A PROPOSED IN SITU SALINITY SENSING SYSTEM 



N. L. BROWN 

 Hytech Division of Bissett-Berman Corporation 

 San Diego, California 



ABSTRACT 



This paper describes a system for the in situ 

 measurement of salinity. The basic components 

 are (l) a salinity bridge composed of an induc- 

 tively-coupled conductivity sensor and a network 

 of temperature and pressure sensors for accurately 

 compensating for the effects of temperature and 

 pressure on the conductivity of sea water and 

 (2) an oscillator whose frequency is accurately 

 and directly controlled by the salinity bridge 

 output voltage to input voltage relationship. 



INTRODUCTION 



Before the design of a salinity system can be 

 considered, it is mandatory to study such par- 

 ameters as the temperature and pressure effects 

 upon the conductivity of sea water and upon the 

 conductivity sensor, the optimum method of sensing 

 the conductivity, methods of compensating for 

 the effect of temperature and pressure on the 

 conductivity of sea water, the effects of marine 

 fouling organisms on the sensors, telemetry 

 methods and, of course, the desired performance 

 with respect to accuracy and other factors . 

 Before selecting the method of sensing the con- 

 ductivity of sea water from an j_n situ instru- 

 ment, the immediate factors to be considered are 

 the electrical, mechanical and chemical effects 

 on the sensor of: (l) pressure, (2) temperature, 

 (3) fouling and (k) corrosion. 



PLATINUM ELECTRODE CONDUCTIVITY SENSORS 



Glass conductivity cells with platinum elec- 

 trodes have been used with considerable success 

 by a number of workers^-' ^, 3 > 4, 5 ^ n laboratory 

 type salinometers and conductivity measuring 

 devices where they have not been subjected to 

 organic and inorganic fouling and extreme environ- 

 mental conditions. In some cases they have been 

 used successfully at seaP>7 but only on instru- 

 ments that are lowered from ships for relatively 

 short periods . 



In all these instruments an accurate check on 

 cell drift is made by the use of standard sea 

 water. However, the problems of electrode 

 fouling and the impracticability of filling an 



electrode type cell mounted on an ill situ instru- 

 ment with standard sea water make them unsuitable 

 for long term use in in situ devices. 



CAPAC IT IVELY- COUPLED CONDUCTIVITY SENSORS 



Capacitively-coupled electrodes have been 

 demonstrated experimentally 8,9 but have not 

 been used in a workable field instrument. This 

 type of electrode does offer freedom from the 

 degrading effects of surface films but requires 

 the use of very high frequencies (typically 

 10 Mcps) which create severe problems when 

 accurate measurements are to be performed in an 

 in situ instrument . 



INDUCT IVELY- COUPLED CONDUCTIVITY SENSOR 



Consideration of the inductively-coupled 

 sensor-'-^' H>3.2, 13,14 indicates that the effects 

 of pressure on the "cell constant" can be made 

 small and highly predictable. Fig. 1 shows a 

 typical sensor using the inductively-coupled 

 principle. Sensors similar to this have been 

 used successfully by the author and others at 

 Woods Hole Oceanographic Institution- 1 -^ to depths 

 of 18,000 feet. 



It can be shown that if the center hole of the 

 sensor is fitted with a Pyrex glass tube arranged 

 to have the same pressure on the outside diameter 

 as on the inside diameter, the change in dimen- 

 sions of this tube due to the bulk modulus, i.e., 

 compressibility, of the glass would be negli- 

 gible. Taking a value of 3 x 10" psi as a mini- 

 mum for the bulk modulus, K, of glass, the total 

 volume change, dV, of a tube would be given by: 



dV 

 V 



(1) 



where P is the ambient pressure, 

 of 1,000 feet, P = kk-0 psi and 



For a depth 



dV 



V 



3 x 



kkO 1 



10 6 = 6,800 



(2) 



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



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