I EXCITING N ' " R " PICKUP | 



iANSFORMER f *Ljr TRANSFORMER 



o[e7 



"u] ° 



e s 



-AAA — AA/Vhi 



Rs f 



INDUCTION 

 CONDUCTIVITY 

 CELL 



Fig. 3- Schematic diagram of transformer bridge. 



Since the voltage ratio is equal to the turns 

 ratio: 



^ N s n s s 



Because the water path forms a single turn 

 around each transformer this becomes : 



w 



Ru- 



R s 



(5) 



It is seen that the sea water resistance is 

 directly proportional to the known resistance. 



For visualizing circuit parameters and for 

 calculating bridge sensitivity the equivalent 

 circuit shown in Fig. h is convenient. The fact 

 that this is a bridge circuit is more readily 

 apparent from the equivalent circuit than from 

 the original circuit. All circuit values are 

 reflected to the output or null circuit; that is, 

 the values are the apparent ones seen from the 

 output terminals of the pickup transformer. 

 X-fj, and Rjjj represent the magnetizing reactance 

 and the equivalent core loss resistance of the 

 pickup transformer respectively. As explained 

 previously the magnetizing reactance and core 

 losses do not affect the null balance but do 

 reduce the gain or sensitivity and therefore must 

 be considered in the circuit analysis. The mag- 

 netizing reactance and equivalent resistance of 

 the exciting transformer are not shown in the 

 equivalent circuit as it is assumed that the 

 power supply source impedance is sufficiently low 

 that these parameters do not affect the null 

 signal. 



n 2 Ru 



AAA^A^A- 



rw © 



Nj 



N s "o 



N-, n 8 e « (rXj 



Fig. k. Equivalent circuit for transformer bridge. 



In the transformer bridge circuit discussed, 

 the variable resistance, 1 R g , is proportional to 

 the water resistance, R u . In many applications 

 it is more desirable to have the variable resis- 

 tance proportional to water conductivity rather 

 than resistance. In low salinity low conductivity 

 water, the resistance, R s , would be extremely 

 large. To obtain a variable null setting 



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