126 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



commutator switch. The difference in transit time is measured by a 

 phase-sensitive detector driven synchronously with the commutator 

 switch. 



Linear velocities in water as low as 0. 1 cm/sec have been measured 

 successfully with this system. The deviation from linearity is indi- 

 cated to be less than 2 per cent. The dynamic response is limited only 

 by the frequency of the synchronous commutator. The system when 

 applied to the outside of the conduit causes no obstructions or dis- 

 continuities in the flow system, but its application is limited to 

 conduits with plastic walls or to large conduits where the transmitter 

 and receiver crystals can be arranged within the conduit. The 

 measurement of flow in metal pipes with an arrangement outside of 

 the pipe is not possible, because of the direct transmission of sound 

 through the metal wall of the pipe, unless special means are used to 

 suppress the influence of this effect. 



1-65. Radioactive-tracer Methods 



Both the electrolytic-tracer method and the electrolytic-dilution 

 method described in 1-61 can be modified b}^ replacing the electro- 

 lyte by a radioactive solution and deter- 

 mining the passage of the cloud or the 

 concentration of the liquid with nuclear- 

 radiation detectors. 



A method suitable for the determination 

 of the velocity with which a liquid runs out 

 of a vessel has been described by Richardson. 1 

 The method is applicable where the velocity 

 profile has the general form shown in Fig. 

 (1-6)8. 



The lower part of a vertical tube is filled 

 with a radioactive-tracer solution, the part 

 above it with a tracer-free solution. As the 

 former runs out and is displaced by the latter, 

 the counting rate measured at a point A 

 outside of the tube decreases. 

 If v v is the linear flow velocity at P and v the velocity in the center 

 of the conduit, their ratio is 



Fig. (1-6)8. Flow-velocity 

 transducer for liquids, 

 radioactive tracer method. 



V, 



y 



Id 



(1) 



1 F. M. Richardson et al., Nucleonics, 13, 21 (July, 1955). 



