with a negative applied voltage. The matrix logic (.Figure 9) is such that 

 only once during a complete scanning sequence is any staff contact selected 

 by the gating circuit while being subjected to a negative voltage from the 

 interrogator. If the contact is above the water level, the negative vol- 

 tage is sufficient to cause the solid state comparator to be held in a 

 non-conducting condition and a binary zero (-0.4 volt) is indicated at its 

 output terminal. If the negative voltage at the staff contact is reduced 

 below a certain level due to the shorting effect of being immersed, the 

 comparator output changes to a binary one and the terminal voltage changes 

 to -5.8 volts. The point at which the change takes place is controlled 

 manually by adjusting a threshold reference potential associated with the 

 comparator. This manual control permits adjustment of the voltage response 

 so that the staff may be made insensitive to salt spray and to water ad- 

 hering to the staff above the measured level. The sensitivity may be 

 increased when tap water is usee in laboratory wave systems. 



A pulse is generated in the output of the NAND gate only if a binary 

 one is present at the output of the comparator during interrogation and 

 selection of the staff contact associated with the particular pulse posi- 

 tion in time. Thus, a train of pulses results when the contacts are 

 scanned in sequence. 



In actual use the wave staff is scanned at a rather high rate - usually 

 sixty-four complete scans of the entire staff each second. The output pulses 

 are maintained at constant width and amplitude, permitting a simple elec- 

 tronic integrating circuit to generate a continuous signal in analog form. 

 This analog signal may be connected to a chart recorder for a visual field 

 record of the wave height. The digital record is formed by selecting a 

 sampling interval, connecting the pulse train to a binary counter for the 

 duration of this interval, and recording the resulting count in binary code 

 on magnetic tape. For digital recording, a timing oscillator within the 

 electronics package is synchronized with the data acquisition system. When 

 only analog recording is desired, the digital data acquisition system is not 

 required. The timing oscillator is self-sustaining, permitting the staff 

 output to be read directly on a standard, damped voltmeter or connected 

 through an integrator to a chart recorder. 



Some of the more important characteristics of the digital wave staff 

 are: 



(1) No calibration is required. The digital read-out is self- 

 def i ni ng. 



(2) The output signal may be transmitted over long distances in 

 a simple three-conductor cable without loss of calibration. 



(3) Calibration does not change with salinity of the water or 

 adherence of saline water to the staff itself. 



(4) Read-out, almost instantaneous, is limited only by the time 

 required for electronic interrogation - not by mechanical 



i nertia. 



Text resumes on page 15. 



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