Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Utility Patent
1998-09-24
2001-01-02
Tung, T. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C204S400000, C204S408000, C204S416000, C204S420000, C205S787500, C205S789500
Utility Patent
active
06168707
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to electrochemical sensors and, more particularly, to temperature-corrected electrochemical sensors characterized by an improved linearized response.
BACKGROUND OF THE INVENTION
Electrochemical sensors are used to detect the presence of ions in solution. A common form of such sensor is the well-known pH electrode which measures the concentration of H
+
ions in solution. Such a sensor is typically formed from a body having a membrane selectively permeable to H
+
ions formed or mounted on a surface thereof for immersion in, or at least contact with, a solution (or semi-solid such as meat, cheese, soil, and the like) to be measured on an exterior face thereof. A filling solution of controlled composition within the body contacts the interior of the membrane and provides a standard potential for an electrode such as a wire which is immersed in the solution. Changes in the pH of the test material (solution, etc.) to which the membrane is exposed change the potential of the electrode with respect to a reference electrode that is also in contact with the test material. The change in potential between the sensing and reference electrodes provides a direct indication of the pH of the test material.
Electrodes of this type, when used with a membrane that is specific to the ion to be measured, are also commonly used to measure other ions such as Na
+
, K
+
, Ca
2+
, F and other mono-valent and divalent cations and anions, and are referred to generally as “ion-selective” electrodes. When available for the particular ion to be measured, such electrodes are very popular, since they usually provide a relatively fast and simple means of measurement with reasonable accuracy.
The readings provided by ion-selective electrodes, however, are strongly temperature-dependent. This is a consequence of the fact that the potential of the electrode is given by the well-known Nernst equation:
E=E
0
+(
RT
F
) ln
a
where E is the potential of the electrode; E
0
is the standard potential of the electrode when immersed in a solution of unit activity; R is the gas constant; T is the absolute temperature; n is the charge on the ion(s); F is the Faraday constant; and a is the activity of the ion(s). Accordingly, it is essential that the readings of such electrodes be corrected for temperature.
Temperature correction is sometimes accomplished by separately measuring the temperature of the test material (e.g., solution) in which the electrode is immersed and thereafter applying a calculated correction factor to readings taken with the electrode. This is the technique used in the Eckfeldt and Scheider patents (U.S. Pat. Nos. 3,662,256 and 4,222,006). In other words, Eckfeldt and Schnieder are measuring the temperature of a test material with a separate device (i.e., thermistor) that is different from the sensing membrane (i.e., gas electrode or pH electrode). Among other drawbacks, this technique is subject to variable error arising from spatial separation between the temperature measuring element and the sensing membrane.
A more successful approach is set forth in U.S. Pat. No. 4,321,544 issued Mar. 23, 1982 to John H. Riseman for “Method And Improved Apparatus For Obtaining Temperature-Corrected Readings Of Ion Levels and Readings of Solution Temperature” and assigned to the predecessor of the present assignee. The disclosure of this patent is hereby incorporated herein by reference.
The invention described in the '544 patent (or Log R patent) makes use of the fact that the logarithm of the resistance of pH sensitive membranes varies inversely with the temperature of the material (e.g., solution) being measured. This fact is used to advantage by applying an externally-generated alternating voltage across the membrane in parallel with the DC voltage generated by the ion concentration differences across the membrane that arise from the material under test. The alternating voltage is used to measure the instantaneous resistance of the membrane and thus provide a correction factor for the measurement. This technique is referred to hereinafter as “the log R” technique.
The method described in the Log R patent was found to provide good temperature compensation above 40 degrees Celsius (40° C.). At lower temperatures, the temperature readings tend to disperse and deviate from actual temperature measurements. Temperature compensation, with the dispersant temperature values is still possible, however, the increase in temperature-dependent dispersion in the readings at least partially limits realization of the full benefits of the log R technique.
SUMMARY OF THE INVENTION
The present invention is directed to an improved linearized, temperature corrected electrochemical meter and measurement system utilizing the same. Preferably, the present invention provides an improved linearized, temperature corrected electrochemical meter that is characterized by a temperature correction factor of narrow dispersion despite the use of membranes of different types and materials and an improved log R temperature measurement system including an improved log R algorithm, an improved circuit that incorporates microprocessor control and measurement, and noise reduction techniques.
More particularly, the present invention provides an improved temperature measurement through the application of an advanced highly stable microprocessor controlled and measured circuit that doesn't require hardware calibration and is not subject to drift and through the application of a more advanced temperature correction algorithm.
It has been discovered that a temperature correction factor that is relatively linear over a wide range of temperatures and that has an unusually narrow range of dispersion despite the use of membranes of varied types and materials may be obtained from a log R type of circuitry by:
(1) establishing a target resistance-temperature (R-T) calibration curve as a function of the calibration curves of a plurality of membranes;
(2) adding a linear resistance term to the log R algorithm to correct the calculated temperature measurement made using the membrane resistance of the electrode to further linearize and provide more accuracy in the temperature correction factor across a wider temperature range;
(3) presetting the resistance-responsive circuitry of a meter capable of providing a log R response to the said target R-T calibration curve;
(4) providing a method to calibrate or adjust the resistance-response algorithm to a specific membrane material to further increase the accuracy and compensate for variations in membrane types, materials and manufacturing;
(5) use of a switched reference and switched capacitor filter to eliminate the difficult, labor intensive hardware calibrations and eliminate the troublesome hardware drift that reduces the measurement accuracy;
(6) use of a microprocessor to control and measure the sensor circuit;
(7) use of a microprocessor to eliminate hardware calibration;
(8) use of a microprocessor to implement more sophisticated temperature measurement algorithm and calibration techniques based on sensor membrane resistance; and
(9) incorporating a conventional thermister temperature measurement circuit and the ability to turn off LogR to allow high accuracy “Laboratory Grade” pH measurements in the same meter that's designed to use LogR.
The target R-T calibration curve may advantageously be formed as the average of the R-T curves of spherical membranes of the type commonly used in pH and other ion-selective membranes.
REFERENCES:
patent: 3662256 (1972-05-01), Eckfeldt
patent: 4222006 (1980-09-01), Schneider
patent: 4321544 (1982-03-01), Riseman
patent: 4495050 (1985-01-01), Ross, Jr.
patent: 4851104 (1989-07-01), Connery et al.
Bronk Peter F.
Caporiccio Robert
Gillette Timothy
Hovis Jeffrey S.
Hrabosky Stefan
Banner & Witcoff , Ltd.
Linek Ernest V.
Orion Research Inc.
Tung T.
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