Method for measuring the conductivity of a low concentration solution

Abstract

The application discloses a kind of low concentration solution conductivity measurement methods, it is related to solution conductivity measurement technical field, MCU utilizes the two-way pulse signals of same period, different phase of on-chip timer TIM output, one way is used to control the closure channel selection of analog switch SW, one way is used to trigger the sampling of on-chip ADC;Analog switch SW is controlled by MCU on-chip timer TIM output pulse, when TIM output pulse signal is low level, the two electrodes of conductivity sensor are connected together by standard resistance R std With analog ground, make two electrodes keep equipotential, when TIM output pulse signal is high level, analog switch state switches, form loop, after a period of time after pulse rising edge, and before the arrival of pulse falling edge, complete the measurement sampling of voltage U1 and U2 between standard resistance, using the pulse of duty cycle <1%, cooperate two-pole conductivity cell, use more simple circuit, realize the accurate measurement of low concentration solution conductivity.

Description

Technical Field

[0001] This invention relates to the field of solution conductivity measurement technology, specifically a method for measuring the conductivity of low-concentration solutions. Background Technology

[0002] In the measurement of solution conductivity, different measurement methods correspond to different measurement ranges. Currently, commonly used methods include:

[0003] 1. In high-concentration solution measurements, to reduce the influence of the solid-liquid junction potential and to avoid electrolysis caused by direct contact between the excitation electrode and the solution, non-contact inductive measurement is generally used. However, when using inductive measurement, environmental electrical noise interference can be severe. When the solution concentration is low, the measurement signal and interference signal cannot be distinguished, so it cannot be used for measuring low-concentration solutions.

[0004] 2. In measuring solutions with low to medium concentrations, to reduce the influence of liquid junction potential, the excitation electrode and the measuring electrode are separated. Multi-electrode conductivity cells are generally used, with four-electrode and seven-electrode cells being the most common. The structure of a conductivity cell is relatively complex. When the solution concentration is very low, a high excitation voltage is required, which can lead to severe polarization of the excitation electrode or even solution electrolysis, resulting in measurement errors.

[0005] 3. When measuring the conductivity of low-concentration solutions, especially in "purified water" or "pure water" systems, a bipolar conductivity cell is generally used. The advantages are: simple cell structure, and the excitation voltage and current are simultaneously the measurement voltage and current. The disadvantages are: a DC excitation source cannot be used during measurement, otherwise electrode polarization will occur, i.e., cations and anions accumulate on the electrode surface, or even electrolysis of the solution occurs, leading to measurement deviation. To reduce the impact of electrode polarization, a low-frequency sinusoidal AC signal is generally used as the excitation signal. Because an AC signal source is used, a relatively complex signal conditioning circuit is required for signal processing.

[0006] Many factors influence the measurement of solution conductivity, with temperature and liquid junction potential being the most significant. Existing technologies have extensively studied the effect of temperature and developed empirical compensation algorithms. This technical solution primarily addresses the influence of liquid junction potential by proposing the use of pulses with a duty cycle of <1% in conjunction with a bipolar conductivity cell, employing a relatively simple circuit to achieve accurate measurement of the conductivity of low-concentration solutions. This solves the aforementioned technical problems. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a method for measuring the conductivity of low-concentration solutions, thereby solving the problems mentioned in the background section.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for measuring the conductivity of a low-concentration solution, comprising the following steps:

[0009] S1. The MCU uses the on-chip timer TIM to output two pulse signals with the same period but different phases. One signal is used to control the closed channel selection of the analog switch SW, and the other signal is used to trigger the sampling of the on-chip ADC.

[0010] S2, the analog switch SW is controlled by the output pulse of the on-chip timer TIM of the MCU. When the TIM output pulse signal is low, the two electrodes of the conductivity sensor are connected through the standard resistor R. std They are shorted together and connected to the simulated ground, which means that the two electrodes are kept at the same potential.

[0011] S3. When the TIM output pulse signal is high, the analog switch state switches: Reference voltage (REF) → Analog switch (SW) → Standard resistor (R) std The circuit is formed by electrode → solution → electrode → analog ground (AGND). The voltages U1 and U2 across the standard resistor are measured and sampled after a delay after the upper edge of the pulse and before the lower edge of the pulse.

[0012] S4. The above circuits are series circuits. According to Ohm's law, the conductivity of the solution can be obtained by converting the cell conductivity coefficient.

[0013] To further optimize this technical solution, to avoid the influence of the ADC input impedance on the measurement, A and B are high-speed operational amplifiers with MOSFET inputs, used as buffers; SW is a single-pole double-throw analog switch; R std It is a reference resistor with a precision of 1‰.

[0014] To further optimize this technical solution, Ohm's law states: Where: R x It is the resistance of the solution.

[0015] To further optimize this technical solution, the pulse signal is kept at a low level for a long time, keeping the two electrodes of the conductivity sensor short-circuited. This is to better release any potential charge imbalance between the electrodes of the conductivity sensor and keep the two electrodes at the same potential. Since the two electrodes are made of the same material and the aqueous solution between the electrodes is isotropic, even if there is a solid-liquid junction potential, it should be equal in magnitude and opposite in direction when equilibrium is reached, so that they can cancel each other out.

[0016] To further optimize this technical solution, a short pulse high-level time is used to complete the signal sampling of U1 and U2 during this period, so as to complete the signal acquisition before the liquid junction potential changes significantly.

[0017] To further optimize this technical solution, software control with strict timing is necessary to complete the measurement. An STM32 microcontroller was used in the experiment. Since the on-chip ADC of the MCU is used, the ADC clock is defined as 12MHz.

[0018] To further optimize this technical solution, the MCU chip is equipped with ADC and TIM resources, making full use of the MCU chip resources to simultaneously realize pulse output and ADC triggering.

[0019] To further optimize this technical solution, the pulse width used is between 30-300µs.

[0020] To further optimize this technical solution, when the analog switch is switched, the presence of resistance, capacitance and inductance in the measurement circuit will cause distortion of the pulse rising edge signal, as well as the phase lag when the operational amplifier is working. Therefore, a certain delay is necessary. If the time is too short, the signal distortion cannot be eliminated. If the time is too long, the liquid junction potential difference between the electrode and the solution will begin to form.

[0021] Compared with the prior art, the present invention provides a method for measuring the conductivity of low-concentration solutions, which has the following advantages:

[0022] 1. This method for measuring the conductivity of low-concentration solutions aims to reduce the influence of liquid junction potential. It proposes to use a pulse with a duty cycle of <1% in conjunction with a bipolar conductivity cell and employs a relatively simple circuit to achieve accurate measurement of the conductivity of low-concentration solutions, which has good market promotion value.

[0023] 2. The method for measuring the conductivity of low-concentration solutions uses a pulse duty cycle of <1%, which reduces the average detection current and minimizes interference caused by the liquid junction potential difference resulting from ion enrichment.

[0024] 3. The method for measuring the conductivity of this low-concentration solution uses a high-speed analog switch with low on-resistance and low leakage current. The analog switch is used to short-circuit the electrodes of the conductivity sensor for a long time to maintain the equipotential between the two electrodes and avoid liquid junction potential difference caused by ion enrichment or electrolytic desorption.

[0025] 4. This method for measuring the conductivity of low-concentration solutions enables low-cost, high-precision conductivity measurement for systems such as "purified water," "pure water," and "boiler water." Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of pulse conductivity measurement, a method for measuring the conductivity of low-concentration solutions proposed in this invention.

[0028] Figure 2 This is a timing diagram of the measurement method for measuring the conductivity of a low-concentration solution proposed in this invention. Detailed Implementation

[0029] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Example:

[0031] Please refer to Figure 1 As shown, this invention discloses a method for measuring the conductivity of a low-concentration solution, comprising the following steps:

[0032] S1. In the figure, the MCU uses the on-chip timer TIM to output two pulse signals with the same period but different phases. One signal is used to control the closed channel selection of the analog switch SW, and the other signal is used to trigger the sampling of the on-chip ADC.

[0033] S2, the analog switch SW is controlled by the output pulse of the on-chip timer TIM of the MCU. When the TIM output pulse signal is low, the two electrodes of the conductivity sensor are connected through the standard resistor R. std They are shorted together and connected to the simulated ground, which means that the two electrodes are kept at the same potential.

[0034] S3. When the TIM output pulse signal is high, the analog switch state switches: Reference voltage (REF) → Analog switch (SW) → Standard resistor (R) std The circuit is formed by electrode → solution → electrode → analog ground (AGND). The voltages U1 and U2 across the standard resistor are measured and sampled after a delay after the upper edge of the pulse and before the lower edge of the pulse.

[0035] S4. The above circuits are series circuits, according to Ohm's law; Where: R x It is the resistance of the solution, and the conductivity of the solution can be obtained by converting the conductivity cell coefficient.

[0036] This method for measuring the conductivity of low-concentration solutions aims to reduce the influence of liquid junction potential. It proposes using a pulse with a duty cycle of <1% in conjunction with a bipolar conductivity cell, employing a relatively simple circuit to achieve accurate measurement of the conductivity of low-concentration solutions. This method has significant market potential. The <1% pulse duty cycle reduces the average detection current and minimizes interference caused by liquid junction potential differences resulting from ion enrichment. The analog switch used is a high-speed analog switch with low on-resistance and low leakage current. Furthermore, the analog switch is used to short-circuit the electrodes of the conductivity sensor for an extended period, maintaining equipotential between the two electrodes and avoiding liquid junction potential differences caused by ion enrichment or electrolytic deposition. This method enables low-cost, high-precision conductivity measurement in systems such as "purified water," "pure water," and "boiler water."

[0037] As a specific optimization in this embodiment, to avoid the influence of the ADC input impedance on the measurement, A and B are high-speed operational amplifiers with MOSFET inputs, used as buffers; SW is a single-pole double-throw analog switch; R std It is a reference resistor with a precision of 1‰.

[0038] Please refer to Figure 2 As shown, the measurement timing sequence is as follows: To complete the measurement, it must be controlled by software with a strict timing sequence, as shown in the figure. Figure 2 As shown: SW line is the control signal of analog switch SW, ADC line is the ADC sampling control signal, T1 is the total pulse period time, T2 is the positive pulse time, S1 is the waiting time from the rising edge of the analog switch signal to ADC sampling, and S2 is the ADC sampling process time.

[0039] As a specific optimization in this embodiment, the pulse signal is kept at a low level for a long time to keep the two electrodes of the conductivity sensor short-circuited. This is to better release the possible charge imbalance between the electrodes of the conductivity sensor and keep the two electrodes at the same potential. Since the two electrodes are made of the same material and the aqueous solution between the electrodes is isotropic, even if there is a solid-liquid junction potential, it should be equal in magnitude and opposite in direction when equilibrium is reached, so that they can cancel each other out.

[0040] As a specific optimization in this embodiment, the short pulse high-level time, during which the signal sampling of U1 and U2 is completed, is to complete the signal acquisition before a large change in the liquid junction potential occurs.

[0041] Among them, the measurement delay time S1: After the analog switch is switched, due to the presence of resistance, capacitance and inductance in the measurement circuit, the pulse rising edge signal will be distorted, and the phase lag when the operational amplifier is working, etc., a certain delay is necessary. If the time is too short, the signal distortion cannot be eliminated. If the time is too long, the liquid junction potential difference between the electrode and the solution will begin to form. Therefore, both too short and too long delay time will affect the true validity of the acquired measurement signal.

[0042] The measurement time S2 is determined by the sampling process of the ADC. The experiment used an STM32 microcontroller. Since the ADC on the MCU is used, the ADC clock is defined as 12MHz. After calculation and testing, a time slice of about 20µs for S2 is sufficient.

[0043] Wherein, the positive pulse time T2: T2=S1+S2.

[0044] Among them, the pulse period T1: In order to ensure that the ions enriched near the electrode surface during the positive pulse have enough time to diffuse and return to the same potential, it is experimentally determined that T1≥100*T2.

[0045] As a specific optimization scheme in this embodiment, the bipolar conductivity sensor used can be a commercially available one and does not require special customization.

[0046] As a specific optimization scheme in this embodiment, the MCU chip has ADC and TIM resources on-chip. By making full use of the MCU chip resources, pulse output and ADC triggering are realized simultaneously. The peripheral circuit is much simpler than the current AC excitation method, and the measurement error caused by the nonlinear distortion of small signals during AC signal processing is avoided.

[0047] As a specific optimization scheme in this embodiment, the pulse width is between 30-300µs, and the data acquisition is completed before the liquid junction potential changes, thus ensuring the authenticity and validity of the acquired data.

[0048] As a specific optimization scheme in this embodiment, the pulse duty cycle is <1%, which reduces the average detection current and reduces the interference caused by the liquid junction potential difference generated by ion enrichment.

[0049] As a specific optimization scheme in this embodiment, the analog switch used is a high-speed analog switch with low on-resistance and low leakage current. The analog switch is used to short-circuit the electrodes of the conductivity sensor for a long time to maintain the equipotential between the two electrodes and avoid liquid junction potential difference caused by ion enrichment or electrolytic desorption.

[0050] As a specific optimization scheme in this embodiment, it is possible to achieve low-cost and high-precision conductivity measurement of systems such as "purified water", "pure water" and "boiler water".

[0051] This method for measuring the conductivity of low-concentration solutions aims to reduce the influence of liquid junction potential. It proposes using a pulse with a duty cycle of <1% in conjunction with a bipolar conductivity cell and employs a relatively simple circuit to achieve accurate measurement of the conductivity of low-concentration solutions. This method has good market potential. The pulse duty cycle of <1% reduces the average detection current and minimizes interference caused by the liquid junction potential difference resulting from ion enrichment.

[0052] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for measuring the conductivity of a low-concentration solution, characterized in that, Includes the following steps: S1. The MCU uses the on-chip timer TIM to output two pulse signals with the same period but different phases. The duty cycle of the pulse signals is <1%. One of them is used to control the closed channel selection of the analog switch SW, and the other is used to trigger the sampling of the on-chip ADC. The liquid junction potential is canceled by a long low level and the sampling is fast by a short high level. S2. The analog switch SW is controlled by the output pulse of the on-chip timer TIM of the MCU. When the TIM output pulse signal is low, the two electrodes of the conductivity sensor are shorted together through the standard resistor Rstd and connected to the analog ground, so that the two electrodes are kept at the same potential to release the charge imbalance. S3. When the TIM output pulse signal is high, the analog switch state is switched, and the reference voltage (REF) → analog switch (SW) → standard resistor (Rstd) → electrode → solution → electrode → analog ground (AGND) form a loop. There is a delay after the pulse rising edge to eliminate signal distortion, and the voltage U1 and U2 across the standard resistor are measured and sampled before the pulse falling edge arrives. S4. The above circuits are series circuits. According to Ohm's law, the conductivity of the solution can be obtained by converting the cell conductivity coefficient.

2. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, To avoid the input impedance of the ADC affecting the measurement, A and B are high-speed operational amplifiers with MOSFET inputs, used as buffers; SW is a single-pole double-throw analog switch; R std It is a reference resistor with a precision of 1‰.

3. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, Ohm's law is: Where: R x It is the resistance of the solution.

4. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, The pulse signal is kept at a low level for a long time to keep the two electrodes of the conductivity sensor short-circuited. This is to better release any possible charge imbalance between the electrodes of the conductivity sensor and keep the two electrodes at the same potential. Since the two electrodes are made of the same material and the aqueous solution between the electrodes is isotropic, even if there is a solid-liquid junction potential, it should be equal in magnitude and opposite in direction when equilibrium is reached, so that they can cancel each other out.

5. The method for measuring the conductivity of a low-concentration solution according to claim 4, characterized in that, The short pulse high-level time, during which the signals of U1 and U2 are sampled, is to complete the signal acquisition before a large change in the liquid junction potential occurs.

6. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, To complete the measurement, software control with strict timing is required. An STM32 microcontroller was used in the experiment. Since the on-chip ADC of the MCU is used, the ADC clock is defined as 12MHz.

7. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, The MCU used has on-chip ADC and TIM resources, making full use of the MCU's on-chip resources to synchronously realize pulse output and ADC triggering.

8. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, The pulse width used is between 30-300µs.

9. The method for measuring the conductivity of a low-concentration solution according to claim 1, characterized in that, When the analog switch is switched, the presence of resistors, capacitors, and inductors in the measurement circuit will cause distortion of the pulse rising edge signal. In addition, due to the phase lag when the operational amplifier is working, a certain delay is necessary. If the time is too short, the signal distortion cannot be eliminated. If the time is too long, the liquid junction potential difference between the electrode and the solution will begin to form.

Images