III. REVIEW OF VELOCIMETERS 



A. Introduction 



The existing instruments belong essentially to two classes: 

 laboratory apparatus and instruments for oceanographic survey work. There 

 have been prototype expendable instruments constructed. The measurement 

 in the laboratory of the speed, possible dispersion, and absorption of 

 sound waves does not concern us here directly, even though such results 

 are of course the basis for the design and in particular for the calibra- 

 tion of all field instruments. Equation (1) describing the speed of sound 

 in sea water as a function of temperature, salinity, and pressure, is the 

 result of laboratory velocimetry and the relative sizes of its terms 

 indicate the precision required in the measurement of the respective 

 parameters. 



Velocimeters for oceanographic survey work, Table III-l, (Ref- 

 erence 26, 27, 28) require almost the same accuracy as the laboratory 

 apparatus, because the results obtained in the ocean are of course to be 

 compared with laboratory data and with other oceanographic data obtained 

 at different locations and times. In addition, scientific velocimetry in 

 situ will in general require the simultaneous measurement of other parameters 

 in equations of the type of (1) . When the speed is being measured and the 

 velocity profile is therefore known, the true depth can of course be ob- 

 tained very accurately from sonic measurement, as for example by inverted 

 echosounder. The instruments must be capable of maximum depths, they must 

 be repairable in the field, and their power supply must be capable of pro- 

 longed operation. On the other hand, the price for such oceanographic 



26 



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