Principle and measurement circuit analysis of seven-electrode conductivity sensor

The basic principles and characteristics of the seven-electrode conductivity sensor are explained. According to the physical characteristics of the sensor itself and the requirements of high-precision measurement, a measurement circuit that can meet the requirements of low temperature drift, high precision and high speed is designed. Integrated circuit chips such as D/A and A/D are used to achieve precise driving and high-speed sampling of the sensor. Compared with traditional methods, the driving frequency and voltage are more accurate and easy to change. High-speed sampling can avoid signal distortion during the conditioning process. The measurement effect of the circuit is verified by experiments.

Conductivity measurement has a wide range of applications in industrial production, environmental monitoring, marine resource development and other fields. Accurate and fast conductivity measurement methods are of great significance to marine research, environmental protection and other aspects. Conductivity sensors can be mainly divided into electrode conductivity sensors and electromagnetic conductivity sensors according to their working principles. Electromagnetic conductivity sensors measure based on the principle of electromagnetic induction, and react to changes in conductivity through changes in induced electromotive force; electrode conductivity sensors measure based on the principle of electrolytic conductivity, and react to changes in conductivity through changes in the resistance of the measured liquid. Electrode conductivity sensors have received widespread attention due to their fast response speed and high precision. This article analyzes the working principle of the seven-electrode conductivity sensor and designs a measurement circuit.

Working Principle and Overview

The seven-electrode conductivity sensor is one of the most accurate electrode-type conductivity sensors, and it has the advantages of both three-electrode and four-electrode. The conductivity cell of the seven-electrode conductivity sensor realizes the separation of the "current" electrode and the "voltage" electrode, which can reduce the electrode polarization impedance, provide a large conduction space, and have a fast response time, thus realizing the rapid measurement of conductivity. There are two grounding electrodes at both ends of the conductivity cell of the seven-electrode conductivity sensor, which can effectively shield the influence outside the conductivity cell, so that the measurement results are not affected by the outside of the conductivity cell, and no water pump is required during the measurement process, which can also ensure high-precision measurement.

1.1 Principle Analysis of Seven-Electrode Conductivity Sensor

The schematic diagram of the seven-electrode conductivity sensor is shown in Figure 1. It consists of seven platinum metal rings embedded in quartz glass. Electrodes 1 to 4 and electrodes 4 to 7 form two groups of measurement units respectively. The current flows in through the common electrode 4 and flows out through the ground electrode 1 and electrode 7. When the current flows through the conductivity cell, it will generate voltage at electrodes 2, 3 and electrodes 5, 6. A constant voltage is added between electrode 4 and the ground. By measuring the change in the current flowing through the common electrode, the change in resistance can be reflected, and then the conductivity of the solution in the conductivity cell can be calculated.

1.2 Measurement system overview

In order to realize the high-precision seven-electrode conductivity sensor measurement circuit, the following aspects are mainly considered when designing the circuit: (1) Reduce the use of analog devices and reduce the noise introduced by the use of analog devices, and use high-speed sampling to extract signal information and perform related calculations. (2) A/D and D/A use the same reference, because conductivity is calculated by the ratio of the driving voltage and the voltage across the sampling resistor. Even if the reference changes to a certain extent, it will not affect the measurement result. (3) The devices used in the system are selected to be low-temperature drift and high-precision devices, especially the op amp and sampling resistor that drive the seven electrodes.

The schematic diagram of the measurement circuit system is shown in Figure 2. The microcontroller STM32F103 controls the D/A to generate a signal with a fixed frequency and voltage. The seven-electrode conductivity sensor is driven by a constant voltage source formed by an integration circuit and a subtraction circuit. The current flows through the sampling resistor, and the voltage value formed reflects the value of conductivity. The voltage is converted by A/D, and the sampled A/D value is processed by an algorithm to obtain the conductivity value.

The system uses STM32F103 as the controller. The STM32F series is a medium-capacity enhanced, 32-bit ARM-based microcontroller with 64 or 128kB flash memory. It has USB, CAN, 7 timers, 2 ADCs, and 9 communication interfaces. It is widely used in various industrial control systems, measuring instruments, etc.