing perforations  128  are opened, in accordance with the ORP reading. 
     The tablet-containing cartridge  61  may optionally be equipped with a small viewing port  144  (FIGS. 7 and 15) formed in the upper end thereof, for alignment with a related site port  148  (FIG. 2) formed in the upper housing plate  46 . With this geometry, the interior of the cartridge  61  can be visually monitored to determine when the tablets  126  therein have dissolved, thereby requiring installation of a replacement cartridge. Such visual monitoring can be enhanced by forming the cap  130  at the lower end of the cartridge  61  from a translucent material or the like, whereupon light can be perceived when viewing through the site port  148  only in the event that the tablets  126  have been dissolved. 
     FIG. 5 shows a sensor unit  150  mounted within the lower housing base  48 , as by removable connection into the underside of a mounting fixture  152 . The sensor unit  150  carries the pair of sensor electrodes  18  and  20  in electrical communication with the controller  16  which activates the electrodes  18  and  20  at programmed times to take water parameter readings such as the pH and ORP readings described above. The sensor electrodes  18  and  20  are positioned within a moisture cup  154  carried as by snap-fit mounting on the lower end of the unit  150 , to capture and retain a quantity of water therein when the monitor unit  10  is removed temporarily from the spa during spa use. Maintaining the sensor electrodes  18  and  20  immersed or in contact with the water has been found to effectively prevent electrode corrosion and degradation which could otherwise shorten operating life. A third electrode comprising the water sensor  72 , such as a conductivity probe, may also be carried by the sensor unit  150  in a position out of contact with water contained within the moisture cup  154 , to signal the controller  16  when the monitor unit  10  is removed from the spa, so that water chemistry readings and responsive dispensing of chemical agents does not occur until the monitor unit  10  is returned to the spa. 
     The spa chemistry monitor unit  10  of the present invention thus provides a relatively compact and self-contained, free floating device for automatically monitoring the quality of the spa water  14 . The monitor unit is programmed to take water chemistry readings at regular intervals, and to respond in an unattended manner by dispensing appropriate chemical agents in the appropriate dosages to maintain water clarity and hygiene. 
     A variety of modifications and improvements in and to the spa chemistry monitor unit will be apparent to those persons skilled in the art. As one example, it will be recognized and understood that the monitor unit may be utilized for larger bodies of water, such as a swimming pool, wherein the chemical-carrying capacity would desirably be increased to accommodate the larger water volume. Moreover, it will be understood that the monitor unit  10  can be adapted to carry additional or alternative chemical agents, such as a flocculent-type water clarifier and others. In one form, the clarifier may be combined with another chemcial agent, such as the acid solution for decreasing the pH level. Other agents such as a selected fragrance may also be dispensed automatically or manually by the monitor unit. Accordingly, no limitation on the invention is intended by way of the foregoing description and accompanying drawings, except as set forth in the append claims.