Water conductivity monitor for a water purification system
This is a continuation-in-part of copending U.S. Ser. No. 08/212,362, filed Mar. 14, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to improvements in water purification systems of the type having a reverse osmosis unit or the like for removing dissolved ionic material and other contaminants from an ordinary supply of tap or feed water. More particularly, this invention relates to a reliable purity or water quality monitor for incorporation into a water purification system, wherein the monitor is responsive to cyclic operation of the reverse osmosis unit to accurately determine and indicate the operational efficiency of the reverse osmosis unit. 
     Water purification systems in general are well-known in the art of the type having a reverse osmosis unit for converting an incoming supply of ordinary tap or feed water into relatively purified water for use in cooking, drinking, etc. In general terms, the reverse osmosis unit includes a semi-permeable membrane through which a portion of the tap water supply is passed, such that the membrane acts essentially as a filter to remove dissolved metallic ions and the like as well as undesired particulate matter from the tap water. In operation, these impurities are removed from one portion of the water flow and concentrated in another portion of the flow which is normally discharged as waste to a drain. The thus-produced supply of relatively purified water is normally passed into a temporary storage reservoir or vessel where it is ready for dispensing and use, typically by operation of an appropriate faucet valve located adjacent a kitchen sink or the like. While the specific construction and operation of the purified water supply system may vary, such systems are exemplified by those shown and described in U.S. Pat. Nos. 4,585,554; 4,595,497; 4,657,674; and 5,045,197. 
     In many instances, it is desirable to obtain an indication of the degree of purity of the purified water produced by the reverse osmosis unit. Alternately stated, it is desirable to obtain an indication of the operating efficiency of the semi-permeable membrane within the reverse osmosis unit. In this regard, the level of water purity will depend upon and thus will vary in accordance with several factors, such as the cleanliness of the reverse osmosis unit membrane and the degree of contamination of the incoming tap water supply in a raw state. The purity level of produced purified water is normally indicated by a measurement of electrical conductivity, wherein a relatively high electrical conductivity correlates with a relatively low resistance and thus reflects a substantial quantity of remaining ionic material which has not been removed by the reverse osmosis unit. Conversely, a relatively low conductivity level indicates that a high proportion of ionic material as well as other contaminants have been removed. A failure of the purified water to meet certain purification criteria indicates that the water supply system may not be operating properly or otherwise that the semi-permeable membrane may need to be cleaned or changed. 
     In the past, test devices and systems have been proposed for use in measuring the conductivity level of the produced purified water in a typical purification system. In some cases, the conductivity of the produced purified water has been compared with the conductivity of the incoming tap water, thereby indicating the operational efficiency of the reverse osmosis unit in proportion to the condition of the tap water inflow. In general terms, such test devices and systems have utilized one or more electrodes for contacting the purified water and, in many designs, for contacting the incoming feed water, to obtain the desired water conductivity readings. These electrodes are coupled to an appropriate operating circuit and source of electrical power to obtain the desired purity level readings which can be indicated, for example, by illumination of one or more indicator lights. 
     Prior water quality monitor test devices have commonly comprised self-contained portable units intended for use by service personnel in testing purified water, as described, for example, in U.S. Pat. No. 3,990,066. More recently, reverse osmosis purification systems designed for under-the-counter use in a typical residence or office environment have been equipped with monitoring circuits integrated directly into the purification system, as shown, for example, in U.S. Pat. Nos. 4,623,451; 4,806,912; 3,838,774; 4,708,791; 5,057,212; and 5,145,575. Indicator lights in such test devices commonly include a red or yellow light energized when the quality of the produced purified water is unacceptable, and a green light energized when the conductivity reading or readings reflect acceptable water quality. 
     While these test devices beneficially provide important information regarding the performance of the reverse osmosis unit, cyclic operation of a typical reverse osmosis unit often results in erroneous test readings. More specifically, many reverse osmosis systems utilize an inlet shut-off valve which responds to the pressure of the produced purified water to turn off the tap water inflow when the storage reservoir reaches a substantially filled condition. The inlet shut-off valve beneficially stops water flow through the system when the reservoir is full, thereby preventing continuous wasted water flow to the drain. However, during the time that the inlet shut-off valve is closed, the pressure differential across the reverse osmosis membrane, necessary for production of purified water, is substantially removed. As a result, some migration or leaching of contaminants across the reverse osmosis membrane can occur, to increase the impurity level of small or isolated volumes of previously produced purified water resident within system flow paths between the membrane and the storage reservoir. Those flow paths provide a convenient mounting site for a pure water electrode, whereby a high and unacceptable conductivity reading can occur if a measurement is taken when the electrode is in contact with one of these isolated volumes of poor quality water. When system water flows resume upon dispensing of a sufficient portion of the purified water via a faucet valve or the like to re-open the inlet shut-off valve, any isolated volume or volumes of poor quality water mix quickly with other purified water so that the overall quality of the water actually dispensed is quite acceptable. However, many test devices respond to faucet valve opening to immediately take a conductivity measurement, whereby the reading can be inaccurate if the inlet shut-off valve has been closed for an extended period of time. Other test devices operate by taking conductivity readings at set time intervals, whereby an inaccurate reading can occur if taken when the inlet shut-off valve is closed or shortly after opening thereof. 
     There exists, therefore, a significant need for further improvements in water quality monitors for testing and indicating the operating performance of a reverse osmosis unit in a water purification system, particularly wherein the water quality monitor is made responsive to cyclic operation of the reverse osmosis unit so that accurate and reliable test readings will result. The present invention fulfills these needs and provides further related advantages. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, an improved water quality monitor is provided for use in a water purification system of the type having a reverse osmosis unit for producing relatively purified water from an incoming tap water supply. The purification system includes a storage reservoir for receiving and storing the produced purified water for immediate dispensing via a faucet valve or the like. A tap water inlet shut-off valve responds to the water level within the storage reservoir to shut off tap water flow to the reverse osmosis unit when the reservoir reaches a substantially filled condition. The water quality monitor includes a test circuit to determine the purity level of the produced purified water in response to cyclic operation of the reverse osmosis unit, as reflected in the preferred embodiment by opening and closure movement of the shut-off valve. The test circuit is designed to take water conductivity readings of the tap water and produced purified water at a point in time which is substantially mid-cycle, namely, substantially after the shut-off valve moves to an open position and substantially prior to return movement of the shut-off valve to the closed position when the reservoir is filled. During this so-called mid-cycle condition, the reverse osmosis unit produces purified water with a substantially optimum efficiency, so that accurate conductivity test readings reflective of the actual quality of the purified water can be taken. 
     In accordance with the preferred form of the invention, the inlet shut-off valve includes a diaphragm-mounted valve member defining differential areas on opposite sides thereof for comparative response to the pressure of the tap water supply and to the pressure of the produced purified water within the storage reservoir. As the storage reservoir approaches a substantially filled condition, the pressure level of the produced purified water increases. When the pressure level of the produced purified water reaches a point reflective of a substantially filled reservoir condition, the shut-off valve moves to the closed position, thereby disconnecting the tap water supply from the reverse osmosis unit to turn the system off. Subsequent dispensing of a sufficient quantity of water from the storage reservoir causes the pressure level of the produced purified water to drop sufficiently so that the inlet shut-off valve moves to the open position. As a result, tap water inflow to the reverse osmosis unit is resumed, to correspondingly resume pure water production. A water purification system of this general type is described in detail in U.S. Pat. No. 5,045,197, which is incorporated by reference herein. Importantly, during the off condition, the normally existent pressure differential across the reverse osmosis unit is removed, such that some migration of previously removed contaminants can leach or migrate to the previously produced purified water. 
     The present invention recognizes that migration of contaminants to the produced purified water, during a system off cycle, can result in localized water volumes having unacceptably high conductivity levels. While these localized water volumes tend to intermix with other purified water so that the water actually dispensed is of acceptable quality, inaccurate conductivity readings may nevertheless occur as these localized water volumes pass conductivity electrodes of the water quality monitor. 
     Accordingly, in the present invention, conductivity readings reflecting water quality of the produced purified water are taken at a point in time substantially after movement of the shut-off valve to the open position, such that localized volumes of unacceptable quality water are permitted to move past the monitoring electrodes before a test reading is taken. Similarly, the present invention insures that test readings are taken substantially prior to reclosure of the shut-off valve. In this manner, conductivity readings are confined to a point in time when the reverse osmosis unit is operating at substantially full pressure differential conditions, and after any localized or isolated volumes of poor quality water have flushed past a pure water electrode. As a result, the conductivity readings will accurately reflect actual operating efficiency of the reverse osmosis unit. 
     The test circuit of the present invention, in accordance with a preferred embodiment, is armed by a reset switch each time the inlet shut-off valve moves from the closed position to the open position to permit resumed system flows and pure water production. In the preferred form, this occurs by movement of a reed switch or the like responsive to opening displacement of the inlet shut-off valve. Return movement of the shut-off valve toward the closed position, which occurs relatively slowly as the reservoir fills, is effective to trigger the test circuit to produce a test pulse utilized to obtain comparative conductivity readings of the tap water and purified water supplies. This test pulse is delivered at a point in time which is substantially mid-cycle between opening movement and subsequent closure of the inlet shut-off valve. The test result is stored in memory for subsequent display, in the preferred form, each time a faucet valve is opened to dispense water. In one alternative embodiment, a selected plurality of test results indicative of unacceptable water quality are required in succession before an unacceptable water quality test result is displayed, thereby further reducing the likelihood of display of false readings. 
     In accordance with other aspects of the invention, the purification system includes a prefilter stage disposed upstream from the reverse osmosis unit for removing particulate from the tap water inflow, wherein such particulate could otherwise cause premature clogging and failure of a reverse osmosis membrane. The test circuit includes a pressure sensor for detecting the pressure drop across the prefilter stage, and for indicating when that pressure drop reaches a level such that a removable prefilter cartridge should be changed. 
     The test circuit further includes means for indicating system failure reflected by the shut-off valve remaining in an open condition for an excessive period of time. A battery power supply is conveniently provided for the test circuit, and means are also provided for indicating low battery power. 
     According to another feature of the invention, a novel flow switch is provided to detect dispensing of purified water by opening of a faucet valve or the like. The improved flow switch includes means for tracking the volume of water dispensed so that a postfilter cartridge can be changed when a predetermined volume of water is dispensed from the system. 
     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 front perspective view, partially in schematic form, illustrating a water purification system and related water quality monitor embodying the novel features of the invention; 
     FIG. 2 is an enlarged fragmented vertical sectional view taken generally on the line 2--2 of FIG. 1; 
     FIG. 3 is an enlarged fragmented vertical sectional view corresponding generally with a portion of FIG. 2, and showing an inlet shut-off valve in a fully open position; 
     FIG. 4 is an enlarged fragmented vertical sectional view generally similarly to FIG. 3, and illustrating the inlet shut-off valve in a partially closed position; 
     FIG. 5 is a circuit block diagram illustrating the water quality monitor for use in the purification system of FIGS. 1-4; 
     FIG. 6, encompassing FIGS. 6A-6D, is a detailed circuit diagram depicting one preferred form of the invention; 
     FIG. 7 is a fragmented sectional view, shown somewhat in schematic form, depicting an improved flow switch for use in the invention; and 
     FIG. 8 is a portion of a circuit diagram reflecting one alternative preferred form of the invention, for implementation into the circuit diagram of FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in the exemplary drawings, a water purification system referred to generally in FIG. 1 by the reference numeral 10 includes a reverse osmosis unit 12 for producing relatively purified water from an ordinary tap water supply 14 or the like. The produced purified water is collected within a suitable storage reservoir 16 and/or dispensed on demand, as by means of a faucet valve 18 or the like. A water quality monitor referred to generally by reference numeral 20 is provided for testing the purity level of produced purified water, and for indicating the purity level as by illumination of appropriate indicator lights (not shown in FIG. 1). The water quality monitor 20 is adapted to operate in response to normal cyclic operation of the reverse osmosis unit 12, to provide accurate and reliable test readings indicating system performance. 
     The water purification system 10 is designed particularly for use in residential and other domestic applications to provide a readily available supply of purified water. As is known in the art, the purification system 10 utilizes principles of reverse osmosis to convert the tap water inflow 14 into a pair of water outflows respectively comprising the relatively purified water having contaminants removed therefrom, and a waste or reject water supply having contaminants and/or impurities concentrated therein. The produced purified water is normally coupled to the storage reservoir 16 for collection and storage, and also to the faucet valve 18 for dispensing on demand for drinking, cooking, etc. One or more parallel-connected dispense paths to a refrigerator ice maker (not shown) and the like may also be provided. The reject water supply, often referred to as brine is normally discharged to a suitable drain. 
     FIGS. 1 and 2 show the purification system 10 in a preferred configuration, in conformance with the water purification system disclosed in U.S. Pat. No. 5,045,197, which is incorporated by reference herein. As shown, the purification system 10 comprises a unitized manifold 22 which supports three processor stages 24, 26, and 28 for converting the tap water inflow 14 into the separated purified and reject water supplies, as described above. In this regard, the first stage 24 comprises an initial filtration or prefilter stage for flow-through passage of the tap water inflow 14 through a prefilter cartridge 30 (FIG. 2) to remove particulate therefrom. The resultant prefiltered tap water passes to the second stage 26, which comprises the reverse osmosis unit 12 and includes a reverse osmosis semipermeable membrane 32 (FIG. 2). In the presence of appropriate pressure conditions as known in the art, the membrane 32 functions to separate the tap water inflow 14 into the purified and reject water supplies. The reject water supply is connected through a suitable drain fitting 34 (FIG. 1) for passage to the drain, whereas the produced purified water is connected to the third or final stage 28 comprising a postfilter stage. The purified water flows through a postfilter cartridge 36 and further through a tank fitting 38 for collection and storage within the reservoir 16, or through a dispense fitting 40 for delivery and dispensing via the faucet valve 18 or the like. 
     An inlet shut-off valve 42 is carried by the manifold 22 and responds to the level of produced purified water within the reservoir 16 to cycle the system between an &#34;on&#34; state and an &#34;off&#34; state. More particularly, as shown best in FIGS. 2-4, the inlet shut-off valve 42 comprises a valve member 44 of differential area mounted on a resilient diaphragm for opening and closing movement to respectively permit and prevent tap water inflow to the reverse osmosis unit 12. In the preferred form, a lower and relatively small area face of the valve member 44 is subjected to the pressure of the tap water supply by direct communication with a discharge port 46 associated with a downstream side of the prefilter stage 24. An upper and comparatively larger surface area of the valve member 44 is exposed to the pressure of produced purified water within a pressure chamber 47 connected via a pressure port 48 to produced purified water discharged from the reverse osmosis unit 12. Accordingly, the pressure within the chamber 47 corresponds to the pressure of purified water within the reservoir 16. The valve member 44 responds to the pressure differential applied thereto, for shutting off tap water inflow when the reservoir 16 reaches a substantially filled condition. By contrast, the valve member responds to this same pressure differential to open and permit resumed tap water inflow when a sufficient quantity of the purified water is dispensed, as by opening the faucet valve 18, to reduce the pressure within the reservoir 16. If desired, a tapered poppet stem 45 shown in dotted lines in FIG. 3 may be provided between the valve member 44 and the port 46 to enhance smooth transition movement of the valve member between the open and closed positions. This poppet stem 45 may be free floating, or attached to the valve member. 
     More particularly, during normal operation of the reverse osmosis unit 12 to produce purified water, with the inlet shut-off valve 42 in a full open position (FIG. 3), the membrane 32 of the reverse osmosis unit 12 is subjected to a substantial pressure differential enabling production of purified water. The produced purified water is coupled for flow into the reservoir 16, wherein the reservoir commonly includes an internal resilient bladder 49 (FIG. 1) which subdivides the reservoir interior into a gas-filled pressure chamber and a water chamber for receiving and storing the purified water. As the reservoir 16 approaches a substantially filled condition, the bladder 49 deforms to reduce the volume of the pressure chamber and thereby result in a progressively increased pressure applied to the purified water. As the pressure applied to the purified water increases, the pressure differential across the reverse osmosis unit 12 gradually decreases. The operating efficiency of the reverse osmosis membrane 32 is in part dependent upon the magnitude of the pressure differential applied thereto. This same pressure differential is applied as described above across the valve member 44. 
     To ensure satisfactory system performance in the production of purified water, the shut-off valve 42 is designed to close when this pressure differential across the membrane 32 reaches a predetermined but still substantial minimum level. For example, in a system subjected to a tap water supply pressure of about 60 psi, the valve member 44 of the shut-off valve 42 is designed with appropriate differential upper and lower surface areas to close when the pressure of the produced purified water reaches a predetermined threshold of about 40 psi. This closure operation is achieved by exposing the larger upper surface area of the valve member 44 to the pressure of produced purified water, in contrast with the proportionately smaller lower surface area of the valve member exposed through the port 46 to the tap water supply pressure. 
     With this geometry, when the pressure of the purified water falls below the level indicative of a substantially filled reservoir 16, the pressure differential across the valve member 44 is sufficient to displace the valve member to an open condition (FIG. 3). The shut-off valve 42 is conveniently designed for snap-action movement to a substantially full-open position by providing an annular valve seat 54 engaged by the lower side of the valve member 44 in the closed position, wherein the valve seat 54 is surrounded by an annular gallery 56 coupled to a flow passage 58 leading to the reverse osmosis unit 12. With this construction, as soon as the valve member 44 moves to a slightly opened state, the cross sectional area exposed to the tap water pressure increases significantly, resulting in effective snap-action movement to the full open position. Supply of the tap water 14 to the reverse osmosis unit 12 is thus resumed to correspondingly resume production of purified water. Moreover, this shut-off valve construction assures that a substantial quantity of the purified water will be dispensed from the full reservoir before the shut-off valve will open, thereby also assuring that the system will thereafter operate for a substantial period of time to produce purified water to re-fill the reservoir. For example, in the illustrative embodiment, the flow area of the port 46 in combination with the gallery 56 provide dispensing of about one gallon from a full three gallon reservoir before the inlet shut off valve opens. 
     As the reservoir 16 approaches a filled condition, the pressure differential across the valve member 44 decreases at a slow rate since purified water production proceeds at a relatively slow flow rate. Eventually, the pressure differential across the valve member 44 approaches a condition causing the valve member 44 to displace slowly toward the closed position. As this pressure differential decreases, and the valve member 44 moves slowly to the closed position, it is known that the operating efficiency of the reverse osmosis membrane 32 also decreases. 
     The water quality monitor 20 of the present invention responds to the above-discussed cyclic operation of the purification system to take water quality test readings within a restricted portion of the system operation cycle when the reverse osmosis unit 12 is substantially at maximum operating efficiency. That is, the present invention recognizes that closure of the inlet shut-off valve 42 removes the pressure differential across the reverse osmosis membrane 32, such that some of the previously-removed contaminants and impurities can leach or migrate across the membrane to contaminate localized portions of the previously produced purified water within pure water flow paths of the system manifold 22. The improved water quality monitor of the present invention thus does not take test readings within a period of time that the inlet shut-off valve is closed, or immediately after opening thereof, to avoid inadvertent and inaccurate reading of a localized pocket of poor quality water. Similarly, the present invention does not take a test reading during the period of time that the shut-off is near the closed condition, since the reduced pressure differential causes the reverse osmosis unit to function at less than maximum operating efficiency. Instead, all test readings are taken during system operation with a substantially optimum pressure differential across the reverse osmosis unit, referred to herein as mid-cycle. The test readings are stored in memory for subsequent and periodic display, for example, each time the faucet valve 18 is opened to dispense water. 
     Operation of the water quality monitor 20, in coordination with cyclic operation of the reverse osmosis unit 12, is obtained by means of a reset switch 60 associated with the inlet shut-off valve 42. As shown in FIGS. 2-4, in accordance with the preferred form of the invention, a shuttle piston 62 carries a magnet 64 in close association with a reed switch 66 mounted above the shut-off valve. A low force spring 68 may be provided to ensure piloted back-and-forth movement of the shuttle piston 62 and magnet thereon with the diaphragm-mounted valve member 44. The reed switch 66 is positioned to be closed each time the valve member 44 moves to the full open position. As the valve member 44 transitions slowly toward the closed position, passing a point about half-way to a fully closed state, the magnet 64 switches the reed switch 66 to an open state. This opening and closure movement of the reed switch 66 is effective to operate the water quality monitor 20, as will be described in more detail. Although the reed switch 66 is moved to the open state when the shut-off valve member 44 is about half-way toward the closed position, it will be understood that this may and will often occur at a point in time which is more than half-way between valve member opening and subsequent closure. 
     FIG. 5 is a schematic block diagram illustrating the structure and functional operation of the water quality monitor. In particular, the reset switch 60 is connected to a convenient and appropriate power source, such as a 9-volt battery 70. When the reed switch 66 is closed upon movement of the shut-off valve 42 to the full open position to resume tap water inflow to the purification system, a charging circuit 72 is connected to the battery 70. The charging circuit 72 functions to develop an appropriate electrical charge which is stored until the reed switch 66 opens. As described above, opening movement of the reed switch 66 occurs in response to movement of the shut-off valve member 44 to a position reflective of a mid-cycle condition of the reverse osmosis unit 12, wherein the system is operating substantially at full pressure to produce purified water. When the reed switch opens, the charging circuit delivers a pulse to a monitor circuit 74 to couple an appropriate test pulse to a tap water electrode 76 and to a pure water electrode 78. These electrodes are conveniently mounted at appropriate positions along flow paths within the manifold 22, as shown in FIG. 2. The monitoring circuit 74 functions to determine the comparative conductivities of the tap water inflow 14 and produced purified water, for purposes of monitoring the operating efficiency of the reverse osmosis unit 12. 
     As shown in FIG. 5, the test reading taken by the monitor circuit 74 is delivered to a memory 80 for storage and subsequent display. Conveniently, the monitoring circuit 74 may take any one of several forms known in the art, particularly such as those monitoring circuits described in U.S. Pat. Nos. 5,057,212 and 5,145,575, which are incorporated by reference herein. The test reading stored in memory 80 is subsequently and periodically displayed by means of an appropriate signal such as indicator lights 82. In accordance with a preferred form of the invention, the display is triggered each time pure water is dispensed such as when the faucet valve 18 is opened, which correspondingly causes a flow switch 84 or the like to trigger the memory 80 for purposes of displaying the stored test reading via energization of the appropriate indicator light 82. As one example, a green indicator light can be illuminated when the conductivity test reading stored in memory indicates acceptable quality of the purified water. By contrast, a yellow indicator light can be illuminated when the test reading indicates unacceptable water quality. When the water quality is unacceptable, proper system performance is typically restored by changing the reverse osmosis membrane 32. 
     The water quality monitor 20 is also designed, in preferred form, to operate the indicator lights 82 in a manner signalling other system conditions for which remedial action may be desirable or necessary. Specifically, the bank of indicator lights 82 is operated by a light driver circuit 86 adapted to receive auxiliary inputs reflective of important system conditions. As shown in FIG. 5, one auxiliary input is obtained from a pressure sensor or switch 88 which indicates the tap water pressure at the downstream side of the prefilter stage 24. When that pressure falls to a predetermined level indicative of a substantially clogged prefilter cartridge 30 (FIG. 2), the light driver circuit 86 responds to operate the indicator lights 82 in a known manner, for example, by illuminating all of the lights simultaneously. 
     The light driver circuit 86 also receives an input from the battery 70. When the detected battery voltage falls below a threshold level, indicating that the battery 70 needs to be replaced, the driver circuit 86 responds to control the indicator lights in a manner reflecting low battery power, such as by preventing illumination of any light when a test reading should normally be displayed. 
     Another input to the light driver circuit 86 is obtained from a counter 90 which counts the number of times that the flow switch 84 is operated by opening the faucet valve 18, without closure of the inlet shut-off valve 12. In this regard, long-term operation of the purification system 10 without periodic closure of the shut-off valve 42 normally indicates a mechanical failure of the shut-off valve 42. The counter 90 thus signals the light driver circuit 86 when this condition occurs so that the indicator lights will be operated in a known manner, such as by illumination of a red indicator light. 
     In addition, if desired, the light driver circuit may include a timer 91 to control the duration of time that the indicator lights 82 are energized. For example, the timer 91 may be set to conveniently extinguish an energized indicator light after a few seconds of illumination, such that the desired indication is provided without excess power drain on the battery 70. 
     FIGS. 6A-6D show the water quality monitor 20 in a preferred circuit implementation in accordance with the functional operating features described above. It will be understood by persons skilled in the art, however, that these functional features can be carried out in a variety of different circuit arrangements. 
     More specifically, initial movement of the inlet shut-off valve 42 from the closed position to a full open position as viewed in FIG. 3, displaces the magnet 64 on the shuttle piston 62 for resetting the reed switch 66 to a closed position. As shown in FIG. 6A, closure of the reed switch 66 connects the monitor circuit to the battery 70 to charge a capacitor C5 to a voltage level corresponding with battery voltage. The circuit is designed to charge the capacitor C5 over a time period sufficiently long so that mechanical rebound or bounce of the shut-off valve does not impact operation of the charging circuit 72. 
     Subsequently, when the reed switch 66 is opened during mid-cycle operation of the reverse osmosis unit 12, as described above, the charge stored by the capacitor C5 is discharged through a NAND gate U5C and a resistor R2, causing the output of the NAND gate U5C to be delivered as a pulse to the set pin of a flip-flop U4A. The set pulse functions to set the flip-flop U4A so that the &#34;Q&#34; output is high and the &#34;not Q&#34; output is low. The &#34;not Q&#34; low output biases a transistor Q3 to an &#34;on&#34; state, resulting in connection of the battery 70 to the input of a voltage regulator U6. This voltage regulator U6, as shown, regulates the voltage applied to the circuit components to a constant level of about 5-volts. The high output at the &#34;Q&#34; terminal of the flip-flop U4A turns a diode D3 to an &#34;off&#34; state, and thereby allows a clock U5B, U5A to switch to an &#34;on&#34; state. A counter U2 is also reset by the same &#34;Q&#34; high output of the flip-flop U4A. 
     The monitor circuit 74 that generates the test pulse for taking conductivity readings is comprised of flip-flops U3A, U3B, and the clock U5B, U5A. The &#34;Q&#34; output of flip-flop U3A is connected to the tap water electrode 76, and the &#34;Q&#34; output of flip-flop U3B is connected to the pure water electrode 78. An output capacitor C8 of the voltage regulator U6 charges, when the reed switch 66 is opened, in a short period of time, about 6 milliseconds before reaching the regulated voltage output. Accordingly, the flip-flops U3A, U3B that generate the test pulse applied to the electrodes are held in a reset condition for approximately 10 milliseconds, comprising the time for a capacitor C1 to charge to a voltage level insufficient to reset the flip-flops U3A, U3B. This short time delay also allows the clock U5B, U5A to turn on and stabilize. At the end of this short interval of about 10 milliseconds, the flip-flops U3A, U3B become active. The first clock cycle causes the &#34;Q&#34; output of the flip-flop U3A to go high while the &#34;Q&#34; output of the flip-flop U3B remains low, thereby making the pure water electrode 78 negative with respect to the tap water electrode 76. The second clock cycle causes the &#34;Q&#34; output of the flip-flop U3B to go high, such that the potential between the &#34;Q&#34; outputs of the two flip-flops U3A, U3B is zero. The third clock cycle causes the &#34;Q&#34; output of the flip-flop U3A to go low, thus making the pure water electrode 78 positive with respect to the negative tap water electrode 76. The fourth clock cycle causes the &#34;Q&#34; output of the flip-flop U3B to go low, making the potential between the electrodes 76, 78 zero. 
     The negative pulse through the electrodes 76, 78 on the first clock cycle is ignored with respect to taking a conductivity reading. When the pulse applied to the electrode 78 is positive, that pulse is also applied to a voltage divider circuit defined by resistors R4, R5 and an adjustable potentiometer TR1. The potentiometer TR1 is set to apply a proportional or comparative voltage to the negative terminal of a comparator U1B, the positive terminal of which is connected to a common electrode 92. When the comparison voltage representing the proportionate conductivity readings of the pure and tap water supplies exceed a reference point selected by adjustment of the potentiometer TR1, the output of the comparator will go high and indicate that pure water production is of unacceptable quality. Conversely, if the comparison voltage applied to the comparator U1B is less than the reference, the output of the comparator U1B is low and indicates that the condition of produced purified water is acceptable. 
     The test reading on the output side of comparator U1B is connected to the data input of a flip-flop U4B for the duration of the positive test pulse. When the test pulse terminates, the &#34;Q&#34; output of the flip-flop U3B switches to the low state, causing a transistor Q2 to turn &#34;off&#34;, which in turn produces a positive clock edge pulse to be delivered to flip-flops U4A, U4B. As a result, the data is stored by the flip-flop U4B. Specifically, if the signal to the data input pin of the flip-flop U4B is low, indicating acceptable water quality, the state of the flip-flop U4B remains unchanged. However, if the data signal is high, indicating unacceptable water quality, the clock pulse changes the state of the flip-flop U4B so that its output &#34;Q&#34; is high and the output &#34;not Q&#34; is low. 
     The clock pulse referenced above is also supplied to the data input line of the flip-flop U4A. The clock pulse causes the &#34;Q&#34; output of the flip-flop U4A to go low and the &#34;not Q&#34; output to go high, thereby removing the bias from the transistor Q3 in order to switch that transistor Q3 to an &#34;off&#34; state. When the transistor Q3 turns &#34;off&#34;, the voltage regulator U6 is disconnected from the battery 70 and the monitoring circuit is now in an &#34;off&#34; state. 
     When water is drawn from the system as by opening the faucet valve 18 to dispense water from the reservoir 16, the flow switch 84 connects the circuit to the battery 70, causing a capacitor C2 to charge. Instantaneously this applies battery voltage to the input of NAND gate U5D. While this voltage exceeds the threshold voltage of NAND gate U5D, the output of the NAND gate U5D goes low and biases the transistor Q3 to an &#34;on&#34; state. Accordingly, once again, the battery 70 is connected by the transistor Q3 to the voltage regulator U6, thereby turning on the voltage regulator and other circuit components. 
     A pair of voltage dividers including resistors R8, R23 and R9, R28 are connected to an amplifier U1D for biasing the display circuitry and also to provide an indication of a low battery voltage condition. In this regard, when the battery has an acceptable voltage level, the output of the amplifier U1D will be low to activate another voltage divider R10, R24. This voltage divider has a junction between R10 and R24 connected to comparators U1A and U1C, to create a reference voltage for the display circuitry. The other terminals of these two comparators are connected respectively to the &#34;Q&#34; and &#34;not Q&#34; outputs of the flip-flop U4B within which the test reading data has been stored, as previously described. When the stored data indicates acceptable water quality, the &#34;Q&#34; output of the flip-flop U4B is low and the &#34;not Q&#34; output of the flip-flop U4B is high. With this configuration, the yellow indicator light is disabled and the green indicator light is enabled to indicate visually the previously stored test reading, namely, that system operational performance is within acceptable limits. Conversely, when the nature of the signals on the outputs &#34;Q&#34; and &#34;not Q&#34; of the flip-flop U4B are reversed, indicating unacceptable water quality conditions, the green indicator light is disabled and the yellow indicator light is enabled, to indicate unacceptable system performance. 
     When the battery voltage decreases to an unacceptable low level, as determined by the voltage dividers R9, R28 and R8, R23, the output of the comparator U1D goes high. This output is connected to the comparators U1A and U1C, resulting effectively in disablement of the yellow and green lights to prevent illumination thereof, regardless of the stored test data, when the faucet valve is opened. Thus, when a test reading should normally be displayed, the absence of an illuminated light indicates that the battery needs to be replaced. 
     The pressure switch or sensor 88 on the prefilter stage 24, which may be located at the discharge port 46 of the prefilter stage 24, closes when the detected pressure indicates that the prefilter cartridge 30 needs to be replaced. The pressure switch connects the circuit common (ground) to the plus terminal of the comparators U1A and U1C, causing both comparators to have a low output and thereby enabling both of the yellow and green indicator lights when the faucet valve is open. When both lights are illuminated, the display thus indicates that the prefilter cartridge 30 should be replaced. 
     The counter 90 is provided as part of the circuitry to count the number of times that the flow switch 84 is closed upon opening of the faucet valve 18, without return movement of the reed switch 66 to a full open position at the conclusion of an off cycle. When the flow switch 84 is closed and the capacitor C2 charges, as described above, the NAND gate U5D produces a low output. As result, a capacitor C6 discharges through a resistor R19, such that a clock input of the counter 90 (U2) increments one count. The counter 90 increments each time the faucet valve is opened, but resets to zero when the reed switch 66 opens upon resumption of tap water flow following a system off cycle. In the preferred form, if the counter 90 increments to 128 prior to reset, a red indicator light is enabled to provide a visual indication that a system failure has occurred. In most cases, this failure is attributable to a mechanical failure of the inlet shut-off valve, with the total accumulated in the counter 90 representing the time that the shut-off valve has remained open, without cycling to the closed position. In other words, the counter essentially acts as a timer to mark opening movement of the shut-off valve, and then clocks the time that the shut-off valve remains open, thereby to provide the appropriate signal if the shut-off valve does not cycle to the closed position in a reasonable period of time. Of course, alternative time methods and devices may be used to provide this system function. 
     A battery switch 73 (FIG. 6A) is conveniently provided for initial system installation, to set the monitor circuit 74 in a condition to indicate acceptable water quality, pending actual conductivity readings at mid-cycle conditions. The battery switch 73 is normally closed, and connects power to the reset pin 10 of the flip-flop U4B via a resistor R30 and a capacitor C9 (FIG. 6C). After a short charge time for capacitor C9, the voltage applied to pin 10 is low, whereby the monitor circuit 74 will energize the green indicator light when the flow switch 84 is closed. During initial system installation, therefore, the circuitry can be set to indicate acceptable water quality by opening and closing the battery switch 73, thus permitting verification that the green indicator light is operational. Of course, during subsequent normal cyclic system operation, the indicator light will be energized according to actual conductivity test readings. 
     FIG. 7 shows one preferred form for the flow switch 84, installed along a dispense flow path leading from the dispense fitting 40 (FIGS. 1 and 2) to the faucet 18. The flow switch 84 includes a spring-loaded poppet 102 which moves to an open position relative to a flow port 104, when the faucet 18 or other dispense device is opened to initiate a dispense water flow. The degree of movement or displacement of the poppet 102 is a direct function of the volumetric flow rate through the dispense flow path. 
     The poppet 102 is slidably carried on a guide stem 106 having a reed switch 108 or the like fitted into a rear side counterbore 110 of the poppet. When the poppet 102 moves to an open position, a magnet 112 carried by the poppet is displaced over the reed switch 108 sufficiently to close the reed switch and supply power from the battery source to a flow volume monitor 114. The thus-energized monitor 114 then receives signals from a poppet position detector 116, such as a Hall effect transducer capable of tracking the poppet by monitoring the position of the magnet 112. Alternate poppet position sensors such as optical sensors may be used. In any case, the position sensor 116 signals the flow volume monitor 114 regarding poppet position when open so that a running accumulation of actual water volume dispensed can be derived. The flow volume monitor 114 can be appropriately coupled to the light driver circuit 86 as viewed in FIG. 7 to energize the indicator lights in a manner indicating that a predetermined volume of pure water has been dispensed. When this occurs, it may be necessary or desirable to replace the filter cartridge 36 of the postfilter stage 28. 
     FIG. 8 shows an alternative preferred form of the monitor circuit, particularly with respect to a modification adapted for implementation into the circuit diagram shown in FIGS. 6A-6D. In this regard, the modified portion of the monitor circuit shown in FIG. 8 includes a labelling scheme which is consistent with FIGS. 6A-6D, with circuit connections broken off and inclusive of identification of the component or components in FIGS. 6A-6D to be connected thereto. In general terms, FIG. 8 discloses circuit means for requiring a selected successive number of unacceptable water quality test readings before the yellow indicator light is actually illuminated. That is, as will be described, the circuit of FIG. 8 will cause the green indicator light to be energized despite the actual test reading stored in memory indicative of unacceptable water quality, unless and until three consecutive test readings indicate unacceptable water quality. With this variation, there is little or no chance that three consecutive false &#34;unacceptable&#34; readings will occur, for whatever reason, such that the yellow light will not be energized unless the reverse osmosis unit is consistently producing water of unacceptable quality. 
     More specifically, with reference to FIG. 8, the flip-flop U4B is coupled to the comparators U1A and U1B through a counter U7 and inverters U8E and U8F. The &#34;Q&#34; output of U4B is unconnected, whereas the &#34;not Q&#34; output is connected to the reset input of the counter U7. When the &#34;not Q&#34; output of the flip-flop U4B is high, reflective of a test reading indicating acceptable water quality, the counter U7 remains in a reset condition and the green indicator light is enabled via the comparator U1C for energization when the faucet valve is opened, in the same manner as previously described. That is, the output Q2 of the counter U7 is connected via the diode D1 to the inverting input of the comparator U1A, thus inhibiting or disabling the yellow indicator light. This output Q2 is also connected to the inputs of the inverters U8E and U8F, with the output of the latter inverter U8F being connected via the diode D2 to the inverting input of the comparator U1C, such that the green indicator light is enabled. 
     When the flip-flop U4B receives a test reading indicative of unacceptable water quality, the &#34;not Q&#34; output switches to a low state to remove the reset from the counter U7. The clock input of U7 is connected through a time delay circuit R31, C10 to the clock input of the flip-flop U4B. Thus, concurrently with the first test reading indicative of unacceptable water quality, the reset condition for the counter U7 is removed. The next consecutive test reading indicative of unacceptable water quality causes the output Q1 of the counter U7 to go high, and the third consecutive test reading of unacceptable water quality yields a high output on the counter output Q2 connected to the comparators U1A and U1C as previously described. The high output on Q2 functions through the inverter U8F and comparator U1C to disable the green indicator light, while turning &#34;off&#34; the diode D1 to enable the yellow indicator light via the comparator U1A. The output of the inverter U8E inhibits the counter U7 from accepting any more counts prior to reset, whereby the yellow light will be enabled unless and until counter reset occurs. Such reset does occur whenever a subsequent test reading reflective of acceptable water quality yields a high &#34;not Q&#34; output from the flip-flop U4B. When the counter U7 is reset in this manner, the green indicator light will be enabled until three successive &#34;unacceptable&#34; water quality readings occur. Of course, the number of &#34;unacceptable&#34; readings in succession needed to enable the yellow indicator light may be varied by appropriate connection of the outputs Q1-Q12 of the counter U7. 
     The invention thus provides an improved water monitoring circuit and system wherein all conductivity test readings are taken during a period of time when the reverse osmosis unit 12 is operating substantially at maximum performance level. By contrast, test readings are not taken during periods of operation at a performance level less than optimum, such as immediately after tap water flow is resumed to the system following an off cycle, or immediately prior to closure of an inlet shut-off valve at the on set of an off cycle. Rather, the conductivity test reading s are taken substantially during a mid-cycle operating condition. The reading is stored and subsequently displayed, in the preferred form, each time a faucet valve is opened to dispense water. A selected number of &#34;unacceptable&#34; water quality readings may be required in succession before an indication of unacceptable water quality is provided. 
     A variety of further modifications and improvements to the improved conductivity monitor of the present invention will be apparent to those skilled in the art. For example, it will be recognized and understood that the monitor 20 can be operated by various trigger signals to take a test reading at a mid -cycle condition of the reverse osmosis unit 12, such as by operating the charging circuit 72 in response to water level detectors within the reservoir. Moreover, alternate means can be provided for displaying the test reading held in memory, such as clock means for displaying the stored test reading at regularly timed intervals. Accordingly, no limitation on the invention is intended by way of the foregoing description and accompanying drawings, except as set forth in the appended claims.