Sec. 1-6] 



MECHANICAL INPUT TRANSDUCERS 



119 



B 



N 

 A 

 \ 



B 



^ 



A 7 ' 



& 



h 3 



y~K_ 



:*) 



The method has been primarily applied for large conduits (order 

 of magnitude of L is 50 to several hundred meters) and for flow rates 

 of the order of several cubic meters per second. The salt-solution 

 injector system for such large channels can be quite complex. 



Errors are caused through differences in velocity of the particles 

 in the conductive cloud (longitudinal growth of the salt cloud as it 

 moves downstream), leading to uncertainties in the determination 

 of the time difference t, since the 

 passage of the cloud through the 

 electrodes AA or BB is not in- 

 stantaneous. Allen and Taylor 

 recommend the elimination of 

 this error by a graphic method in 

 which the time difference t is 

 measured between the center of 

 gravity of the current-time areas 

 as indicated in Fig. (1-6)1. Fur- 

 ther errors may arise through 

 inhomogeneous distribution of 

 the particle velocity and variation 

 of concentration over the cross 

 section. De Hallerin a theoretical 

 study 1 indicates that this error 

 diminishes with increased turbu- 

 lence (increased Reynolds num- 

 ber). The error for laminar flow is considerably larger than for 

 turbulent flow. In order to reduce the error resulting from the vari- 

 ation of velocity over the cross section, de Haller proposes the use 

 of a point electrode positioned at a distance of 0.75 to 0.8R (R = 

 internal radius of the conduit) from the center of the conduit if the 

 velocity profile is axially symmetric, or of a ring electrode of 0.777? if 

 the velocity profile is asymmetric. Other authors consider screens 

 covering the entire cross section of the conduit as the best form of 

 electrodes. 



Experimental studies of the salt-velocity method have been 

 carried out by Miiller. 2 In a comparative study on flow meters, 

 Kirschmer and Esterer 3 found that the total error of the method is 

 between 1.2 and 6 per cent. The error is always negative, i.e., the 

 actual flow rate is larger than that indicated by the method. The 



1 P. de Haller, Helv. Physica Acta, 3, 17 (1930). 



2 K. E. Miiller, Schweiz. Bauz., 87, 41 (1926). 



3 O. Kirschmer and B. Esterer, Z. Ver. deut. Ing., 74, 1499 (1930). 



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

 for liquids: (a) electrolytic tracer 

 system; (b) current-time function at the 

 electrodes A A and BB. 



