TIMERS, COUNTERS AND RATE MEASUREMENT 



{Figure 34.28). The total is indicated digitally in a small window up to 

 a maximum of 9,999. These devices are electromagnetic and have coils 

 wound for a variety of impedances; some are only of a few ohms and are 

 not of much use in conjunction with electronic apparatus except via a relay. 

 There is, however, a 500 ohm model which operates on a current of 100 mA, 

 suitable for inclusion in the anode circuit of a small thyratron, and a 2,300 

 ohm type which is operated by 30 mA, appropriate for control by a power 

 valve. 



Drop counters 



Drops are counted either conductimetrically or photoelectrically. In 

 the former method drops are used to bridge a pair of electrodes enabling 

 a current to flow which causes, directly or indirectly, the operation of the 

 indicating device. A simple direct system, intended for kymograph marking, 

 is shown in Figure 41.6, in which the indicator is a high-resistance post 



Cl 



o 



a 

 o 



Q. O 



O =s 



o4H 



a 

 o 



1111 



Inter-electrode 

 resistance 



120V. HT 

 batteries 



Figure 41.6 



Amplifier- 

 output 



"V — r 



y 



Time 

 Figure 41.7 



office relay with a light pointer attached to the clapper. A similar arrange- 

 ment, but mains driven, has been demonstrated by Bernstein and Betts^. 

 The difficulty with simple counters such as these is the damage done by 

 electrolysis to the body fluid being measured, rendering it unsuitable for 

 recirculation or analysis. In addition, mechanical obstruction arises in 

 the counting cell due to the generation of foam by electrolysed gases. More 

 sophisticated counters employ a valve or thyratron to control the indicator; 

 in this way the current passing through the fluid can be made much smaller. 

 The mechanical design of drop-counter cells is quite difficult. It must be 

 realized that the device has to accept fluids of a wide range of viscosities 

 and delivery rates. Typically, trouble may be experienced from a drop 

 bridging the electrode gap before its predecessor has fallen off". Campbell 

 and Gilmour^ overcame this by arranging the electronic circuit to respond 

 to rate of change of inter-electrode resistance rather than to the resistance 

 itself. They achieved this by using a capacitor-coupled valve amplifier with 

 a time constant shorter than the shortest interval between drops {Figure 

 41.7). For applications where even the minutest current through the fluid 

 cannot be tolerated the photoelectric drop counter may be used. Hilton 

 and Lywood'^ may be consulted. These authors report that their apparatus 

 successfully counts drops even of transparent, colourless fluids. 



646 



