To calculate the second resistor value: 



30 volts ^ 1,667 ohms - 1,579 = 88 ohms. 



18 milliamperes 



To calculate the third resistor: 



30 volts 



17 milliamperes 



1,765 ohms - 1,667 ohms = 98 ohms, 



Such calculation shows that the resistors will not be of equal value 

 to obtain equal increments of pen movement. The individual resistor values 

 will be higher at the left and lower on the right side of the resistor 

 string in Figure 6. 



The circuit in Figure 6 applied to a practical circuit for measuring 

 wave heights in fresh water is shown in Figure 7. 



The major difference between the two circuits (Figures 6 and 7) is 

 that the water path is now used to activate the changes in the variable 

 resistor. Another change is the addition of a variable calibration re- 

 sistor to adjust for differences in the conductivity of fresh water and 

 differences in penmotors. Analysis of this circuit shows that the bottom 

 resistor in the gage circuit is connected to the ground rod. This con- 

 nection is necessary due to the electrical resistance of the residual water 

 path on the epoxy resin when the gage submergence is small. Further analy- 

 sis of the circuit shows that the water path has a resistance of its own, 

 therefore, some current will flow from each submerged metal sensing tip to 

 the ground rod; such flow will aid in reducing the effective resistance 

 of the water path. 



Using the 30-volt source previously calculated as required to provide 

 full-scale penmotor operation in the practical gage circuit in Figure 6, 

 it is found that the recorder penmotor will not rise to full scale. This 

 is due to the added resistance of the water path. To compensate for the 

 increased resistance, it is necessary to increase the voltage from the 

 constant d.c. voltage source. The increased resistance path will also 

 change the direct logic used for Figure 6 in calculating the resistors for 

 the gage. 



Design experience has resulted in the selection of a d.c. voltage 

 source of 46 and 54 volts as the best value for most fresh-water applica- 

 tions. This same experience has resulted in the resistor values shown in 

 Tables I and II for 20- and 25-foot series-type gages. If a 10-foot gage 

 is desired, it is recommended that the spacing of the sensing tips be used 

 to 0.1 foot and the resistor values for the 20-foot gage be used. 



There will be some electrolytic action in the water path due to the 

 use of direct current. This action usually causes a hard powder to form 

 on the lead sensing tips of the gage. Rate of coating formation depends 

 on the mineral content of the local water. The gage must be cleaned of 

 the deposit to obtain the most accurate operation. Frequency of cleaning 



Text resumes on page 16 



