Ph sensor measurement

Abstract

To provide a method of measuring impedance of a pH sensor electrode.SOLUTION: A method disclosed herein comprises applying a positive voltage pulse to a pH electrode, and acquiring a plurality of samples of the sensor voltage response, and estimating impedance of the sensor electrode from the samples. Then, a negative voltage pulse width is dynamically adjusted to cancel out induced charges from the positive voltage on the pH sensor, so that the sensor quickly recovers to read subsequent media pH levels.SELECTED DRAWING: Figure 10

Description

Technical Field

[0001] The present disclosure relates to pH sensors, and more particularly to techniques for measuring the impedance of glass pH sensor electrodes.

[0002] Background Art The term pH represents a quantitative measure of the acidity or basicity of an aqueous or other liquid solution. The term pH = -log[H+] typically converts the value of the concentration of hydrogen ions in the range of about 1 to 10 -14 gram equivalents per liter to a number between 0 and 14. In pure water, which is neutral (neither acidic nor alkaline), the concentration of hydrogen ions is 10 -7 gram equivalents / liter, which corresponds to pH 7. Solutions with a pH less than 7 are considered acidic, and solutions with a pH greater than 7 are considered basic or alkaline.

[0003] A probe capable of measuring pH consists of two electrodes, namely, a sensor electrode, also known as a glass electrode, embedded in a special formulated glass, and a reference electrode. A voltage is generated by ion exchange. Ion exchange occurs on the inner surface of the glass electrode. Since the acidity of the potassium chloride inside the electrode is different from that of the solution being measured, the activity of hydrogen ions is different, resulting in a difference in charge. When this occurs, a potential difference is generated between the side of the glass electrode and the reference electrode, which is proportional to the acidic or alkaline level of the medium solution. For each 1 pH change, the potential, also known as the slope, changes by 59.16 mV. The reference is maintained at an ideal zero potential. The potential difference between the two electrodes is measured and converted into a reading of the pH level.

[0004] Since the potential difference to be measured is generated across the pH glass, all due to the high glass impedance in the range of 50 MOhm to 500 MOhm at 25°C, special means must be taken to properly measure the voltage.

[0005] When the temperature is below 25°C, the glass impedance increases, and when the temperature rises, it decreases.

[0006] The quality of pH measurement strongly depends on the state of the glass.

[0007] The aging of a pH electrode can potentially cause changes in the electrical characteristics of the electrode over time or during harsh use or environments. Electrode aging causes an increase in glass / reference impedance, measurement response time, a decrease in voltage-pH gradient especially in the alkaline region, and / or a shift in the asymmetric potential. As the electrode deteriorates, the ability of the probe to accurately measure pH also deteriorates, resulting in inaccurate and / or inconsistent pH level measurements.

[0008] An increase in glass electrode impedance can indicate changes in the chemical composition of the membrane glass, stable growth of the inner gel layer within the membrane, or mechanically induced damage to the outer gel layer of the membrane during measurement and cleaning. When the glass is damaged, pH measurement is impaired.

[0009] For all of the above, being able to measure glass impedance is good practice for reliable pH measurement. U.S. Patent No. 9,488,611 to Rezvani et al. discloses a method for detecting the impedance of a pH electrode.

[0010] SUMMARY OF THE INVENTION A method for measuring the impedance of a pH sensor electrode is provided. The method includes applying a positive voltage pulse to the pH electrode, acquiring a plurality of equal-time samples of the sensor voltage response, and estimating the impedance of the electrode from those samples. A negative voltage pulse is then applied to cancel the induced charge from the positive voltage on the pH sensor, whereby the sensor quickly recovers to read the media pH level.

[0011] These and various other features and advantages will become apparent from the following detailed description when read in conjunction with the exemplary embodiments described. This summary and abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. Further, the claimed subject matter is not limited to embodiments that solve any or all of the disadvantages noted in any part of this disclosure.

[0012] Further application areas will become apparent from the description provided herein. The description and specific examples in this summary are for illustrative purposes only and are not intended to limit the scope of the disclosure.

[0013] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible embodiments, and are not intended to limit the scope of the disclosure.

[0014] Corresponding reference numerals indicate corresponding parts throughout several views of the drawings.

Brief Description of the Drawings

[0015]

Figure 1

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[0016] Embodiments for Carrying Out the Invention Referring to FIG. 2 here, the pH probe electrical model of the same probe as shown in FIG. 1 is similar to a high resistor (50M - 500M) and a parallel capacitor.

[0017] A voltage source (V1) and a fixed divider (R2) are provided to measure R_glass.

[0018] The voltage across the pH probe can be solved by the following equation:

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[0019] Circuit concept: The voltage source (V1) in FIG. 3 is designed using a differential operational amplifier (U1) so as to be able to generate bipolar pulses. Another operational amplifier (U2) is used as a buffer for signal conditioning of the pH input. As shown in FIG. 3, bipolar pulses (p1 and p2) are sent to the pH probe. When applied alternately, the pulses are applied to two sides of the glass. The positive pulse is used for glass impedance measurement, and the negative pulse is used to cancel the positive charge induced on the pH probe during glass impedance measurement, so that the pH reading can be quickly recovered. The amplitude of the pulse is controlled for the minimum residual energy in the glass for mV level measurement of the minimum pH. A predetermined time pattern (D) is applied to the glass impedance measurement period.

[0020] As shown in FIG. 4, due to the capacitive element of the glass, if a negative value is not applied to the probe, it takes a considerable amount of time for the probe to return to a correct pH reading, so the pH reading is not immediately accurate. Accordingly, an embodiment of applying a negative pulse (p2) is provided. Also, the pulse width of p2 is dynamically adjusted by a logic device under different operating conditions.

[0021] A logic device such as a controller integrates the total energy of the electrode voltage response after transmitting the pulse P1, and then transmits a negative pulse (P2) having a width adjusted by a logic device adjusted to match the energy induced by the positive pulse P1. The width is determined by dividing the total energy of the electrode voltage response by the voltage amplitude of P1. Since the total energy of the probe response can vary under different operating conditions, the negative pulse width is adjusted accordingly.

[0022] As shown in FIG. 6, three

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[0023] In FIG. 1

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[0024] Actual calculation: The graph in FIG. 7 is the probe glass impedance output voltage when a pulse is applied from the actual circuit (FIG. 3). For the calculation, three uniform time samples of 0.09090465 volts at 0.1245 seconds, 0.125465114 volts at 0.3485 seconds, and 0.14543876 volts at 0.5725 seconds are used. Using Equations 5 and 6:

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[0025] In summary, especially referring to Figure 9, the method of the present invention can significantly shorten the time frame for determining the pH of the glass electrode in the pH detection probe. Instead of waiting for the electrode voltage response to reach a steady state such as point C in Figure 9 (which may take up to 30 seconds), the present invention takes three equal-time samples before the electrode voltage response reaches an asymptote represented by line B. The electrode voltage response is shown by the solid line A in Figure 9. From these samples, the pH of the glass electrode is estimated very quickly, generally in less than 250 ms. Then, a negative voltage pulse is applied to cancel the induced charge from the positive voltage on the electrode, so that the electrode can quickly recover to read the subsequent medium pH level.

[0026] The present invention shows an improved method for measuring the glass impedance of a pH probe by using least squares fitting to an inverse exponential function. Some of the improvements are as follows: · The pulse is applied simultaneously with the current for measuring the mV value of the electrode before pH treatment, so that the only interfering energy consists of the pulses applied to the glass, which is significantly lower than when applied with respect to the zero ground mV level. · Since this method applies a consistent method of pulsing the glass at a predetermined timing, the measurement becomes more reliable. · By applying a predicted value of the RC level, this method requires only a very short time (200 milliseconds) compared to the 10 - 30 seconds required for the electrode voltage response to reach the final value, so it does not sacrifice the pH measurement time. In this way, the impact on runtime pH measurement is reduced to 1 - 2 seconds compared to 10 - 30 seconds. · To further shorten the glass recovery time and return to the normal measured value of pH, the opposite energy (applied from the other side of the glass) helps to further shorten the recovery time. This reverse energy pulse is calculated using the energy of the integrated positive pulse.

[0027] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. The individual elements or features of a particular embodiment are generally not limited to that particular embodiment and, where applicable, are interchangeable and can be used in the selected embodiment even if not specifically illustrated or described. This may also be modified in many ways. Such modifications should not be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

**Claim 1** A method for measuring the impedance of a pH sensor electrode, comprising: applying a positive voltage pulse to the pH electrode; acquiring a plurality of samples of the electrode voltage response to the positive pulse, wherein the samples are equally time-aligned with each other during the application of the positive pulse; estimating the impedance of the electrode from these samples; applying a negative voltage pulse to cancel the induced charge from the positive voltage on the electrode so that the electrode quickly recovers to read the medium pH level. A method comprising the above steps. **Claim 2** The method according to claim 1, wherein the samples are acquired within less than 250 ms from the start of the positive pulse. **Claim 3** The method according to claim 2, wherein three samples are acquired at equal time intervals. **Claim 4** The method according to claim 3, wherein the three samples are acquired before the asymptote of the curve of the electrode voltage response. **Claim 5** The method according to claim 1, wherein the negative pulse is dynamically adjusted as a function of the electrode voltage response. **Claim 6** The method according to claim 5, wherein the width of the negative pulse is dynamically adjusted. **Claim 7** The width of the negative pulse is: determining the energy of the electrode voltage response after applying the positive pulse; generating a negative pulse that substantially matches the total energy of the electrode voltage response; dynamically adjusted by the above steps. The method according to claim 6. **Claim 8** The method according to claim 7, wherein the pulse width of the negative pulse is determined by dividing the total energy of the electrode voltage response by the voltage amplitude of the positive pulse.

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