171 



The instrument is extremely portable and well suited to use in small 

 boats or as a pack load. A single case contains all of the meters, switches, and 

 batteries as well as storage space for the electrodes and cable. A calibration 

 curve shows its most useful range to be from °/oo to 20 °/oo salinity in which 

 range the probable error in salinity is approximately\+^ 0.2 °/oo. 



Parrack and Jensen, referred to in a Texas A. and M. progress report 

 (1952), give a brief description of a recording salinity meter which uses a con- 

 ductivity cell as the sensing element. 



"The electrode system consists of four monel rings fixed in a glass tube 

 within the water line. A regulated A.C. voltage is impressed across a 

 resistance in series with the two outer electrodes such that the im- 

 pressed voltage is divided between this series resistor and the resis- 

 tance of the water column. This division is controlled by the conduc- 

 tance of the water column between these outer electrodes. The two in- 

 ner electrodes measure the IR drop across the water column between 

 them by use of a vacuum tube voltmeter. The variation in signal is 

 eventually read as a current on an Esterline-Angus Recording Milliam- 

 meter. It is not feasible to cover the entire salinity variation in a 

 single range, consequently four range resistors are used so as to divide 

 the variation over four ranges". 



The accuracy and long range stability of the instrument is not known, as 

 a full report has not yet been published. The electrode arrangement, similar 

 to that described by Shedlovsky (1930), achieves partial separation of the sys- 

 tem applying a potential to the cell from the system measuring the IR drop. The 

 stability of monel electrodes is questionable for copper is known to slowly dis- 

 solve from monel in sea water. 



More important than details of the construction of these in situ instru- 

 ments is the consideration of the fundamental design principles. Each of the 

 instruments mentioned gains its primary information from the electrical resis- 

 tance between a pair of electrodes placed in the water. The measurements with 

 these instruments can only be as reliable as the ability of the electrodes and in- 

 dicating devices to measure the true resistance of the solution between them. 



In a sense, many of the problems encountered in the design and use of 

 in situ conductivity instruments are similar to those met in the studies that gave 

 the absolute values of the conductance of reference solutions such as potassium 

 chloride that are now used as standards for the calibration of conductivity cells. 



Jones and his co-workers (1931, 1933), Parker and Parker (1924), and 

 Shedlovsky (1930) give detailed descriptions of the fundamentals of conductimet- 

 ric measurements. In all high precision studies, much emphasis is placed on 

 the treatment of the conductivity cell not only during platinization but also during 

 the cleaning, washing, and drying of smooth platinum electrodes prior to making 

 the conductivity measurements. For example, Morgan and Lammert (1923) 

 note that the apparent resistance of a cell having smooth platinum electrodes 

 varied by two to three per cent depending on the method used for cleaning the 

 cell. They attribute at least a part of the variation to the formation of a gal- 

 vanic cell composed of the cell electrodes and plated or absorbed contamina- 

 tions. In any event, the use of a conductivity cell over long periods without 

 calibration brings up many stability problenns not evident when such a cell is 

 used with frequent calibration. 



In the design of in situ conductance devices, many changes must be made 



