Measurement of ocean wave height using a capacitance-based liquid level sensor

Abstract

A digital liquid level measuring gauge has been designed, constructed and laboratory tested. The prototype gauge is specially made for ocean wave height measurement, but the underlined principle can be extended to level measurements in other situations. The gauge consists of a field unit anchored at the location where the ocean wave height is to be measured, and a base unit stationed in land.. The base unit can be connected to a computer to download wave height data for analysis. The field unit (see Fig.) consists of (1) an air-filled torus-shaped fully submerged platform anchored to sea bed, (2) a level sensing column hinged to the platform, and (3) a control box with electronic circuits, a radio transmitter, and rechargeable batteries. The anchoring is done in such a way that the mean sea level is roughly at the centre of the level sensing column. The sensing columns should be longer than twice the amplitude of sea waves. The control circuits perform many tasks: it converts the variation of capacitance between the two vertical tubes in the sensing column to variable frequency radio signal; it determines the sampling periods and sampling rates; it also communicates with the base unit by radio signals. The field unit is designed to consume very little power and the rechargeable batteries alone can power the unit for 10 days. The base unit consists of a radio receiver, control circuits, and an interface. This unit can receive radio signals from the field unit which must be within 2 km. It decodes radio signals and obtains digital wave height data for transmission to a computer using the parallel port.