120 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



method of injecting the salt solution (single and multiple injectors, 

 symmetric and asymmetric) is of negligible influence. 



The electrolytic-tracer method has been used by Prausnitz and 

 Wilhelm 1 for measuring the turbulent concentration fluctuations 

 and, thus, the variation with time of turbulence in a liquid. An 

 electrolyte (hydrochloric acid-methanol mixture) is continuously in- 

 jected into a stream of tap water. The conductivity fluctuations are 



measured downstream by a very 

 small probe consisting of two ends 

 of platinum wire (1.2 mm long and 

 1 mm apart) protruding from the 

 end of a tapered glass tube. The 

 Y output signal (variation of resist- 

 ance with time) is electronically 



r 2 



Fig. (1-6)2. Plow-velocity transducer processed and furnishes informa- 



for liquids, electrolytic dilution method. tion for the study of turbulence. 



b. Electrolytic-dilution Method. 

 A salt solution of a concentration C 1 is introduced at a steady and 

 known flow rate F x into a stream of a liquid of low conductivity 

 (water) flowing with the flow rate F z . After mixing, the concen- 

 tration is 



^2=^1^ (1) 



The concentration C 2 is determined at a point downstream by elec- 

 .trolytic-resistance measurement (or by any other chemical or optical 

 method), and the unknown flow rate F z is determined from Eq. (1). 



The method has been investigated by Kirschmer and Esterer, op. cit., p. 1505, 

 and found to be very accurate; the error for large flow rates was found to 

 vary between 0.03 and 0.06 per cent. 



1-62. Liquid-flow Induction Transducer 



The liquid, which should have a conductivity of at least 10 -5 

 ohm _1 -cm _1 , flows through an insulating tube as shown in Fig. (1-6)3. 

 The tube lies in a magnetic field of a flux density B. Two electrodes 

 are inserted into the tube, their surfaces flush with the inner surface 

 of the tube. If the liquid flows through the tube with the average 

 velocity v (averaged over the cross section of the tube), charges 

 within the liquid are moved to the electrodes, and a potential dif- 

 ference arises between both electrodes which is 



e = Bdv X 10" 8 volts (1) 



1 J. M. Prausnitz and R. H. Wilhelm, Rev. Sci. Instr., 27, 941 (1956). 



