Section III. PARALLEL-TYPE, STEP -RES I STANCE GAGE FOR SALT WATER 



1. Theory of Operation of Parallel-type, Step-Resistance Gage 



Due to the low resistance path created by a film of salt water 

 on the epoxy wave gage sections, the gage design for fresh water cannot be 

 used in the ocean. To compensate for the low-resistance, salt-water film 

 and the increased electrolytic action in salt water, it is necessary to 

 provide low resistance values in the wave staff and to use alternating 

 current and low voltage in the sensing circuit. The circuit in Figure 16 

 was evolved to permit the use of low-voltage, low-value resistors, and 

 alternating current in the wave-sensing circuit. Analysis of this circuit 

 (Figures 16 and 17) shows that a standard 115-volt, 60-cycle line is con- 

 nected to a constant-voltage transformer. The output of the transformer 

 is a constant 115 volts ± 1 percent for powerline variations between 95 and 

 130 volts. This removes variations in the record that might be caused by 

 a change in line voltage. Output of the constant-voltage transformer is 

 applied to an autotrans former which provides a means for varying the volt- 

 age applied to the wave-gage circuity. This feature of voltage adjustment 

 permits calibrating the wave gage for full-scale indication on the strip- 

 chart recorder. A voltmeter is used to monitor the voltage out of the 

 autotrans former. 



The selected voltage from the autotrans former is connected to a 

 stepdown transformer which further reduces the line voltage to a value 

 suitable for wave-sensing resistors. The stepdown transformer also 

 isolates the powerline from the wave staff. The secondary winding of the 

 stepdown transformer is connected in series with the variable-resistance 

 circuit provided by the parallel-resistor, water-conducting path, and the 

 primary winding of a step-up transformer. The step-up transformer is 

 identical to the stepdown transformer except that its windings are used 

 in a reverse manner. 



Output voltage from the step-up transformer is applied to a bridge 

 rectifier and low-pass filter to convert the varying amplitude (caused 

 by changes in water level) of the 60-cycle a.c. source to a d.c. signal 

 suitable for driving the recorder penmotor. The stepdown, step-up trans- 

 former units and the rectifier- filter unit have fixed resistors incorpor- 

 ated in them to aid in getting a linear signal from these units. 



This circuit utilizing the parallel resistance circuit of the wave 

 staff and the low- voltage windings of the stepdown, step-up transformers 

 is a low-voltage, current-sensitive circuit. Such a circuit must have 

 low resistance electrical connections. It is mandatory that the step-up, 

 stepdown transformers be placed physically close to each other and as 

 near as possible to the wave gage staff sections. 



Design experience has evolved the resistor values for gages of 20 and 

 25 feet as listed in Table V. These values operate with the other elec- 

 trical component's listed in Table VI, and should provide a gage with good 

 operational features and good linearity. Changes in components or re- 

 sistor values may cause nonlinear gage response, and require circuit 

 modifications. 



28 



