Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Chemical analysis
Reexamination Certificate
1998-05-13
2001-04-24
Assouad, Patrick (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Chemical analysis
C324S439000, C137S392000
Reexamination Certificate
active
06223129
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention relates generally to conductivity measurement systems and the low-cost probes thereof, such as used in commercial dishwashers, and particularly to conductivity measurement systems which compensate for probe contamination.
2. Description of Related Art
Industrial dishwashers use conductivity measurement systems to maintain proper detergent concentrations in the dishwashers' wash water. Conductivity measurement systems are well known and typically include a probe that has first and second electrodes submerged in the wash water. A signal from a source circuit is applied to the electrodes to induce a current between the electrodes. This current, which is mirrored in the source circuit, is determined by dividing the voltage in the source circuit by the impedance of the source circuit. The conductivity of the wash water is then determined by dividing the current between the electrodes by the voltage across the electrodes.
Current flow in an aqueous solution, e.g., the wash water, is facilitated by the flow of ions between the electrodes. In an industrial dishwasher, the ions are provided by the detergent. Thus, increasing the detergent concentration results in a corresponding increase in the conductivity of the wash water. The relationship between wash water conductivity and detergent concentration for a particular detergent is typically stored in a look-up table, thereby allowing detergent concentration to be easily derived from wash water conductivity.
As current is induced between electrodes in an aqueous solution, ions begin accumulating on one of the electrodes. The ions accumulating on the electrode surface each occupy a finite space such that after a time period t there is no more available surface area on the electrodes on which ions may accumulate. This phenomena, known in the art as polarization, reduces current flow between the electrodes and may result in erroneous conductivity measurements which, in turn, lead to erroneous detergent concentration measurements. Thus, when the electrodes become polarized, the detergent concentration of the wash water is perceived by the dishwasher to be too low, thereby leading to the addition of detergent to wash water that may, in reality, already be of a desired detergent concentration.
Further, when used as described above, the electrodes undesirably accumulate non-conductive particles thereon which, in turn, reduce the effective area of the electrodes. As a result, contamination of the electrodes speeds the above-described polarization of the electrodes and, therefore, diminishes the useful life of the electrodes.
In theory, the effect of polarization upon conductivity measurements can be eliminated by calculating conductivity the instant current is induced between the electrodes, since at time t=0 ions have not yet accumulated on the electrodes. Here, the voltage between the electrodes must be measured just as the source signal that induces current in the wash water is asserted. Unfortunately, such an approach is not feasible. First, there are significant characteristic variations between ion species during the first 1-2 microseconds of aqueous current flow. Since the ion species of the detergent is typically unknown, measurements taken within the first 1-2 microseconds are unreliable. Second, it is very difficult to fabricate a circuit which can produce a source pulse and then immediately capture an analog reading produced by the source pulse.
U.S. Pat. No. 4,756,321 discloses an industrial dishwashing system in which a continuous AC signal is applied to first and second electrodes submerged in wash water to induce a current between the electrodes. The resulting current is measured over time, and then used to calculate the conductivity of the wash water. Conductivity is then converted into a logarithmically scaled detergent concentration. Here, the continuous current flow between the electrodes results in a continually increasing polarization of the electrodes. As a result, the electrodes must be either cleaned or replaced at regular intervals. The servicing of the electrodes is not only expensive, but also reduces operating efficiency of the dishwasher. Further, this system's inability to measure or predict electrode contamination makes it even more difficult to optimize the useful life of the electrode.
Another approach involves driving the electrodes with a pulsed DC signal as described, for instance, in U.S. Pat. No. 4,119,909. In that system, the pulsed DC signal induces short pulses of current between the electrodes in the wash water. Use of short current pulses reduces polarization and, thus, increases the useful life of the electrodes, as compared to the averaging technique disclosed in U.S. Pat. No. 4,756,321. However, conductivity measurements provided by this approach are nevertheless influenced by polarization. Further, this system, like that disclosed in U.S. Pat. No. 4,119,909, is unable to measure or predict electrode contamination. It is therefore difficult to accurately determine when or at what rate the measured conductivity deviates from the actual conductivity and, as a result, the accuracy with which this approach maintains the detergent concentration at a target level is compromised. It is thus also difficult to maximize the intervals at which the electrodes are cleaned or replaced and, therefore, difficult to maximize the useful life of the electrodes.
SUMMARY
An apparatus and method for measuring conductivity of an aqueous solution are disclosed which compensate for polarization and provide warning of electrode contamination. In accordance with the present invention, one or more DC pulses are applied to first and second electrodes submerged in an aqueous solution such as, for instance, the wash water of an industrial dishwasher. The voltage at the first electrode is measured at first and second predetermined times after initiation of the one or more DC pulses. The difference between the respective first and second measured voltages is calculated and then compared to a predetermined threshold value. If the difference voltage exceeds the predetermined threshold value, thereby indicating that the electrodes are sufficiently contaminated so as to soon require cleaning or replacement, an alarm signal is asserted. In this manner, present embodiments maximize the useful life of the electrodes and, thus, minimize servicing costs.
Further, present embodiments provide conductivity measurements compensated for polarization. Linear regression of the first and second measured voltages is used to calculate the voltage at the first electrode at the beginning of the one or more DC pulses, i.e., at time t=0. The resistivity of the solution is calculated using Ohm's Law, and then converted into conductivity according to the known K factor of the solution. In some embodiments, conductivity is provided in logarithmically scaled measurement units, known in the art as Beta units. Since the conductivity of the solution is calculated according to the electrode voltage at the beginning of the DC pulse, the measured conductivity of the solution is not influenced by polarization. In this manner, present embodiments effectively compensate for polarization, and thereby produce a more accurate conductivity measurement, as compared to the prior art. As a result, present embodiments greatly reduce the likelihood of incorrect detergent concentrations resulting from erroneous conductivity measurements.
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Chan Wai Yin Cedric
Livingston James W.
Assouad Patrick
DiverseyLever, Inc.
Pennie & Edmonds LLP
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