APPENDIX C 



DESIGN AND DEVELOPMENT OF THE RECORDING DEPTH GAGE 



INTRODUCTION 



It was proposed that an accurate instrument for measuring the depth 

 at various points along a beach profile be designed and developed as part 

 of contract number DACW72-72-C-0020. The instrument would be designed 

 to satisfy the need for accurate measurements just seaward of the break 

 point of waves where fathometers and lead line sounding are inaccurate. 



This instrument was conceived as an application of the very accurate 

 absolute pressure sensors presently available to detect small changes in 

 water level. The concept of design involved placing such a pressure 

 sensor on the bottom and sensing the change in water level over a pre- 

 determined period of time. The high frequency oscillations of the water 

 surface due to waves would be electronically filtered out of the pressure 

 sensor output to produce a single average reading for the depth. 



PRELIMINARY DESIGN MAY-NOVEMBER 1972 



The instrument is designed around a Statham Model PA506-53, 13 to 

 53 psia (absolute) pressure sensor which can accurately detect changes 

 in water level of ±0.08 feet (2.4 cm) in water depths as great as -58 feet 

 (-17.7 m) . Preliminary design of the instrument involved putting the out- 

 put of this sensor through an amplifier low-pass filter and a variable 

 gain amplifier onto a digital display. This electronic circuitry was 

 intended to make an accurate measurement of the depth by filtering out 

 the high frequency oscillations caused by waves. Filtering was achieved 

 by using a capacitator which had a time constant of 3 minutes. The 

 output from the filter was then amplified and read on the digital panel 

 meter. 



A prototype of this instrument was constructed in September 1972, 

 and tested by progressively lowering the pressure sensor into a labora- 

 tory deep tank. This test indicated that the instrument gave unstable 

 readings and tended to drift from zero reset. The initial change in 

 design was to replace the intergrated circuit used for amplification 

 with a more stable unit. This resulted in the circuit design shown in 

 Figure B-l. Also, the prototype instrument had a two and one-half 

 digital panel meter for recording the data which was found to be of in- 

 sufficient accuracy. 



CONSTRUCTION, DECEMBER 197 2 - MAY 1973 



The completion of the electronic circuit design and initial testing 

 resulted in an instrument that was believed to be ready for field use. 



97 



