ABSTRACT 



This report describes briefly the salient features of new instrumentation 

 for measuring and recording the wave amplitudes and wave forms of water waves. 

 This instrumentation was developed by the Electronic Circuit Development Branch* 

 of the Electronic Engineering Division of the David Taylor Model Basin, at the re- 

 quest of the Hydromechanics Laboratory. Because of the increased accuracy and 

 flexibility afforded by the instrumentation described, it has replaced other tech- 

 niques which were formerly used. 



The gaging element of this system consists of extremely small insulated 

 wire, suspended rigidly upright in the water. The conductor acts as one plate of 

 a capacitor and the water in which the gaging element is partially submerged acts 

 as the other; the insulation material on the wire forms the dielectric of the capac- 

 itor. The capacity of this "condenser" (measured between the conductor and 

 water) is directly proportional to the linear length of the submerged portion of 

 the wire. This capacitor is connected in one arm of a resonant-bridge circuit, 

 which may be balanced for the quiescent level of the water. Variations in water 

 height which occur as waves pass the gaging element produce capacitive unbal- 

 ance of the a-c bridge in each sense, and the direction of unbalance is recovered 

 by a more or less conventional phase sensitive demodulator circuit incorporated 

 in the circuit of the wave-height recorder. 



This report includes schematic wiring diagrams of the instrument, its 

 pov/er supply, and a direct-coupled amplifier suitable for driving the recording 

 galvanometer in a Sanborn direct-writing recorder. 



INTRODUCTION 



For the accurate measurement of small amplitude water waves, it is, of course, desir- 

 able to use a gaging element whose presence will least affect the height or form of the wave; 

 ideally, measurement of amplitude should be made at one very small point or along an imagi- 

 nary vertical line, normal to the still surface of the water. One previously used technique 

 for measuring wave height and profile at the David Taylor Model Basin was to photograph a 

 grid alongside the basin wall. It was found that small imperfections along the wall or even the 

 engraved grid lines created a considerable adverse effect on the shape of the wave. Moreover 

 because of viscosity and surface tension effects in the proximity of the wall, it was found 

 highly desirable to measure wave profile at some distance from the wall. A further disadvan- 

 tage of the photographic method is the necessity for development of the film, which requires 

 considerable time, and, at best, one obtains only the amplitude and profile of one particular 



*Now the Metric Systems Branch of the Instrumentation Division. 



