Design of the Smart ph Sensor Based on Ion Selection Electrode

Transcription

1 Sensors & Transducers 2014 by IFSA Publishing, S. L. Design of the Smart ph Sensor Based on Ion Selection Electrode Xuelun Hu, * Yaoguang Wei, Yingyi Chen, Chunhong Liu College of Information and Electrical Engineering, China Agricultural University, Beijing , China Tel.: , fax: weiyaoguang@gmail.com Received: 19 November 2013 /Accepted: 27 December 2013 /Published: 14 March 2014 Abstract: In aquaculture ph is one of the important indicators which affect cultivation objects healthy growth, whether ph value is normal or not would affect the survival of cultured objects. The traditional ph detecting method has poor stability and reliability, a smart ph sensor which is based on ion selective electrode has been designed in this paper, it adopts positive and negative double power to solve the problem of power supply, the self-excited prevented and impedance matching circuit has been designed to eliminate the selfexcited oscillation circuit's influence on the measurement accuracy. The filter circuit has been designed to prevent the interference of the power frequency noise signal effectively, the zero and range adjustment circuit has been designed to expand the scope of the sensor. The temperature compensation correction model has been proposed to solve the problem of temperature compensation. The experiment results have shown that the developed sensor has good stability, reliability, and suitable for ph monitoring in aquaculture water quality. Copyright 2014 IFSA Publishing, S. L. Keywords: ph, Ion selective electrode, Water quality testing. 1. Introduction China is an aquaculture superpower, the gross production of China has ranked first in the world for many years, and the breeding production accounts for more than 70 % of the world. But the aquaculture industry in China mainly adopts the traditional management ways which has little content of science and technology and consumes a large number of resources [1]. The actual aquaculture production of domestic is still lack of real-time monitoring of water quality parameters, farmers mostly rely on their own experiences to evaluate the quality of aquaculture water through water s features and observation of aquatic animals behavior [2]. The ph value is an important parameter to measure whether the water quality is safe or not and also affects the growth and reproduction ability of breeding biology. Smart ph sensor can achieve real-time online monitoring of water ph. The excellence of short measurement time and high measuring accuracy mean it can deal with sudden changes of water quality rapidly. Therefore, the research and development of smart ph sensor, which has many advantages including low cost, environmental protection and implementation of online monitoring, has very important practical significance. There are many methods to detect ph value at home and abroad, these methods generally can be divided into artificial colorimetric method and chemical sensors [3]. Artificial colorimetric method belongs to the traditional ph value detecting method, 92 Article number P_SI_546

2 the advantage of it is simple, but the precision is relatively low, so it is always applied to the off-line environment. Chemical sensor is a method of detecting ph in water by using a variety of chemical sensors, most of the research about chemical sensor at home and abroad are electrode ph sensor, fiber ph sensor, chemically modified electrode ph sensor and ion sensitive field effect transistor (ISFET) ph sensor [4]-[5]. The key technology of the intelligent sensor compensation is the combination of traditional sensor and microprocessor [6], the technology processes the sensor data by making full use of the computing ability and storage capacity of the microprocessor. This paper designs a ph sensor based on ion selective electrodes. Due to the characteristics of ph electrode, including high impedance and vulnerable to interference, and the most suitable ph value is in aquatic environments [7], we design the filter circuit, amplifying circuit and zero adjustment circuit, after the calibration experiment, accurate ph value can be measured. hydration layer. Bulb layer also has potential, due to the inner hydrogen ion, concentration is invariant, the outer layer of hydrogen ion concentration is transformed, therefore, potential caused by outer sphere also changes. The potential difference between two electrodes and the concentration of hydrogen ions in solution follows the Nernst equation, thus we can determine the solution ph by Nernst equation. At 25 C, the each unit change of ph value will cause the change of 59 mv potential, and this is the basis that glass electrode can be the indicator electrode of hydrogen ion. The ph sensor used in this paper is a kind of composite ph glass electrode, the glass electrode and the reference electrode are combined in a single electrode, as shown in Fig. 1. The composite ph glass electrode is the ordinary ph glass electrode with a glass layer as the outside sleeve, the glass spacer sleeve is filled with a certain concentration of KCl solution, put the AgCl-Ag electrode immersed in KCl solution so as to constitute the reference electrode. Because the Cl- activity in the solution is certain, the AgCl-Ag electrode potential is constant. 2. The Measurement Principle and Method of ph Value Ion selective electrode (ISE) is the analysis tool which is relatively simple and cheap in many chemical analysis instruments. It is a kind of electrochemical sensor which determines the activity or concentration of ions in the solution by using the electrode membrane potential, and is commonly known as the electrode sensor. Ion selective electrode is a measuring system of two electrodes, ion selective membrane is located between two liquid phases, Ag-AgCl working electrode is placed in internal filling solution which contains the electrode response ion, the reference electrode is linked together with samples through the salt bridge [8]. The cell potential Emf is the superposition of several potentials which are internal reference electrode potentials, ISE inner membrane potential, ISE outer membrane potential, external reference electrode potential. When the ion activity in the sample changes, only the outer membrane potential is variable, the rest are fixed components. They can be classified as the Nernst equation ' 0 constant E, so the cell potential can be expressed as: Emf E ' lg (1) n The head bulb of ph glass electrode is made of special sensitive film, it is the main part of the electrode and only sensitive to hydrogen ions. When the electrode is immersed in the solution to be tested, the measured hydrogen ions in the solution will carry on ion exchange with electrode bulb surface Fig. 1. Composite ph glass electrode. 3. Hardware Development of Intelligent ph Sensor 3.1. The Overall Structure of Intelligent ph Sensor The analog circuit design diagram of sensor is shown in Fig. 2. The sensor consists of a ph glass electrode probe, temperature sensor, analog board and CPU board. Because the resistance of the glass sensitive membrane is very large, it is usually up to hundreds of megabytes ohm in a normal temperature, so the first level of amplifying circuit must use a high input impedance amplifier to match the impedance. Thus we can get a voltage signal which is positive in acid solution and negative in alkali solution. This signal through the two order 50 Hz low-pass filter so as to filter the signal above 50 Hz and then through a polarity reversal, low-pass filter with a magnification, thus we can get a voltage signal which is negative in 93

3 acid solution and positive in alkali solution. Then this signal through a zero adjustment circuit with a magnification of signal so as to raise the zero point of the output voltage. Finally, we can get a positive voltage which is small in acid solution and big in alkali solution. After calibration we can measure the ph value of the solution by this voltage signal The Hardware Circuit Design of Intelligent ph Sensor The circuit needs to supply positive and negative double power for the operational amplifier TLC27L4. This makes the operational amplifier have a positive and negative output and have better low frequency characteristics. 1) The 3.3 V power module. The core chip of power supply module is LP2981, which is a is a fixed output voltage regulator, the voltage can be fixed at 1.8 V, 2.5 V, 2.7 V, 2.8 V, 2.9 V, 3 V, 3.2 V, 3.3 V, 3.6 V and 5 V, and the voltage regulator includes over-current, and over-heating protection. Power module circuit is shown in Fig. 3. As shown in the figure, the capacitor C 16 is a bypass capacitor, which is a maintaining power providing energy for local devices. It can make the regulator output uniform, thus reducing the load demand. Bypass capacitor C 16 should be as close as possible to the power pin and the ground pin. This can be very good to prevent the ground potential rise and noise caused by oversize input. Capacitor C 17, C 18 are decoupling capacitor, they played the role of battery and can meet the change of driving circuit of current, so as to avoid coupling interference. The decoupling capacitor can bring interference output signal as the filter object, so as to prevent interference with the signal back to the power supply. At the same time, they also play the role of energy storage. Inductance L 2 plays the role of filtering AC signal. Fig. 2. Circuit structure of sensor. Fig V power module. 2) The -3.3 V power module. The core chip of -3.3 V power supply module is FPK1, which property between the traditional linear regulator and switching regulator. The external storage element is a capacitor, so as to alleviate the potential problem of electromagnetic interference. As shown in Fig. 4. 3) Self-excited prevented and impedance matching circuit. Because the resistance of the glass sensitive membrane is very large, it is usually up to hundreds of megabytes ohm in a normal temperature, so the first level of amplifying circuit must use a high input impedance amplifier to match the impedance. The core chip of impedance matching circuit is CA3140, which is a high input impedance amplifier developed by Radio Corporation America and has excellent performance. It s input impedance up to 1.5 TΩ and has a very low input current. The working power supply of CA3140 is a wide range from 4 V to 36 V. 94

4 Fig V power module. Despite that there is no input signal in the input terminal of the amplifier, the output terminal will still have a signal with certain amplitude and frequency, so we need to add a self-oscillation prevented circuit. First of all, we need to connect a 15 M resistance in the positive output terminal of the glass electrode, and connect a polystyrene capacitor 1000 pf in the negative output terminal, then access them to the positive input terminal of CA3140. As shown in Fig. 5, the circuit uses the RC delay compensation so as to offset the effect of input distributed capacitance and eliminate the self-excited oscillation. By shortening the external connection wire and increasing the power supply to the decoupling capacitor, the circuit can further eliminate the oscillation. Fig. 7. The signal damping to 42 db in 50 Hz, so the circuit has good filtering effects. Fig. 8 is the waveform for this filter, the input is 100 mv DC signals add a 50 Hz AC signal which amplitude is 200 mv. After filtering the AC signal was basically removed, and the amplitude of output fluctuation is less than 3 mv [9]. Fig. 6. The low-pass filter in the transmitter circuit. Fig. 5. Self-oscillation prevented circuit. 4) The low-pass filter circuit. Through the experiment we found that, the output signal from the electrode probe is easily affected by 50 Hz interference signal, so we increased the low pass filter in the signal conditioning module. The low-pass filter circuit used in this design is composed by a two order low pass filter and a voltage follower, as shown in Fig. 6. Through the simulation of TINA, the signal s amplitude frequency characteristics as shown in Fig. 7. Amplitude frequency characteristics of low pass filter. 5) Low pass filter circuit with magnification. As shown in Fig. 9 is a first-order low-pass filter with magnification, the circuit can reverse the polarity. Capacitor C 21 is an AC feedback element. For electric capacity, the capacitance is smaller while the frequency of the signal is high, thus the amount of feedback is large and the magnification is smaller, so the circuit can magnify the low-frequency signal and suppress the high frequency signal. 95

5 Fig. 8. Waveform of the low-pass filter. Fig. 9. Low pass filter circuit with magnification. 6) Zero and range adjustment circuit. Because of the operational amplifier circuit has offset voltage and offset current, the working process of circuit would have zero drift. In order to suppress the zero drift, this design adds a zero adjustment circuit to introduce a compensation to offset voltage through changing the resistance of the sliding rheostat RX 1. At the same time, adjusting RX 1 can change the input bias current so as to adjust the symmetry of the circuit and realize zero output while static state. The circuit also has the function of range adjustment, the formula of amplifier's gain is: G = 1+ R18/[R14 + RX (right)], (2) By adjusting the sliding rheostat RX 1, the range is changed while the resistance value is changed. We can improve the measurement precision of the instrument through adjusting the zero and the range of the circuit. As shown in Fig. 10 is zero and range adjustment circuit. Fig. 10. Zero and range adjustment circuit. 96

6 4. The Experiment of ph Sensor Performance Test 4.1. Calibration of ph Sensor Put the sensor into A, B,C three standard solution to measure the ph, after each measurement the sensor must be carefully washed by sucking up water droplets, and then we could measure the next sample. Recording after the numerical stability, we got 413 mv in ph standard solution A (ph=4.008, 25 C), 1277 mv in ph standard solution B (ph=6.865, 25 C), 1863 mv in ph standard solution C (ph=9.180,25 C). We could get the calibration curve y=0.0036x as shown in Fig. 11 based on these data. The R 2 of the curve is , so we figured out that the linearity of the ph sensor is very good. Because the output is a linear graph y=ax+b, we could identify A, B after calibration. The A, B values were stored in the microprocessor program, so the micro-controller would set the output voltage into the calibration formula y=ax+b automatically, the calculated y is the ph value of the solution Analysis of the Performance of the Sensor 1) Accuracy analysis. At room temperature (25 C), added distilled water or sample into the above three kinds of standard solution so as to get the ph value of solution of 5.00, 10.00, respectively. Put the sensor into the sample solution, measure these sample solution eight times. The measurement results are shown in Table 1. Fig. 11. The calibration curve of Ph sensor. Sample Table 1. The accuracy of measurement. Measurement result Average value Absolute error Fractional error % % % 2) Precision analysis. Precision refers to the discrete degree of the values measured by the same method in the same experimental condition for the same sample repeated, generally with the relative standard deviation (RSD) to represent. The calculation formula of precision is as follows: RSD n i 1 X X i n 1 X 2 100% (3) X is the single sample measurement, X is the average value of n parallel sample survey data, n-1 is the degree of freedom. The smaller the relative standard deviation (RSD) means the determination results are more concentrated and the precision is better. Deliquated the standard solution C with distilled water and measured this sample ten times continuously. The measurement results are shown in Table 2. Micro-controller would calculate RSD every ten measurements. When the RSD is less than 0.005, this indicates that the precision of this group of data is very good, then micro-controller would put these data into internal memory. The results show that the sensor has high accuracy. 3) Repeatability. Drawing a scatter diagram by using the above four groups of data, as shown in Fig. 12. It can be seen from the figure that the repeatability of the ph sensor is very good. 97

7 Table 2. The precision of measurement results. The number of measurements average value RSD Measured value % Fig. 12. The measurement of repeatability. 4) Stability. Added distilled water into standard solution A so as to get the ph value of solution of 5.00 and 6.00 respectively. Put the sensor into the sample solution to measure the stability of ph sensor. Recorded the data every hour and measured ten times continuously. The results are shown in Table 3. The results of data analysis indicate that the stability of the sensor is good and the measured values are in the range of error. 5)Temperature compensation of ph sensors According to the Nernst equation, the ph changes when the temperature increases 1 C. If the ph sensor is calibrated at room temperature, difference in temperature will be within ±20 C in the practical application, so ph would change ±0.06. Using three kinds of standard solution, we did the experiment temperature effect on ph sensor by using test box of RGDJ-100 which had function of alternating high and low temperature. The results are shown in Fig. 13. Aiming at the existing problems, this paper designed the temperature compensation of software for measurements of the intelligent ph sensor [10]. Firstly, a group of temperature value can be measured. Then the temperature signal as one of the multiple sampling switch signal is sent to the microcontroller [11]. MCU receives the voltage signals of temperature X and voltage signal of ph sensor Y via the serial interface, and stores the data provisionally. After signal sampling, acquired data can be analyzed. Ultimately, an equation about the relationship of temperature and ph voltage signal is achieved: y = ax2 + bx + c, (4) Because the difference of making process in each ph glass electrode, the characteristics of output would be variant, so diverse value of a, b, c can be acquired according to each situation. Collect the temperature voltage signal in 25 C and put the signal into the fitting equation then calculate the ph value. After each measurement of the sample, MCU would operate the compensation program of temperature so as to compensate temperature error of the sensor. This method can realize temperature compensation of the ph sensor measurements quickly and accurately. The flow chart of temperature compensation of ph sensor is shown in Fig. 14. Table 3. The stability of measurement results. Sample Measurement result Average Fractional value error % % 98

8 negative 3.3 V power module, so as to solve the problem that the output of the glass electrode is positive in acid solution and negative in alkali solution. This paper also designed the impedance matching and self-excited prevented module, so as to solve the problem that the resistance of ph glass electrode is very large and the output voltage is small and too susceptible to disturbances. And this paper designed low-pass filter module so as to solve the problem of the 50 Hz frequency interference. This paper also designed the zero and range adjustment module so as to solve the problem of zero drift and the output voltage is out of range of the scope of A/D. Through the signal conditioning circuit, we can get out the output voltage of glass electrode accurately and magnify the voltage to a certain extent. After calibration of the ph sensor, the experiment data shows the ph sensor has good linearity, accuracy, precision, repeatability and stability, it can meet the demand of ph detection on aquaculture which is intensive and numerical [12]. Acknowledgements Fig. 13. Measurement result of temperature: (a) Relationship of temperature and ph value, (b) Relationship of temperature and ph deviation. This work was supported by Special Fund for Agro-scientific Research in the Public Interest ( ). References Fig. 14. The flow chart of temperature compensation program. 5. Conclusion In view of the existing problems in the ph measurement, this paper designed the positive and [1]. Yujuan Xiang, Research on intelligent measuring instrument with multi parameter in wastewater treatment, Master's Graduation Thesis, Beijing University of Chemical Technology, Beijing, [2]. Shihong Xie, Xinrong Wu, Shitao Xie, et al., Effect of fishery water quality analysis and monitoring in aquaculture, Jiangxi Fisheries Science and Technology, 4, 2005, pp [3]. Hong Jiang, Study on ammonia measuring instrument, Master's Graduation Thesis, Tianjin University, Tianjin, [4]. Qian Liu, Daoliang Li, Kun Ma, et al., Design of intelligent transmitter aquaculture water ph value, The Chinese Society of Agricultural Engineering, 24, 2, 2008, pp [5]. Hua Yang, Xiaoming Huang, Design of automatic and continuous monitoring system of water quality, Software Guide, 5, 2006, pp [6]. Mingrui Zhu, The research on the automatic monitoring system for online aquaculture parameters plant, Master's Graduation Thesis, Shanghai Fisheries University, Shanghai, [7]. Zhenhua Wang, Jian Gu, et al., Study on several alkaline reagent in the recirculating aquaculture system ph regulation, Chinese Agricultural Science Bulletin, 26, 01, 2010, pp [8]. Shengmin Dong, Chengyu Wang, Yukun Pan, et al., Present situation and development of ph glass electrode, The Glass and Enamel, 32, 02, 2004, pp [9]. Qisheng Ding, Research of Smart Water Quality Sensors in Aquaculture Based on Electrochemistry PhD's graduation thesis, China Agricultural University, Beijing, 5,

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