Dissolved oxygen measurement systemMy diploma thesis, "Dissolved oxygen measurement system" is dealing with the development and design of a dissolved oxygen measurement system based on Clark's type polarographic oxygen sensor. The measurement of dissolved oxygen concentration is important in various fields. For example, it is performed in wastewater management and water quality monitoring. Oxygen measurements are also common in biology, chemistry and medicine. The proposed dissolved oxygen measurement system consists of a Clark’s cell, analog preamplifier for signal conditioning, NTC thermistor, a Cypress PSoC configurable mixed signal array and personal computer as presented in Fig. 1. Clark's cell is a linear amperometric oxygen sensor based on an externally polarized electrochemical cell. The output current of the Clark's cell is proportional to the partial pressure of oxygen in the sample. The preamplifier is realized in two stages and converts the current signal from the output of the Clark's cell into a voltage signal, which is then amplified in the second stage of the preamplifier. The first stage of the preamplifier is realized by a TLC 271 operational amplifier and is also used as a source of external polarization voltage for Clark's cell. The second stage of the preamplifier, realized by an LF 411 operational amplifier, amplifies the voltage signal and enables cancelling of the voltage offset in the system. The Cypress PSoC ("Programmable System on Chip") configurable mixed signal array chips enable the design of microcontroller-based systems with a wide range of integrated analog and digital peripheral circuits on a single chip. The PSoC mixed signal array consists of an 8-bit CPU core, programmable analog and digital blocks, system resources and programmable I/O ports. In a CY8C26233 PSoC chip, a programmable gain amplifier (PGA) module, a 12-bit AD converter and a UART module are implemented. Voltage signal from the preamplifier output is sampled, digitized and sent by a UART module to the computer. Dissolved oxygen concentration is highly temperature dependent; therefore it is crucial to measure the sample temperature. An externally connected NTC thermistor and reference resistor are designed for the purpose of integrated temperature measurement chain in PSoC. The sample temperature is calculated from the voltage measured on the NTC thermistor, and the temperature data are sent by UART module to the PC. The software with graphical interface developed on the PC enables measurement data storage and analysis. In this circuit only the basic measurement functions of the system are implemented. Therefore, this work is only the basis for further dissolved oxygen measurement system development by designing temperature regulation and sample stirring subsystems, development of calibration procedures and software upgrades.
Mario Hrgetic
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