| With the continuous improvement of living standards, more and more ion content problems come out. People pay more attention to the ion content problems. ion content will be seriously affected industrial production, food security, environment, people's health and so on. Ion concentration control has been more and more attentioned. However, the current water quality monitoring equipment has also adopted the basic laboratory analysis equipment, heavy workload, low sampling frequency, complex operation, high cost and very expensive, which can not satisfy the total quantity control of water quality testing requirements. Therefore, this study developed a sodium ion concentration detector which is cost-effective, simple operation, easy to carry .This Na+ ion concentration testing instrument's function is real-time detection of sodium ion concentration in the water and display the result. This detector takes ion exchange method,it can detect Mg2+,K+,F+ with the same hardware conditions,only by change the corresponding ion electrodes.In this paper, designed a sodium ion concentration detector based on the Nernst equation, given the concentration of the principle of detection and temperature compensation, and the relevant formula is derived. From both hardware side and software side to describe the design process in detail. Finally, through experiments to verify the relevance and reliability of the detector.Now to introduce the detector from hardware side and software side:1, Hardware.The detector is composed of voltage conversion circuit, amplifier circuit, AD circuit, single-chip and its subsidiary circuit,temperature sensor, keyboard, LCD and so on. The concentration of Na+ ion will be turn to voltage. The voltage is transferred to STC89C58RD+ to deal after dealed by the amplifier circuit and analog-to-digital conversion circuit.Then the ruslut will be display on the LCD.In the part of hardware design, take STC89C58RD+ as the control core, This single-chip has more resources, and with E2PROM internal, and has a relatively large storage space. In addition, it's instructions is fully compatible with the traditional code of the MCS-51, as a result of the high-speed core, its processing speed is faster. At the same time, it is a single-chip which is super low power consumption and anti-interference, it is suitable for the needs of the system. In addition,the auxiliary circuit is also carried out a special deal, in order to enhance the stability and reliability of system.Amplifier circuit uses INA116. INA116's input bias current is very low, typically 3fA. Great resistance value for the level M?, and much greater than the electrode's resistance. In addition, it has a very low drift voltage (<2mV) and high (> 84dB) of the common mode rejection ratio. Magnification can be directly adjustment in a wide range with an external resistor, it suits the application of high-precision measuring ion concentration measuring instruments.AD circuit uses ADS1100. Its greatest advantage is for the I2C interface,it has a simple form, you can save a great deal hardware resources of the interface.In addition it can simplify the hardware design.Human-Computer Interaction module using the four buttons to complete the functions of measurement, storage, switching,return and so on. With reasonable design, the four keys can complete all data input and order entry.In addition,use 12864 to show each interface. In general, human-computer interaction module is simple,clear,and easy to operate.This system selects 18B20 as the temperature sensor which is produced by DALLAS U.S. company , this device does not require an external circuit, which can transmit data with one line, the hardware circuit is simple.AC 220V is the power of power supply part, this power supply part can supply voltage like +9 V,-9V, +5 V,through the reasonable design. This part is security, stability, working to meet the requirement of equipment.2, software.The software of this detector uses KeilC51 to develop. Uses C language which is better readability, portability and reliability to programe. through the serial port to write procedures to single-chip microcomputer. Made the entire implementation process,the measurement of ion concentration measurement, display, file storage and other operations come true.Software is composed with main program and subprogram. The various subprogram is controled by the main program according to a predetermined way, it can finish its function when the trigger conditions are ripe. Different function has different subprogram is necessary.It plays a good role to make software be rigorous logic and context clearly. In addition, new features to increase it's easy to add new features. This is the basic principles to be followed when design project.Subprogram include: choice of key functional subroutine, display subroutine, measurement subroutines, calibration subroutine, E2PROM subroutine, storage subroutines and so on.The part of software can finish this main function: Read the conversion value of ADS1100 through the I2C interface, and restore their terms, have been amplified voltage values, And restore its terms,then get the amplified voltage values. Read DS18B20 conversion value through the single bus interface, and restore their calculation, get the temperature of test solution. According to the principle of temperature compensation read the needs value from the E2PROM memory, according to the temperature compensation to Calculate the ion concentration of testing solution, and displayed on the LCD screen.At the end of hardware and software design,through the test solution to verify the detector's accuracy and reliability. Finally, an analysis of error sources and proposes ways to reduce the error are given.The key technology of the Na+ ion concentration testing instrument is to deal with the signal which is collected by electrode and derive the temperature compensation formula.1, Deal with the signal which is collected by electrodeThe signal which is collected back by the electrode is very sick, for the class mV, so the early work of signal processing is very important, it's necessary to avoid interference of orther signal. First of all, because of the high resistance of the electrode, so we need to select a high-input impedance amplification chip to support. In addition, the lead electrode signal using coaxial cable and the outer shield of coaxial cable need to connected with the earth, so that electromagnetic interference can be effectively inhibited. Finally, the sick signal sent to the amplifier directly,the pin connected to the input pin and not pass through the PCB board, however, which is the same role in order to reduce the interference.The above job can not be omitted for the accuracy of the results .2, Derive the temperature compensation formulaIn the process of measurement of Na+ ion concentration, the temperature impacts the result,. Without a certain degree of temperature compensation will be affected the Na+ ion concentration measurement accuracy and stability.In the actual measurement of Na~+ ion concentration always design like this: express a certain concentration of sodium ion as a voltage value. But, the result of electrode system of the potential at the same concentration, is different when the temperature is different. Therefore, it is necessary to make a certain temperature as a standard. However, the measured temperature of the solution are usually deviate from this standard, thus eliminating the effect of temperature on the measurement improve the accuracy is the premise. The equipment will use the temperature compensation formula to eliminate the impact of temperature.Low-cost, reliability and stability is emphasised when the equipment was designed.The whole system has been successful.At the same time,because of the detector can be used as other ion concentration replacement only by a different plasma electrode and change the magnification of amplifier circuit.it can achieve a wide range of ion concentration detected, At the same time it has a portable and multi-functional characteristics. therefore the entire system has enormous application.
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