Impedance measurement of a pH electrode

• US 8,063,644 B2
• Filed: 08/17/2007
• Issued: 11/22/2011
• 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
• Primary Examiner(s): NATALINI, JEFF WILLIAM
• Application Number:
  US11/894,038
• Publication Number:
  US 20080042665A1
• Time in Patent Office: 1,558 Days
• Field of Search: 324/438
• US Class Current: 324/438
• 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 wherein applying a test current includes 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;

measuring a voltage response comprising 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;

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

determining whether the calculated impedance exceeds a maximum allowable impedance; and

selectively generating a fault signal based upon the step of determining whether the calculated impedance exceeds the maximum allowable impedance; and

calculating a source voltage of the pH electrode as an average of the first voltage response and the second voltage response, the source voltage being indicative of pH of a solution.

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, wherein measuring a voltage response comprises sampling a plurality of voltages across the pH electrode.

4. The method of claim 3, 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.

5. The method of claim 1, 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.

6. The method of claim 5 wherein the first and second time durations are substantially similar.

7. The method of claim 1, 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.

8. The method of claim 1, 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.

9. The method of claim 8, wherein each of the first and second plurality of voltage samples comprises three voltage samples taken across the pH electrode.

10. The method of claim 9, wherein the plurality of voltage samples are averaged over multiple line voltage periods.

11. 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.