1.0 



0.9 



ce 



Pd 0-8 



in 



2 0.7 



% 0.6 

 Q 0-5 

 N 0.4 



O 



z 



0.3 



0.2 



0.1 







10 20 30 40 50 60 70 80 90 



p (DEGREES) 



Figure 7. Directional response of Marine Advisers Inc. ducted current meter Q-8 

 (data taken from Marine Advisers polar plot, operating instructions manual). 



SUMMARY 



The use of impellor meters for shallow-water oceanographic measurements 

 is not recommended. Nonlinear outputs give rise to false dc levels when filtered. 

 Threshold and calibration changes at low velocities, caused by fouling, would 

 probably be serious, and the noncosine response is a disadvantage. The use of 

 two or three meters in the impellor system would result in hydrodynamic interference 

 between the meters. 



Electromagnetic 



Electromagnetic flowmeters, or induction meters, have been towed from 

 ships for oceanographic measurements or used to measure flow in channels. 

 One possible oceanographic application concerns the flow in tubes with artifi- 

 cially generated magnetic fields, where the tube gives the meter some directional- 

 ity. A typical transverse magnetic-field configuration is shown in figure 8. Elec- 

 trodes are mounted flush with the inside of the pipe wall and are in contact with 

 the fluid. The line joining the electrodes is perpendicular both to the magnetic 

 field and the flow. Magnetic fields may be ac or dc, although with electrolytic 

 solutions such as sea water, ac fields reduce electrode polarization. 



Analytical work with these meters assumes that the fluid is nonmagnetic 

 with the same permeability as a vacuum; conductivity of fluid, ct, is assumed 

 isotropic. The Hall effect is neglected, and variations in conductivity and 

 thermoelectric effects are assumed negligible. The conductivity of the pipe walls 



15 



