Optical Sensing Techniques for Liquid Level Gauging.

Available from UMI in association with The British Library. The level of a liquid may be measured in a number of ways. Traditionally, electronically based sensors are used, however in many applications, such as inflammable liquids, optically based sensors are not only safer but may also be operated remotely. In recent years many optical sensors have been developed for a number of measurands, many using optical fibres not only as transmission link but also as the sensing medium. In the work presented for this thesis several optically based sensors have been developed for use in liquid level monitoring systems. The three main objectives have been to develop a sensor which has an accuracy comparable with more traditional methods, has an extended unambiguous range, and a reduced temperature cross-sensitivity. The most sensitive optical sensors developed are those employing interferometric techniques. Three interferometric strain gauges have been investigated, two using fibre sensing elements and the third using bulk samples of fused quartz. These sensors may be used in conjunction with a restrained float to monitor liquid levels indirectly. A liquid level may also be monitored directly using optical AM radar. Although this technique is not optically coherent it is electronically coherent and may be seen to be analogous to the interferometric techniques. Each of the sensors employs a different signal processing to extend its unambiguous range: combined interferometric and polarimetric, dual interferometric, frequency modulation, and dual wavelength.