7. Linear output. 



a. Dc levels not connected with swell amplitudes. Because non- 

 linearities are a function of amplitude, the swell magnitude 

 can generate different, false dc signals in some cases. This 

 is particularly important when looking at diurnal signals. 



b. Intermodulation distortion from nonlinearities, where more than 

 one strong frequency is present in the natural spectrum, should 

 be low. 



8. Long-term stability. The ability to check and adjust (or record) the 

 zero and sensitivity is a necessary feature. 



9. Electrical output. 



10. Size that will permit testing in tow tanks. 



FLOWMETERS 



Acoustic 



Acoustic flowmeters are of two basic kinds: (1) those that measure time 

 difference of sound travel between two points,'^''* and (2) those that measure the 

 Doppler shift. 5 '6 



TIME DIFFERENCE 



Two transducers, alternately radiator and receiver, permit finding the dif- 

 ference in transmission time for the two directions (fig. 1). Two radiators and two 

 receivers allow pulses to be sent simultaneously in both directions (fig. 2); two 

 closely spaced, parallel acoustic paths are used. 



A desirable feature of acoustic meters is their lack of moving parts. Out- 

 put linearity is better than 1 percent and with this type is independent of the speed 

 of sound in the medium assuming identical acoustic paths. With meters using only 

 two transducers, this assumes the same acoustical path for a time duration greater 

 than the double transmission time. In two parallel paths, this assumes identical 

 conditions over the enclosed space during the simultaneous transmission time. 

 Under conditions of strong thermal gradients, instantaneous velocity errors up to 

 10 centimeters per second may occur.'* 



