A PROBE TYPE INDUCTION CONDUCTIVITY CELL 



E. E. AAGAARD and R. H„ vanHAAGEN 

 Oceanic Instruments, Inc. 

 Houghton, Washington 



ABSTRACT 



OPERATING PRINCIPLE 



The salinity of sea water may be calculated 

 from measurements of water electrical conductivity 

 and temperature in situ . A compact pressure- 

 protected induction conductivity cell and a ther- 

 mistor temperature sensor were mounted in a probe 

 to form a salinity transducer. Constructed for 

 installation in the nose of a deep-sea remote 

 controlled underwater vehicle, the probe is read- 

 ily adapted to other applications such as mounting 

 on or towing behind a research ship. The con- 

 ductivity cell is of the induction type, using 

 two toroidal transformer cores to avoid the polar- 

 ization and fouling problems of exposed electrodes . 

 The toroidal cores are connected in a transformer 

 bridge circuit which is adaptable either to direct 

 or to servo actuated null balance readout. The 

 probe has been proof -tested at pressures up to 

 3,000 psi. 



INTRODUCTION 



One measure of the salinity of sea water is 

 its electrical conductivity. A conductivity cell 

 for measurements of salinity may be calibrated 

 in a sample of sea water of known salinity. The 

 temperature of the sample must be carefully con- 

 trolled since conductivity varies considerably 

 with temperature. For in situ measurements of 

 salinity the temperature of the water as well as 

 the electrical conductivity must be accurately 

 measured. The salinity transducer probe, as 

 shown in Fig. 1, uses an induction conductivity 

 cell and a thermistor temperature sensor. Con- 

 structed for use in the nose of a deep-sea remote 

 controlled vehicle, the transducer posed a major 

 design problem because of the crushing pressure 

 at great depths. 



In the past, accurate determination of con- 

 ductivity has been a serious problem because 

 fouling and polarization of exposed electrodes 

 in a sea water environment can introduce serious 

 errors in the results. To eliminate this 

 inherent problem with exposed electrodes, the 

 conductivity probe design was based on the induc- 

 tion technique described by Gupta and Hills . 

 The measure of conductivity is the coupling pro- 

 vided by a sea water path between two toroidal 

 transformers . 



The transformer cores are encapsulated on a 

 common axis in a toroidal housing which allows 

 the free flow of sea water through the common 

 hole in the two cores. The arrangement is shown 

 in Fig. 2. A primary winding on the exciting 

 transformer core is connected to an alternating 

 current power supply. A single turn secondary 

 winding is fbrmedby the conducting sea water threading 

 through the hole in the toroidal housing. The 

 return current path is the infinite liquid medium 

 in which the unit is immersed. The current 

 through the liquid path will be proportional to 

 the electrical conductivity of the medium. The 

 sea water current path is also the single turn 

 primary winding for the pickup transformer core. 

 The output signal is taken from a secondary 

 winding on this core. 



By placing on each core an additional winding 

 connected in a series circuit with a variable 

 resistance, the induction conductivity cell 

 becomes a transformer bridge. The setting of the 

 variable resistance required to give a null signal 

 in the secondary winding of the pickup core is a 

 measure of the electrical conductivity of the 

 liquid path. An outstanding advantage of the 

 transformer bridge circuit is that the null adjust- 

 ment is essentially independent of variations in 

 the transformer core losses due to temperature 

 or pressure. 



TRANSFORMER BRIDGE 



o 

 Calvert et al. have described the operation 



of the transformer bridge in some detail. Only 

 a brief discussion of its application in the 

 induction conductivity probe will be given here. 

 Fig. 3 is a schematic diagram of the circuit. 

 R u represents the unknown conductivity of the sea 

 water path. The circuit is adjusted to null by 

 the variable resistance, R . The capacitor tunes 

 the null signal to the power supply frequency. 

 Tuning is desirable since it filters out noise 

 above and below the power supply frequency and 

 eliminates null signal phase shift with respect 

 to the exciting voltage. Phase shift is undesir- 

 able where the null signal is compared with the 

 exciting voltage as in applications using a phase 



sensitive detector or servo actuated null bal- 

 ancing system. 



Superior numbers refer to similarly numbered references at the end of this paper. 



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