Optically Powered Oil Tank Level Measurement System Based On Differential Capacitance Sensing

Crude oil tank is important equipment for storing oil in oil field. Oil tank liquid level is one of the important parameters of storing and transmitting crude oil technology. Precise liquid level measurement is the dependence of managing. But the measurement site is inflammable and explosive, it is difficult to detect in such severe site. Therefore high precision and automatic measurement of the oil liquid level play a great role in oil industry.Considering the requirements of the measurement site under specific working condition, this paper presents a scheme of oil tank liquid level measurement system based on differential capacitance sensing. In view of hydrostatic tank gauging method, the probe is optically powered, and then the information is transmitted through optical fiber to Micro-control unit far from the site. So the inflammable and explosive problem can be solved, the system is safe in nature.Three differential capacitance sensors are used to measure the applied pressure, and one thermistor is used to compensate the effect that the temperature changes have on measuring precision of the capacitance sensors. The mathematical model of measuring liquid level is established, the characteristics of differential capacitance and thermistor are conveyed, and the relationship between pressure and capacitance is deduced in the paper. The theory above is the basis of the system measurement and calibration.By analyzing the characteristics of basic photo-electric elements, optical fiber, light source and photo detectors can be proper chosen which construct the optically powered channel and fiber-optic transmission channel. The design of driving circuit of LD and photo detector ensures that stable and reliable operation voltage of the probe can be given. The design of micro power consumption probe is the key technique in the optically powered system. The probe uses complete CMOS circuit to realize pulse-width modulation to pulse-position modulation of differential capacitance and thermistor. The multiplex electric signal can be converted to light pulse and then betransmitted to micro control unit (MCU) through a single fiber.The pulse signal can be demodulated directly by high performance MCU. The capture and measurement of pulse-width and ratio processing of measured signal, parameters calculation and display, warning are accomplished by hardware and software.The experiments of oil tank liquid level measurement system based on differential capacitance sensing are carried out, such as experiments of pressure-capacitance relationship, calibration of temperature and liquid level, etc. The results prove that the system has high accuracy and stability, and is immune to electro-magnetic interference.