Ph sensor measurement

Abstract

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 and acquiring a plurality of samples of the sensor voltage response and estimating an impedance of the sensor electrode from the samples. The negative voltage pulse width is then dynamically adjusted to counteract the induced charge from the positive voltage on the pH sensor such that the sensor rapidly recovers to read subsequent medium pH levels.

Description

Technical Field

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

[0002] The term pH represents a quantitative measure of the acidity or alkalinity of an aqueous or other liquid solution. The term pH = -log[H+], which converts the concentration value of hydrogen ions - which typically ranges from about 1 to 10 -14 gram equivalents per liter - into 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 per liter, which corresponds to a pH value of 7. Solutions with a pH value less than 7 are considered acidic; solutions with a pH value greater than 7 are considered alkaline or basic.

[0003] A probe capable of measuring the pH value consists of two electrodes: a sensor electrode, also known as a glass electrode embedded in a special formulated glass, and a reference electrode. Ion exchange generates a voltage. Ion exchange occurs on the inner surface of the glass electrode. Since the acidity of potassium chloride inside the electrode is different from that of the solution being measured, the activity of hydrogen ions will also be different, resulting in different charges. When this happens, a potential difference appears between the sides of the glass electrode and the reference electrode, and this potential difference is proportional to the acidity or alkalinity level of the medium solution. For every 1 pH value change, the potential (also known as the slope) changes by 59.16 mV, and the reference voltage remains at an ideal zero potential. The potential difference between the two electrodes is measured and converted into a pH level reading.

[0004] Since the potential difference to be measured is generated across the pH glass, special measures must be taken to correctly measure the voltage, all due to the high glass impedance, which ranges from 50 MOhm to 500 MOhm at 25°C.

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

[0006] The quality of pH measurement depends largely on the condition of the glass.

[0007] The pH electrode ages, which causes the electrical characteristics of the electrode to change over time or in harsh applications or environments. Electrode aging can lead to an increase in glass / reference impedance, a long measurement response time, a decrease in voltage-pH slope (especially in the alkaline region), and / or a shift in the asymmetric potential. As the electrode degrades, the ability of the probe to accurately measure the pH value also degrades, resulting in inaccurate and / or inconsistent pH level measurements.

[0008] An increase in the impedance of a glass electrode can indicate a change in the chemical composition of the membrane glass, a stable growth of the internal membrane gel layer, or mechanically induced damage to the external gel layer of the membrane during measurement and cleaning. When the glass breaks, the pH measurement is compromised.

[0009] For all of the above reasons, being able to measure the 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. SUMMARY OF THE INVENTION

[0010] 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 equally timed samples of the sensor voltage response, and estimating the impedance of the electrode from these samples. Then, a negative voltage pulse width is applied to cancel the induced charge from the positive voltage on the pH sensor, such that the sensor quickly recovers to read the medium pH level.

[0011] These and various other features and advantages will be apparent by reading the following detailed description with reference to the exemplary embodiments described therein. The Summary of the Invention and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used to assist in determining the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that solve any or all of the disadvantages noted in the Background.

[0012] From the description provided herein, further fields of applicability will be apparent. The description and specific examples in the Summary of the Invention are intended for illustrative purposes only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0014] Throughout several views of the drawings, corresponding reference numerals indicate corresponding parts.

[0015] Figure 1 Shows an example of a pH sensor probe having a pH glass electrode, the pH value of which can be measured;

[0016] Figure 2 is a circuit model;

[0017] Figure 3 is a circuit for measuring the glass impedance of a pH electrode;

[0018] Figure 4 is a pulse diagram;

[0019] Figure 5 is a waveform showing the delay that may be encountered in typical pH measurement techniques;

[0020] Figure 6 is a voltage-versus-time waveform during measurement according to a preferred embodiment of the present invention;

[0021] Figure 7 is a voltage-versus-time waveform of the pH glass electrode during measurement;

[0022] Figure 8 is a voltage-versus-time waveform of the pH glass electrode during measurement;

[0023] Figure 9 is a voltage-versus-time waveform of the pH glass electrode during measurement, showing the estimated measurement voltage according to the teachings of the present invention; and

[0024] Figure 10 is a circuit diagram employed in the method of the present invention. DETAILED DESCRIPTION

[0025] Now referring to Figure 2 , a pH probe electrical model of a probe similar to the probe shown in Figure 1 is similar to a high-value resistor (50M to 500M) and a parallel capacitor.

[0026] To measure R_glass, a voltage source (V1) and a fixed voltage divider (R2) are provided.

[0027] The voltage across the pH probe can be solved by the following formula:

[0028]

[0029] To further simplify the equation:

[0030] Let It is a unitless ratio

[0031] Equation 1 can be simplified to the following equation:

[0032]

[0033] Now, Equation 2 is the same as the standard exponential growth equation, as follows:

[0034] y(t) = a[1 - exp(-b*t + c)] Equation 3

[0035] Equation 3 can be further simplified to:

[0036] y(t) = g + h*exp(-b*t) Equation 4

[0037] where g = a, h = -a*exp(c)

[0038] Extract three samples y(t1), y(t2), y(t3) from the y(t) data points before the asymptote region B( Figure 9 ) where t3–t2 = t2–t1

[0039]

[0040] Now use y(t) = g + h*exp(-b*t) and solve for h using any two evenly - timed samples, then solve for g and c.

[0041]

[0042] Now the steady state of y(t) = a[1 - exp(-b*t + c)] can be estimated by entering a long time constant in “t”.

[0043] Circuit concept:

[0044] Figure 3 The voltage source (V1) in Figure 3 is designed with a differential operational amplifier (U1) such that it can generate bipolar pulses. Another operational amplifier (U2) is used as a buffer for signal conditioning of the pH input. As

[0045] As Figure 4 illustrated, due to the capacitive component of the glass, the pH reading is not immediately accurate because if no negative pulse is applied to the probe, the probe will take a long time to return to the correct pH measurement. Therefore, an implementation of applying the negative pulse (p2) is provided. Additionally, the pulse width of p2 is dynamically adjusted by a logic device under different operating conditions.

[0046] A logic device such as a controller integrates the total energy of the electrode voltage response after sending pulse P1 and then sends a negative pulse (P2) with a width adjusted by the logic device, and the width is 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. The total energy of the probe response can change under different operating conditions, so the negative pulse width is adjusted accordingly.

[0047] AsFigure 6 As shown in the figure, three Vs glass Samples are carefully selected from the curve and are evenly distributed in time, as shown. By applying three samples in Equations 5-7, V is accurately estimated glass steady state.

[0048] Since the values of V1 and R1 ( Figure 1 ) are already known, the probe impedance can be solved using the following Equation 1:

[0049]

[0050] Equation 1 is further expanded to Equation 2 because V1 is replaced by a differential operational amplifier and the addition of U2 in Circuit 2.

[0051]

[0052] Where:

[0053]

[0054] (Note: V diff is the output generated by the differential operational amplifier)

[0055] Actual calculation:

[0056] Figure 7 The curve in is the probe glass impedance output voltage when a pulse is applied from the actual circuit ( Figure 3 ). Three samples with average timing are used in the calculation, 0.09090465 volts at 0.1245 seconds, 0.125465114 volts at 0.3485 seconds, and 0.14543876 volts at 0.5725 seconds. Using Equations 5 and 6:

[0057]

[0058] Using the substitution method in Equation 4, h can be solved:

[0059] y(t3) = g + h*exp(-b*t3); 0.14543876 = g + h*exp(-2.447519293*0.5725)

[0060] y(t2) = g + h*exp(-b*t2); 0.125465114 = g + h*exp(-2.447519293*0.3484)

[0061] h = -0.111042361

[0062] Rearranging Equation 4, g can be solved for using any one of the three samples. Sample 2 is used for this calculation;

[0063] g = y(t) - h * exp(-b * t)

[0064] = 0.125465114 - (-0.111042361 * exp(-2.447519293 * 0.3485) = 0.17278856

[0065] Using Equation 7, c can be solved for:

[0066]

[0067] Now y(t) = a[1 - exp(-b * t + c)]

[0068] = 0.17278856[1 - exp(-2.447519293 * t - 0.442156893)] in the steady state can be plotted Figure 8 in.

[0069] Figure 10 The output of the differential amplifier in (which is similar to Figure 3 , with a value added)

[0070]

[0071] The probe impedance can be solved for using the following formula

[0072] In summary and with particular reference to Figure 9 , the method of the present invention can significantly shorten the time range for determining the pH value of the glass electrode in a pH sensing probe. Instead of waiting for the electrode voltage response to reach a steady state, such as Figure 9 point C in (which may take 30 seconds), three equally timed samples are taken before the electrode voltage response reaches the asymptote represented by line B. The electrode voltage response is shown by the solid line A in Figure 9 . The pH value of the glass electrode is estimated very quickly from these samples, typically in less than 250 ms. Then, a negative voltage pulse is applied to cancel the induced charge from the positive voltage on the electrode, enabling the electrode to quickly recover to read subsequent medium Ph levels.

[0073] The present invention shows an improved method for measuring the glass impedance of a pH probe by using least squares fitting to invert an exponential function. Some of the improvements are:

[0074] ·Apply a pulse at the current synchronous value of the original mV value of the electrode pH; thus, the only interfering energy consists of the pulse applied to the glass, which is significantly lower than the interfering energy that would be generated when applying the pulse with reference to the zero, ground mV level.

[0075] ·The method applies the pulse to the glass in a consistent manner for a predetermined time, so the measurement becomes reliable.

[0076] ·By applying the predicted RC level value, the method takes a very small amount of time (200 milliseconds) compared to the 10 - 30 seconds required for the electrode voltage response to reach the final value, so the pH measurement time is not sacrificed. In this way, compared to 10 - 30 seconds, the impact on the running - time pH measurement is reduced to 1 - 2 seconds.

[0077] ·To further reduce the glass recovery time and return to normal pH measurement, the opposite energy (applied from the other side of the glass) will help to further reduce the recovery time. This reverse energy pulse will be calculated using the integrated positive pulse energy.

[0078] For purposes of illustration and description, the foregoing description of the embodiments has been provided. 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, but, where applicable, are interchangeable and can be used in selected embodiments even if not specifically shown or described. This can also vary in many ways. Such variations 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

1. A method for measuring the impedance of a pH sensor electrode, comprising: Apply a positive voltage pulse to the pH electrode; acquiring a plurality of samples of the electrode voltage response to the positive pulse, the samples being equally timed with respect to one another during application of the positive pulse; The impedance of the electrode was estimated from these samples; as well as A negative voltage pulse is applied to counteract the induced charge from the positive voltage on the electrode, causing the electrode to recover quickly to read the media pH level.

2. The method according to claim 1, wherein: The samples are acquired less than 250ms after the start of the positive pulse.

3. The method according to claim 2, wherein: The three samples were acquired equally spaced in time.

4. The method according to claim 3, wherein: Three samples were taken before the asymptote of the curve of the electrode voltage response.

5. The method according to claim 1, wherein: The negative pulse is adjusted dynamically as a function of the electrode voltage response.

6. The method according to claim 5, wherein: The width of the negative pulse is adjusted dynamically.

7. The method according to claim 6, wherein: The width of the negative pulse is dynamically adjusted by: determining the energy of the electrode voltage response after application of the positive pulse; and A negative pulse is generated that substantially matches the total energy of the electrode voltage response.

8. The method according to claim 7, wherein: The pulse width of the negative pulse is determined by the total energy of the electrode voltage response divided by the voltage amplitude of the positive pulse.

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