The Design And Implementation Of The Capacitive Sensor For Thickness Measurement Of Water Film

Water film thickness is the key performance index to evaluate the heat exchanged capacity of falling-film evaporation and the surface form of the water film fluctuation state. With the deepening of the research on the falling-film evaporation heat transfer theory, the water film thickness usually reaches micrometer.Thus, the detection of water film thickness must be very high accuracy and dynamic frequency response to meet the requirements of water film measuring. Therefore, it is necessary to develop a sensor to realize the high precision measurement of water film thickness.By comparing several different measuring method of water film thickness, this research builds a capacitance working model, which is based on electrostatic field theory. The research developed a capacitive sensor for thickness measurement of water film, this capacitive sensor is a composite structure, outside the sensor surface increases with a layer of dielectric coating of PTFE. In order to prevent water mist spraying effect on the sensor measurement accuracy, the coating isolated the surface of water film from the sensor probe. The sensor converts the change of the water film thickness into capacitance value, this research designed non-electricity conversion circuit; and capacitance signal is converted to a voltage signal for acquisition and display; the hardware circuit realized the high precision detection of capacitance signal of PF scale. Under the Lab VIEW software environment, the research developed a PC platform for thickness measurement of water film and display. Using A/D acquisition card high-speed collect output voltage signal of non-electricity conversion circuit and uploaded to the PC. The LabVIEW software processing voltage signal, converted to the corresponding water film thickness values. What’s more, we developed a transplantable user application program, which is convenient for on-line measurement of industrial heat exchange equipment.Finally, we set up an experiment to make an evaluation of the index of the linearity and sensitivity of sensor by data analysis. Through capacitance sensor installation and operation in falling film evaporation equipment, give an analysis of the element that causes the sensor measurement error, and put forward corresponding relevant solutions.