I. Field of the Invention
This invention relates generally to water treatment apparatus, and more specifically to a new and improved method and apparatus for indicating the need for regeneration of the ion exchange resin bed of a water softener.
II. Discussion of the Prior Art
In most present day residential and commercial water softening installations, regeneration is made to occur based upon elapsed time only and independent from the volume of water treated. In such conventional systems, an electromechanical timer mechanism is set to initiate the regeneration cycle at fixed times during each week, irrespective of the particular water demand during the period between successive regeneration cycles. It is, of course, wasteful of both water and brine solution to regenerate the ion exchange bed when that bed is not substantially exhausted. For example, in the event that a homeowner forgets to turn off the timer unit when departing on a vacation trip, the softener unit may repeatedly go through its regeneration cycle even though the ion exchange resin bed is fully active.
A more preferable way of initiating the regeneration cycle is to measure the condition of the bed and only initiate regeneration when the bed is close to being fully exhausted. Various approaches have been described in the prior art for achieving this result. Typically, means are provided in the ion exchange bed for sensing the change in conductivity between two test probes. When a voltage is impressed across the test probes, the amplitude of the current flowing is related to the condition of the bed. A relatively high current flow is established in the presence of sodium ions in the resin, whereas, as the sodium ions are replaced with magnesium and calcium ions, the conductivity of the resin bed decreases and the current flow is accordingly reduced. Such an approach has not been altogether successful in that the electrode probes tend to become contaminated with iron deposits or the like over a period of use which causes the system control point to drift erratically and adversely affects the ability to get accurate readings of the conductivity factor of the bed. Then too, municipalities periodically introduce different chemicals into the water system which affects the conductivity of the electrolyte existing between the sensing probes. As such, it is difficult to provide an electronic control circuit for monitoring the conductivity and providing a reliable signal which indicates that depletion is imminent and that regeneration should take place. Because the conductivity sensing approach involves the application of an external voltage across an electrode gap, electrolysis takes place leading to the generation of hydrogen gas which could be undesirable. A further drawback of such a conductivity sensing approach is that immediately re-triggering of the regeneration system can occur due to the presence of hard water in the tank left by the rinse cycle. The problem is especially acute where the high volume rinse operation results in channeling of hard water in the center or core of the resin bed. Repeated triggering is, of course, to be avoided because it is unnecessary and wasteful of both brine and water.
In the Tejeda U.S. Pat. No. 3,869,382 there is described a technique in which the exhaustion of the resin exchange bed is determined by measuring an internally generated potential which undergoes a sharp change as exhaustion is approached. That is, if a pair of electrodes is located in the flow path at positions spaced in the direction of flow with at least a portion of the ion exchange resin material interposed between the electrodes, a potential is electrochemically generated so long as the water is flowing. This potential is present between the electrodes and it experiences a rather marked change in level when the portion of the column between the electrodes changes from its active state to its exhausted state. The change in potential is periodically manually sensed while the water is flowing and used to indicate the need to initiate the manual regeneration cycle.
The method and apparatus of the Tejeda '382 Patent also may not be used in a building's central water softening system or a residential system, but is limited to the use on individual faucets in that building as disclosed. The reason for this is that in practical residential, commercial and industrial installations, existing electrical codes typically require that the inlet pipes and the outlet pipes of the system be electrically bussed together to provide a continuous earth ground, since the plumbing is often used as an electrical earth return to ground potential. Because the water softening tank is usually fabricated from a fiberglass reinforced plastic material or the like (an insulator), electrical codes require that a conductive strap be connected between the water inlet pipe to the softener tank and the water outlet pipe leading to the building's supply mains. Under these conditions, with no water flow, the internally generated potential observed in the Tejeda system is reduced to the point where the potential cannot reliably indicate the need for regeneration.
The present invention makes use of some of the principles set forth in the above-referenced Tejeda Patent but is considered to be an improvement thereover in that not only is the change in potential between the electrodes continuously monitored, but so is the rate of change of that potential with respect to time. Thus, even when the flow of water through the ion exchange bed is intermittent, the passage of the hardness front to a location intermediate the spaced apart electrodes will be detected. Were it not for the sensing of the rate of change of potential, it is conceivable that an intermittent usage of water in the building could result in the travel of the hardness front through the interelectrode zone without producing a sufficiently high potential to be detected by the external sensing circuitry. As such, a system using only the approach described in the Tejeda Patent could result in complete exhaustion of the bed without the desired initiation of a regeneration cycle.