Conditioning of signal from a ph probe and its calibration

Transcription

1 Conditioning of signal from a ph probe and its calibration Toshita Khandare, Prashant Meshram Department of Electronics and Telecommunication Engineering Vishwakarma Institute of Information Technology, Maharashtra, Pune Abstract Design of a signal conditioning ph meter with the help of a ph probe having high output impedance is described. For this design we require impedance matching circuit to transfer total voltage generated, along with an amplifier and span zero circuit to bring it to a desirable range. I. INTRODUCTION ph is one of the most important parameters of a solutions. Kinetics and equilibrium of virtually every reaction occurring in a solution depends on the ph of the solution, and these reactions are not only responsible for a typical chemical application (like compound synthesis), but also for the way plants absorb nutrients from the soil, water animals grow their shells, our bodies regulate breathing and so on. In the case of biological systems ph change in the range of few hundredths can have a significant effect. That in turn means knowing ph of the solutions is usually very important if we want to control the situation. ph meter works by measuring potential difference between known reference electrode and the measuring ph electrode. Potential of the ph electrode depends on the logarithm of the concentration (or more precisely activity) of hydrogen ions. This dependence is described by the Nernst equation, thus once the potential has been measured we can directly calculate the ph of the solution. In this project we are mainly developing ph meter for its application in High Performance Liquid Chromatography (HPLC). II. ELECTRICAL DESIGN In order to obtain the value of ph of liquids, the design of electrical circuit is as shown in Fig1. Fig1: Block Diagram 32

2 1) Linear Power supply: Specifications: - Input supply= 230V (A.C) Output voltage = +12V and -12V. Current = 500mA Fig2: Simulation of power supply Regulator ICs 7812 and 7912 are used to obtain +12 and -12V, respectively. Fig.2 Shows the simulation of designed power supply in which center tapped transformer is used for supply voltage, the bridge rectifier(d1,d2,d3,d4 coverts the supplied A.C. into rectified D.C. voltage, followed by ripple capacitor(c1,c2)which reduces the ripple voltage and the noise in the circuit then the voltage regulator(u3,u4) converts rectified D.C. into a constant D.C. voltage. The capacitors(c3,c4) are connected to prevent abrupt changes the output voltage due to fluctuations in the A.C. supply line. Ripple is the small unwanted residual periodic variation of the direct current (DC) output. This ripple is due to incomplete suppression of the alternating waveform within the power supply.. Fig.3 Ripple voltage 33

3 Fig 3 shows the ripple voltage fluctuations i.e. 12mV which is quite good. This small amount of ripple won't affect our circuit. These readings are taken with the help of Tektronix DSO mention in the reference [4]. a) Load regulation curve (Fig. 3) indicates that the power supply can deliver a current of 0.5 A = 12V/24Ω. Fig 4: Load regulation +12V power supply b) Measuring the output voltage of our power supply with and without load with the help of, we got the following results. c) These readings are taken with the help of Tektronix DMM4040 mention in the reference [5]. Considered Load Resistance = RL = 24Ohms. VNL = Voltage with No Load = 12.04V VFL = Voltage with full load = 11.74V Load Regulation = = *100 =2.5ᵒ/0 c) Output Voltage: +12V & -12V Output current: 500mA; 2) ph SENSOR: Range of sensor: -414mV to

4 = +. VJER-Vishwakarma Journal of Engineering Research 1) A typical ph probe is a combination electrode, which combines both the glass and reference electrodes into one body. 2) Internal solution is usually a ph=7 buffered solution of 0.1 mol/l KCl for ph electrodes. 3) Body of electrode is made from non-conductive glass or plastic. 4) The bulb, which is the sensing part of the electrode is made from a specific glass which is sensitive to hydrogen ions. Relation between ph and potential developed is given by: 35

5 Where, R - Gas constant = 8.314(Joules/ Kelvin. mole). T is the temperature in Kelvin. F - Faraday's constant = 9.65*10 4 (Coulombs/ mole). E 0 is the cell constant in volts. H + - Hydrogen ion concentration (Unit - mol/lit) [ + ] As we are using the sensor whose output impedance is very high which is in the range of 500MΩ, we require an impedance matching circuit to transmit total power. So we have used LF356 as an impedance matching buffer circuit. 3) IMPEDANCE MATCHING: Impedance matching is the most important step in power transfer. In order to achieve impedance matching the output impedance of the sensor should be almost equal to the input impedance of the electronic circuit. Fig 6: Schematic of impedance matching circuit We can achieve impedance matching by using a high input impedance IC for example LF356 8 pin DIP OPAMP IC has been used as shown in Fig.6. Common features of LF356: --High Input Impedance: Ω --High Common-Mode Rejection Ratio: 100 db --Large DC Voltage Gain: 106 db Due to impedance matching we can carry forward the exact output signal obtained from the ph sensor to the rest of the electrical circuit reducing the power loss. The voltage at the output of ph sensor is of very small amplitude and neither is in suitable range regarding the further processing. Therefore, we have to use an amplifier circuit to level-up the amplitude of this output voltage from the sensor 4) AMPLIFIER + SPAN-ZERO CIRCUIT: Range of ph: 0 ph to14 ph Range of output of the ph sensor: -414mV to +414mV.. 36

6 We have used an instrumentation IC AD8222 to serve the purpose of amplification, plus we have to bring the amplified voltage in a specific range so that it can be processed. Hence, in an attempt to minimize the PCB components we have used a DUAL-OP-AMP AD8222 IC. Fig 7: Schematic of amplifier and span-zero circuit. Fig.7 shows the schematic of AD8222, which is a dual OP-AMP instrumentation IC, using this we have designed a amplifier which has single resistor gain control and also a span zero circuit according to our required range of 0.5V to 5V. Resistor RG1 is the gain control resistor, as the gain is 10 we kept the resistor value to be 5.49KOhm. Whereas another op-amp is used to design the span-zero circuit which shift's the voltage to a required value with the help of 3 resistors R3, R5, R6 which helps us to keep the voltage in the range of 0.5V to 5V. Fig.8 shows the internal structure of AD8222. Fig 8: Internal structure of AD8222 Features:- 1) Gain is set with 1 resistor per amplifier (G = 1 to 10,000). 2) CMRR = 106dB minimum (with gain= 10). 3) Temp range= -40 to 125 C. All the readings have been observed and recorded on Tektronix 6 1/2 -DMM4040, and have been plotted using Root software. a) Response of electrical circuit for different solutions. III. RESULTS 37

7 Fig 9: response of circuit for solution of ph 4. Fig.9 plot shows that, for ph4 we get the voltage in the range of 2.231V to 2.237V. These readings have been plotted in the form of histogram below from which we get the mean value of these readings as 2.235V. Fig 10: response of circuit for solution of ph 7. Fig 10 plot shows that, for ph7 we get the voltage in the range of V to V. These readings have been plotted in the form of histogram below from which we get the mean value of these readings as 3.003V 38

8 Fig 11: response of circuit for solution of ph 9. Fig 11 plot shows that, for ph9 we get the voltage in the range of 3.483V to V. These readings have been plotted in the form of histogram below from which we get the mean value of these readings as 3.484V. b) Histogram plot: The histogram plot gives us an idea of the mean value of voltage for a solution of particular ph. The peak indicates value of the mean voltage of the solution. It also gives us the error i.e. sigma and the RMS value. Fig 12: Plot for solution of ph4 In Fig.12 for ph 4 the mean value obtained is 2.235V, error is given by sigma which is

9 Fig 13: Plot for solution of ph7 In Fig.13 for ph 7 the mean value obtained is 3.003V, error is given by sigma which is Fig 14: Plot for solution of ph9 In Fig.14 for ph 9 the mean value obtained is 3.484V,error is given by sigma which is c) Calibrati 40

10 Fig 15: plot of average values corresponding to solutions of ph4, ph7, ph9 In Fig.15 the mean value obtained for different ph is been plotted. The curve is found to be reasonably linear. The slope is 0.24 ±0.01 V/pH and intercept is 1.285±0.07 V. IV.CONCLUSION We have performed impedance matching of the signal obtained from the ph probe and transferred it to the electrical circuit with minimum power loss. Also, we have spanned the output voltage from the range of - 414mV to +414mV into 0V to 5V using the span-zero circuit. Moreover, we have recorded the readings using Tektronix 6 1/2 -DMM4040 and plotted the output of the electrical circuit in the form of histogram plot. V. FUTURE SCOPE In future this ph meter will be used to measure the alkalinity of solutions in dynamic conditions and will be fit for various industrial applications. ACKNOWLEDGEMENT We express our deep gratitude to Mr. M.R.Sanghvi for giving us the opportunity of working on this unique project. The initial driving force in the form of Prof. (Dr.) C. S. Garde our teacher and mentor. We thank our guide Prof. K. J. Raut for his support and guidance through various stages to complete our tasks. We would like to thank our senior Ravi Kesharwani for his support and guidance. REFERENCES:- [1] K. Sesha Maheswaramma, Mridula Chugh, Book of Engineering Chemistry [2] Instructional and service manual for ph meter Equiptronics. [3] Datasheets of AD8222, OP07, IC7812, IC7912, [4] Tektronix Digital Storage Oscilloscope TDS2000C series. [5] Tektronix 6 1/2 DMM4040, 41