wave. In the adjustment of water-wave generators it is advantageous to be able to observe 

 the effect of various adjustments on a direct-writing recorder which is producing a continuing 

 record of each wave as it passes the measurement point. A still further disadvantage of the 

 photographic method lies in the relative difficulty of moving the measurement point out near 

 the center of the basin or to other points along the length of the basin at which m.easurements 

 are required. 



On 18 January 1952, the Electronics Engineering Division received a request from the 

 Hydromechanics Laboratory to develop reliable instrumentation of this type which preferably 

 should possess none of the disadvantages met in the application of photographic methods then 

 in use. The following performance requirements were stated in lieu of specifications: 



The instrumentation should be capable of measuring and recording amplitudes of water 

 waves in the range of to a maximum of 2 ft; the lengths of the waves to be recorded lie with- 

 in the range of 2 ft to 30 ft, with corresponding periods of 0.564 to 2.43 sec; frequency re- 

 sponse should be such as to show no more than il percent variation in amplitude from to 

 at least 2 cps. A maximum error of ±1/4 in. in 2 ft of wave height was to be allowable. 



A study of various techniques employed at other laboratories engaged in work of the 

 same general nature was made. ^'^ All systems investigated by the author were found unsatis- 

 factory for this particular purpose because of their inability to satisfy the accuracy and fre- 

 quency-response requirements or their dependence on the chemical composition of the water. 

 Float systems usually fail to respond rapidly enough to "follow" rise and fall of extremely 

 short waves due to the inertia of the float or the friction or mass of the pickup unit attached; 

 moreover it is difficult to design a float which will approach, dimensionally, the requirement 

 for measuring the wave height at a single point. Systems which depend on the variation of 

 electrical resistance between two probes immersed in the water approach the dimensional re- 

 quirement but suffer from their dependence on the mineral content of the water, corrosion, or 

 oxidation of the probes and cleanliness of the water surface. 



The capacitive-wire gage system described herein meets or exceeds all the require- 

 ments and avoids the difficulties stated in the preceding paragraph; further advantages which 

 influenced the choice of this system over the many other types of systems in current use 

 included the fact that there are no critical tolerances or spacing adjustments, it has excellent 

 linearity characteristics, is easily cleaned or replaced, and is inexpensive. 



MEASUREMENT SYSTEM 



The measurement system described in this report is known at the Taylor Model Basin 

 as the type 145-A Dynamic Wave-Height Recorder. For discussion, it may be divided into 

 three major functional parts-the gaging element, the electronic circuitry, and the direct- 

 writing recorder. (See the semi-pictorial sketch. Figure 1). The principle of operation may 



References are listed on page 13. 



