The basic principle and key features of the seven-electrode conductivity sensor are explained. Based on the physical characteristics of the sensor and the demands for high-precision measurements, a measuring circuit has been designed that ensures low temperature drift, high accuracy, and fast response. The use of integrated circuits such as D/A and A/D converters allows for precise drive signals and high-speed sampling. Compared to traditional methods, this system offers more accurate control over frequency and voltage, making it easier to adjust. Additionally, high-speed sampling helps prevent signal distortion during the conditioning process. Experimental validation confirmed the effectiveness of the circuit design.
Conductivity measurement plays a vital role in various fields, including industrial production, environmental monitoring, and marine resource development. Accurate and rapid conductivity measurement is crucial for marine research and environmental protection. Conductivity sensors can be broadly categorized into electrode-type and electromagnetic-type based on their working principles. Electromagnetic sensors rely on electromagnetic induction, where changes in conductivity are reflected by variations in induced electromotive force. Electrode-type sensors, on the other hand, measure conductivity through electrolytic conduction, where resistance changes in the liquid are detected. Due to their fast response and high accuracy, electrode-type sensors have gained widespread attention. This article explores the working principle of the seven-electrode conductivity sensor and presents the corresponding measurement circuit design.
Working Principle and Overview
The seven-electrode conductivity sensor is a high-precision electrode-based device that combines the advantages of three-electrode and four-electrode configurations. It separates the "current" and "voltage" electrodes, reducing polarization resistance and allowing for a larger flow path and faster response time, enabling rapid conductivity measurements. Additionally, two grounding electrodes at both ends of the sensor help shield external interference, ensuring that the measurement remains unaffected by external factors. This design also eliminates the need for a pump during measurement, further enhancing precision.
1.1 Principle Analysis of the Seven-Electrode Conductivity Sensor
Figure 1 shows the schematic of the seven-electrode conductivity sensor, which consists of seven platinum rings embedded in quartz glass. Electrodes 1–4 and 4–7 form two sets of measurement units, with current passing through the common electrode 4. When current flows from the ground electrode 1 to electrode 7, currents are generated in electrodes 2, 3, and 5, 6. By applying a constant voltage between electrode 4 and ground, changes in the current flowing through the common electrode can be measured, reflecting resistance changes and ultimately calculating the solution’s conductivity.
[Image: Principle and Measurement Circuit Analysis of Seven-Electrode Conductivity Sensor]
1.2 Overview of the Measurement System
To achieve high-precision measurement using the seven-electrode conductivity sensor, several design considerations are essential. First, minimizing the use of analog components reduces noise and improves signal integrity, allowing for accurate data extraction through high-speed sampling. Second, using the same reference for both A/D and D/A conversion ensures that even if the reference voltage fluctuates, the measurement result remains unaffected, as conductivity is calculated based on the ratio of driving voltage to sampled resistor voltage. Third, the system employs low-temperature-drift and high-precision components, especially for operational amplifiers and sampling resistors used to drive the seven electrodes.
Figure 2 illustrates the measurement circuit. The STM32F103 microcontroller controls the D/A to generate a fixed frequency and voltage signal. The sensor is driven by a constant voltage source created using an integration and subtraction circuit. Current passing through the sampling resistor generates a voltage proportional to the conductivity, which is then converted to digital form via A/D. The resulting data is processed by an algorithm to determine the final conductivity value.
[Image: Principle and Measurement Circuit Analysis of Seven-Electrode Conductivity Sensor]
The system uses the STM32F103 microcontroller, part of the STM32 series, which features a 32-bit ARM core, 64 or 128 KB of flash memory, USB, CAN, 7 timers, 2 ADCs, and 9 communication interfaces. It is widely applied in industrial control systems and measurement devices due to its versatility and performance.
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