UNCC ECGR 6185 - Designing a pH Data Acquisition and Logging Device using an Inexpensive Microcontroller

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Designing a pH Data Acquisition and Logging Device using an Inexpensive Microcontroller Chaitanya S. Misal University of North Carolina at Charlotte [email protected] James M. Conrad University of North Carolina at Charlotte [email protected] Abstract Accurate measurement and analysis of pH data is required for a multitude of applications ranging from the agricultural sector to clinical laboratories. In many of these applications an inexpensive hand-held unit is needed. This paper describes a low cost and low power pH sensing and data logging system using a Renesas microcontroller and pH sensor. This paper describes the sensor data acquisition and data logging system including details of its construction, capabilities and applications. 1. Introduction A pH value is the function of the Hydrogen ion concentration in a solution. Many important properties of a solution can be determined from accurate measurement of pH, including acidity of the solution and the extent of reaction in the solution [6]. The pH scale ranges from 0 to 14 with pH 7 as neutral, pH 0 as highly acidic, and pH 14 as highly basic. 1.1 Motivation Accurate measurement and analysis is very important for many applications ranging from industrial operations to biological processes. Many of these applications demand a low power, low cost portable device. Some of the prominent applications include [1]: • Biochemistry. • Agronomy: The availability of nutrients for plant, depend upon the pH content of the soil as well as the activity of soil bacteria. Knowledge of the pH value helps determine the necessary measures to be taken to ensure the health of plants. • Food Science: Food products need to have an optimum pH level; hence knowledge of pH will help in maintaining quality of food products. • Chemical Research and Engineering: Researchers use the knowledge of pH to determine and thus understand the chemical reaction rate. Engineers use this information to develop practical commercial processes (i.e. nuclear plants, process industries). • Environmental Research and Pollution Control: pH knowledge of a lake or a river is important to maintain proper ecological balance. Extremes in pH can affect the flora and fauna adversely. Also, protecting our waterways requires constant monitoring of industrial effluent. pH measurements are used as a guide to the proper neutralization of these industrial wastes, as well as to monitor the final effluent quality. Therefore, accurate pH measurement helps with proper management of cost saving techniques. Any developed system would also need to be cost effective compared to similar products available in the market from YSI Corporation (current cost: $380) [3]. However, the existing product does not have storage capability. 1.2 System Overview This system provides a small, inexpensive, and easily deployable pH data acquisition and data logging system. Figure 1 summarizes the components and functionality of the designed system. The pH sensor measures a solution and gives an output in the form of millivolts (positive voltage for pH above 7 and negative voltage for pH below 7). This small voltage is fed to signal conditioning circuit which was designed to account for both positive and negative voltages and give a corresponding output from 0 to 5 Volts. Further, it is fed as input to a 10-bit Analog-to-Digital converter which is an integrated peripheral on the Renesas M16C/62P microcontroller chip. The converted digital value is used by a C code program which performs a table look-up to obtain the equivalent pH value. The pH value is displayed on the LCD display and logged on to the 4Kb on-chip flash Fig. 1: System Block Diagrammemory block (non-volatile data logging). Use of the on-chip memory’s (EEPROM) reserved area helps to save the cost of having external memory and also reduces the size of the system. The timers are set in the system to take readings at regular intervals and log the data in the memory. The timer period can be set to longer intervals to improve system power consumption. This data can be observed in real time as well as offline by downloading the EEPROM memory. The distribution of pH values over a period of time can be plotted for further analysis and study. 2. Data Acquisition Data acquisition includes the process of extracting, transforming and transporting data from the source system and external data sources to the microcontroller for storage. 2.1. pH Sensor The sensor selected was a Sensorex general purpose epoxy pH electrode (S200C) [8]. It is a standard laboratory-use electrode which, when inserted in a solution, produces a small voltage (mV) proportional to Hydrogen ion concentration [4]. The output voltage for known standard pH values were measured for the Sensorex electrode and compared with a known standard pH meter. The recorded values were used to build the look-up table used to pH meter operation. 2.2. Analog Circuitry Figure 2 shows the analog circuit used to condition the small voltage signal. The zener diode provides constant voltage at the pin2 of operational amplifier, while pin3 measures the output from the sensor. The amplifier is used in an inverting configuration with a gain of 10. The design is such that when the solution is neutral (i.e. pH 7) the output voltage is 2.5 Volts (range is 0-5 Volts). The designed circuit is inexpensive, accurate and reliable. Fig. 2: Signal Conditioning Circuit (“Analog Circuitry” in Figure 1) 2.3 Renesas Microcontroller (M16C/62P) The Renesas M30626P microcontroller is a single-chip 16bit CMOS microcomputer. It operates using sophisticated instructions featuring a high level of instruction efficiency. The M30626P has a 1 Mbytes address space and is capable of executing instructions at up to 24MHz. It has a multiplier and DMAC which combined with fast instruction processing capability makes it suitable for communication and industrial equipment which require high speed arithmetic/ logic operation [6]. The pH sensing system was built using a Renesas SKP16C/62P Evaluation Board (Figure 3).


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UNCC ECGR 6185 - Designing a pH Data Acquisition and Logging Device using an Inexpensive Microcontroller

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