Study On A Medium-independent Capacitive Liquid-level Measurement

There are a lot of liquid level measurement methods. In those methods, the capacitive measurement has been widely used in industrial, agricultural, health care and other fields because of low price and simplicity. But its applications are constrained because that the dielectric constant must be known before measurement and the periodic calibration of the dielectric constant must be done.Based on the analysis of the problems in the present capacitive level measurement, combined with capacitive liquid level measuring principle and disadvantages of present solving measures, the improved sensor structure and a new type of capacitive liquid level measurement method are presented in this thesis. And based on differential, it realized the capacitive level measurement with medium independency. Its main application is in dynamic liquid level measurement, such as automobile fuel liquid level measurement.The basic concepts of measurement and compositions of measurement system related to the study in this thesis are discussed. As the key part of this system, sensor is analyzed in detail.The principle of capacitive level measurement and capacitive structures in the conducting medium and the non-conducting medium are analyzed carefully. The reasons of getting dielectric constant before measurement and periodic calibration in traditional capacitive liquid level measurement are summarized. Then a new type of capacitive measuring structure is put forward in the thesis to effectively measure the change of liquid level and relationship between capacitor value and dielectric constant. And two signals are processed by differential. Then the medium independent liquid level measurement is realized.To validate the correctness of analysis, the capacitive structure and the whole measurement system are designed according to the measurement principle. The system includes the capacitance-voltage conversion circuit, data acquisition (A/D conversion), data processing and display on computer.An experimental device of liquid level measurement and the corresponding measurement program are developed. In order to simulate different medium, water and gasoline are used in these experiments and the precision is 2.16 mm and 4.65mm respectively in the range of 130.8mm. The results show that the medium independent liquid level measurement is realized, and the correctness of this study is validated.The research results of this thesis have some reference value and can guide the improvement and perfection of traditional liquid level measurement.