134 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



pulse length. In air (oxygen) the spark method furnishes ionization 

 predominantly of a negative sign and strong enough for convenient 

 measurements; but the electrodes projecting into the gas-flow con- 

 duit are likely to alter the flow pattern. Ionization by a particles 

 (modulated by rotating or oscillating shutters), by high-energy elec- 

 tron beams, soft X rays, ultraviolet radiation, and corona discharges 

 have been considered or tried but have apparently not led to practical 

 results. 



Two different types of detectors are used, the ion collector, i.e., 

 an electrode that is charged by a supply voltage and discharged by 

 the ions which it collects, and the induction electrode pickup, in 

 which, through electrostatic induction, a voltage is induced by the 

 passing ion cloud. Both types can be mounted either protruding into 

 the gas stream or flush with the wall. Figure (1-6)14 shows an ion 



, ~ 



Shield 



, ' , ' , ::: . .. : •' . , ' , 



-Insulated 

 collector 



i 



Fig. (1-6)14. Ion collector for flow-velocity 

 measurements in gases. 



collector protruding into the gas conduit. It is initially charged to a 

 high potential and discharged by the ions it attracts. Its main dis- 

 advantage is the fact that it imparts a velocity to the ions which is 

 different from that of the neutral gas molecules. The output signal 

 from such a collector (when connected to an amplifier with 10 MQ. 

 input impedance) is of the order of 1 mV. The duration of the pulse 

 is about 0.1 msec. The flush-mounted induction pickup is generally 

 preferred, its influence upon the flow pattern is negligible, and its 

 output signal permits accurate definition of the passage of the ion 

 cloud over the pickup electrode. It is insensitive, however, to ion 

 clouds having exactly balanced positive and negative charge distri- 

 bution. The electrode can be formed either by a section of the pipe 

 or by a metal plate or button set flush with the wall and insulated 

 from the rest of the pipe, as shown in Fig. (1-6)13. 



The signal definition depends upon the width of the electrode in 

 the flow direction. For the cvlindrical electrode, a width of 20 to 30 



