118 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



signal is of particular importance when the time average of an inter- 

 mittent or pulsating flow velocity is to be measured. If the flow 

 rate is measured by means of a nonlinear system, such as an orifice 

 and a transducer which measures the pressure difference p before 

 and in the orifice, and if the flow rate varies with time, the average 

 flow rate F can be expressed by 



F = c(V;p)av 



Since there is always an inertia in the measurement of p, it is likely 

 that not the instantaneous pressure p but the average pressure p av is 

 measured, so that 



F' = cVpw 



F' is always larger than F, sometimes by as much as 40 per cent. 



Some of the methods for measuring liquid flow can be used for the 

 flow-velocity measurement of gases as well, e.g., the thermal-, the 

 sonic-, and the radioactive-tracer methods. Anrep and Downing 1 

 have used a gas flow-velocity transducer such as described in 1-66 for 

 liquid flow-velocity measurements. The liquid enters a flask and 

 displaces the air from the flask. The velocity of the escaping air is 

 measured with a thermal transducer. 



1-61. Resistive Transducer Systems for Liquid Flow Velocity 



a. Electrolytic-tracer Method. This method, which is suitable only 

 for discontinuous measurements of the linear flow velocity and the 

 volume flow rate of water, has been described by Allen and Taylor 2 

 and is illustrated in Fig. (1-6)1. The water flows through a conduit 

 with the average linear velocity v. At the point i\ T , a shot of a salt 

 solution is injected into the stream, causing a cloud of electrolyte with 

 a conductivity higher than that of the surrounding water. The cloud 

 moves with the water; the passage of the conductive cloud causes an 

 increase of an electric current through the electrodes A A and, later, 

 the electrodes BB, which are spaced from the electrodes AA by a 

 distance L. The time difference t between these passages is measured 

 and the velocity is determined from v = L/t. If V is the volume of 

 the water between the two pairs of electrodes, the flow rate F can be 

 determined from F = V/t. The method can be modified by measur- 

 ing the time difference between the injection of the solution and the 

 passage of the solution cloud through one pair of electrodes. 



1 G. V. Anrep and A. C. Downing, J. Sci. Instr., 3, 221 (1925). 



2 C. M. Allen and E. A. Taylor, Trans. ASME, 45, 285 (1923). 



