The basic principle and characteristics of the seven-electrode conductivity sensor are described. According to the physical characteristics of the sensor and the requirements of high-precision measurement, a measuring circuit capable of satisfying low temperature drift, high precision and high speed is designed. Using D/A, A/D and other integrated circuit chips to achieve accurate drive and high-speed sampling of the sensor, compared to the traditional way, the frequency and voltage of the drive is more accurate, and easy to change, high-speed sampling can avoid the signal in the conditioning process Distortion in the middle. The measurement results of the circuit were verified by experiments.
Conductivity measurement has a wide range of applications in industrial production, environmental monitoring, marine resource development and other fields. Accurate and rapid conductivity measurement methods are of great significance for marine research and environmental protection. The conductivity sensor can be mainly divided into an electrode conductivity sensor and an electromagnetic conductivity sensor according to the working principle. The electromagnetic conductivity sensor is measured according to the principle of electromagnetic induction, and the change of the conductivity is reflected by the change of the induced electromotive force; the electrode type conductivity sensor is measured according to the principle of electrolytic conduction, and the change of the conductivity is reflected by the change of the measured resistance of the liquid. Electrode conductivity sensors have received wide attention due to their fast response and high accuracy. The article analyzes the working principle of the seven-electrode conductivity sensor and designs the measurement circuit.
Working principle and overviewThe seven-electrode conductivity sensor is one of the higher precision of the electrode type conductivity sensor, and has the advantages of three electrodes and four electrodes. The conductivity cell of the seven-electrode conductivity sensor realizes the separation of the "current" electrode and the "voltage" electrode, which can reduce the polarization resistance of the electrode, the large flow guiding space and the fast response time, and realize the rapid measurement of the conductivity; the seven-electrode conductivity sensor There are two grounding electrodes at both ends of the conductivity cell, which can effectively shield the influence outside the conductivity cell, so that the measurement result is not interfered by the outside of the conductivity cell, and no pump is needed 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 Fig. 1. It consists of seven platinum metal rings embedded in quartz glass. Electrodes 1 to 4 and electrodes 4 to 7 respectively form two sets of measurement units, and current flows through the common electrode 4. When the ground electrode 1 and the electrode 7 flow out, a current is generated in the electrodes 2, 3 and the electrodes 5, 6 when a current flows through the conductivity cell. By adding a constant voltage between the 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 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 should be considered in designing the circuit: (1) reduce the use of analog devices, reduce the noise introduced by using analog devices, and extract the information of signals by high-speed sampling. Make relevant calculations. (2) A/D and D/A use the same reference because the conductivity is calculated by the ratio of the driving voltage to the voltage across the sampling resistor. Even if the reference has a certain change, it will not affect the measurement result. (3) The devices used in the system use low-temperature drift and high-precision devices, especially the operational amplifiers and sampling resistors 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 fixed frequency and voltage signal. The seven-electrode conductivity sensor is driven by a constant voltage source formed by the integration circuit and the subtraction circuit. The current flows through the sampling resistor, and the formed voltage value reflects The value of the conductivity, the voltage is analog-digital transformed by A/D, and the sampled A/D value is processed by an algorithm to obtain a conductivity value.
The system uses STM32F103 as the controller. The STM32F series is a medium-capacity, 32-bit ARM core-based microcontroller with 64 or 128kB flash memory. It has USB, CAN, 7 timers, 2 ADCs, and 9 communication interfaces. Widely used in various industrial control systems, measuring instruments, etc.
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