170 



Temperature 



BRIDGE PHASING 

 NETWORK 



600a BALANCING 

 POTENTIOMETER 



NICKEL WIRE 

 THERMOMETER 



Conductivity 



BRIDGE PHASING 

 NETWORK 



CABLE PHASING 

 CAPACITOR 



BALANCING 

 POTENTIOMETER 



CONDUCTIVITY 

 CELL 



CELL a CABLE 

 PHASING CAPACITOR 



Fig. 2. Schematic equivalent cir- 

 cuits of CTI temperature and conduc- 

 tivity units. Commercial components 

 have been used where possible. 



in construction. Three CTI instru- 

 ments have been constructed and have 

 been in use for about three years. The 

 operating characteristics of the CTI are 

 discussed in a later section. 



Von Arx (1947) described A Sal- 

 inometer for Use in Brackish Water. 

 The instrument is unique in that its re- 

 sponse to variations in salinity depends 

 upon the behavior of a pair of electrodes 

 that have been subjected to extreme but 

 controlled polarization when immersed 

 in sea water. The electrodes, one of 

 copper and the other of zinc, become 

 coated with relatively insoluble salts of 

 the metals during polarization, ZnC03 

 on the zinc and mixture of basic carbo- 

 nate and hydroxide on the copper. Thus 

 a "battery" is formed, the potential of 

 which is controlled by electrode reac- 

 tions of the type; 



Zn-l:^ Zn*^-*- 2e and Cut:; Cu'''^2e. 

 Since the composition and solubility of 

 the solids formed during polarization 

 are nearly independent of salinity over 

 a wide range of dilution, the potential 

 of the "battery" will be nearly constant 

 regardless of the salinity of the water 

 in which polarization occurred. 



When used to measure salinity, 

 the "battery" is connected across the coil of a milliamnneter and with this con- 

 stant external load the current through the circuit is limited by the resistance 

 of the sea water path between the electrodes. Thus, the current drawn from 

 the cell can be used as a measure of salinity, since variations in the latter con- 

 trol the internal resistance of the cell. 



A plot of current vs. time for the discharge of a pair of polarized elec- 

 trodes resembles the familiar discharge curve of a storage battery. The cur- 

 rent is initially high, drops rapidly to a plateau, and then falls rapidly again. 

 The magnitude of current in the plateau region has been shown to be an exponen- 

 tial function of salinity. Unfortunately, the duration of the current plateau is 

 short, one to two minutes depending on the salinity. It becomes necessary then 

 to repolarize the electrodes frequently during use. Irreversible changes in the 

 electrodes may occur unless repolarization is controlled. Ideally, the elec-^ 

 trodes should be "backed up" until the discharge current during the first part of 

 a current reading corresponds to the lower part of the first knee of the dis- 

 charge curve. 



The effects of variations in temperature have been very nearly eliminat- 

 ed by placing a resistance in series with and mounted directly on the electrodes. 

 The compensating resistance was constructed to have a temperature coefficient 

 of equal magnitude but of opposite sign to that of the electrical resistance of sea 

 water. It seenns reasonable to assume that the current plateau in the discharge 

 curve might be extended in time by providing a means of mechanically entrap- 

 ping polarization products at the surface of the electrodes. 



