O-EIUG SEAL 



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6LECTCICAL COUUECT HOLE 

 ^■TdEAHL-ltLjeO STEUT 



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Fig. 1. Conductivity sensor for in situ salinity system. 



Since the dimensional changes are equal in all 

 directions, the change in length and circumfer- 

 ence will each be one-third the proportional 

 change in volume . Since the "cell constant " is 

 proportional to area/length, the change in "cell 

 constant" will be one-third the proportional 

 change in volume, i.e., 1 part in 20,000 at a 

 depth of 1,000 feet. Obviously the use of a 

 glass tube in this manner would mean that a direct 

 electrical path between the outside of the glass 

 tube and the inside of the insulated steel liners 

 (Fig. l) would have to be prevented. This tech- 

 nique is the one used successfully at the Woods 

 Hole Oceanographic Institution to depths of 

 18,000 feet. 



If the glass tube is made from Pyrex, the 

 effect on the "cell constant" due to changes in 

 dimension caused by changes of temperature would 

 also be directly related to the linear changes. 

 Taking a value of 3-2 x 10"°/°C as the tempera- 

 ture coefficient of Pyrex, the linear changes, 

 and consequently the "cell constant," over a tem- 

 perature range of ±15°C will be ±1+8 parts per 

 million, i.e., approximately ±1 part in 20,000. 



Fouling due to the deposition of thin films, 

 on the surfaces of the sensor have no significant 

 effects. Fouling due to the growth of marine 

 organisms in the center hole of the sensor would 

 be a problem if it became so gross as to change 

 significantly the effective dimensions of the 

 center hole. It should be noted that any system 

 yet devised senses the conductance of a par- 

 ticular geometrical configuration of the con- 

 ducting medium and not its specific conductivity. 

 This means that any type of sensor will suffer 

 from the effects of gross fouling. However, 

 electrode type sensors are affected by minute 

 surface films on the metal electrodes whereas 

 the capacitively and inductively-coupled sensors 

 do not have this problem. Also, the inductively- 

 coupled types have better electrical 



characteristics if the center hole is large, 

 whereas the electrode cells suffer from an 

 unfavorable ratio of "polarization" resistance 

 to overall resistance if the inside diameter of 

 the cell is made very large. The use of larger 

 diameter center holes in inductively- coupled 

 sensors make them less affected by fouling 

 organisms and easier to maintain in a clean con- 

 dition than would be the case if a small hole 

 were used. 



EFFECT OF TEMPERATURE OH THE CONDUCTIVITY OF SEA 

 WATER 



The effect of temperature on the conductivity 

 of sea water is very large. For a temperature 

 change from to 30°C the conductivity of sea 

 water approximately doubles. This means that the 

 temperature effect must be accounted for very 

 accurately if precise salinity information is to 

 be obtained. The following are some of the tem- 

 perature compensation techniques that are to be 

 considered. 



Computation of Salinity 



The first method used to measure salinity in 

 situ involved computation of salinity from mea- 

 sured values of conductivity and temperature. 

 However, the accurate determination of salinity 

 requires a very accurate measurement of tempera- 

 ture as well as conductivity. Since the compu- 

 tation is rather complex, a digital computer 

 would be necessary to handle even small amounts 

 of data. 



Thermistor Compensation 



The earliest methods of automatic temperature 

 compensation used a thermistor-resistor network 



20 



