Research On Technology Of Liquid Level Detection Based On Capacitive Measurement

Liquefied natural gas(LNG) plays an important role in the modern energy industry, and the liquid fuel volumetric monitoring system, which is an essential part during the storage and transportation of LNG, should be guaranteed to be precise and real-time reliable. Owing to its simple structure, low cost and reliability in low temperature, the capacitive liquid level sensor is widely used in the industry of LNG tank. Therefore, it is of great research significance to upgrade the precision and the reliability of liquid level sensing technique to a higher level.In this thesis, the latest liquid level sensing technique in the whole world is reviewed first. Then the advantages and disadvantages of the principle and the method of the capacitive liquid level sensing technique over those of other sensing technique are analyzed, and the novelty of the capacitive sensing technique has been studied. Thereafter, a capacitive sensing system has been designed. This system can monitor the change of liquid level by the precise measurement of capacitance, and the reliability of this system operated in ultra-low temperature has been improved substantially. In this work, based on the architecture of the sensor, the hardware system with capacitance-to-digital-convertor PCAP01 and MSP430 as platform has been designed. This hardware system include both signal acquisition circuit and display circuit, which enable it to acquire signal from the sensor, to control the sensor, and to display the result digitally. The software programming for the signal acquisition and display has also been done respectively. In terms of the performance of the circuit and the architecture of the sensor, the precision, sensitivity and easy operation have been shown in this design.In order to test the performance of the liquid level sensing system, diesel oil and liquid nitrogen which have similar dielectric constant with LNG have been employed to do great amount of simulation experiments at room temperature and ultra-low temperature(-169°C). By comparing the results and analyzing errors, the performance of the capacitive liquid level sensing system designed in this work has been proved. Based on the results from diesel oil experiment and liquid nitrogen experiment, standard curve of LNG measurement has been fitted.Basic functions of capacitive liquid level sensing system, such as to acquire data in ultra-cold environment, to upload and download data, and external control of buttons, have been realized by the design in this thesis work and user experience has been improved to some extent.