but the basic principles of many of them (in each category) are simi- 

 lar. For example, the principles of the step-resistance gauge developed 

 by the Beach Erosion Board have been used in the construction of 

 fixed wave gauges of all sizes. The transducers of bottom pressure 

 gauges are usually strain gauges or Bourdon tubes. Most of the 

 instruments described below are used to obtain data on a routine 

 basis. No laboratory wave gauges are discussed, nor any stereo-photo- 

 graphic techniques; however, references on both of these instrumenta- 

 tion techniques are given at the end of this Section. 



1. Bottom pressure instruments 



a. Wiancko pressure measuring system 



This pressure measuring system was designed by the U. S. 

 Navy Mine Defense Laboratory to measure and record water pressure 

 changes of 0.1 inch to 80.0 inches to a depth of 200 feet. As much as 

 12,500 feet of type WF8/G cable may be used if certain precautions 

 are observed. The pressure measuring system includes three basic 

 units which are shown in Figure VII- 1. The underwater unit consists 

 of a differential pressure gauge in a housing which also contains a 

 hydraulic filter to compensate for static pressures and a calibration 

 relay circuit. The differential pressure gauge produces an electrical 

 signal which is proportional to the pressure variation by changing the 

 ratio of two inductances. The electronic unit contains two resistances 

 which complete a bridge circuit with the two inductances and produce 

 a d.c. voltage proportional to the pressure variations. The recording 

 unit is a recording milliammeter which serves as the indicating device 

 for the pressure variation. 



The calibration is accomplished by paralleling one leg of the gauge 

 coil •with a capacitor which will unbalance the bridge the same amount 

 as would 32 inches of water pressure change. The a.c. bridge current 

 is provided by a 3,000 c.p.s. oscillator in the electronic unit. The 

 bridge is operated at a point of deliberate unbalance, but the electronic 

 unit is arranged to subtract off a signal voltage equal to that caused 

 by the normal amount of deliberate unbalance; therefore, meter 

 deflections are proportional to changes from this normal degree of 

 unbalance. (See Reference VII- 30.) 



b. Acoustic system Mark I, Mod 4 



The acoustic system Mark I, Mod 4, designed by the Naval 

 Ordnance Laboratory, measures and records very low-frequency sounds 

 from the low audio ranges down to static pressures. The pressure- 



VII- 4 



