Sec. 1-6] 



MECHANICAL INPUT TRANSDUCERS 



12] 



where B is the flux density in gauss, d the distance in centimeters 

 between the electrodes, i.e., the inner diameter of the tube, and v the 

 average linear velocity of the liquid in centimeters per second. Equa- 

 tion (1) can be transformed into 



BF 4 



~T x 



a tt 



io- 8 



volts 



(2) 



Electrode 



where F is the average flow rate of the liquid in milliliters per second. 



The method has been perfected primarily by the work of A. Koliu and 

 collaborators. See literature references until 1951 in W. (I. James, Rev. Sci. 

 Instr., 22, 989 (1951). 



The magnetic field is usually of the order of 1,000 oersteds and may 

 be continuous (direct current) or alternating (alternating current). 

 In the first case the output voltage 

 e will be a d-c voltage; in the 

 latter case it will be an a-c voltage, 

 and e will denote the instantan- 

 eous value if B denotes the in- 

 stantaneous value of the flux den- 

 sity. The a-c method reduces the 

 danger of electrolytic polarization 

 at the electrodes if the frequency 

 is sufficiently high and permits the 

 use of a-c amplifiers in subse- 

 quent stages. At the same time, it 

 introduces in the output parasitic 

 a-c voltages from capacitive a-c 



pickup and from asymmetric induction in the liquid and in the leads. 

 The magnitude of these voltages increases with the frequency. 



The transducer can be used for flow rates from the order of 10 -3 

 mm/sec up to the highest values. According to Thurlemann 1 and 

 Kolin, 2 the transducer output is independent of the type of flow 

 (laminar or turbulent) and of the form of the velocity profile. How- 

 ever, Shercliff 3 has shown that any axial asymmetry may severely 

 influence the output. If the flow velocity near one electrode is appre- 

 ciably higher than the average velocity, the sensitivity of the trans- 

 ducer (S = Ae /Av) can be increased by a factor as high as 2, and if 

 the flow is concentrated near the wall between both electrodes, the 



1 B. Thiirlemann, Helv. Physica Acta, 14, 383 (1942). 



2 A. Kolin, Rev. Sci. Instr., 16, 109 (1945). 



3 J. A. Shercliff, J. Appl. Physics, 25, 817 (1954), and J. Sci. Instr., 32, 441 

 (1955). 



Electrode 



Fig. (1-6)3. Flow-velocity transducer 

 based upon electromagnetic induction 

 in liquids. 



