Development Of High Precision Conductivity Instrument Based On Embedded System

Conductivity is an important electrochemical parameter for describing the characteristic of a solution.The application of the conductivity meter has been gradually expanded from traditional hydrological testing,agricultural irrigation,and aluminum plant process control to other new issues such as liquor identification,salinity testing,and Chinese medicine extraction.Meanwhile,the accuracy requirements of the conductivity meter are constantly increasing,with wider measurement range in practical applications.In the conductivity measurement technology,the electromagnetic measurement method is mainly suitable for high conductivity detection due to system reasons,and the electrode measurement method is more widely used in practical production applications.This paper analyzes the development status of conductivity meters and electrode sensors at home and abroad,studies the influencing factors of solution conductivity during measurement,and proposes a bipolar pulse method for high-precision measurement.Simultaneously A high conductivity electrode sensor and a low electrode conductivity sensor are designed for the measurement of different conductivity solutions.By using a bipolar pulse to stimulate the conductance cell to reduce the effects of polarization and capacitance in the measurement process,temperature compensation is used to improve the adaptability of the conductivity meter at different temperatures.The conductivity meter uses the STM32F407 chip as a microprocessor,has functions such as data display,timing storage,and sharing data with other devices.The hardware circuit is divided into the control board and the transmission board.The control board mainly includes the clock,communication,display,storage and other circuits.The transmission board is mainly composed of the conductance signal detection,control signal isolation,and conductance signal processing circuits.The conductivity meter exchanges data with other devices through serial communication and CAN communication,and uses a capacitive touch screen to provide human-computer interaction.The microprocessor controls the on-off of the high-speed electronic switch through PWM,and the ±5V voltage is periodically loaded on both ends of the conductance tank to realize bipolar pulse stimulating effect.The positive and negative excitation signals are separately sampled,and each half cycle signal is equivalent to a direct current signal in the amplifier processing,which reduces the signal processing difficulty.Using high-precision,low-noise AD7124-8 analog-to-digital converter of ADI company as the signal acquisition unit,its variable gain programming provides great convenience for measuring conductivity in different ranges.The PT1000 with good stability is used as a temperature sensor,and the circuit adopts a four-wire connection method to eliminate the error of the lead resistance.After hardware and software debugging,the conductivity meter has high measurement accuracy and good system repeatability.