Liquid level measurement using a coplanar transmission line

Bibliography: leaves 93-94.The coplanar line has been used very successfully as an element in microwave circuits. Small size, high Q-factor, and accurate reproduction, are some of its many advantages. The coplanar transmission lines discussed in this report, are targeted at liquid level measurement, and are typically 30 cm. long. Their operating frequencies are consequently much lower than those of microwave coplanar waveguides, but they have common advantages. The factor which separates the coplanar line from similar liquid level sensors, is that it makes use of the electrical component of the electromagnetic fringe field, setup between its inner conductor, and the surrounding ground plane. The line is effectively a sharply tuned resonator, incorporated as the frequency controlling element of an electronic oscillator. The output frequency falls as a dielectric material penetrates the fringe field. An impressive sensitivity is accomplished by using very thin conductors, thereby ensuring that the fringe field energy is maximised. The most important feature of this sensor is its ability to operate non-intrusively when used with non-conducting vessels, or if employed in a metal tank, the unit can be encased in a dielectric material where the line is non-contacting (the liquid does not penetrate the unit). This combined with its excellent mechanical and electrical stability, and an accuracy better than 1 percent, makes the coplanar line a strong competitor in the field of liquid level measurement. The research began with a theoretical approach, and used lines machined from an Aluminium plate for characteristic impedance measurement. An empirical relation between the gap width, the line thickness, and the characteristic impedance of the line is presented. To assist with the design of the sensor, a lumped capacitance model of the line was developed. Various geometries were tested, and modified until a near linear response to water level was achieved. An advanced engineering model of the level sensor has been developed, which incorporates a stable digital output display, user calibration from the line's end points, and temperature compensation. A T-shaped line, which concentrates the field around its open end, was used for other applications such as, evaporation monitoring, measurement of slurry settlement, and to observe the effect of acids, bases, and salts in water. Various applications of the different coplanar line designs are proposed