Design & Implementation of Optical Fiber Liquid Level Sensor Using He-Ne Laser

Abstract

One of the difficult duties in chemical industrial units is the determination of the level of liquid for real – time monitoring. Determination of this parameter is useful in process control loop. Hence present study is devoted for this purpose by employing microbend based optical fiber sensor. In optical fibers the microbending is considered as a one of the unwanted properties in optical communications due to its effect on attenuating and distorting the transmitted signals. Nevertheless, it's possible to exploit this property in modulating the light intensity and hence using the fiber as liquid level sensor. In this work, in order to continuously monitor liquid level in petroleum and chemical industries, an optical fiber sensor based on microbend effect was designed and manufactured. The system is consist of a sensor that is composed of a microbend modulator, sensing fiber, emitting / detecting devices, in addition to liquid container unit, and an electronic circuit was used to control the liquid level. Four threaded aluminum blocks with different spatial period, and one flat, are used to generate microbending in a multimode step index fiber. These blocks (one flat and another threaded for each case) cross the fiber and pressed by the liquid container unit, liquid pressure is converted into small displacement of portable block, and then the sensing fiber is bent and the through light intensity is reduced. By detecting the output light power, liquid level can be obtained according to the exerted pressure. A relation between the resulting laser output in term of voltage and load in term of liquid height, and spatial period (number of teeth) are obtained in each case. So the conclusion is that the mechanism of microbending can be used in optical fiber to produce a liquid level sensor, and then control of that level. Using pressure measurement to determine liquid level is particularly useful for applications with foaming or bubbling liquid, where other level measurement technologies have difficulties. The results show that the laser technique is both accurate and immediate. Test results show that this sensor is suited for dangerous applications of liquid level measurement especially in fields where electrical isolation and / or electromagnetic interference (EMI) resistance are strictly required.