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FIELD OF INVENTION 
     This disclosure relates to water purification particularly with respect to water containing vessels such as spas, hot tubs, whirlpools, pools and the like and to a chlorinator or oxidizer generator suitable for such purpose. 
     RELATED ART 
     Portable spas have become quite popular as a result of their ease of use and multiplicity of features such as varied jet and seating configurations. Maintaining appropriate water chemistry and sanitation is of course important to enhancing the spa user experience. 
     SUMMARY 
     The following is a summary of various features, aspects, and advantages realizable according to various illustrative embodiments of the invention. It is provided as an introduction to assist those skilled in the art to more rapidly assimilate the detailed discussion which ensues and does not and is not intended in any way to limit the scope of the claims which are appended hereto in order to particularly point out the invention. 
     An illustrative embodiment of a portable spa drop-in chlorinator includes a housing having an inlet at a first end, wherein an electrode assembly is mounted so that spa waters flows through the electrodes and out of a second end of the device. When an appropriate voltage is applied, the electrodes interact with the fluid within the chlorinator to generate various oxidizing agents. In one embodiment, the chlorinator is cylindrical and is sized to fit within the central opening of a filter element located in a filter compartment of a portable spa. 
     In one embodiment, respective outer electrodes comprise titanium, while inner electrodes comprise doped diamond particles embedded in a plastic mesh substrate. In other illustrative embodiments, the doped diamond surface comprises the surface of a whole diamond electrode. In other illustrative embodiments, the diamond coated substrate may be selected from one of the group including titanium, niobium, silicon, platinum, or stainless steel. The electrodes may be solid metal plates or a mesh, the latter providing increased surface area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a drop-in chlorinator according to an illustrative embodiment; 
         FIG. 2  is a top view of the chlorinator of  FIG. 1 ; 
         FIG. 3  is a sectional view of the chlorinator of  FIG. 1  taken at  3 - 3  of  FIG. 1 ; 
         FIG. 4  is a schematic perspective view of an illustrative electrode assembly embodiment; 
         FIG. 5  is a top end perspective view of a drop-in chlorinator illustrating an electrode assembly according to  FIG. 4  encapsulated in the device; 
         FIG. 6  is a top view of a second electrode assembly embodiment; 
         FIG. 7 . is a top schematic view illustrating one implementation of the electrode assembly of  FIG. 6 ; 
         FIG. 8  is a top schematic view illustrating a second implementation of the electrode assembly of  FIG. 6 ; 
         FIG. 9  illustrates one method of fabricating the assembly of  FIG. 6 ; 
         FIGS. 10-12  are side schematic views illustrating various applications of chlorinators according to the illustrative embodiments; and 
         FIG. 13  is a side exploded view of a drop-in chlorinator assembly useful in the application of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3  depict an illustrative embodiment of a compact drop-in chlorinator  11 . The chlorinator  11  has a cylindrical housing  13 . An electrode assembly  15  comprising electrodes  25 ,  27 ,  29 ,  31  is disposed vertically through the interior of the housing  13  and retained in the housing  13 , for example, by surrounding epoxy potting compound  17 . In an illustrative embodiment, epoxy  17  fills the interior of the cylinder  13  except for the space occupied by the electrode assembly. An electrical cable  19  supplies the device  11  with power and is also encapsulated by the epoxy potting compound  17 . Respective end caps  16 ,  18  enclose the opposite ends of the housing  13  and assist in shielding the electrode assembly  15  from foreign matter, and are optional in various embodiments. 
     In one embodiment, spacers  20  may be used to space the electrodes apart. As seen in  FIG. 5 , the epoxy potting may overlap the spacers  20  and edges of the electrodes  25 ,  27 ,  29 ,  31  to hold the assembly  15  in position. 
     As illustrated in  FIG. 4 , the electrode assembly  15  comprises a pair of outer electrodes and a number of inner electrodes. In the illustrative embodiment of  FIGS. 1-4 , an outer electrode pair  21  and two inner electrodes  23  are provided. In this embodiment, the outer electrode pair  21  comprises a pair of rectangular titanium electrodes  25  and  29 , while the inner electrodes  23  comprise rectangular diamond electrodes  27  and  31 . Electrical leads L 1 , L 2  emanating from the cable  19  are welded or otherwise electrically connected to the respective titanium electrodes  25 ,  29 . The inner electrodes  27 ,  31  float electrically, i.e., are not connected to ground. Additional inner electrodes, for example, up to twenty, may be provided in alternate embodiments. 
     In one embodiment, the titanium electrodes  25 ,  29  comprise titanium coated with ruthenium iridium. The diamond electrodes  27 ,  31  may comprise 0.250 micron boron doped diamond crystals embedded in a teflon sheet (plastic matrix) such that diamond protrudes from each side of the sheet. The plastic matrix can be polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene or other suitable materials. In alternate embodiments, the diamond electrodes can comprise either a coating on a substrate or whole diamond designed to be self supporting. 
     In a second electrode assembly embodiment  18  shown in  FIG. 6 , a single central rectangular diamond electrode  41  is positioned between respective titanium outer electrodes  43 ,  45 . In one embodiment illustrated in  FIG. 9 , the electrode assembly  18  of  FIG. 4  is tightly wrapped in a solid plastic film or tape  49  to keep the epoxy potting material out of the assembly  18  during fabrication. Again, the electrodes  41 ,  43 ,  45  may be separated and positioned by nonconductive, e.g. plastic mesh spacers  55  ( FIG. 7 ) or individual plastic spacers  57  ( FIG. 8 ). The central diamond electrode  41  floats electrically, i.e., is not connected to ground. The ends of the plate electrodes  43 ,  43  may be passivated, e.g., ruthenium iridium coated to avoid corrosion and calcium scale. 
     Illustrative uses of a drop-in chlorinator are shown in  FIGS. 10-12 .  FIG. 10  shows an “over the bar top” application where the electrode cable  19  extends over the top edge of the spa  101  and suspends the chlorinator  11  in a floating position in the spa water. 
       FIG. 11  illustrates an embodiment wherein the electrical cable  19  passes through a pass through seal  107  in the sidewall  104  of the spa  101  into the spa tub or filter compartment, suspending the chlorinator  11  in one of those areas. The “dry” side  103  of the cable  19  may be located in the electrical equipment area of the spa  101  where it may interface with the spa controller circuitry as hereafter described in more detail 
     In the embodiment of  FIG. 12 , the electrical cable  19  enters the filter compartment  105  and is dropped down the central cylindrical opening  106  of a filter element  107 . In this position, spa water is pulled through the electrode assembly, e.g.  15 , of the unit  11  by the pump of the spa water circulation system. Thus, the diameter of the cylindrical chlorinator  11  is selected to fit down the internal pipe of the filter element  107 . The chlorinator  11  may of course be located elsewhere in the circulation path of the spa. While a snug fit between the chlorinator  11  and internal filter pipe is shown in  FIG. 12 , a looser fit is preferred, for example, providing a difference of 0.25 inches between the respective diameters of the two parts. In one illustrative embodiment, the drop-in chlorinator may be 1.3 inches in diameter and six inches in length or otherwise properly sized to fit down a filter stand pipe. 
     A drop-in chlorinator assembly particularly useful in the embodiment of  FIG. 12  is illustrated in  FIG. 13 . That assembly includes a chlorinator  11 , a stand pipe cap assembly  109 , and a pass through assembly  111 . The diameter of the pipe section  113  of the cap assembly  109  is selected such that it fits snugly into the central cylindrical opening in the filter element  107 , while the diameter of the rim  115  of the cap portion  117  is such that it abuts the top surface  121  of the filter element  107 . A strain relief device  119  is further provided and, when assembled, is attached to the cable  19  in the interior of the cap assembly  109 . The chlorinator  11  is thus suspended with the filter element  107  at a position determined by the length of L 3  of the cable  19 . The pass through assembly  111  includes a strain relief providing nut  122 , a pass through fitting  123  and first and second 9-rings  125 ,  127 . 
     In various alternate embodiments, the electrodes are rectangular in shape and each comprise a boron doped synthetic diamond electrode tailored to flow rate. Such electrodes may be formed, for example, by chemical vapor deposition (CVD) of a very thin coating of boron or nitrogen doped diamond onto a niobium substrate. Such electrodes may be fabricated, for example, by Adamant, Chauxde-Fords, Switzerland. Other substrate materials may be used such as titanium, silicon, platinum or stainless steel. Embodiments may also be constructed of self-supporting diamond without using a substrate, such as may be obtained, for example, from Advanced Oxidation, Cornwall, U.K. In various embodiments, the substrates may either be solid plates or mesh, the latter providing increased surface area. 
     In operation of illustrative embodiments in an illustrative portable spa environment, a constant current mode of operation of the device  11  may be employed. In such case, a selected current flow through each electrode pair in the range of 1-5 amps, for example, 2 amps, may be used with a floating voltage across the outer electrode pair of 5-24 volts. In such embodiments, flow rates through the cell  11  may range from ½ gallon to 5 gallons per minute. An advantage of the chlorinator according to embodiments above is that it has low salt level requirements (0 ppm to 1000 ppm) vs. typical 3500-5000 ppm. Electronically, a constant current AC/DC transformer supplying 1 to 5 amps at 5 to 24 volts D.C. may be used along with a microcontroller to control activation of the chlorinator  11 . 
     In such embodiments, hydroxyl radicals are generated directly off the electrode plates. The hydroxyl radicals then oxidize organic waste in the process water or react with water and dissolved salts to produce various oxidizers. These include but are not limited to, ozone (O3), hydrogen peroxide (H2O2), sodium hypochlorite (NaHOCI/OCl), chlorine dioxide (ClO2), sodium persulfates (NaHSO5) and sodium percarbonate (Na 2 CO 3 ). This broad spectrum of oxidizers is capable of neutralizing organic and other contaminants which may be present. 
     A chlorine generator system according to an illustrative embodiment may operate in an open-loop mode using scheduled and timed generation of chlorine. The length and interval of daily generation is typically a function of the spa size, bather load, and water salinity. In such a system, the cell  11  may produce a constant stream of 0.1 to 0.60 ppm (parts per million) chlorine in a 4 gpm flow (0.5-2 amp &amp; 1000-2000 ppm salt). To maintain the chlorine level in the water, the cell  11  must operate longer for a large spa than for a small spa. Additionally the cell  11  must run longer with a higher expected bather load. The salt level has a strong direct relationship to the quantity of chlorine produced. 
     In an illustrative open loop system the user inputs three variables to the system at start-up. The first is the SPA SIZE or (SPA). A size code may be used (e.g. 1-8). The anticipated USE LEVEL or (USE) (1-5) is the second variable. Use level “(1)” corresponds to minimal use and vacation mode. A higher level should be entered if more bathing is expected. The user preferably adjusts the use level over the course of use. The third start-up variable input is the water hardness (Hd). This parameter controls the polarity reversal cycle timing used to clean the electrodes. This variable may not be employed in alternate embodiments. 
     As an additional input feature, a manual chlorine addition (Add) or BOOST command may be implemented. This command instructs the system to generate enough chlorine to add 2 ppm to the spa. This chlorine Add temporarily overrides scheduled operation times. 
     The manual Add or BOOST command dictates that the system run for a length of time sufficient to add 2 ppm Chlorine. The amount of time needed to bring the water to 2 ppm is highly dependent on the amount of bather load in the water. A standard 24 hour dose or longer may be needed to completely bring the water up. In one implementation of the Add or BOOST command, the system switches from 2 amps to 4-4.5 amps to rapidly generate chlorine. One run cycle every six hours may be used to maintain uniform around the clock treatment. 
     In one embodiment, salt is measured each time the unit  11  generates chlorine as well as when requested by the user. The system measures the salt level of the water by means of measuring the voltage and current across the cell  11 . The voltage reading is then compared against allowable limits. The salt concentration is normalized, and displayed on the user interface. A voltage higher or current lower than specified returns a low salt error and a voltage less or current higher than specified returns a high salt error. 
     If there is a low salt condition, an error may be sent to the spa controls, triggering a “water care” icon to flash. The unit  11  may be allowed to continue to generate chlorine in this condition. The spa controls or controller modulates available voltage or current to a regulated limit to automatically compensate for low salt or conductivity situations. If there is a high salt condition, an error will be sent to the spa controls, again triggering the water care icon to flash. In this case, the unit  11  will not generate chlorine until the salt level has been corrected. 
     To prevent mineral scale on the electrodes  53 ,  55 ,  57 ,  59 , polarity reversal may be used. The time period of the reversal is a function of water hardness and is preferably made adjustable to a user input hardness reading. Rapid cycling of the electrodes will cause premature electrode failure. Therefore a dead band in the cycle may be implemented to allow the electrodes to discharge prior to the polarity reversal. The dead band interval may be, for example, a minimum of 10-20 seconds. 
     At either initial start-up or at a maintenance event, the spa water should be manually balanced. Once the spa water has been balanced it should be super chlorinated (5 ppm). Super chlorination prepares the system for operation and immediate spa usage by cleaning the spa after a period of nonuse. After super chlorination, salt is added to the water. The spa control system may operate such that the water care icon is blinking to indicate that the salt level is low and/or the unit has not been initialized or programmed. Salt should be added slowly into the filter compartment while all of the jets are operating. The jets should operate an additional 10 minutes after the salt is fully added. An example of a target salt concentration is 1000 ppm. High demand users can add up to 2000 ppm salt, which will lower the hours required to generate chlorine and therefore lower the USE level. A salt level reading is preferably taken every time the unit begins a generation cycle to ensure proper salt levels at start-up and during the time between water changes. 
     Typical operation of an illustrative system preferably requires a weekly chlorine and water quality check to ensure that the system is working correctly. Although the user is not required to enter the chlorine concentration, the value is needed to determine the use level. Over the course of the first month, the user may determine their Use Level by taking a reading of the water before they enter the spa. If the chlorine level is low, e.g., “1” or less, the user will want to increase the use level by one to increase the output. If the user finds that the chlorine level is 5 or higher, the user will want to drop the use level by one and retest in a few days or a week. If the bather load is predictable, the use level may only need occasional adjustments. 
     If the bather load is sporadic, the user may want to perform a manual addition. In such case, the user may enter the spa control menu and confirm an addition (Add or Boost). The addition operation turns the system on immediately and operates the specified amount of time determined to elevate the chlorine level by 2 ppm (this depends on bather load and time and cannot be guaranteed). If the water is overly polluted such that the actual bather load far exceeded the anticipated bather load, a manual dichlor/MPS dose may be used and is compatible with the system. 
     Typically, the spa will require a monthly manual shock with MPS or dichlor to eliminate any accumulated waste. The oxidizer level should be brought to and held at 5 ppm while all jets configurations and pumps are operated for 30 minutes each. It is important to monitor pH at this time as well to ensure that the water remains balanced. 
     Over time the water level in spa typically drops from evaporation or splash out. When fresh water is added to the spa, it is important to rebalance the water and monitor the salt concentration. The system may employ a conductivity sensor to determine the amount of salt in the water and whether it is too high or too low. A water care icon may be arranged to blink to indicate that the salt is low and that more salt is needed. Salt should be added in 0.25 lb (100 g) increments to ensure that it is not over dosed. 
     While the apparatus and method have been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the disclosure is not limited to the disclosed embodiments. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims.

Summary:
An oxidizer generating apparatus comprising a cylindrical housing and an electrode assembly attached at one end of the housing comprising at least three vertically disposed electrodes, the electrodes being spaced apart so as to define a water flow path between them, the electrodes comprising titanium outer electrodes and at least one inner diamond electrode.