INTRODUCTORY REMARKS 



FOR 



PAPERS ON ELECTRICAL CONDUCTIVITY, SALINITY AND DENSITY 



R. G. PAQUETTE, Session Chairman 



Defense Research Laboratories 



General Motors Corporation 



Santa Barbara, California 



As an introduction to the papers presented on 

 electrical conductivity, salinity and density, 

 it seems appropriate to dwell briefly on accuracy 

 required of transducers for measuring these 

 parameters. The term "accuracy" is used in its 

 scientifically accepted sense as associated with 

 a 95% probability. 



SALINITY 



When asked about his requirements for accuracy 

 in oceanic measurements of salinity, the oceanog- 

 rapher usually will give almost a stock answer 

 of iO.Ol parts per thousand. Since the salinity 

 of the ocean is about 35 parts per thousand this 

 represents an accuracy of about 1 part in 3; 500. 

 Actually, for many purposes, a lesser accuracy 

 is permissible; in fact, until the advent of the 

 salinity bridge in 1956 the accuracy was about 

 0.03 parts per thousand or less although few 

 oceanographers clearly recognized that it was so 

 poor. For other purposes, especially any studies 

 in the deep ocean where gradients and temporal 

 changes are extremely small, 0.01 parts per 

 thousand accuracy may be marginal. In large 

 deep estuaries accuracies of about -0.1 parts 

 per thousand are desirable and in estuaries 

 strongly influenced throughout most of their 

 depths by fresh water, accuracies of *0.5 parts 

 per thousand may be permissible. 



temperature is equivalent to salinity changes of 

 about O.'j- parts per thousand salinity per degree 

 C, precise control or measurement of temperature 

 would be required. 



ELECTRICAL CONDUCTIVITY 



The electrical conductivity of sea water is of 

 little interest in itself; it is normally used to 

 determine the salinity. The electrical conduc- 

 tivity of sea water in millimhos is roughly the 

 same as the salinity in parts per thousand, 

 nominally 30 with a marked dependence on tempera- 

 ture. The conductivity increases with tempera- 

 ture by about 2% to k$ per degree C. To obtain 

 an accuracy of id. 01 parts per thousand in 

 salinity requires a temperature measurement 

 accuracy, or at least a long term reproducibility, 

 of about 0.01°C if all the error is due to tem- 

 perature. The existence of other errors forces 

 distinctly higher requirements on the temperature 

 accuracy. Again, it should be evident that there 

 is not much hope in making absolute measurements 

 and that differential measurements referred to a 

 stable temperature-compensating reference stan- 

 dard will be required. The electrical conduc- 

 tivity also has a pressure coefficient of the 

 order of 0.0005% per psi so that at any great 

 depth provision must be made for a pressure 

 correction. 



DENSITY 



CONCLUSION 



It is sometimes implied that the oceanographer 

 measures salinity only to obtain density. This 

 is not true. If he could measure density to the 

 required precision he could deduce circulation 

 patterns by the geostrophic method, but there 

 are other oceanic processes which would still 

 require salinity measurement for their descrip- 

 tion. Consequently, there is not as much 

 inducement to produce an in situ density probe 

 as a salinity probe. If density is to be 

 regarded as an indirect route to the measurement 

 of salinity, note must be taken of the fact that 

 an accuracy of 0.01 parts per thousand in 

 salinity corresponds to an accuracy of 7 x 10" 

 gm/cm3 in density. Such accuracies probably 

 can only be obtained by differential measure- 

 ments . Since the density dependence on 



,-6 



These are difficult requirements, all the more 

 so because they must be met under rugged ship- 

 board conditions and in the hands of technicians 

 who are not experts in the art of measurement. 

 I should like to remind you that in demonstrating 

 the utility of a measuring instrument there is a 

 great deal of difference between 20 measurements 

 in a laboratory and a year of carefully docu- 

 mented testing at sea. There is also an impor- 

 tant difference between the instability and incon- 

 venience which will be tolerated by the enthusi- 

 astic inventor and that which will be accepted 

 by the working scientist at sea. I hope we can 

 begin to see more of thoroughly demonstrated 

 instruments in distinction from those which are 

 so inadequately tested that they are merely 

 interesting. 



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