TM No. 377 



*5 ~***L' (H-28) 





The ratios of X divide out, since the same impeller is used in "both air 

 and water. Thus : 



T^M-Kv^ - (n-29) 



For air at 20° C, J A = I.83 x 10 poises. For sea water at 15 C, 

 7 W = 1.00 x 10"2 poises. Thus, for a time constant of 3.5 seconds in 

 air, equation 11-29 "becomes T (W) *<* 6k milliseconds. Five estimates were 

 made of the impeller time constants, involving response of the u and w 

 meters to flow in both directions. The results are tabulated in table II-4. 



Average value T r (W) = 66.5 ms. 

 Standard deviation 0^_= + 8.9 ms. 



These values seem reasonable in view of the previous estimates of T 

 for OMDUM I and II, and are well within the desired range for measurements 

 of ocean wave oscillations. 



Amplitude of Induced Voltage as an Analog of Flow. Turning again to 

 the principle of the voltage output of the wave meters, let <pM be the 

 number of lines of magnetic flux per unit area emanating from each magnet 

 inbedded in the impeller blade of a wave meter. If the miniature coil of 

 the meter contains N turns, then the voltage pulse generated by the magnet 

 passing the potted coil is (according to Faraday's law): 



d( t>t> (n-30) 



£(*) = « -g 



In other words, the instantaneous voltage generated is proportional to the 

 rate at which the magnet passes the coil. This rate, in turn, is propor- 

 tional to the angular velocity of the impeller idfc). Thus : 



41 



