Pressure probe with built-in temperature sensor and conductivity cell design and realization of the prototype

In this thesis, a monolithic three-parameter sensor that can measure the pressure, Electrical Conductivity (EC), and temperature values of any liquid and send the measured values with universal data outputs has been designed and prototyped. In this context, different pressure measurement methods have been investigated. It has been found appropriate to use piezoresistive pressure sensors using the gauge measurement method. Different temperature sensors have been tested for temperature measurement and PT-100 temperature sensor has been decided to be used. Different materials and different cell models have been tried for electrical conductivity measurement. For these measurements, 2 active and 2 passive graphite electrodes have been used to obtain the cell with a wider EC measurement range. In addition, single-pole / double-throw (SPDT) switches have been used to send smooth square waves to active electrodes. An adjustable voltage is applied to active electrodes of EC cell with the help of microcontroller. Furthermore, the frequency of the voltage to be applied can be controlled by the microcontroller. These controls allow the sensor to be used in different liquids. Two ADCs of the microcontroller are used to measure the temperature together with pressure and EC. This helps to reduce the influence of temperature on EC and pressure measurement. The three parameter sensor is designed to have industrial norms in the framework of this thesis. In this context, the parts of the PCB and sensors that do not come into contact with water are coated with electronic gel for sealing. The outer cover of the sensor, which will remain in liquid continuously, is made of corrosion resistant 316 stainless steel. Since the prototype sensor has a diameter of 22 mm, a 4-layer PCB is designed. Due to the low usable area of the PCB, the PCB is designed in such a way that materials can be placed on both surfaces. The sensor can read the pressure, temperature and EC raw values with the microcontroller it contains, and record it in the desired range to the flash memory. When the request to read data is received, it can convert the raw values into readable format and send them over RS-485. The completed prototype sensor was kept in the trial pool for 20 days and the accuracy of the results has been observed. In addition, the sensor was sent to an independent accredited laboratory and tested, so the accuracy of the measurement values was proven. It has been seen that the designed sensor can operate more accurate than most of the sensors of same type. Keywords: pressure measurement, electrical conductivity measurement, temperature measurement, MEMS ,4 pole EC cell design, water quality