AN ELECTRONIC GAGE FOR MEASUREMENT OF SMALL WAVES AND RIPPLES 



by 

 Francis W. Kellum 

 Research Division - Beach Erosion Board 



In laboratory wave action studies there occasionally are times 

 when the wave heights become so low that with commonly used types 

 of wave measuring equipment such as the step-resistance gage, the 

 parallel-wire gage and the capacity gage, substantial errors in the 

 recorded data are caused by the meniscus resulting from surface 

 tension. The only way that the meniscus effect can be eliminated 

 completely is to have some type of measuring device that does not 

 touch the water surface. Such a gage has been developed, and is 

 discussed below. The action of the gage depends on variation of 

 the dielectrical capacity in the air space between the water surface 

 and a probe, placed a short distance above, with changes in water 

 level. This capacity is part of the tuning circuit of an oscillator. 

 A variation of the water level changes the frequency of the 

 oscillator. The output of this oscillator is mixed (heterodyned) 

 with a constant frequency oscillator. The resulting beat frequency 

 is fed into a tuned amplifier which is tuned so that any change of 

 the beat frequency causes a rapid change in amplification. 



A variation of the water-probe capacity caused by a change in 

 the water level then results in a change in the beat frequency and 

 a change in output of the amplifier. 



The output of the amplifier is rectified and used to drive a 

 Brush amplifier and recorder. The output can be made practically 

 linear over a 0.2-foot water level variation, if the components of 

 the circuits are properly adjusted. 



Referring to the block diagram (Figure 1), the sections designat- 

 ed fixed oscillator and variable oscillator are two radio frequency 

 generators. The fixed oscillator generates an alternating current 

 of approximately 8.50 megacycles per second, and the variable os- 

 cillator approximately 8.93 megacycles per second, the difference 

 being about 430 kilocycles per second, or the beat frequency. 



The variable oscillator frequency can be made to vary by changing 

 its capacity to ground. Referring to the variable oscillator (Figure 2) 

 the dashed portion of the drawing, marked A in that section, re- 

 presents the changing capacity when the gage is in operation. This 

 changing capacity is formed by the probe acting as one plate and the 

 water surface as the other plate of a condenser. When the water rises 

 and falls the capacity changes due to the changes in spacing between 

 the acting plates of the condenser. 



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