R2 >> Rp 

 R2>> RTH 



'THERMISTOR 1-PRESSURE POT 



Fig. 7- Pressure compensating circuit. 



BRIDGE INPUT TOROIO 



BRIDGE OUTPUT TOROID 



TEMPERATURE COMPENSATION 

 CIRCUIT 



E, = Ej R F] = R F2 

 Nl = N 2 RT1 = Ru 



!4_J 



PRESSURE COMPENSATION CIRCUIT 



Fig. 8. Complete salinity bridge. 



20,000-foot depth, i.e., 8,800 psi and 0°C, the 

 increase in conductivity due to pressure is 

 approximately 5-5$. Therefore, at this depth 

 and pressure, the uncertainty due to pressure 

 compensation errors is ±0.02 parts per thousand. 

 At shallower depths the uncertainty is reduced 

 in proportion. 



COMPLETE SALINITY SENSING BRIDGE 



The salinity bridge circuit complete with tem- 

 perature and pressure compensation circuits is 

 shown in Fig. 8. A current, I w , is induced to 

 flow in the sea water loop by the application of 

 Ej_ to the toroidal transformer, T]_. 1^ which 

 is proportional to the conductivity of the sea 

 water loop, sets up a magneto-motive force on 

 the magnetic circuit of T 2 . A counter mmf pro- 

 portional to the difference between I}Nt_ and 

 I2N2 is set up by the combined outputs of the 

 pressure and temperature compensating circuits . 



At one particular value of salinity (depending 

 on R s ) these mmf 's will be in balance and E will 

 be zero. Any change in I w at constant salinity 

 due to temperature or pressure changes is bal- 

 anced by a similar change in the output of the 

 temperature or pressure compensating circuits, 

 thus maintaining a balance dependent only on 

 salinity. 



Fig. 9. Block diagram of salinity oscillator. 



SALINITY OSCILLATOR 



In a simple system the salinity bridge shown 

 in Fig. 8 could be balanced by a servo system 

 acting on R s . R s could be a precision potenti- 

 ometer coupled to a shaft encoder for digital 

 readout. However, this system would require con- 

 siderable amounts of power in an underwater 

 package and would have the disadvantage of larger 

 size and lower reliability due to the number of 

 moving parts . An improved system might utilize 

 the output voltage to input voltage relationship 

 of the salinity bridge to control the frequency 

 of a special phase shift oscillator. A block 

 diagram is shown in Fig. 9- 



It can be shown that with suitable design the 

 error voltage, E , from the salinity bridge is 

 either in phase or 180° out of phase with the 

 bridge input voltage, E^. If the error voltage, 

 E , is added to a voltage, E , which is 90° 

 out of phase with Ei, then the phase of the 

 resultant E + Eq = E r , will shift as the bridge 

 balance changes with changes in salinity. The 

 resultant, E r , is amplified and then applied to 

 a phase shifting network consisting of R^, Rg, 

 Ca and Cjj. The output of the phase shifting net- 

 work is amplified in A 2 and applied to the input 

 of the bridge, thus closing a complete loop. The 

 loop will oscillate at a frequency at which the 

 sum of the phase shift between E r and E^ plus the 

 phase shift in the phase shifting network amounts 

 to 180°. Experimental oscillators of this type 

 have shown that the salinity uncertainty due to 

 supply voltage and temperature variations on the 

 electronics is not worse than ±0.003 parts per 

 thousand in salinity for an oscillator covering 

 a range of 5 parts per thousand. 



The quadrature voltage circuit shown in Fig. 9 

 has to be temperature compensated because when 

 the bridge is off balance, E Q will not be zero 

 and will vary with temperature at a given salinity. 

 This means that E should vary with temperature 

 by the same proportional amount as E . However, 

 the accuracy with which E_ is compensated becomes 

 less critical as the bridge approaches balance 

 and is completely unimportant at the balance point. 

 Consequently, the accuracy with which E is 



23 



