INTERSECTED VOLUME 



TRANSDUCERS 



Figure 3. Doppler acoustic meter. 



Transmitted frequencies must be very stable; with typical values from 3 to. 

 10 megacycles per second (Mc/s), the resulting Doppler shift is only a few hundred 

 cycles per second (c/s). The method is capable of determining direction as well 

 as magnitude. In view of the meter measuring the velocity some distance from it- 

 self, little hydrodynamic disturbance might be expected, but this is only so when 

 the meter is used in unidirectional flow and oriented to point into the flow. In 

 oscillating flow, large distortions can result from disturbance of the flow about 

 the case. Multiple sets of transducers mounted in a symmetrical case could con- 

 ceivably be used, but would complicate matters considerably. 



Typical accuracies of the Doppler meter are 1 to 2 percent of full scale, 

 and ranges are from a few tenths centimeters per second to several hundred. The 

 computed flow rates are a function of the speed of sound, and corrections are 

 needed. The directional response is unknown. The meter will respond to high- 

 frequency fluctuations. 



SUMMARY 



The time-difference and Doppler-shift acoustic flowmeters are more suita- 

 ble for constant-temperature, imidirectional flow, laminar or turbulent, than for the 

 required need. Temperatui'e effects (zero drift and multipath propagation); inability 

 to measure and set the zero in some cases; hydrodynamic interference with the 

 flow; and uncertain direction response pose serious problems. 



