TM NO. 377 



the initial goal is to make simple measurements with simple sensors without 

 having to solve intricate problems of elaborate instrumentation development. 

 This should come later. 



There are four general categories of sensors from which to choose : sea 

 water emf induction systems (Bowden and Fairbairn, 1956); acoustic flow meters 

 (Chelupmik and Green, 1962, and Lester, 1961 )j force deflection or thrust systems 

 (Flow Corp*, 1965); and the common impeller devices. 



Each of these instrument categories has definite drawbacks. Both the 

 induction and acoustic systems are relatively untried. They have met with 

 difficulties involving the stability of the electronics and are very expensive 

 to fabricate. The force response meters (i.e., drag or thrust spheres and 

 deflecting plate systems) are difficult to calibrate and expensive. Although 

 they have been successfully developed for the atmosphere (see Boe, 1963)5 they 

 have met with electronic difficulties when placed in the turbulent ocean 

 medium (Zeigler, 19&3)* Use of these three types of systems in the measurement 

 of oceanic turbulence is certainly not precluded, but a major development effort 

 would be required. 



The impeller devices offer the attributes of rapid and atable response 

 combined with simple and inexpensive electronic packaging. A continuous 

 recording of fluid flow can be made with the impeller flow meter by coupling 

 the propeller magnetically to a magnetic =pickup amplifier circuit or to an 

 induction coil. The pickup coil must be isolated electrically (but not 

 magnetically) from the sea water. 



The most common types of rotor or impeller devices are the Von Arx meter 

 (Von Arx, 1950) and the Roberts-type current meters, all of which are really 

 electronic variations of the mechanical Ekman meter (Ekman, 1926). These 

 instruments utilize an impeller mounted on a horizontal axis. An electrical 

 swivel and a tail -vane arrangement (figure II-l) permit the axis of the impeller 

 to seek the current direction. The speed of flow is registered by the frequency 

 of the impeller revolutions. The magnitude of the emf (or, in the case of the 

 Roberts meter, the frequency of the inductive voltage pulses) provides an analogous 

 relation for speed of the current. The relationship of the angular velocity of 

 the impeller (and hence, the induced emf) to the flow speed is usually linear. 

 In some meters the direction of flow is detected by an electrical analog 

 comparison of the compass heading with the vane heading. 



Analysis of a current vector in two dimensions (in the XY plane) requires 

 a pivoting=vane type instrument that can rotate in response to the oscillatory 

 motions of the waves. Since available pivoting impeller devices are 50-80 cm 

 in length, the directional time response of the instrument would be strongly 

 limited by the large moment of inertia. Moreover, there would be no response 

 to oscillatory motions having a scale diameter less than the length of the 

 instrument. It is quite obvious, therefore, that without extreme modifications 

 and miniaturization, pivoting impeller-type meters would be undesirable for 

 measuring wave motions. 



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