187 



the mass characteristics of shallow inshore waters, where lateral and vertical 

 gradients are large and change quickly as a result of water movement. At a 

 particular location and depth, changes of salinity of as much as a few parts per 

 mille may occur within as many minutes. An instrumental accuracy of a few 

 tenths of a part per mille of salinity is more than adequate to track the movement 

 of these "fronts" between water parcels, and the information immediately avail- 

 able on the oceanographic vessel can be used to guide the search pattern.* Slow- 

 er conventional techniques using reversing bottles, although affording higher ac- 

 curacy for a particular impounded sample, may not be any more representative 

 of the surroundings and may be inadequate because of the length of time involved. 

 On the other hand the STD with its limited accuracy is not suited for measuring 

 the detailed structure and temporal changes in the deeper water of inshore basins 

 or the open sea where changes of a few hundredths of a part per mille may be 

 quite significant. Dr. Carritt has stressed the importance of vessel motion and 

 accelerations on instruments taken to sea. Let us also keep in mind that the 

 motions and accelerations within the sea itself may seriously affect the measured 

 values. Both must be considered in tailoring an instrument for a particular sea- 

 going job. Further, greater precision generally means more time required to 

 reach equilibrium, greater instrumental fragility, less stability and considerably 

 greater cost. The equipment should not be expensively "overdesigned" for pre- 

 cision where changing ambient conditions or other variables nullify or seriously 

 cloud the interpretation of the more precise values. 



From the operator's standpoint, Dr. Carritt has stressed the need of 

 simplicity in seagoing instruments, and has implied in his statement of a "free 

 hand to serve as an anchor" a greater potential operator fatigue at sea than in 

 the shore-based laboratory. That this greater fatigue is very real can be attest- 

 ed to by all carrying out chemical measurements at sea and particularly those 

 suffering sea sickness. Considering the operator as well as the scientific ob- 

 jectives I would like to amplify the comments on the use of the Wenner bridge to 

 determine salinity. Measurements by the Coast Guard in routine operations at 

 sea show a precision of about j^O.004 °/oo in salinity as reported by Soule and 

 Barnes (1950). In a series of 38 double measurements on Copenhagen Standard 

 water, batch P-15, a precision of +0.00Z o/oo was obtained. The accuracy de- 

 pends largely upon the constancy of the ratio of the conductivity to chlorinity at a 

 given temperature, good temperature control, the uniformity of the slide wire, 

 and the accuracy of the titrations upon which the calibration of the slide wire is 

 obtained. The usual Knudsen titration is less precise than the conductivity 

 measurements, but these can be made in port and the bridge checked over long 

 periods at sea using the Copenhagen Standard water and oil sealed sub-standard 

 sea water. 



The latter if suitably stored in five gallon lots has been found to drift at 

 a rate not exceeding about 0.002 °/oo salinity per month. Operations at sea are 

 simplified over any titration nmethod in that only one solution, the sea water, is 

 involved in a single measurement and its volume does not need to be precisely 

 measured. Enlisted men of the Coast Guard were trained to run the bridge in a 

 small fraction of the time required to teach titrations; they would carry out 

 nneasurements in alnnost any sea conditions, and operator error was almost en- 

 tirely eliminated. An individual measurement now requires about 3j nninutes 

 but could be cut about 25 per cent by the use of additional cells. Balancing the 

 modified Wheatstone Bridge through the use of a servo system would make the 

 operation automatic to the extent that only the samples would need to be added 

 and removed and the readings recorded. Although, as Dr. Carritt innplies. 



* - Editorial note: See conaments by Dr. Carritt immediately following. 



