Apparatus and method for conductivity measurement including probe contamination compensation

• US 6,223,129 B1
• Filed: 05/13/1998
• Issued: 04/24/2001
• Est. Priority Date: 05/13/1998
• Status: Expired due to Term

• Current Assignee: Diversey Incorporated (Sealed Air)
• Original Assignee: Diverseylever Incorporated (Unilever PLC)
• Inventors: Livingston, James W., Chan, Wai Yin Cedric
• Primary Examiner(s): ASSOUAD, PATRICK J
• Application Number:
  US09/078,400
• Time in Patent Office: 1,077 Days
• Field of Search: 702/30, 702/32, 702/50, 324/439, 324/691, 137/392, 137/5, 733/350.5, 422/82.02
• US Class Current: 702/30
• CPC Class Codes: A47L 15/4297 Arrangements for detecting or measuring the condition of the washing water, e.g. turbidity
• CPC Class Codes: G01N 27/06 of a liquid involving electrolysis G01N27/26
• CPC Class Codes: Y10T 137/7306 Electrical characteristic sensing

Abstract

A conductivity measurement system provides one or more DC pulses 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 DC pulse(s). Linear regression of the first and second measured voltages is used to calculate the voltage at the first electrode at the beginning of the DC pulse(s), i.e., at time t=0. The resulting voltage at time t=0 is then used to calculate the conductivity of the solution, thereby compensating for the effects of polarization. Further, the difference between the respective first and second measured voltages is compared to a predetermined threshold value to determine whether the electrodes are so contaminated that polarization compensation is no longer feasible, thereby signaling that the electrodes should be cleaned or replaced.

Claims

1. A method of measuring representations of the electrical conductivity of an aqueous solution which compensates for polarization, said method comprising the steps of:

(a) providing first and second electrodes within said solution;

(b) generating, using a measurement circuit, a DC pulse to induce a current between said first and second electrodes;

(c) sampling the voltage at said first electrode at a first predetermined time interval after the generation of said DC pulse and generating a first voltage value in response thereto;

(d) sampling the voltage at said first electrode at a second predetermined time interval after the generation of said DC pulse and generating a second voltage value in response thereto;

(e) calculating, using linear regression and said first and second voltage values, the voltage at said first electrode contemporaneous with the generation of said DC pulse and generating a third voltage value in response thereto;

(f) producing, in response to said third voltage, a first signal representing the conductivity of said solution.

2. The method of claim 1, wherein said first predetermined time interval is approximately 10 μ

s.

3. The method of claim 2, wherein said second predetermined time interval is approximately 15 μ

s.

4. The method of claim 3, wherein said DC pulse has a duration of approximately 20 μ

s.

5. The method of claim 1, wherein said step (f) further comprises:

(f1) determining the current in said measurement circuit and generating a second signal in response thereto, said second signal representing the current in said solution;

(f2) dividing said second signal by said third voltage to generate said first signal representing the conductivity of said solution.

6. The method of claim 5, wherein said solution contains a detergent having a concentration, said method further comprising the steps of mapping, using a look-up table, said first signal representing the conductivity of said solution to a third signal representing the concentration of said detergent.

7. The method of claim 1, further comprising the steps of:

(g) subtracting said first voltage value from said second voltage value to generate a difference voltage;

(h) comparing said difference voltage to a predetermined threshold voltage; and

(i) sounding an alarm if said difference voltage exceeds said predetermined threshold voltage, said alarm indicating that said electrodes are contaminated.

8. The method of claim 1, where:

in step (b) said DC pulse comprises first and second DC pulses;

in step (c), said first electrode is sampled at said first predetermined time interval after the generation of said first DC pulse to generate said first voltage value; and

in step (d), said first electrode is sampled at said second predetermined time interval after the generation of said second DC pulse to generate said second voltage.

9. The method of claim 1, further comprising the step of:

(c1) generating, using said measurement circuit, a second DC pulse to induce current between said first and second electrodes;

wherein step (d) said first electrode is sampled at a second predetermined time after the generation of said second DC pulse.

10. A method of measuring representations of the electrical conductivity of an aqueous solution which compensates for polarization, the method comprising:

generating a DC pulse to induce a current between first and second electrodes positioned within the solution;

sampling a voltage at the first electrode at a first predetermined time interval after the generation of the DC pulse and generating a first voltage value in response thereto;

sampling the voltage at the first electrode at a second predetermined time interval after the generation of the DC pulse and generating a second voltage value in response thereto;

calculating, as a function of the first and second voltage values, a third voltage corresponding to the voltage at the first electrode at a time contemporaneous with the generation of the DC pulse; and

generating a conductivity signal as a function of the third voltage, the conductivity signal representing the conductivity of the solution.

11. The method of claim 10, wherein generating the conductivity signal includes:

determining a current flowing between the first and second electrodes and generating a current value in response thereto, the current value representing the current flowing in the solution;

generating the conductivity signal as a function of the third voltage and the current value.

12. The method of claim 11, wherein the solution contains a detergent having a concentration, the method further comprising:

mapping, using a look-up table, the conductivity signal to a concentration signal representing the concentration of the detergent in the solution.

13. The method of claim 11, further comprising:

subtracting the first voltage value from the second voltage value to generate a difference voltage;

comparing the difference voltage to a predetermined threshold voltage; and

generating an alarm signal if the difference voltage exceeds the predetermined threshold voltage, the alarm signal indicating that the electrodes are contaminated.

14. A method of measuring representations of the electrical conductivity of an aqueous solution which compensates for polarization, the method comprising:

generating first and second DC pulses to induce a current between first and second electrodes positioned within the solution;

sampling a voltage at the first electrode at a first predetermined time interval after the generation of one of the first and second DC pulses and generating a first voltage value in response thereto;

sampling the voltage at the first electrode at a second predetermined time interval after the generation of another one of the first and second DC pulses and generating a second voltage value in response thereto, wherein the one and the another one of the DC pulses are not the same DC pulse;

calculating, as a function of the first and second voltage values, a third voltage corresponding to the voltage at the first electrode at a time contemporaneous with the generation of either one of the DC pulses; and

generating a conductivity signal as a function of the third voltage, the conductivity signal representing the conductivity of the solution.

15. The method of claim 14, wherein generating the conductivity signal includes:

determining a current flowing between the first and second electrodes and generating a current value in response thereto, the current value representing the current flowing in the solution;

generating the conductivity signal as a function of the third voltage and the current value.

16. The method of claim 15, wherein the solution contains a detergent having a concentration, the method further comprising:

mapping, using a look-up table, the conductivity signal to a concentration signal representing the concentration of the detergent in the solution.

17. The method of claim 15, further comprising:

subtracting the first voltage value from the second voltage value to generate a difference voltage;

comparing the difference voltage to a predetermined threshold voltage; and

generating an alarm signal if the difference voltage exceeds the predetermined threshold voltage, the alarm signal indicating that the electrodes are contaminated.

18. Apparatus for measuring electrical conductivity of an aqueous solution, comprising:

a probe having first and second electrodes, for submersion in a tank containing the solution;

a circuit for generating a DC pulse to induce a current between the first and second electrodes within the solution; and

a controller programmed to;

sample voltage at the first electrode at first and second predetermined time intervals after the generation of the DC pulse;

generate first and second voltage values in response to the sampling of the voltage at the first electrode at the first and second predetermined time intervals, respectively;

calculate, as a function of the first and second voltage values, a third voltage corresponding to the voltage at the first electrode at a time contemporaneous with the generation of the DC pulse; and

generate a conductivity signal as a function of the third voltage, the conductivity signal representing the conductivity of the solution.

19. The apparatus of claim 18, wherein the controller includes software executed by the controller for determining a current flowing between the first and second electrodes and generating a current value in response thereto, the current value representing the current in the solution, and for generating the conductivity signal as a function of the third voltage and the current value.

20. The apparatus of claim 19, wherein the solution contains a detergent having a concentration, and the controller includes software executed by the controller for mapping, using a look-up table, the conductivity signal to a concentration signal representing the concentration of the detergent in the solution.

21. The apparatus of claim 19, wherein the controller includes software executed by the controller for:

subtracting the first voltage value from the second voltage value to generate a difference voltage;

comparing the difference voltage to a predetermined threshold voltage; and

generating an alarm signal if the difference voltage exceeds the predetermined threshold voltage, the alarm signal indicating that the electrodes are contaminated.

22. Apparatus for measuring electrical conductivity of an aqueous solution, comprising:

a probe having first and second electrodes, for submersion in a tank containing the solution;

a circuit for generating first and second DC pulses to induce a current between the first and second electrodes within the solution;

a controller programmed to;

sample voltage at the first electrode at a first predetermined time interval after the generation of one of the first and second DC pulses and generating a first voltage value in response thereto;

sample the voltage at the first electrode at a second predetermined time interval after the generation of another one of the first and second DC pulses and generating a second voltage value in response thereto, wherein the one and the another one of the DC pulses are not the same DC pulse;

generate first and second voltage values corresponding to the sampled voltages;

calculate as a function of the first and second voltage values, a third voltage corresponding to the voltage at the first electrode at a time contemporaneous with the generation of either one of the DC pulses; and

generate a conductivity signal as a function of the third voltage, the conductivity signal representing the conductivity of the solution.

23. The apparatus of claim 22, wherein the controller includes software executed by the controller for determining a current flowing between the first and second electrodes and generating a current value in response thereto, the current value representing the current in the solution, and for generating the conductivity signal as a function of the third voltage and the current value.

24. The apparatus of claim 23, wherein the solution contains a detergent having a concentration, and the controller includes software executed by the controller for mapping, using a look-up table, the conductivity signal to a concentration signal representing the concentration of the detergent in the solution.

25. The apparatus of claim 23, wherein the controller includes software executed by the controller for:

subtracting the first voltage value from the second voltage value to generate a difference voltage;

comparing the difference voltage to a predetermined threshold voltage; and

generating an alarm signal if the difference voltage exceeds the predetermined threshold voltage, the alarm signal indicating that the electrodes are contaminated.

26. Apparatus for measuring electrical conductivity of an aqueous solution, comprising:

a probe having first and second electrodes, for submersion in a tank containing the solution;

a circuit for generating one or more DC pulses to induce a current between the first and second electrodes within the solution;

means for sampling voltage at the first electrode at a first predetermined time interval after the generation of a respective DC pulse of the one or more DC pulses, and for sampling the voltage at the first electrode at a second predetermined time interval after the generation of a respective DC pulse of the one or more DC pulses; and

means for generating first and second voltage values in response to the sampling of the voltage at the first electrode at the first and second predetermined time intervals, respectively, calculating, as a function of the first and second voltage values, a third voltage corresponding to the voltage at the first electrode at a time contemporaneous with the generation of any one of the one or more DC pulses; and

generating a conductivity signal as a function of the third voltage, the conductivity signal representing the conductivity of the solution.

27. The apparatus of claim 26, wherein the generating means includes means for determining a current flowing between the first and second electrodes and generating a current value in response thereto, the current value representing the current in the solution, and for generating the conductivity signal as a function of the third voltage and the current value.

28. The apparatus of claim 27, wherein the solution contains a detergent having a concentration, and the generating means includes means for mapping, using a look-up table, the conductivity signal to a concentration signal representing the concentration of the detergent in the solution.

29. The apparatus of claim 27, wherein the means for generating includes means for:

subtracting the first voltage value from the second voltage value to generate a difference voltage;

comparing the difference voltage to a predetermined threshold voltage; and

generating an alarm signal if the difference voltage exceeds the predetermined threshold voltage, the alarm signal indicating that the electrodes are contaminated.