'RADIATOR/ 

 RECEIVERS 



'^'igure 1. Time-difference acoustic meter, one path. 



Figure 2. Time-difference acoustic meter, two paths. 



Such temperature gradient effects would be serious if the meter were used 

 to measure velocities associated with internal waves. Apparent velocity changes 

 could be caused by the temperature gradients associated with the internal waves. 

 The use of a meter with two transducers and a single path reduces this error. The 

 typical ranges are from a few tenths of a centimeter per second to several hundred 

 and overall accuracies 1 to 2 percent of full scale. However, there is a requirement 

 for stable, expensive electronics because the differences in transit time are gener- 

 ally less than a microsecond. The instrument is inherently capable of sensing 

 direction as well as magnitude of flow, but its use in oscillating flow can be 

 severely limited by flow disturbance. Vortex shedding from the transducers pro- 

 duces high-frequency velocity fluctuations. Some meters may be zeroed in the 

 field in the presence of flow, a necessary requirement for long-term measurements. 



DOPPLER SHIFT 



The Doppler acoustic meter depends on volume reverberation and a result- 

 ing frequency shift (fig. 3). Acoustical energy is transmitted in a narrow beam 

 from one transducer. A receiving transducer having a similar beam pattern is posi- 

 tioned so that the two beams overlap. Typical volumes intersected are a few cubic 

 centimeters at a distance of 50 centimeters from the transducers. The frequency 

 of the reflected signal from the intersected volume filled with natural small scat- 

 terers is compared with the transmitted frequency. The average velocity of the 

 scatterers is assumed to be that of the fluid. The frequency difference is propor- 

 tional to the fluid speed in the direction of the beam. 



