A Capacitive Wave Profile Recorder

Research studies at the St. Anthony Falls Hydraulic Laboratory necessitated the development of a device for measuring and recording the profile of surface waves. The initial phase of these studies involved waves with periods ranging from 1/3 to 1 sec and with heights ranging from 1/2 to 3 inches. Existing methods of measuring and recording the profiles of these waves were considered to be not entirely satisfactory. A method has been developed whereby wave heights are measured electrically with a recording oscillograph; the deflections correspond to the depths of submergence of an insulated wire into the water. This insulated wire acts as a small capacitor whose capacity varies directly with the wetted area of the wire. The system has a linear calibration. Thus, accurate and continuous wave profiles can be recorded. Hysteresis effects due to surface tension are about 0.003 ft. A sensitivity up to 1/2 cm pen deflection per O.OO1-ft variation in water level is possible. The method may be used for greater wave heights, however some adjustment in oscillator may be required in extreme instances.