Impedance measurement of a PH electrode

• US 20080042665A1
• Filed: 08/17/2007
• Published: 02/21/2008
• Est. Priority Date: 08/18/2006
• Status: Active Grant

• Current Assignee: Rosemount Analytical, Inc. (Emerson Electric Company)
• Original Assignee: Rosemount Analytical, Inc. (Emerson Electric Company)
• Inventors: Feng, Chang-Dong, Rezvani, Behzad, Lomibao, Jeffrey
• Granted Patent: US 8,063,644 B2
• Time in Patent Office: Days
• US Class Current: 324/713
• CPC Class Codes: G01N 27/4165 for pH meters

Abstract

A method of measuring impedance of a pH electrode is provided. A test current is applied to the pH electrode for a time duration that is less than 50 percent of a time constant that is associated with electrical characteristics of the pH electrode. A voltage response of the pH electrode is measured when the test current is applied to the pH electrode. An impedance of the pH electrode is calculated as a function of the voltage response.

Claims

1. A method of measuring impedance of a pH electrode, the method comprising:

applying a test current to the pH electrode for a time duration that is less than 50 percent of a time constant that is associated with electrical characteristics of the pH electrode;

measuring a voltage response of the pH electrode when the test current is applied to the pH electrode; and

calculating an impedance of the pH electrode as a function of the voltage response.

2. The method of claim 1, and further comprising:

determining a time constant of the pH electrode as a function of an approximated circuit equivalent of the pH electrode, the approximated circuit equivalent being indicative of electrical characteristics of the pH electrode.

3. The method of claim 1, and further comprising:

calculating a source voltage of the pH electrode as a function of the voltage response.

4. The method of claim 1, wherein measuring a voltage response comprises sampling a plurality of voltages across the pH electrode.

5. The method of claim 4, wherein sampling a plurality of voltages comprises taking a plurality of voltage samples across the pH electrode during a time duration that is less than approximately three seconds.

6. The method of claim 1, wherein applying a test current comprises:

applying a first test current to the pH electrode for a first time duration and applying a second test current to the pH electrode for a second time duration, the second test current being substantially equal to but opposite in polarity to the first test current; and

wherein measuring a voltage response comprises measuring a first voltage response when the first test current is applied to the pH electrode and measuring a second voltage response of the pH electrode while the second test current is applied to the pH electrode.

7. The method of claim 6, comprising:

applying the first test current to the pH electrode for a first time duration that is less than 25 percent of a time constant that is associated with electrical characteristics of the pH electrode; and

applying the second test current to the pH electrode for a second time duration that is less than 25 percent of a time constant that is associated with electrical characteristics of the pH electrode.

8. The method of claim 7 wherein the first and second time durations are substantially similar.

9. The method of claim 6, wherein calculating the impedance of the pH electrode comprises:

calculating an impedance of the pH electrode as a function of the first voltage response and the second voltage response.

10. The method of claim 6, and further comprising:

calculating a source voltage of the pH electrode as a function of the first voltage response and the second voltage response.

11. The method of claim 6, wherein measuring the first voltage response comprises acquiring a first plurality of voltage samples across the pH electrode when the first test current is applied to the pH electrode, and wherein measuring the second voltage response comprises acquiring a second plurality of voltage samples across the pH electrode when the second test current is applied to the pH electrode.

12. The method of claim 11, wherein each of the first and second plurality of voltage samples comprises three voltage samples taken across the pH electrode.

13. The method of claim 12, wherein the plurality of voltage samples are averaged over multiple line voltage periods.

14. The method of claim 1, wherein calculating an impedance of the pH electrode as a function of the voltage response is performed within a duration that is less than about 50 percent of the time constant that is associated with electrical characteristics of the pH electrode.

15. A method of measuring impedance of a pH electrode, the method comprising:

determining a circuit equivalent based on electrical characteristics of the pH electrode;

generating an approximated circuit equivalent as a function of the circuit equivalent, wherein generating an approximated circuit equivalent includes selecting an arbitrary non-zero frequency component approximation;

applying a test current to the pH electrode;

measuring a voltage response of the pH electrode while the test current is applied to the pH electrode; and

calculating an impedance of the pH electrode as a function of the voltage response and the approximated circuit equivalent.

16. The method of claim 15 wherein selecting an arbitrary non-zero frequency component approximation comprises selecting an arbitrary approximation for a component associated with a solution junction layer of the pH electrode.

17. The method of claim 15 wherein the solution junction layer is a gel layer of the pH electrode.

18. The method of claim 15 and further comprising:

determining a time constant of the approximated circuit equivalent, wherein measuring a voltage response of the pH electrode is based on the time constant.

19. The method of claim 18, wherein measuring a voltage response of the pH electrode comprises taking a plurality of voltage samples across the pH electrode over a time duration that is less than 50 percent of the time constant.

20. The method of claim 18, wherein applying the test current to the pH electrode comprises applying the test current for a time duration that is less than 50 percent of the time constant.

21. The method of claim 20, wherein applying the test current comprises

applying a first test current to the pH electrode for a first time duration and applying a second test current to the pH electrode for a second time duration, the second test current being substantially equal to but opposite in polarity to the first test current, and wherein measuring a voltage response comprises

measuring a first voltage response when the first test current is applied to the pH electrode and measuring a second voltage response of the pH electrode while the second test current is applied to the pH electrode.